1. Introduction
The Qinghai–Xizang Plateau serves as an important ecological security barrier for China. However, the relatively weak transportation infrastructure in Xizang severely hinders the high-quality and rapid development of the Qinghai–Xizang Plateau region. Accelerating the construction of the transportation infrastructure in Xizang, increasing the areas accessible by road, and establishing a comprehensive railway and highway network in the Qinghai–Xizang Plateau region are of great significance for promoting sustainable development and long-term stability in Xizang. Owing to natural and historical factors, the transportation infrastructure in the Qinghai–Xizang Plateau region remains undeveloped. Taking the Xizang Autonomous Region as an example, by the end of 2022, the total road mileage was only 1.201 × 105 km, with a road density of 10.01 km per 100 km2, which is much lower than the Chinese national average (55.01 km per 100 km2). The railway infrastructure is even scarcer, with a total railway length of 1200 km and a railway density of 0.1 km per 100 km2, which is 1/16 of the Chinese national average (1.61 km per 100 km2). With the continuous advancement of major national strategies such as “A Country with Strong Transportation Network” and “Western Development Strategy,” a large number of significant transportation infrastructure projects are gradually being implemented in the Qinghai–Xizang Plateau and the surrounding Qinghai–Xizang areas.
Tunnel engineering has been widely utilized in the development of transportation infrastructure because of its safety and environmentally friendly characteristics. In line with ongoing and future development projects, a considerable number of ultra-long deep tunnels urgently need to be constructed in the Qinghai–Xizang Plateau region. For example, in the construction of a high-altitude railway, tunnel sections account for as much as 82.6%. The Qinghai–Xizang Plateau region is undergoing the construction of numerous ultra-long deep tunnels in various national strategic water conservancy and hydropower projects, including the west route of the South-to-North Water Diversion Project and a hydropower project in the downstream of the Yarlung Zangbo River.
Table 1 lists some representative ultra-long deep-buried plateau tunnels and tunneling projects that have been completed or are under construction in the Qinghai–Xizang Plateau region. As shown in
Table 1, the longest of these tunnels is over 16 km in length, with the greatest burial depth of 2525 m. This is comparable with other ultra-long deep tunnels across the world, as presented in a comprehensive list summarized by Zhu et al.
[1]. In their list, deep tunnels have a length of over 10 km and a burial depth of over 500 m. For example, the Koralm Tunnel in Austria has a length of 32 km and a burial depth of over 1250 m, whereas the Brenner Base Tunnel in Italy has a length of over 55 km and a burial depth of up to 1600 m. Moreover, the Gotthard Base Tunnel in Switzerland is the longest railway tunnel in the world, with a length of over 57 km and a burial depth of 2450 m.
Although the lengths and buried depths of the tunnels listed in
Table 1 are similar to those of international tunnels, the tunnels built on the Qinghai–Xizang Plateau are located at higher altitudes, generally above 3500 m. High altitudes lead to a series of complex problems in tunnels, such as ventilation and oxygen supply issues. Moreover, the strong coupling of internal and external forces on the Qinghai–Xizang Plateau results in extremely complex geological conditions, highly active crustal movement, rapid geomorphic processes, and frequent geological disasters. These extreme environmental factors severely limit tunnel construction projects. Challenges in tunnel construction can be separated into three categories: environmental, geological, and engineering challenges.
1.1. Environmental challenges
The Qinghai–Xizang Plateau, which is often referred to as the “Roof of the World” or the “Third Pole”
[2], is the most extensive plateau in China and the highest plateau in the world. In terms of its origin, the plateau is a complex product of convergence and collision between the Indian and Eurasian Plates, which has resulted in the intense uplifting and compression of the relatively ductile Qinghai–Xizang region to the north
[3]. The broad Qinghai–Xizang Plateau comprises two parts: ① the flat highland located in the central region, which is the true plateau and is referred to as the Central Plateau; and ② the surrounding area with massive mountain ranges, such as the Himalaya, Longmen, Altun, Qilian, Kunlun, and Hengduan Mountains. The average elevation of these mountain ranges exceeds that of the Central Plateau. Although there is a significant difference in elevation between the two regions, they are collectively known as the Himalaya–Qinghai–Xizang orogen, with most areas within the orogen being situated at an elevation range of 3000–5000 m above sea level.
Fig. 1 shows the diverse climate of the Qinghai–Xizang Plateau region. During summer, monsoons bring moisture, whereas the winter months are characterized by dry westerly winds
[4],
[5]. Moreover, the terrain of the Qinghai–Xizang Plateau exhibits significant variations in elevation, with relative height differences between ridges and valleys often ranging from 2000 to 3000 m and occasionally exceeding 5000 m
[6]. This intricate topography not only affects the movement of air masses but also induces vertical motion in the atmospheric boundary layer. The coupling of various factors results in considerable fluctuations in temperature and humidity
[7],
[8] in the region.
Fig. 2 [9],
[10] shows the dynamic changes in the annual precipitation, annual average temperature, oxygen content, and atmospheric pressure at different elevations on the Qinghai–Xizang Plateau.
As shown in
Fig. 2, all four parameters exhibit a notable decrease as the elevation increases. When the elevation rises above 4000 m, the temperature is close to 0 °C, and the oxygen content and atmospheric pressure drop to approximately 60% compared with those in regions at sea level. Moreover, there is significant variability in the precipitation data, reflecting the strong meteorological fluctuations of the Qinghai–Xizang Plateau.
1.2. Geological challenges
The extreme environment of the Qinghai–Xizang Plateau is reflected not only in external atmospheric factors, such as extreme cold and low oxygen, but also in its geological characteristics. The Qinghai–Xizang Plateau is located in a compressional–collisional zone between the Indian Plate and the Eurasian Plate. The relatively rigid Indian Plate continues to exert strong northward and eastward thrust, resulting in successive subduction and closure of the oceanic crust, which forms complex continental blocks and structures
[11],
[12],
[13]. Simultaneously, the relatively ductile Xizang region on the northern side of the plate collision zone experiences intense uplift under strong horizontal compression, which creates extremely high crustal stress features. Moreover, the geostress distribution is complex
[14]. The “Y-shaped” distribution of crustal stress in the region demonstrates significant anisotropy in both the magnitude and orientation of the stress field
[15].
Fig. 3(a) shows the vertical and horizontal principal stress data obtained from borehole surveys in the region. As shown in
Fig. 3(a), the principal stresses exhibit a significant linear increasing trend with increasing depth. The lateral pressure coefficient (
Kav) can be calculated from the stress data obtained at various depths, as shown in
Fig. 3(b). Analysis of the data indicates that the
Kav value in this region significantly exceeds the standard value predicted by the Hoek–Brown criterion and approaches 1 at a depth of approximately 500 m.
The strong unloading effect from high crustal stress in rock formations is a challenge for tunnel construction in the Qinghai–Xizang Plateau. In addition, intense seismic activity and frequent large earthquakes in the region pose hidden risks to the safe operation of engineering projects
[16],
[17]. According to related records, from 1713 to 2013, at least 18 earthquakes with magnitudes of 7 (M7) or higher occurred along a certain high-altitude railway, as shown in
Table 2.
The data from
Table 2 further confirms the frequent occurrence of large earthquakes in the Qinghai–Xizang Plateau region
[18]. Taking a high-altitude railway under construction in that region as an example, the railway line is generally located in a high-intensity seismic zone, with peak ground accelerations of seismic motion ranging from 0.1
g to 0.4
g [19]. Among them, the proportion of areas with peak ground accelerations over 0.2
g is 53.57%. This is due to the multiple active fault zones along the route, which include the Longmen Mountains, Lancang River, and Nujiang faults, in addition to active Holocene fault zones
[20].
The hydrogeological conditions in the Qinghai–Xizang Plateau region are also crucial factors to be considered in construction projects. The distribution and occurrence of surface water and groundwater are typically controlled by climate, tectonics, lithology, and topography, resulting in considerable complexity
[21]. Coupled with the varying seasons and climatic challenges of extreme cold, low rainfall, and heavy snowfall, the intense tectonic activity and numerous active fault zones in the region create favorable conditions for the migration and enrichment of groundwater. This leads to the formation of a unique plateau-type karst. Moreover, high altitudes and significant elevation differences directly contribute to the development of high-pressure and ultra-high-pressure water systems in the Qinghai–Xizang Plateau
[22]. Furthermore, the groundwater in the region not only serves as a direct source of high-pressure water but also acts as a propagation medium for convective hydrothermal systems
[23]. The Qinghai–Xizang Plateau is situated in one of the four major global geothermal belts—the Mediterranean–Himalaya geothermal belt—and is characterized by geothermal resources. In most areas, the underground temperature range is 40–60 °C. Groundwater is a carrier of geothermal energy and directly affects the geothermal gradient, as shown in
Fig. 4. Abundant hot water has emerged along major active fault zones, creating a widespread distribution of high-temperature thermal springs.
Fig. 5 shows the monitoring data in 2022 for the temperature and outburst of water in the No. 5 cross-passage of the Layue Tunnel along a high-altitude railway. It can be observed that the temperature of the outburst water inside the tunnel mostly exceeds 70 °C, leading to an ambient temperature of 40 °C inside the tunnel, which is higher than the normal physiological temperature of humans.
Table 3 lists the temperature and outburst water characteristic data from March to June in 2022 at the No. 5 cross-passage of the Layue Tunnel. During this period, the outburst water temperature remained above 70 °C for 4 consecutive months, with the highest temperature being recorded at 92.7 °C.
1.3. Engineering challenges
The undulating mountain ranges of the Qinghai–Xizang Plateau indicate that tunnels constructed in the region will inevitably be ultra-long and deeply buried. Varying criteria exist for categorizing tunnels based on length. In China, highway tunnels longer than 3000 m are considered to be ultra-long, whereas the relevant railway standard classifies tunnels longer than 10 000 m as ultra-long. Moreover, different countries have different definitions of tunnel depth. According to
JTG D70–2004: Code for Design of Highway Tunnel [24] and
TB 10003–2005: Code for Design on Tunnel of Railway [25], the critical depth of deep-buried versus shallow-buried tunnels is determined based on whether the top layer of the tunnel can form a pressure arch. In general, tunnels with burial depths exceeding 50 m are classified as deep tunnels for railways and highways. This standard has been used in other countries. In addition, deep tunnels can be categorized based on the measured
in situ geostress, according to the British Tunnelling Society’s
Specification for Tunneling [26]. Considering a high-altitude railway that is currently under construction in the Qinghai–Xizang Plateau, there are as many as 15 tunnels with lengths exceeding 20 km. Of these, the longest tunnel has a length of 42.4 km, which accounts for over 50% of the cumulative length, as shown in
Fig. 6(a). Moreover, 31 tunnels have maximum burial depths greater than 1000 m, collectively constituting more than 70% of the cumulative length, as shown in
Fig. 6(b).
During the planning and demonstration of the downstream Yarlung Zangbo River hydropower project, a development scheme was adopted to avoid significant geological hazards in the deep valley section. This scheme involves the construction of six long and large underground cavern groups and no fewer than five large three-level underground powerhouses. The total tunnel length is approximately 46 km, with both a drop and a maximum burial depth of 2200 m. In addition, according to the planning and demonstration of the west route of the South-to-North Water Diversion Project, the project crosses the Garze–Litang active fault zone, Xianshui River fault zone, and Shaluli Shan tectonic mixed-rock zone. The total length of the individual tunnels is 218 km, with a maximum burial depth of 2200 m. This burial depth and length far exceed the standards set in the current specifications. Compared with traditional medium–short tunnels, the deep-seated and complex geological environment differs significantly from shallow rock conditions; in combination with the significant increase in tunnel depth and length, this gives rise to distinct engineering characteristics for deep tunnels. Tunnel construction in these environments presents technical challenges in two main areas: issues related to rockburst, high temperature, and water influx; and issues related to ventilation and oxygen supply.
1.3.1. Rockburst, high temperature, and water influx
An increase in tunnel burial depth directly leads to higher crustal stress, ground temperature, and underground water pressure. Under the influence of high crustal stress, when a tunnel passes through hard rock formations, such as granite and gneiss, the accumulated strain energy within the rock and the brittleness of the rock make the tunnel highly susceptible to rockburst disasters. When a tunnel passes through soft rock formations with a low elastic modulus, such as mudstone, shale, and slate, the surrounding rock undergoes significant time-dependent creep deformation failure. Rockbursts and large deformations can cause severe damage to tunnel linings and construction machinery and may even result in casualties.
In addition, as the depth from the surface increases, the formation temperature also increases, giving rise to high ground temperatures. This can lead to deterioration of the working environment during construction, lower labor productivity, and danger to the health and safety of workers. The additional thermal stress caused by the high ground temperature can result in cracks in the initial tunnel support and secondary lining, affecting structural safety and durability.
In addition to the aforementioned issues, sudden influxes of water are another problem that must be addressed during the construction of deep tunnels. Once a water influx disaster occurs, it can result in minor losses to life and property or even force changes in the tunnel alignment. Moreover, the water resources and environment around the tunnel may be adversely affected, which may trigger secondary disasters in some cases, such as ground collapse or subsidence.
Tunnel disasters in traditional tunnels are relatively simple and occur less frequently because of their shorter lengths and shallower burial depths. However, as the tunnel burial depth and length increase, more complex working conditions are encountered while traversing different geological layers. This complexity is reflected not only in the diversity of rock formations but also in the frequent occurrence of active faults, structural impact zones, and coupled multiple hazard forms. Regarding high crustal stress, rockburst disasters in traditional short and shallow tunnels are mostly instantaneous and occur immediately after rock unloading, which allows for a certain degree of predictability and prevention. However, under the influence of ultra-high crustal stress in a deep tunnel, the magnitude and genesis of rockburst disasters may undergo significant changes, shifting from instantaneous rockburst to delayed rockbursts, where rockbursts sometimes occur after tunnel excavation and unloading. The uncertainties associated with this type of rockburst make it difficult to predict, and the ultra-high crustal stress conditions make the energy released during the rockburst much greater. Therefore, compared with instantaneous rockbursts in traditional tunnels, this type of delayed rockburst can cause much greater damage and pose more significant hazards.
Previous tunnel data have shown that large deformations typically fall within the range of a few centimeters. However, the strong abrasiveness of rock formations in the Qinghai–Xizang Plateau region, coupled with the ultra-high crustal stress, can cause the deformation of surrounding rocks to increase several-fold. Furthermore, under extremely high-temperature conditions and the influence of external tectonic stresses, the characteristics of large deformations in the surrounding rocks of tunnels become even more complex, potentially leading to extreme deformation disasters induced by hard-rock creep and deformation controlled by tectonic stresses.
With respect to the high ground temperature, the formation temperature increases significantly as the depth increases. As shown in the temperature gradient variation curve in
Fig. 4, when the tunnel burial depth reaches 1000 m, the ground temperature exceeds 80 °C. Considering the widespread distribution of deep tunnels along high-altitude railways, it can be inferred that deep tunnels in the Qinghai–Xizang Plateau region will experience ultra-high ground temperatures. The continuous ultra-high ground temperature not only prevents construction personnel from working inside the tunnel but also causes short circuits in construction machinery due to the water vapor carried by high-temperature outbursts, severely affecting construction efficiency. Moreover, the complex coupled environment of high ground temperature and crustal stress inside the tunnel significantly influences the constitutive models, mechanical characteristics, and disaster modes of rockbursts and large deformations. New types of thermomechanical coupling disasters, such as high-temperature rockbursts and high-temperature creep deformation of the surrounding rocks, may occur.
High water pressure is caused by increased burial depth; when the water pressure exceeds 0.5 MPa, outbursts of water exhibit high water pressure characteristics. Owing to the extreme burial depth of the proposed tunnels, a significant difference in water levels will result in a water pressure of several megapascals or even orders of magnitude higher, producing an ultra-high-pressure water flow. After tunnel excavation, ultra-high-pressure water will flow rapidly along the widely distributed fractures and cause existing fractures to expand rapidly to both sides. Consequently, the surrounding rock conditions will become more fragile. Ultra-high-pressure water flow not only worsens the surrounding rock conditions but also accelerates the destruction of the tunnel lining. This is primarily because, under high temperatures and seismic loads, the tunnel lining will develop numerous cracks. The ultra-high-pressure water flow will deepen the existing cracks, accelerating the destruction of the lining. Simultaneously, a high ground-temperature environment will produce high-temperature and high-pressure water outbursts, which will cause even more severe disasters during tunnel construction.
1.3.2. Ventilation and oxygen supply
Adequate ventilation and oxygen supply is a pertinent challenge of ultra-long tunnels. The typical oxygen content and natural ventilation are sufficient for the survival and operational requirements of medium-to-short tunnels. Even in long-distance tunnels, existing ventilation and oxygen supply systems can fully meet the needs of personnel and work operations. However, for ultra-long tunnels spanning several tens of kilometers, meeting the ventilation and oxygen supply requirements specified in regulations is challenging.
Ventilation maintains air circulation inside and outside a tunnel by providing fresh air inside the tunnel in a timely manner and removing dust and toxic gases generated by fires or explosions. If an existing ventilation layout, such as transverse ventilation, is used, the leakage rate and ventilation resistance per hundred meters will exponentially increase. This leads to a severe ventilation insufficiency in ultra-long tunnels. In the event of emergencies, such as fires or explosions that force trains to stop inside a tunnel, insufficient ventilation prevents harmful gases from being expelled, posing a serious threat to passenger safety.
Oxygen is essential for the health of onsite personnel and the regular operation of construction machinery. Under normal conditions, the oxygen concentration inside a tunnel gradually decreases with increasing depth. However, some field tests and theoretical works have proven that the decrease in oxygen concentration with depth is not a simple linear relationship but follows a complicated polynomial function declining tendency, which is also influenced by the tunnel altitude value. Therefore, it can be predicted that the oxygen concentration in ultra-long deep tunnels spanning tens of kilometers may approach 0. Moreover, in high-altitude areas at the same depth, the oxygen content inside the tunnel is even lower, leading to an extension of the oxygen supply cycle and a significant increase in the required oxygen supply volume. Furthermore, extremely hot and humid adverse environmental conditions increase the oxygen requirements of construction workers and machinery, and low oxygen levels result in very low work efficiency or even work stoppages, leading to project delays. Therefore, more effective measures are required to ensure a concentrated oxygen supply.
2. Technical challenges
Based on the 14th Five-Year Plan and the 2035 Vision Plan, China intends to undertake a range of major ambitious national strategic projects in the Qinghai–Xizang Plateau and its surrounding regions. These projects will involve the development and construction of larger and more intricate infrastructure, taking into account the unique geological characteristics of the area. The Qinghai–Xizang Plateau and its neighboring regions are known for their complex crustal structure, intense plate movement, rapidly changing and steep terrain, and extreme mechanical environment, which is characterized by diverse three-dimensional (3D) tectonic stress and deformation fields that are unparalleled in the world. The construction of ultra-long deep tunnels and the assurance of their long-term safe operation have presented significant and demanding challenges to the scientific and engineering community of China. The primary technical issues encompass various possible disasters such as active-fault-zone, high-geostress, high-pressure water-surge, high ground temperature, low airflow and oxygen, and multi-hazard coupling disasters.
2.1. Active-fault-zone disasters
The phenomenon wherein an active fault transitions from a state of static friction to dynamic sliding is commonly referred to as stick-slip behavior. This brittle rupture process entails the sudden release of significant amounts of energy, resulting in an earthquake. Creep-slip faults are characterized by relatively gradual movement, in contrast to the stick-slip behavior of active faults
[26]. The presence of active fault zones poses a significant risk of catastrophic events to the tunnels that traverse them. For example, the seismic event that occurred in Wenchuan in 2008 resulted in significant structural impairment in the tunnels situated within the affected region. In particular, the Longdongzi Tunnel, which traverses the Longmen Mountains fault zone, experienced significant seismic damage, including circular misalignment of the tunnel section, collapse of the top portion of the sidewall, and failure of the secondary lining of the roof arch. The Longxi Tunnel, situated between the Yingxiu fault and Longxi fault, experienced an earthquake that led to a vertical displacement of approximately 1.0 m, misaligned deformation, and collapse of the lining arch. Consequently, the functionality of the tunnel was completely compromised. On January 8, 2022, the Daliang Tunnel of the Lanzhou–Xinjiang high-speed railway experienced significant structural damage as a result of the Menyuan earthquake, which had a magnitude of 6.9. The tunnel body was displaced by a maximum offset of 1.78 m and experienced a maximum elevation of 0.68 m. Consequently, train operations were disrupted for a duration of 8 months. Furthermore, taking an under-construction plateau railroad as an illustration, numerous fractures along the railway exhibited both slip and backlash activities. These fractures demonstrated a discernible annual average displacement rate with a maximum value of 21–22 mm per year. Active faults exhibit the mechanical characteristics of viscous slip faults, which are associated with abrupt and intense seismic events. An example of a significant seismic event is the earthquake with a magnitude of 8.6 that took place in 1950 in the Medog–Zayu region of China. To date, this is the most powerful earthquake documented in China. Its impact provoked landslides downstream of the Yarlung Zangbo River.
The potential disasters that may be encountered by tunnels traversing active faults are enumerated in
Fig. 7 and include the misalignment of tunnel openings, cracking of the lining, dislodging of the arch, structural misalignment, and uplifted arch bulging, which result in significant losses in both construction and maintenance. Hence, it is imperative to employ tunnel designs, rational construction techniques, and stringent safety-management protocols during the construction of tunnels on the Qinghai–Xizang Plateau in order to manage and mitigate the potential hazards associated with viscous slip and creep slip that may arise in tunnels intersecting active fracture zones.
2.2. High-geostress disasters
Under high
in situ stress, tunneling projects are highly susceptible to rockbursts and large deformation disaster risks
[27],
[28]. A rockburst is a geohazard event in which the dynamic rupture of intact or jointed hard rock occurs, resulting in significant human casualties and property damage. Rockburst incidents are mostly characterized by a moderate to strong intensity, often accompanied by stripping and ejection phenomena. A deep-drilling test with a depth of 1400 m was conducted at the lower Yarlung Zangbo River hydropower station over the past several years. The test findings indicated a stress value of 56 MPa, although it was postulated that the maximum stress value of the lower cutoff tunnel situated at a burial depth greater than 600 m would be approximately 100 MPa. This observation suggests that the
in situ stress in the area is exceptionally elevated.
The safety of deep tunneling projects is significantly compromised by the presence of high
in situ stress rockbursts and the associated risks of large deformation hazards. A significant rockburst event occurred at the Chenglan Railway Ping’an Tunnel, wherein varying degrees of rockburst phenomena were observed within the tunnel, extending for 4875 m. The rockburst location exhibited an unevenly formed crater, measuring approximately 1.0–3.0 m in size. Similarly, the Jinping Hydropower Station, which has an average burial depth of approximately 2000 m, experienced a rockburst event during its construction phase. This rockburst was induced by significant stress redistribution due to the disruption produced during the excavation of the cavern. Consequently, the earth exhibited cracking with a maximum displacement of 10 cm. Moreover, taking a finished tunnel in the Lhasa–Nyingchi section of a plateau railroad as a case study, it was found that approximately 94% of the tunnel segments had significant incidences of rockbursts during the building phase. The occurrence of rockburst leads to a significant number of rockfalls, which are accompanied by audible noise and the impact of gas waves. These events have detrimental effects on the excavation surface, equipment, and workers, as depicted in
Fig. 8(a). Most of the aforementioned rockburst disasters are classified as instantaneous rockbursts—a term that refers to the occurrence of a rockburst disaster shortly after tunnel excavation. The occurrence of rockbursts following a period of excavation is characterized by a time lag in the opposite direction. The prevailing conditions throughout this period are represented by an increased level of risk owing to the presence of personnel and construction equipment engaged in ongoing activities. A time-lag rockburst was observed during the operation of a tunnel with a length of 11.6 km, resulting in a deformation of the tunnel floor, specifically the arch, by 38.9 cm, and a disruption in the tunnel’s length by 70.8 m. Consequently, traffic was interrupted for 11 days.
In addition to rockbursts, excessive levels of stress can induce deformation in the weak surrounding rock of tunnels, thereby presenting a significant hazard to tunnel safety. As depicted in
Fig. 8(b), the Muzhailing extra-long highway tunnel experienced a major catastrophic deformation during construction in the soft carbonaceous slate surrounding rock, with a maximum deformation exceeding 2 m and an arch dismantlement rate of 30%.
The Qinghai–Xizang Plateau undergoes plate subduction, which is distinguished by many ophiolite tectonic mélange zones. The presence of intricate rock formations and well-developed porous structural surfaces leads to significant rock abrasion, resulting in substantial tunnel deformation in this area. These massive deformations present high danger levels. The ongoing construction of a plateau railway line is anticipated to encounter significant risks from large-deformation disasters in tunnel projects. The projected length of the section that may be affected by this disaster has been estimated to be 159 km, with a substantial portion (21.7 km) being particularly susceptible to strong, large deformations. Typically, structures with low strength and highly abrasive rock formations exhibit increased susceptibility to large-scale creep-type deformations. A significant number of extensive deformation disasters have occurred within the suture zone in the presence of exceedingly high
in situ and tectonic stresses. In addition, the rock belt, fracture shear zone, and other related areas exhibit a higher degree of complexity and distinctiveness in the Qinghai–Xizang Plateau. A plateau railroad tunnel situated within the eastern Himalaya tectonic knot zone is subject to the influence of a multifaceted tectonic stress environment. The observed phenomena led to the occurrence of creep in the hard rock, which subsequently caused deformation and cracking of the original support and distortion of the steel frame, as shown in
Fig. 8(c). Furthermore, a plateau railroad tunnel situated on the northwest side of the Yarlung Zangbo suture zone has experienced the combined impacts of ongoing plate tectonic activity, a highly intricate
in situ stress environment, and frequent seismic events in the area, which has led to internal joints and fissures within the rock. The tunnel was created using an open-type tunnel boring machine (TBM), and instances of the hidden unloading of joint dumping blocks were observed multiple times. In addition, the surrounding rock has undergone crushing and deformation due to the influence of the significant
in situ stress, resulting in the immobilization and obstruction of the TBM shield, as depicted in
Fig. 8(d). The occurrence of significant deformations poses a substantial risk to the advancement of tunnel construction, the safety of individuals, and the protection of property. Researchers continue to grapple with significant challenges in mitigating these dangers.
2.3. High-pressure water-surge disasters
The Qinghai–Xizang Plateau has undergone multiple phases of change, resulting in distinctive features, such as vast glacial lakes and plateau-type karst formations. Alpine rainfall and snow climates have also distinctively contributed to the formation of these unique surface features. However, the presence of broken rock bodies on a plateau increases the potential risk of abrupt waterslides occurring within tunnels. Inrush water disasters emanating from water-rich strata pose a formidable challenge for tunnel construction in the Qinghai–Xizang Plateau region of China.
Fig. 9 shows the hazards caused by the inrush of water into a highland railroad tunnel. The Wushaoling Tunnel, located in the Qilian Mountains tributary along the Lanzhou–Xinjiang railway line, encountered a significant water surge during its construction phase. In the Maoxian Tunnel, the average daily water surge was recorded at 3.0 × 10
4 m
3, but the daily pumping of water reached 1.0 × 10
4 m
3. Such a significant disparity between the water surge and the pumping volume often leads to severe consequences, including delays, injuries, economic losses, environmental damage, and potential abandonment or rerouting of tunnels to alternative locations. The Malujing Tunnel on the Yichang–Wanzhou Railway experienced a remarkably high daily average surge volume of 1.728 × 10
7 m
3, which resulted in significant disruptions and numerous injuries.
2.4. High ground temperature disasters
Xizang has a significant abundance of geothermal resources, exhibiting ground temperatures in the range of 40–60 °C. In certain regions, the ground temperatures can reach as high as 90 °C
[29]. As shown in
Fig. 10, the current construction of a plateau railroad is encountering numerous challenges posed by more than 50 high-temperature hot springs. These springs have detrimental effects on construction processes. In addition, approximately 15 tunnels are potentially exposed to high-temperature thermal hazards. One particular railroad tunnel experienced a high-temperature hot-water gush that measured 92.6 °C during the construction phase. The intrusion of geothermal energy into the peripheral rock contact zone may lead to a thermal catastrophe within the tunnel, necessitating reconfiguration of the tunnel environment. This can result in deterioration of the performance of the supporting materials and a decrease in the stability and longevity of the tunnel structure. Furthermore, elevated temperatures can lead to an increased heart rate, decreased physical energy, dehydration, and potential fainting in humans. These circumstances significantly diminish construction productivity and jeopardize the safety and well-being of construction personnel.
2.5. Low airflow and oxygen in plateau tunnels
Compared with tunnel construction on plains, the unique climate and environment of the Qinghai–Xizang Plateau necessitate distinct technological approaches for tunnel ventilation and oxygen provision. The reduced air density at higher altitudes necessitates an increased air volume for construction employees and internal combustion construction machinery. The conventional drilling and blasting method and the TBM approach generate significant quantities of dust and toxic gases during various processes such as drilling, blasting, slurry spraying, and ballast discharge. Consequently, it is imperative to ensure adequate ventilation in order to mitigate the associated risks
[30]. Estimates suggest that, at altitudes of 3000 and 4000 m, the ventilation demand for tunnel construction increases by 2.46-fold and 3.02-fold, respectively. As a result, it is difficult for existing ventilation systems to ensure sufficient ventilation within ultra-long deep tunnels. Furthermore, in a low-oxygen environment, the levels of dust produced by drilling, blasting, slurry spraying, ballast discharge, and other activities, in addition to the emission of harmful gases such as carbon monoxide (CO), nitrogen monoxide (NO), and nitrogen dioxide (NO
2) from internal combustion machinery, increase substantially. In fact, these concentrations can surpass normal levels by up to tenfold.
Fig. 11 shows the ventilation issues encountered during long-distance tunnel construction at high altitudes. The prospective high ground temperature and humidity in deep tunnels not only force internal combustion engines to operate under high-load conditions but also exacerbate the low-pressure and low-oxygen construction environment. This can result in hypoxia and other physiological reactions among construction personnel, reducing their efficacy and severely impeding progress.
2.6. Multi-hazard coupling disasters
As tunnels increase in depth and length, the geological conditions become progressively more complex, leading to an increased likelihood of various geological dangers such as rock explosions, significant deformations, high-pressure water, and elevated ground temperatures. These multiple types of disasters can often exert compounded and mutually influential effects. The generation of additional thermal strain arises from non-uniform heat transport and thermal expansion in the surrounding rock. The increase in high ground temperature during the stress-release process in the surrounding rock can lead to an accelerated stress concentration rate in the surrounding rock, thereby expediting the occurrence of rock explosions. Consequently, a positive correlation between tunnel temperature and rock explosion magnitude exists. However, the post-excavation low-temperature wind flow disrupts the temperature field of the surrounding rock, resulting in a significant temperature gradient in the shallow portion of the rock along the radial direction. The combined effects of thermal stress generation and excavation unloading result in the peripheral rock slab cracking of structural surfaces. This phenomenon leads to an increase in the maximum tangential and main stresses. Furthermore, rockbursts accelerate under the influence of thermal coupling rockburst
[31].
Fig. 12 shows a common hazard associated with the coupling of high-temperature surge water on the Qinghai–Xizang Plateau. The hydrothermal activity occurring on the Qinghai–Xizang Plateau is primarily governed by the presence of extensive and profound fractures and secondary fractures. The groundwater, heated by either ambient ground temperature or latent heat sources, ascends through these fractures, forming hot springs and high-temperature regions. Hence, tunneling within this area is challenging, given the elevated temperatures and humidity resulting from surges of high-temperature water and the potential structural deficiencies in surrounding rock support systems. Therefore, a single-hazard risk assessment for the management of ultra-long deep-buried underground tunnels is insufficient; instead, a thorough multi-hazard assessment is required for tunnel construction.
When a region experiences simultaneous hazards, the interplay between these hazards exhibits intricate interactions influenced by elements such as the timing, extent of impact, and nature of their effects. Disregarding these relationships can undermine the severity of catastrophe coupling, compromising the accuracy of the complete risk assessment outcomes. Hence, exploration of the multi-hazard coupling mechanism and complete evaluation of its associated disaster risks hold significant scientific research value and technical importance.
3. Research progress
The geological environment of the deep rocks on the Qinghai–Xizang Plateau is complex, with geological features such as high seismic intensity, geostress, and environmental gradient disturbances. This causes ultra-long deep tunnel projects to suffer from a series of technical problems, such as active fractures, soft rock deformations, rockbursts, sudden water surges, high ground temperatures, insufficient ventilation, and inadequate oxygen supply
[32],
[33]. Thus far, numerous experts and scholars have conducted extensive research on the complex technical problems encountered by ultra-long deep tunnel projects. The relevant scientific research results can be separated into two categories: ① disaster-causing mechanisms and ② control techniques.
3.1. Disaster-causing mechanisms
The study of tunnel disaster-causing mechanisms mainly focuses on analyzing the geological environment, the formation mechanisms of tunnel disasters, and the dynamic response of supporting structures to the disaster, in order to reveal the laws, characteristics, and causes of tunnel disasters, along with their destructive effects on structures. Relevant research results can lay a theoretical foundation for the ongoing improvement and development of disaster prevention and control technologies.
At present, research on tunneling across active fault zones is primarily concerned with the fault slide mechanism and the response mechanism of the structure. To study the slip mechanism of active faults, many scholars
[31],
[32],
[33],
[34] have used model tests and numerical simulations to investigate the roughness, sliding rate, sliding mode, and evolution of the characteristic parameters of the interrupted level in the slip process, revealing the slip damage form of the fault surface from various perspectives. Researchers have conducted onsite monitoring, modeling tests, and numerical simulations to further investigate the deformation, stress distribution, and extent of damage to tunnel structures under various fault slip characteristics and other reaction mechanisms. Notably, experts have conducted extensive studies in this field
[35],
[36],
[37], analyzing the sensitivity of key parameters such as the tunnel span, seismic intensity, and surrounding rock level. These researchers have proposed the concept of reasonable overburden equivalent height and made appropriate modifications to the static method
[38],
[39],
[40],
[41],
[42].
Fig. 13 [38] demonstrates the principle of their proposed longitudinal generalized reaction displacement method, which significantly improves the theory of the seismic dynamics of tunnel support structures. Furthermore, the researchers created a large-scale (1:35) multi-inclination, wide-speed-control, and high-precision oblique slip misalignment test device for tunnels crossing active faults (
Fig. 14 [41]). Based on this device, a lining damage modeling test under the creep-slip action of an oblique-slip fault was systematically conducted. Moreover, based on the theories of cohesive fracture damage and concrete damage plasticity, the creep-slip deformation law of the tunnel lining and the damage deterioration characteristics of the lining structure under the influence of multiple tilting-slip fault factors, such as tunnel depth and fault angle, were investigated. More importantly, the 3D damage pattern evolution of the tunnel structure was revealed, from cracking damage to crack extension and then to staggered crack damage under different fault forms and segmental defense lengths.
Regarding high geostress rockburst disasters, many scholars have conducted extensive research on the definition and classification of rockbursts
[43],
[44],
[45],
[46],
[47] by employing experiments, numerical simulations, and other technological methods to study the mechanical properties of rock bodies, such as the rupture characteristics of high-energy rocks under stress, the process of crack expansion, and the final forms of rock body damage
[48]. For example, Liu and Dai
[49] conducted a comprehensive review of the latest advancements regarding the deformation and failure mechanisms, as well as the fatigue constitutive relationships, of rocks under cyclic loading over the past six decades. Wang and Park
[50] studied the comprehensive prediction of rockbursts. Feng et al.
[51] conducted an early prediction study on rockbursts. Zhang et al.
[52] conducted a careful investigation of the geological conditions after rockbursts.
In their paper, Feng et al.
[51] independently developed the world’s first hard rock high-pressure true triaxial aging rupture process testing device (
Fig. 15) and conducted key research on fine-sensing technology and process discrimination for hard rock aging rupture, as well as control technology. Furthermore, they developed the first international high-precision microseismic intelligent monitoring system and used it to conduct a series of studies on the evolution of the microseismic patterns of rockbursts induced by different methods.
Fig. 16 [48] shows a typical application of the microseismic-event-based quantitative rockburst warning method and assessment theory in a tunnel. As shown in
Fig. 16, accurately visualizing the probability of rockburst makes it possible to reveal the mechanism of chain rockbursts induced in tunnels with extremely high stress, allowing a database of tunnel geology and rockburst control cases to be formed. In recent years, researchers have conducted research on the damage characteristics of rock bodies under dynamic conditions. They have conducted explosion tests and vibration monitoring to study the influence of explosion shock waves on the crack extension, energy transfer, and form of the damaged surface of the rock mass. For example, Wang and Park
[50], Cho et al.
[53], and Bagde and Petroš
[54] and so forth performed cyclic loading and unloading experiments under various conditions and proposed rockburst prediction indexes, such as the brittleness coefficient, rockburst susceptibility index, elastic energy index, and residual energy index, which reveal the intrinsic properties of rocks that tend to undergo rockbursts and predict the possibility of rockbursts.
Regarding large deformation disasters in weak surrounding rock, researchers have primarily investigated the mechanical features of rock bodies, the concept of large deformation, and the novel support theory
[55],
[28],
[56]. Because the mechanical properties of the surrounding rock body are the main factors controlling large deformations, many experts and scholars have performed exhaustive research on the strength of the rock body, deformation characteristics, and rock crack expansion by means of field investigation and indoor tests to elucidate the mechanical behavior of weak rock bodies and their impact on the large deformation of a tunnel
[57],
[58],
[59],
[60],
[61], revealing the load-transfer mechanism of the support-enclosure system and the overall structural damage mechanism induced by local failures. To understand the mechanical properties of rock bodies, investigators have analyzed the occurrence of large deformations in soft rock. Li et al.
[62] developed a large-scale true 3D multi field coupled physical simulation test technology and equipment system for deep and complex disaster-conceiving environments, as shown in
Fig. 17. They used the system to conduct a large number of physical simulation tests on the spatial deformation of weak and fractured surrounding rock in sectional tunnels, which revealed the strong spatial and temporal effects of controlling factors—such as softness, fracture, low strength, and rheological properties—on the deformation of the surrounding rock. Furthermore, a newly developed support theory has been fundamental to the research and development of large-deformation disaster prevention and control technology for soft rock in high-geostress tunnels
[63],
[64],
[65],
[66]. Through theoretical analyses and mechanical constitutive model development, numerous scholars have proposed new representative soft rock large-deformation support systems and theories. In particular, Sui et al.
[67] presented the concept of constant-resistance large-deformation support, established a structural mechanics model of constant-resistance large-deformation support material, and deduced the energy-balance equation of the interaction between the engineered rock mass and the engineered rock mass, which provides a new reference for large-deformation support.
Before the disaster mechanism of high-geothermal tunnels can be studied, an analysis of the origin of high geothermal temperatures in tunnels is required. Researchers typically conduct fieldwork to examine tunnel heat sources by analyzing hot springs and using techniques such as borehole and fluid thermometry
[68]. For example, Wei and He
[69] analyzed the distribution characteristics of high geothermal temperatures along the Lhasa–Nyingchi Railway based on regional tectonic and high-temperature geological engineering data. Furthermore, studying the heat transfer mechanism of the surrounding rock of a tunnel at multi-grade heat source temperatures is fundamental to examining high-geothermal tunnel catastrophes and the development of thermal hazard mitigation technologies
[70],
[71],
[72],
[73],
[74]. Zeng et al
[72] developed a large-scale 3D physical modeling test platform for heat exchanger-envelope rock-cavity airflow coupling (
Fig. 18). Multiple tests, such as peripheral rock heating, heat exchanger water heat transfer, and tunnel ventilation heat transfer tests, were conducted to determine the dynamic change pattern of peripheral rock heat transfer at various cross-sectional locations of the tunnel under complex working conditions involving water transfer and ventilation.
High temperatures can affect the mechanical behavior of the surrounding rock and supporting materials of tunnels, such as the lining, which in turn can induce tunnel catastrophes. In order to analyze the surrounding rock, researchers have used field measurements and numerical simulations to study the thermal dynamic response of the rock mass under high geothermal temperatures and have explored the thermal expansion, thermal stress distribution, and heat conduction characteristics of the rock mass, thereby revealing the mechanism of catastrophes caused by high geothermal temperatures. Researchers have also investigated the performance changes and adaptability—including the stability, thermal expansion resistance, and high-temperature resistance—of different types of support materials in high-geothermal environments through indoor experiments, field observations, and numerical simulations
[75],
[76],
[77],
[78]. Under high-temperature conditions, heat transmission from the surrounding rock to the lining leads to fractures in the aggregate–mortar bond in the concrete, resulting in high-ground-temperature disasters in tunnels. Wang et al.
[79] conducted onsite measurements for high-geothermal railroad tunnels, explored the shifting laws governing the temperatures of the surrounding rock and lining structure, and proposed an ontological model of temperature damage at the shotcrete–rock interface. In addition, Cui et al.
[80], Orosz et al.
[81], Villagrán Zaccardi et al.
[82], and Tang et al.
[83] tested and analyzed the mechanical properties of indoor concrete under high-temperature curing conditions using modern techniques such as electron microscope scanning and nuclear magnetic resonance.
Regarding high-pressure surge water disasters, understanding the water source and hydraulic characteristics is fundamental in determining the mechanism of high-pressure surge water disasters
[84]. Many experts and scholars have utilized hydrogeological experiments and numerical simulations to study hydraulic characteristics, such as the water source type, water level change rule, and hydraulic gradient, in order to better understand the hydrological background and hydraulic conditions that contribute to surge water disasters
[85]. The formation of seepage channels in a rock mass under high osmotic pressure is at the core of surge flooding studies. Based on indoor experiments and numerical simulations, researchers have investigated the permeability and seepage paths of rock bodies in detail to reveal the formation mechanism of seepage channels in surge water. In these two studies, Li et al.
[86] and Wei et al.
[87],
[88] started from the formation of water sources, combined with differences in the permeability characteristics of the rock body under different stress states, and ultimately determined the formation mechanism of rock body surge water.
Fig. 19 [87] shows how water influx levels in tunnels can be predicted by combining rock properties and utilizing inverse imaging mechanisms, which can be useful for preventing water influx and outflow disasters. Macroscopically, the rock mass is isotropic; however, at the microscopic level, it exhibits a significant degree of anisotropy, which is mostly observed in cracks and joints. Therefore, researchers have focused on the influence of distribution, openness, connectivity, and other fault and fissure parameters through geological experiments and rock mechanics tests. For example, Liu et al.
[89], Xu et al.
[90], Zhang et al.
[91], and Liu et al.
[92] revealed the influence laws and intrinsic mechanisms of rock fracture mechanical behavior based on indoor experiments and theoretical derivations.
Most studies on tunnel ventilation and oxygen supply have focused on the tunnel airflow theory and ventilation systems
[93],
[94],
[95],
[96],
[97],
[98]. Through indoor experiments, theoretical analyses, and numerical simulations, the effects of sensitive parameters, such as air velocity, temperature distribution, and turbulence structure, on gas diffusion and the thermal environment in tunnels have been investigated. For example, Zeng et al.
[99] proposed a method for calculating the air leakage rate by combining experimental tests and theoretical analysis, which considered the effects of the static pressure difference and air volume change. Li et al.
[100] systematically summarized comprehensive technologies such as ventilation control standards, fan characteristics, and construction oxygen supply technology in high-altitude tunnels in low-pressure, low-oxygen, and low-temperature environments. In addition, owing to the complexity of such environments, the combustion characteristics, rescue conditions, emergency management, and fire control in tunnels in plateau areas will be significantly different from those in plain areas. To ensure rapid evacuation and rescue in fire accidents, in-hole cave rescue stations have been designed for super-long railroad tunnels and tunnel groups in the Qinghai–Xizang Plateau, including four structural types: single-hole two-lane tunnels, parallel guide pits, single-hole single-lane tunnels, and double-hole single-lane and parallel guide pits. Moreover, a smoke exhaust mode and a personnel evacuation plan have been developed for the rescue stations. However, because the current train combustion heat-release characteristics and smoke-production mechanisms in plateau tunnels have not yet been explored, rescue station evacuations and smoke exhaust programs remain at the theoretical level. In addition, the smoke exhaust, ventilation, and smoke control equipment lack a coordinated design.
3.2. Control techniques
Currently, tunnel disaster prevention and control technology is progressing toward a more comprehensive approach, shifting from the management of individual disaster points to the integrated management of points, lines, and surfaces. Geological disaster prevention and control have transitioned from passive to active. Active control technology for extremely long and deep tunnel disasters on plateaus has progressively gained attention in domestic and international research. Such technologies use sophisticated safety measures to mitigate potential hazards and prevent catastrophic incidents. A methodology has been devised to recognize the geological environment of tunnel catastrophes before the occur based on the mechanistic and evolutionary principles of high-energy environmental disasters in extremely long and deep tunnels in plateau regions. The development of active control technologies for typical geological disasters—such as high ground stress, large deformations, rockbursts, high permeability, thermal damage, and crossing active faults—will ensure the safety of tunnel construction. Significant progress has been made in four major technical fields in response to typical disasters in ultra-long deep tunnels, as outlined below.
3.2.1. Stability control technology for the surrounding rocks of high in situ stress tunnels
At present, tunnel construction primarily focuses on the strong support functions of the initial support and secondary lining. In complex geological environments, such as high
in situ stress, weak surrounding rock, or hard rockburst strata, passive protection measures that only strengthen the supporting parameters are nonoptimal for regulating the deformation of the surrounding rock, as demonstrated by practical experience. Tunnels are susceptible to a variety of engineering catastrophes such as large deformations, rockbursts, steel arch distortion, spalling of sprayed concrete, and bolt failure. Thus, the use of active support technology to control the deformation of the surrounding rock has been proposed. By artificially intervening in the tunnel deformation process, the stiffness of soft rock can be actively increased, or the stress state of the surrounding rock can be rapidly improved. The final displacement can be controlled to an ideal target value such that the surrounding rock support structure system can attain a stable, coordinated, and safe long-term health status
[101]. Based on the concept of active deformation control of surrounding rock with high
in situ stress, the proposed support measures can be roughly divided into four categories: ① radial active support force, which is applied to the surrounding rock, such as via negative-Poisson’s-ratio (NPR) high-constant-resistance large-deformation anchor bolts/cables (
Fig. 20 [102]), prestressed anchor cables, or prestressed anchor bolts; ② active modification of the surrounding rock, such as via advance small pipe/anchor grouting and radial grouting of the surrounding rock; ③ active removal of high
in situ stress, such as through an energy absorption anchor, pressure relief support (
Fig. 21), advance hydraulic fracturing, or pressure relief drilling; and ④ using high-performance sprayed concrete to enhance the deformation constraint effect of the surrounding rock, such as steel-fiber-sprayed early-age high-strength concrete.
3.2.2. Disaster control technology for tunnels with high ground temperatures
The thermal damage levels and types of ultra-long deep tunnels in the Qinghai–Xizang Plateau vary. Therefore, when applying active thermal damage prevention and control technology, it is necessary to determine specific technical measures based on the thermal damage type and level, in order to accomplish targeted treatment, classified prevention, and control. Ventilation is the most frequently employed chilling measure for preventing and controlling thermal damage. When the level of thermal damage is low, it can be prevented and controlled by optimizing the ventilation system and method. As the environment outside a Qinghai–Xizang Plateau tunnel is typically cold, this external cooling capacity can be applied for active cooling inside the tunnel by enhancing the ventilation system and airflow. To address dry-heat high-temperature tunnels, based on existing heat-insulation technology, researchers have developed many new heat-resistant insulation materials and have proposed effective laying methods for heat insulation layers. For hydrothermal high-ground-temperature tunnels, researchers have considered the combined influence of ultra-high-temperature rock mass and fissure water, developing new high-temperature and corrosion-resistant water-insulation materials based on heat-insulation technology. Many newly proposed water-insulation technologies can be combined with high-temperature fissure water drainage and sealing measures.
In environments with extreme temperature and humidity, the advantages of existing artificial or non-artificial cooling technologies should be fully exploited, and multiple technologies should be made to work together. At present, a more advanced technology should use large-scale loop heat pipe self-driven antifreeze technology (
Fig. 22), self-regulating absorption refrigerator cooling technology in tunnels based on the heat source grade (
Fig. 23), and other technologies for recycling thermal damage around the tunnel to make full use of geothermal energy in the surrounding rock, thus achieving heat dissipation and efficient energy conversion. The next steps of active prevention and control technologies require clarifying the applicability of human functional protective equipment in high-temperature working environments, establishing a reasonable cyclic operation mechanism, and developing control and emergency rescue technologies for sudden human physiological thermal damage in high-temperature tunnels.
In conclusion, using the current tunnel construction on a plateau railway as inspiration, the policy of deepening geology, controlling heat sources, blockage removal, monitoring and warning, and system support can effectively achieve the comprehensive cooling of high temperature and thermal damage tunnels in different sections.
3.2.3. Active control technology for water inrush in high-osmotic-pressure tunnels
Effectively managing the correlation between drainage and blockage is crucial in attaining effective control over water influx disasters in ultra-long deep tunnels in plateau regions. A specific response strategy should consider the geological conditions and strata types for categorization and governance. In plateau karst strata, construction prevention and control optimization schemes utilizing data from multiple sources have been developed. These sources include various karst forms, water pressure, volume parameters, and outburst-prevention safety distances. In addition, a technology that integrates water and mud outburst prevention and control during construction with a disaster control decision model has been proposed. For a fault fracture zone, a classified control and localized blocking grouting reinforcement technology system has been proposed that considers multiple dimensions, such as the stability of the surrounding rock in the fault fracture zone and the performance evaluation of the anti-outburst structure. Foundational technology has been devised for the collaborative active prevention and control of water and mud inrush in fault fracture zones. This technology combines advanced drainage and pressure reduction with advanced grouting, water-sealing, and reinforcement.
3.2.4. Earthquake damage prevention and control technologies for tunnels crossing active faults
Active fault zones are widely distributed in the Qinghai–Xizang Plateau, and ultra-long deep tunnels in this region are threatened by fault creep, stick-slip, and sudden strong earthquakes. The principal approach to address these issues implements a deformation-adaptive structural system within tunnels situated in a creeping active fault zone. In addition, the adoption of active control technology to mitigate major earthquake disasters is recommended for tunnels that traverse viscous sliding active fault zones. An understanding of the specific fracture zone location is the foundation of tunnel earthquake disaster prevention and control technology. A targeted rock drilling jumbo developed by Li et al.
[62] incorporates seismic detection technology, a borehole electromagnetic directional detection mode, and an antenna prototype. It also includes tunnel disaster information, internal and external perception modes, and patrolling equipment. These advancements have enabled the practical imaging of faults and broken zones, offering a reliable method for detecting submeter disasters with precision. Experiments have been performed to verify the control effect of deformation adaptation measures, which include the appropriate expansion of the tunnel section, reservation of deformation and reinforcement space, strengthening of local lining, development of fault-resistant hinge design (
Fig. 24), and the use of foam concrete as a damping layer (
Fig. 25). Moreover, crucial structural design parameters have been proposed to meet the requirements of tunnel fracture resistance in creep slip zones.
Investigations
[35] have also been conducted on the seismic performance of various components, including isolation layers, seismic joints, encircling rock reinforcement, and multilayer linings. Prominent seismic measurement parameters appropriate for active-fault tunnels with sticky slips have been suggested. Moreover, applying new energy-releasing materials such as vibration-reducing fiber concrete, foam concrete, backfilling light rock and soil, setting rubber plates, and pressure injection asphalting materials can help control intense earthquake damage. The antivibration effects of these new energy-releasing materials on the lining and surrounding rock have been conducted.
Previous research has shown that there has been significant progress in understanding and controlling these disasters. However, these studies have mostly focused on individual disasters, with few considering the coupled effects of multiple disasters. For ultra-long deep tunnels, the coupling of multiple disasters alters the disaster occurrence mechanism and damage patterns, significantly increasing their complexity and harm. Consequently, traditional research is insufficient to meet the requirements of disaster control and prevention in this context. Therefore, in addition to conducting research on the incubation mechanisms of multiple disaster couplings, developing more advanced and effective technical means to reduce and prevent tunnel disasters at the source is necessary. This requires comprehensive and precise early warning detection and disaster risk assessment. In addition, mechanized, rapid, and intelligent construction of tunnels should be performed to effectively reduce the harm caused by disasters. Moreover, research on post-disaster emergency support technologies is lacking.
4. Future prospects
4.1. Tunnel disaster prevention and integrated disaster warning techniques
Significant developments have been achieved in tunnel disaster prevention and control techniques, including risk assessment, quantification, intelligent monitoring and early warning, proactive prevention and control, and intelligent construction and operation. Active tunnel disaster prevention and control technologies have made remarkable progress thus far; however, future efforts are urgently needed in precise disaster detection, proactive avoidance, monitoring, evaluation, and comprehensive early warning techniques. The primary areas requiring technological research and development include the following.
4.1.1. Sophisticated probing techniques in extreme mechanical environments
The Qinghai–Xizang Plateau is the most intricate continental crustal region in the world. However, owing to insufficient fundamental geological surveys and explorations, in addition to a lack of comprehensive and systematic regional research, the current understanding of the extreme geological dynamic processes and disaster-formation mechanisms in this area is severely limited. Hence, conducting basic regional geological surveys, geophysical exploration of the deep crustal structure, and controlled deep borehole detection in regions designated for imminent large national engineering projects is imperative. Moreover, extensive geological survey results from railways, highways, and hydropower projects should be fully utilized to establish a 3D “Transparent Earth” geological information platform in order to achieve a crustal structure visualization of the Qinghai–Xizang Plateau. Furthermore, an integrated Earth dynamics sensing system that incorporates satellite-based comprehensive remote sensing and ground-based hydrological, meteorological, geological, and seismic monitoring, along with deep crustal stress observation, should be established to provide geological assurance for risk avoidance, early warning, and control measures in constructing ultra-long deep-buried underground projects.
4.1.2. Regional geohazard mitigation routing techniques
Geohazard mitigation routing techniques, which avert massive catastrophes by using scientific and rational methods to select routes, are essential for proactive disaster prevention and control. Technically, disaster reduction route selection involves three key aspects: disaster identification, digital route selection, and overall disaster risk reduction design. Rapid geological hazard identification and route selection are critical technological challenges that urgently need to be addressed in the planning of railway routes in complex and hazardous mountainous areas. To achieve digital route selection, it is crucial to incorporate catastrophic data into a digital model of the virtual environment. This can be achieved by using intelligent route-selection algorithms to quickly generate different routes and engineering possibilities. These options can then be evaluated and screened based on various criteria. After establishing suitable route plans, further efforts must be made to coordinate railway engineering schemes, optimize disaster risk-reduction designs, and implement effective and dependable engineering measures, such as tunnel bypassing and bridge crossing, to avoid geological hazards. These measures will facilitate the safe construction and long-term operation of ultra-long deep tunnels.
4.1.3. Quantitative disaster risk assessment under intense internal and external dynamic coupling
Risk quantification is an indispensable component in tunnel disaster risk reduction. Nevertheless, the assessment of ultra-long deep tunnel disasters in the Qinghai–Xizang Plateau is hindered by the intricate interplay between internal and external dynamics. Challenges include the presence of various disaster types, reliance on a limited set of evaluation indicators, and the need to consider multiple factors simultaneously. Consequently, achieving rapid, precise, and reliable disaster assessment is difficult. Therefore, establishing a geological disaster database that considers the strong interactions between internal and external forces on the Qinghai–Xizang Plateau is essential. This should be accompanied by the development of collaborative computing methods based on data and models, and the creation of a high-precision intelligent technique for identifying images of rock mass joints and fractures, allowing the early detection and accurate handling of geological disaster risks. In addition, when investigating rapid quantification assessment techniques for disaster risk, it is vital to consider the interaction effects of multiple factors. Such research should aim to uncover the failure mechanisms of tunnel structures, establish quantitative vulnerability-assessment models, and propose methods for water-thermal multifield coupling risk-quantification analysis. These efforts will contribute to the development of quantitative and sophisticated disaster risk assessment.
4.1.4. 3D integrated intelligent monitoring and early warning of tunnel geohazards
Geological disaster monitoring and early warning systems are imperative for ensuring the safe construction and long-term operation of ultra-long deep tunnels. At present, there are several technical challenges pertaining to monitoring accuracy, equipment integration, data transmission, and intelligent early warning systems. In the near future, these systems will evolve to become intelligent, automated, comprehensive, and exact. Therefore, it is necessary to conduct research on technology for comprehensive sky, space, subsurface, and underground intelligent monitoring and early warning of geological disasters; to propose multidimensional monitoring techniques; and to develop multi-source data fusion, intelligent processing, and scene construction technologies. This will make it possible to establish an emergency service and multisource cross-scale information-sharing platform, ultimately achieving proactive control of geological engineering disasters
[103].
Moreover, establishing comprehensive advanced geological forecasting and imaging technology for tunnels can assist in the prediction of adverse geological conditions. This is critical for ensuring safe and rapid tunnel construction, conducting disaster warnings, and implementing proactive prevention measures
[64]. For ultra-long deep tunnels, it is essential to fully consider the typical geological disaster development characteristics. In addition, designing an advanced tunnel detection and observation system with a three-way offset distance for drilling and blasting methods and developing portable active seismic wave advanced forecasting technology and equipment will aid in achieving long-distance imaging and disaster warning of causative structures, as shown in
Fig. 26.
Intelligent classification technology based on multidimensional geological information is a future research focus for intelligent monitoring and early warning of tunnel disasters. The development of this technology requires the standardization of calibration methods for drilling parameters such as rock drilling jumbos, anchor trolleys, and geological drilling rigs; the formulation of data standards for high-definition digital images of tunnel faces, high-resolution underground water videos, and high-density 3D point clouds of tunnel faces (
Fig. 27); the development of an automated intelligent collection system for multidimensional geological information of surrounding tunnel rock; the creation of intelligent identification methods and analysis techniques; and the establishment of a dynamically expandable database for the intelligent evaluation of surrounding rock quality samples based on drilling parameters. This will involve the construction of intelligent evaluation models and algorithms for the quality of the surrounding rock of a tunnel based on the drilling parameters. In addition, it is important to establish refined intelligent classification and feedback techniques that are precise in two dimensions and longitudinal in three dimensions for the rocks surrounding the tunnel face based on multidimensional geological information. A software system for the automatic intelligent grading of a tunnel’s surrounding rocks using drilling and blasting methods, as shown in
Fig. 28, should also be developed. Other crucial steps include constructing a theoretical system, engineering methods, and a database for digital underground space; integrating tunnel-lifetime data collection, processing, representation, and analysis; and establishing a dynamic 3D design technology for rock mass tunnel support based on digital twin technology. The development of these technological measures will permit the automation and integration of data collection, data transmission, pre-disaster information processing, early warning level analysis, and automatic warning information release.
4.2. Mechanized rapid tunnel execution and intelligent construction techniques
Traditional tunnel construction methods have relatively low levels of mechanization and lack information, making it challenging to ensure the construction quality, safety, and progress of ultra-long deep tunnel projects. This is particularly the case under the extreme conditions of the Qinghai–Xizang Plateau, which include low temperatures, air pressure, and oxygen levels, and extremely complex geological conditions. In such environments, the efficiency of traditional drilling and blasting methods is significantly reduced, leading to a higher labor intensity and increased construction risks for workers. Moreover, existing mechanical equipment fails to meet the requirements for quality control, safety assurance, and disaster response. Thus, there is a pressing need for mechanization and intelligent construction in large deep-buried underground engineering projects. To address these challenges, research on pivotal technologies and on equipment for intelligent tunnel construction is necessary. This research should be centered on reducing worker numbers and achieving unmanned construction. Future research should primarily focus on three main areas, as described below.
4.2.1. Systematized full-process mechanized intelligent equipment and robot-aided construction
The research and development of systematized and mechanized construction equipment and robotic construction technology are essential for the transformation of drilling and blasting methods in ultra-long deep-buried mountainous tunnel construction. Achieving equipment systematization, reducing human involvement, enabling information integration, implementing integrated scheduling, and ensuring overall safety control are key. First, the impact of various factors—such as the environment, geology, construction methods, and cross-sectional dimensions—on the efficiency of drilling and blasting tunnel construction equipment should be explored. Research on adapting systematic mechanical equipment to complex surrounding rock conditions, different physical spaces, and various excavation methods should be conducted, along with the establishment of technical schemes for systematic full-process construction equipment configurations for tunnels in complex high-altitude geological conditions. Considering different construction methods, various cross-sectional shapes, and multiple special conditions, a modular equipment configuration database should be established, and systematic equipment suitable for high-altitude drilling and blasting tunnel construction, as shown in
Fig. 29, should be developed.
Furthermore, research should concentrate on developing precise scene-positioning technology for high-altitude tunnels, precise and flexible control technology for arm frames, long-distance control and transmission technology, and rapid construction technology for upgraded anchor net support systems based on integral drilling and grouting machines. Methods for equipment usage with fewer personnel and unmanned operations under remote real-time monitoring, along with reliable takeovers, should be established. In addition, research is needed on the development of highly mobile tracking platforms, the investigation of unmanned parallel operation strategies for special robot-assisted processes, and the optimization of mechanized operation processes for the main and auxiliary processes in tunnel construction. Moreover, a collaborative information-management platform for intelligent tunnel construction equipment should be established. Research should also focus on studying the dynamic interactive mode of data flow in intelligent tunnel construction and defining the data interaction and linkage processes between equipment groups, equipment and the environment, and equipment and the surrounding rock. This will enable data sharing among equipment groups based on a collaborative management platform. These efforts will provide technical support for the information-based collaborative scheduling of tunnel construction equipment, the improvement of construction organization management, overall safety control, and the lifelong maintenance and upkeep of tunnel construction.
4.2.2. Intelligent TBM construction techniques for high-energy and complex geological environments
Intelligent TBM construction technology is a recent development trend for mountainous tunnels. However, in the complex and high-energy geological environment of the Qinghai–Xizang Plateau, TBMs often risk getting stuck or trapped. Therefore, major research and development is required for full-face TBMs and their construction technology for ultra-long deep tunnels. First, TBM remote control, monitoring and measurement, and construction risk-monitoring systems based on big data for complex high-energy geological environments—including ultra-hard rocks, high
in situ stress strata, and active fault zones—should be developed. Also, an Earth/tunnel/machine/signal/human intelligent collaborative management and control system, as shown in
Fig. 30, should be established. Second, optimizing TBM cutting tools and configurations requires the development of theories and the investigation of the spatiotemporal effects and rock-breaking mechanisms of TBM tunnel excavation under conditions with ultra-hard rocks and high
in situ stress. It also requires the optimization of excavation parameters in high-energy geological environments with low temperatures and oxygen levels. This will facilitate the development of dual-mode/multimode TBM excavation equipment and support devices suitable for tunnel construction in different geological environments, including high-altitude tunnels with ultra-hard rocks, high-energy geological formations, active fault zones, low temperatures, and low oxygen levels.
Because the lining must provide effective support during TBM tunnel construction to ensure safety, single-layer energy-absorbing initial support structures, assembly lining structures against large deformation strata and active fracture zones, extruded concrete linings, and other rapid support and construction technology systems should be developed. Furthermore, to prevent extreme situations, research on emergency handling measures for TBM tunnel excavation under complex geological conditions—such as high in situ stress, high water pressure, and active fault zones—should be conducted. These measures should include dealing with rockbursts, large deformations, subsidence, water inrushes, gases, and other emergencies. Moreover, techniques for TBM radius variation and cutter replacement in trapped situations should be studied in soft rock and fractured zones.
4.2.3. Ventilation and integrated oxygen-supply techniques for intelligent tunnel construction
Ensuring tunnel safety and speedy construction requires appropriate high-altitude ventilation and oxygen supply conditions. In the construction of high-altitude ultra-long tunnels, there are common challenges in providing adequate ventilation and oxygen supply, such as long ventilation distances, multiple work faces, ventilation systems with complex construction, multiple oxygen-supply points with dynamic changes, various types of ventilation pollutants with complex flow patterns, and the coordination of multiple fan operations and equipment layouts
[102]. To address these technical challenges, there is a need for an intelligent ventilation monitoring system and equipment that consider the above factors throughout the entire construction period and perform real-time dynamic monitoring and precise evaluation of ventilation effectiveness. In addition, coupled with the numerical simulation and analysis of ventilation systems, intelligent control of fan equipment should be realized. Moreover, based on an investigation of the coupled effects of high altitude and high ground temperature on ventilation during the construction of high-altitude tunnels, technologies such as joint ventilation for multiple work faces in a single shaft, air leakage control in long-distance high-pressure high-volume ventilation ducts, and frequency-variable speed-adjustable fans should be developed.
Regarding the oxygen supply in high-altitude tunnels, the optimization and improvement of variable-pressure adsorption oxygen production processes in high-altitude environments are necessary. Other essential tasks that are required include determining the air quality and oxygen volume fraction at tunnel faces, establishing scientific oxygen-demand standards for personnel during construction stages, proposing phased scientific and dynamic designs for tunnel oxygen supply plans, and developing comprehensive oxygen supply technologies. Developing advanced intelligent ventilation and oxygen supply equipment for construction, including new energy construction equipment and efficient air-purification systems, is essential for achieving energy conservation, emission reduction, and environmentally friendly low-carbon tunnel construction.
4.3. Long-term operational safety and emergency rescue techniques for tunnels
The extreme mechanical environments in the Qinghai–Xizang Plateau present significant challenges for the long-term safe operation and durability of large and deep-buried underground engineering projects. Nevertheless, the current research and development of ground engineering maintenance theories and technologies under extreme mechanical conditions are still in the initial stages, both domestically and internationally. Therefore, it is necessary to further study extreme mechanical behaviors and disaster intensity and to systematically investigate the incubation of hidden defects and the mechanisms of disaster evolution in ultra-long deep tunnels. Long-term support theories under extreme mechanical conditions should be explored, and new durable materials should be developed. In addition, flexible and resilient linings that can adapt to structural changes during long-term operation and engineering maintenance methods need to be proposed. Furthermore, research should focus on developing multidimensional inspection and monitoring techniques, and intelligent diagnostic systems. This includes establishing a reliable and integrated approach for the multidimensional perception of high pressure, high in situ stress, high-temperature water, and other disaster sources. These methods should be integrated into construction processes to achieve unmanned and intelligent geological forecasting. Research on the 3D detection of disaster situations, the perception of disaster status information, and chain-based accurate disaster prediction and warning techniques, along with the development of green restoration materials and rapid intelligent treatment technologies and equipment, will contribute to the establishment of a comprehensive infrastructure security technology system in China. This technology will optimize the long-term efficiency and durability of the infrastructure, thereby prolonging its operational lifespan.
Trains traveling through ultra-long tunnels present a significantly elevated danger of fire, and the consequences of accidents can be catastrophic. Moreover, because of the complex conditions of ultra-long deep tunnels in the Qinghai–Xizang Plateau, conducting rescue operations outside is difficult. High-altitude regions are characterized by low pressure, oxygen levels, and temperatures. These challenging external conditions lead to longer response times for personnel, decreased physical endurance, and substantial increases in evacuation time. To address disaster prevention and rescue in high-altitude ultra-long railway tunnels, future efforts should focus on systematically studying the combustion characteristics of train fires in low-pressure and low-oxygen environments within tunnels. In addition, it is crucial to understand the impact of high-speed train travel on heat release and smoke production during combustion, how critical parameters of railway tunnel train fires change with altitude and train speed, the flow and diffusion mechanisms of fire smoke when trains stop at rescue stations, and the combined effects of the smoke exhaust design, smoke exhaust fan capacity, and the arrangement of smoke control devices on smoke exhaust effectiveness. Based on this, the design scale and key parameters of the evacuation and smoke exhaust design at rescue stations should be optimized, and safe and efficient smoke exhaust solutions should be proposed.
5. Concluding remarks
With the continuous advancement of China’s 14th Five-Year Plan and the national strategies of “A Country with Strong Transportation Network” and “Western Development Strategy,” infrastructure construction in the western regions of China—particularly in the Qinghai–Xizang Plateau—is entering a rapid development stage. This growth has inevitably involved the construction of numerous tunnel projects. The high mountains and deep valleys of the Qinghai–Xizang Plateau require tunnels to be ultra-long and deeply buried. However, the complex climate and active geological structures in the plateau region pose severe technical challenges for these tunnels, such as high seismic intensity, high ground stress, high-pressure water surges, extreme geothermal conditions, and inadequate ventilation and oxygen supply. These challenges threaten the safe construction and long-term operation of such tunnels.
This paper provided a comprehensive review of the complex environmental features, geological conditions, and geological tectonics of the Qinghai–Xizang Plateau, along with its strong internal and external dynamic coupling. In addition, this paper analyzed the main challenges encountered in completed and ongoing tunnel projects in the region, conducted a detailed analysis of the causes and characteristics of each technical challenge, and systematically summarized the latest scientific research progress and technological achievements, with a focus on typical disaster-causing mechanisms in high-altitude tunnels and relevant proactive disaster prevention and control technologies. More importantly, this paper proposed mechanization, intelligent approaches, and information collection as the leading principles for construction technologies, as well as long-term safety and emergency support technologies, with the aim of providing technical support and theoretical references for the efficient management of complex hazards in ultra-long deep tunnels on the Qinghai–Xizang Plateau.
Currently, the construction technology for high-altitude ultra-long deep tunnels in China is still in its early stages and there is a lack of engineering experience and systematic technical systems. Numerous gaps in the fundamental theories, technical processes, platform equipment, and other areas need to be filled. Many technical challenges must be addressed in engineering construction, disaster prevention and control, and operations and maintenance. Therefore, in the future, innovative forces in the field of tunnel construction—both domestic and international—must come together to develop solutions for high-altitude ultra-long deep tunnel construction and operation utilizing cutting-edge scientific and technological innovations, such as next-generation intelligent equipment, digital information collection, advanced communication technologies, big data algorithms, cloud/edge computing methods, digital twins, and green construction technologies. These efforts will help to achieve breakthroughs in the safe construction and long-term service of ultra-long deep tunnels on the Qinghai–Xizang Plateau.
Acknowledgments
The authors gratefully acknowledge Academician Xiangsheng Chen, Academician Shucai Li, Academician Xiating Feng, Academician Hehua Zhu, Academician Chuan He, and Prof. Dingli Zhang for providing valuable materials and organizing relevant research achievements. Prof. Zhiqiang Zhang, Prof. Yanhua Zeng, Dr. Chuan Zhang, and PhD students Tingyu Zhu, Xiaolong Liao, and Junnan Ren from Southwest Jiaotong University are appreciated for their assistance in part of writing, translation, and proofreading.
Compliance with ethics guidelines
Yong Zhao, Yanliang Du, and Qixiang Yan declare that they have no conflict of interest or financial conflicts to disclose.
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