1. Introduction
The Ceneri Base Tunnel (CBT) is the southernmost portion of the New Railway Link through the Alps (NRLA) crossing the Swiss Alps from north to south [
1]. The client is the AlpTransit Gotthard Ltd., on behalf of the Swiss government. The NRLA is designed to create a continuous flat-rail connection from Basel to Milan, which will reduce travel times, increase the efficiency and sustainability of freight traffic, and connect Switzerland to the European high-speed railway network (
Fig. 1) [
1]. Along with the CBT, the main elements of the NRLA consist of the Lötschberg Base Tunnel (34.6 km total length), which was constructed from 1999 to 2006 and opened in 2007, and the Gotthard Base Tunnel (57 km total length), which was begun in 1999 and opened in December 2016.
The CBT is characterized by a twin tube system with a single-track railway tunnel with a length of 15.4 km linked through cross-passages that are spaced approximately 325 m apart (
Fig. 2) [
1]. The total excavation length amounts to approximately 40 km and had an excavation volume of approximately 4 × 10
6 m
3. The cost of the works was about 2.1 billion Euro.
The construction works started in 1997 with the drill-and-blast (D&B) excavation of the geological exploratory tunnel (3.1 km in length), which was located approximately in the middle of the alignment (Sigirino). In 2008, an intermediate access adit (2.3 km)—running almost parallel to the exploratory tunnel—was excavated using a gripper tunnel boring machine (TBM) (
Fig. 3(a)). Two large caverns (with a cross-section of about 270 m
2) were excavated for logistical and operational reasons. The caverns were connected to the end of the intermediate access adit and of the exploratory tunnel.
The excavation of the main tunnels started in 2010 from the intermediate heading of Sigirino. The north (approximately 8.3 km) and south (approximately 6.3 km) tunnels were excavated simultaneously using D&B method (Lot 852, about 90% of the total excavation). The remaining portion of the tunnel was completed in backward driving lots at the north (Lot 853) and south (Lot 854) portals. In order to minimize the time and cost of construction, a total of eight excavation faces were operated simultaneously.
The Sarè junction caverns (
Fig. 3(b)) are located in south of CBT and extend for a length of approximately 400 m. They are characterized by a variable geometry of the section, which consists of a conical pattern with a maximum cross-section of about 260 m
2. At the end of the Sarè junction caverns, there are two stub tunnels of approximately 150 m each, which will allow the construction of the future extension of the NRLA project through Italy.
The excavation of the CBT finished in March 2015 and January 2016 with a breakthrough at the south and north portals, respectively. The civil works are currently under completion, and the installation of the railway infrastructure systems began in the summer of 2017. The CBT is scheduled to be opened in 2020.
The cross-section of the CBT (
Fig. 4) has an excavation surface area of 48-87 m
2 and an optimized horseshoe shape [
2]. The shape is designed to meet the multiple requirements that result from the operational needs of the railway line, along with other requirements related to the project execution and its geomechanical environment. A double lining is constructed along the entire CBT; the external lining provides temporary stability during construction, while the inner lining provides long-term stability for the tunnel for the entire project life (100 years).
The inner lining is mostly constructed using non-reinforced concrete, and is only reinforced over the tunnel portions that are characterized by poor geological conditions or geometrically complex zones (e.g., connections with cross-passages). A drainage system is placed between the two linings and waterproofing is placed at the arch [
3].
Groundwater is collected from the lateral drains to a central drain, which evacuates the water toward the north portal.
The inner lining was cast far from the excavation face. This allowed the rock mass to experience larger deformations; as a result, based on the ground response curve, this procedure reduced the final load on the inner lining (thus allowing a reduction in the thickness of the inner lining and in its reinforcement). Static validations of the inner lining were systematically performed during construction, using the crosscheck information gained during excavation.
2. Geological investigation campaign and geological conditions expected before excavation
The entire CBT is situated in the crystalline bedrock of the Southern Alps [
4], as shown in
Fig. 5. The overburden of the tunnel varies from about 10 m to a maximum of 850 m. A detailed survey campaign with a total bored length almost equal to that of the main tunnel, and with single boreholes of lengths up to 700 m, was executed during the period 1991-2008. Based on geological and geotechnical conditions, it was possible to distinguish and characterize 47 homogeneous sections in order to perform an assessment of probable risk scenarios [
5], as shown in
Fig. 6.The lithostratigraphic units crossed by the tunnel can be divided into two areas: in the north, the Ceneri Zone (10.4 km); and in the south, the Val Colla Zone (5 km). The Ceneri Zone is primarily formed of gneiss and, to a lesser extent, of basic and ultra-basic rocks (e.g., amphibolites and serpentinites) that have all undergone considerable metamorphism (amphibolite facies). The whole area is also affected by complex tectonics, of which the main consequence is a high dip angle for the schistosity, lithological contacts, and axial plane folds. The Val Colla Zone also includes a series of paragneiss and orthogneiss, combined with intermediary basic rocks (hornblende schists). Above all, it is their structural geometry that sets these two areas apart. In the Val Colla Zone, the planar structures follow sub-horizontal trends, while they are more vertical in the Ceneri Zone. The interface between these two areas, which is 600 m in thickness according to forecasts, is formed by the “Val Colla Line (VCL),” which is mainly composed of mylonites (fine-grained schists, often with a high mica content) resulting from intense pre-Alpine ductile deformation, probably dating back between 290 million and 240 million years. Diaclases run through the entire rock mass and the project crosses some fault areas (fragile Alpine tectonics, especially Insubric phase). Although the rock mass is covered by alluvial, detrital, fluvio-glacial, and glacial Quaternary sediments, the digging for the base tunnel only goes through loose materials in the areas around the north and south portals. During the construction works, only minor water inflows were expected and the permeability of the rock mass was found to be generally low.
3. Excavation methods
The excavation methods were based on the geological prognosis results and on other requirements such as the environmental impact, logistics, and the timing/investment ratio. Besides the two shorter conventional tunneling works at the portals, the main excavations were carried out from the underground cavern located at the intermediate heading of Sigirino. Based on extensive geological investigations, the final design phase was closed with a high confidence in the selected excavation methods of the CBT. To summarize these methods, TBMs were planned for the tubes (a single-shield TBM for the south direction and a double-shield TBM for the north direction), while D&B method was limited to a short connection from the Sigirino access adit to the Val Colla Line, and the cross-passages.
In the review phase of the final design, the excavation of the northern portion of the CBT by means of TBM was abandoned due to the high risk of shield jamming (estimated tunnel radial convergence up to 30 cm) in correspondence to the fault zones (22 fault zones, having a possible extent ranging from 10 to 110 m). For the excavation of the southern portion, both TBM and D&B methods were tendered as possible excavation methods. For economic reasons, the contractor who was awarded the construction of the main tunnels (Lot 852) selected the D&B method for the excavation of the entire tunnel (this option was about 10% cheaper than the TBM option). The owner and the contractor mutually decided to use a highly mechanized D&B method using efficient and modern heading installations [
6], as shown in
Fig. 7. Each installation was planned to lead financial and technical improvements and consists of a ventilation platform that can be moved independently, a heading platform, an invert platform, a crusher, conveyors, and a single-track suspension rail [
7].
4. Rock support classes
A total of 10 standard rock support classes (see the cross-sections in
Fig. 8) were designed in order to deal with the geotechnical conditions that were expected along the tunnel drive of the CBT. The rock support classes range from SPV 1 to SPV 10; the SPV 1 to 4 classes are characterized by flat invert slab, frictional bolts, and shotcrete reinforced with fibers; the SPV 5 and 6 classes are characterized by curved invert slab, fully grouted bolts, shotcrete reinforced with mesh or fibers, and reinforcing ribs; and the SPV 7 to 10 classes are characterized by a pseudo-circular shape, radial and face injected bolts, shotcrete reinforced with mesh, and reinforcing ribs with yielding elements [
8], [
9].
It should be noted that the distance between the location of the rock support installation and the tunnel face depends on the rock support class; in general, the higher the rock support class, the smaller the distance between the rock support installation and the tunnel face. The rock support classes SPV 3-accelerated (SPV 3acc) and SPV 6A were conceived during construction; the standard rock support class SPV 3acc is an intermediate class between SPV 2 and SPV 3 that allowed a faster advance rate (in particular toward the north), while SPV 6A is characterized by the installation of Toussaint-Heintzmann (TH) ribs and a larger over-excavation. The larger over-excavation allowed larger rock mass convergences (15 cm at the tunnel crown and 23 cm at the wall); this reduced the load on the lining, and thus decreased the need to install the heavier—and more time consuming—rock support classes SPV 7 to 10.
Fig. 9 shows the daily advance rate and costs for each standard rock support class, as well as a comparison between the cumulated tunnel lengths of installed rock support classes as foreseen in the design phase and those installed during construction. Due to the encountered geotechnical conditions, the lighter rock support classes SPV 1 and 2 were not adopted during construction; the “medium-to-heavy” rock support classes (SPV 4 to 10) were installed over the majority of the tunnel length.
A selection of the most appropriate rock support is essential to ensure the safety of the construction works while complying with the project time schedule and costs (i.e., minimization of the project risks). The flow chart in
Fig. 10 shows the phases of the decision-making process that were adopted for the CBT; this process was aimed at mitigating and managing risk scenarios while ensuring the optimal choice of rock supports, according to Ref. [
10]. The decision-making process involves the designer, the construction site manager, and the contractor.
The six phases of the decision-making process shown in
Fig. 10 are described as follows:
(1) Planning. This entails an interpretation of the geological survey (along the profile and from the surface), laboratory investigation results, previous monitoring data, and results of numerical calculations in order to evaluate the most probable risk scenarios.
(2) Standard cross-section choice. This is based on the considerations of the design phase, and the standard cross-section is chosen. The choice is made in agreement with the designer, construction site manager, and contractor, and all the information relating to the excavation (i.e., type and number of installed safety measures, risk scenarios encountered, tunnel advance length, etc.) is collected in the module for safety measures.
(3) Excavation. This is performed according to the standard cross-section chosen.
(4) Check of the correct standard cross-section chosen. This is done through interpretation of geotechnical survey and monitoring data. In the case of successful testing, it is possible to proceed with the selected standard cross-section. In the case of too-conservative or too-light supports, the standard cross-section must be updated.
(5) Verification of structural safety and serviceability. If the standard cross-section is too light, leading to an increase in deformations and possible cracks in the concrete, etc., the structural safety must be checked. If the structural safety is not compromised, advance work can continue with a new, stronger cross-section. If structural safety is not achieved, replacement actions and the installation of additional supports must be carried out before excavation can continue. The choice of rock supports is made in agreement with the designer, construction site manager, and contractor.
(6) Design and execution of replacement measures. The works must be performed in the shortest possible time while ensuring optimal safety conditions. Tunnel excavation can only continue once the replacement measures and installation of additional supports are completed.
During tunnel construction, the geological conditions ahead of the tunnel face were evaluated by means of systematic probe drilling. Core drilling was carried out only before entering known major fault zones. A detailed face mapping was carried out after each round length.
5. The tunneling experience
In general, the rock mass conditions that were encountered during the excavation agreed well with the ones expected in the design phase (based on the extensive survey campaign). However, due to the intrinsic complexity of a long and deep tunnel and the complex behavior of some rock formations, unexpected events occurred during excavation. The following sections describe the major challenges that were encountered during excavation.
5.1. Squeezing conditions and strong asymmetric convergences in the intermediate heading of Sigirino and CBT north
The excavation of the logistics caverns and the neighboring CBT toward the north was performed within the Giumello gneiss (GiuG) formation, under a high overburden with maximum values of approximately 800 m.
Using the two-dimensional (2D) distinct element code UDEC™ [
11], the stress-strain behavior of the different types of rock mass encountered was evaluated. The main geological and geotechnical structures (joints, fault system, and disturbed zones) were defined on the basis of the forecast for each macro-structural area and on the basis of the geometrical characteristics (i.e., tunnel direction and overburden).
For the CBT excavation, according to the numerical results, maximum displacement would occur close to the fault zones and should amount to about 10 cm.
Higher deformations than those predicted were observed during the excavation in the geological formation of GiuG [
12]. Compared with the prognosis, the geological response of the rock mass showed behavior that was strongly influenced by the fault zones (i.e., faults and shear zones formed by cataclasites of up to 1 m thickness, or cataclasites with kakirite of up to 20 cm thickness) subparallel to the direction of excavation and to schistosity that significantly increased the structural anisotropy (
Fig. 11). The displacement monitoring performed by three-dimensional (3D) stations and multipoint extensometers showed high values of deformation of 30-40 cm for the CBT’s northwest advance. A strongly asymmetric pattern was found, with velocity values that were not assumed to decrease even at an outstanding distance from the face (
Fig. 12) [
13]. As a consequence, important cracks in the first lining occurred (
Fig. 12). To achieve an optimal understanding of the deformation behavior of the rock mass, several back-analyses [
14] were performed using numerical calculations (
Fig. 13). The latter allowed a correct validation of the mitigation measures as well as the redesign of the first lining and of the second lining. Due to the structural anisotropy of the rock masses, asymmetrical convergences frequently occurred during the excavations of the CBT north (leading to the local formation of cracks in the primary lining). The convergences were higher in the east side of the tunnel crown and in the west side of the tunnel wall. The asymmetric convergences required the installation of longer bolts (compared with the standard rock support classes) and/or the increase in the bolting density over 15%-20% of the overall north CBT excavation.
5.2. Excavation through a major fault zone under high overburden in CBT south: The Val Colla Line
The major fault zone that was expected and encountered during the CBT excavation is the so-called VCL [
15], [
16], an intensely tectonized zone with a length of 658 and 529 m in the east and west tubes, respectively. During excavation, the VCL was encountered about 200 m earlier than expected (
Fig. 14). The major issue related to the deviation between the geological forecast and the encountered geology was related to the higher overburden (approximately 450 m instead of 350 m), and thus to the higher stress state. The encountered rock mass formations were intensely tectonized, and were as follows:
• Formation 1a: an alternation of phyllites, mylonite, and mica schist;
• Formation 1b: gneiss;
• Formation 2: a heterogeneous mixture of lithotypes 1a and 1b with kakirite-type fault breccia and fault-gauge; and
• Formation 3: a kakirite-type fault-gauge.
The most unfavorable geotechnical section—which consisted of alternating fault breccia and fault-gouge-type kakirite, mylonite, and cataclasite, which was several meters thick—was found over a length of about 250 m. For the excavation of rock mass formations 2 and 3, the SPV 7 to 10 cross-sections were adopted; these are characterized by a pseudo-circular shape, radial and face injection bolts, shotcrete reinforced with mesh and reinforcing ribs, and yielding support with the presence of slots.
The combination of extremely poor rock mass conditions and high overburden led to strongly anisotropic deformations (peak values up to about 80 cm,
Fig. 15), loosening and, finally, to a major face instability (about 150 m
3 of material). To overcome the section where the collapse occurred, piles, shotcrete reinforced with mesh, reinforcing ribs (type TH 29), and radial and grouted face bolts were installed (
Fig. 16). Analytical calculations were also performed in order to check the tunnel face stability (based on Ref. [
17]).
Over the remaining portion of the VCL, the encountered geotechnical conditions were more favorable than expected, as shown in
Table 1.To achieve an optimal understanding of the deformation behavior of the rock mass, several back-analyses were performed using finite-element method (FEM) analyses. The latter allowed a correct validation of the mitigation measures as well as the redesign of the first and second linings [
18].
5.3. Excavation through a major fault zone under high overburden in CBT north: The Val Mara Zone
For the north advance, the most critical geotechnical section was represented by the Val Mara Zone (VMZ). The VMZ was characterized by a length of about 150 m, a maximum overburden of about 800 m, and the widespread presence of kakiritic and cataclastic levels embedded in mixed gneiss and intensively tectonized.
In this section, as forecast in the design phase, the SPV 7 to 10 cross-sections were adopted; these are characterized by a pseudo-circular shape, radial and face injection bolts, shotcrete reinforced with mesh and reinforcing ribs, and yielding support with the presence of slots.
During excavation, the VMZ was encountered approximately 140 m (
Fig. 17) earlier than expected. The poor ground conditions led to strongly anisotropic deformations (with peak values up to 40 cm) and finally to a face instability of 250 m
3 (
Fig. 18). Umbrella pipes, shotcrete reinforced with mesh, reinforcing ribs (type TH 29), and radial and face injection bolts were installed before restarting the excavation (
Fig. 19). Several back-analyses were performed using FEM calculations in order to check the external lining and the consequences on the inner lining; an analytical calculation was also performed in order to check the tunnel face stability (based on Ref. [
17]).
5.4. Excavation of large caverns under difficult rock mass conditions in CBT south: The Sarè junction
The Sarè junction, which is located in CBT south (
Fig. 2), is characterized by a twin tunnel system of 400 m in length, a cone shape, a 260 m
2 maximum cross-section area, and an overburden of 150 m. At the end of Sarè caverns, there are two stub tunnels of approximately 150 m each; these allow the excavation of the future extension through Italy, while minimizing the disturbance to the CBT. Based on core boreholes from surface, laboratory, and
in situ investigation, gneiss and orthogneiss formations and two disturbed zones were forecast (
Fig. 20). Numerical calculation with a distinct element method code was implemented and a maximum displacement of 80 mm was forecast; no problem was therefore expected on the external ring. In the execution, shortly after the first 80 m of excavation, a strong anisotropic rock mass with a low dip angle sequence of layers consisting of cataclastic fault-rock alternating with schistose gneiss with better rock matrix characteristics was found [
19]. Compared with the analysis in the design phase, higher deformations and cracks in the first lining occurred during the full-face tunnel crown excavation (
Fig. 21). Through the back-analysis of massive deformation monitoring and geomechanical data gained from horizontal core boreholes, an accurate geomechanical model was built (
Fig. 22). With this new model, a comparative numerical analysis was performed with full-face tunnel crown excavation and with the tunnel crown excavated in sectors. From these results, it was decided to carry out the remaining excavation with the tunnel crown excavated in sectors (
Fig. 23). The supports installed comprised a shotcrete lining of 35 cm reinforced with two mesh layers; unlike the design phase, injection bolts of 10 m and 8 m and lattice girders of four
ϕ32 mm spaced 1.2 m apart were adopted. Further analysis was made to excavate peculiar cross-sections of the twin tunnels that were separated by a rock pillar only 3 m wide, corresponding with the terminal part of the Sarè junction. The external lining for the stubs and the CBT tunnel consists of 25 cm of shotcrete reinforced with fibers; unlike the design phase, injection bolts of 8 m together with TH 25 reinforcing ribs (spacing of 1 m) were installed. The caverns were excavated with a mechanical method (i.e., hammer) in order to minimize the effect of nearby excavations. Due to the supports installed and the excavation technique adopted, there were no significant cracks in the shotcrete lining and the maximum convergence measured was about 3 cm. Due to the new tunnel supports and excavation methods that were designed, the excavations started in March 2013 and were successfully completed in October 2013. In spite of geological uncertainty, a strategic application of the observational design [
10] ensured the economic and safe execution of a large cavern, while remaining in compliance with the schedule.
5.5. Excavation in an urban area and interaction with an existing road tunnel: The south portal
The south portal included in Lot 854 was located in a particularly critical urban area close to Vezia (Lugano), with the presence of buildings (minimum overburden of 11 m), including a historical building; it was situated in a narrow corridor surrounded by the A2 Swiss highway, the Vedeggio-Cassarate road tunnel (Lugano ring road), and the existing federal railway line (
Fig. 24).
The underground works involve the D&B excavation of the opposite tunnel, which has a very low overburden ranging between 5 and 35 m. The west and east tubes have a length of 303 and 340 m, respectively, and the cross-section is approximately 80 m2. The geological formation in the project zone, which was prospected by means of exploratory boreholes and seismic methods, takes the form of Stabiello gneiss. This consists of a heterogeneous jointed sequence of schistose gneisses and sericitic-chloritic schists.
The new structure crosses the Vedeggio-Cassarate road tunnel, bypassing Lugano [
20] and this tunnel’s escape and rescue galleries, at a vertical distance of about 4 m to a few tens of meters from the portal; the final lining of the road tunnel was completed at the time of the beginning of the CBT excavation. A predictive study of the behavior of the interfering structures and the use of special excavation and support methods together with complex tunnel-monitoring systems for subsidence and vibration control were required for the assessment of this interference (
Fig. 25). In the design phase, numerical simulations with the distinct elements 2D UDEC™ code allowed the forecasting of the stress-strain behavior of the rock mass and the consequences of the excavations at the surface level; a good comparison between the monitored deformations (6-7 mm) and those obtained from the numerical simulation (5-10 mm) was recorded. To manage the risk of blast vibrations [
21], the following measures were imposed: limits even more restrictive than those indicated by the Swiss standards (i.e., a minimum value of 6 mm·s
−1 for frequencies < 30 Hz); a complex monitoring system through geophones that was continuously jointly assessed; and technical restrictions on the D&B process (i.e., drilling and shot-firing only during certain periods of the day, general restriction of the quantity of explosive per initiation stage at < 0.25 kg·delay
−1, subdivision of the excavation cross area, and reduction in round length). Due to the use of advanced technical and organizational solutions, along with favorable geological and geotechnical conditions, the excavation was successfully completed about ten months ahead of the projected date that had been agreed on in the program of works, while ensuring environmental and economic sustainability.
5.6. Underpassing a major highway with very low depth of cover and challenging ground conditions: The north portal
The north portal (Lot 853) is characterized by the construction in the entrance zone of a large cavern underpassing the five-lane A2 Swiss highway at an acute angle (around 30°) [
22].
The highway remained continuously in service during the tunneling works. A single-track rail tunnel for the Lugano-Bellinzona railway “Bretella” has also been constructed near and parallel to the cavern. After the excavation of around 50 m in soft ground, the excavation of Lot 853 advanced toward the south with the conventional D&B method for about 800 m. The minimum overburden in soft ground was 8 m and the maximum overburden in rock was 150 m. The most critical factor was the design and construction of the cavern in soft ground consisting of the motorway embankment, with a maximum span width of 24 m and a height of 17 m (
Fig. 26) [
22]. Extensive studies performed with analytical and FEM numerical calculation were carried out before the following design concept was worked out: ① drilling of the side tunnels with an excavated area up to 60 m
2; ② vertical single fluid jet grouting columns for abutment supports; ③ construction of the concrete abutments within the side tunnels; ④ crown drive in stages with jet grout canopy and shotcrete shell with excavated area up to 160 m
2; and ⑤ bench and invert excavation with an excavated area of 140 m
2. Due to the presence of natural ground with poor characteristics, the concrete abutments for the crown were founded on a body of vertical jet grouting columns (
Fig. 27). Through these elements, the load acting on the crown is transferred into the concrete abutments and subsequently diverted directly into the solid bedrock through the jet grout foundation body. The face support was guaranteed by grouting columns reinforced with steel bars; the steel anchors provided reliable support in the tunnel’s longitudinal direction and the jet grout bodies a shear resistance in the sliding surfaces of potentially unstable blocks.
The analytical stability calculations were performed according to Ref. [
17].
Risk management was based on the following three elements: quality control of the works, deformation monitoring of the structure, and closure of the driving lane when the excavation was progressing underneath.
The most important factor of the risk management was the jet grouting work, which was analyzed with every single horizontal jet grouting test field; this testing was not only aimed at finding the jetting parameters, but also analyzed the installation sequence and the heave-and-settlement movement on the surface caused by high-pressure injections. Different parts of the jet grouting test field were analyzed. The density, compression strength, and cohesion were verified to meet the design requirements. When the jet grouting work started, some unexpected settlements occurred, due to geological reasons (i.e., the injection of very soft ground) that required particular attention during all stages of the execution. The operational and organizational processes of the horizontal jet grouting operations were improved with the creation of a flow chart, in which the tasks of the contractor were divided into providing instruments and methods, and the tasks of the on-site supervision were divided into providing controlling and monitoring; this allowed the designers to ensure that the design was fully respected at the construction site. For the face displacement monitoring, the recently developed reverse head (RH) extensometer was used. In order to monitor the settlements, a number of continuous monitoring systems of the motorway embankment were provided, including two total stations, inclinometers, piezometers, extensometers, and RH extensometers (
Fig. 27).
Due to methodical quality control during the execution of the large number of jet grouting columns, the surface settlements were restricted to a certain degree, as expected on the basis of the structural calculations. The settlements were diffused over an area with a diameter of approximately 100 m, which did not restrict the passage of approximately 60 000 daily vehicles passing on the A2 Swiss highway. In addition to the design activities, the systematic monitoring of the works and the optimization of the jet grouting columns at the face and in the crown were of primary importance.
The works for Lot 853 began in 2008 and were successfully completed in 2015 (
Fig. 28), as agreed on in the working program, while ensuring environmental and economic sustainability.
6. Conclusions
The main conclusions that were gained from the construction experience of the CBT are as follows:
(1) The geological conditions encountered during the excavation of the CBT were characterized by high heterogeneity, resulting in great variability in the occurrence of risk scenarios. These factors caused some unexpected events during the excavations.
(2) Due to the adoption of adequate investigation techniques during the design and construction phases, it was possible to develop a reliable geotechnical model already during the design phase of the project, which made it possible to elaborate an optimal design of the project.
(3) The design tools that were provided allowed the project managers to face unforeseen events without stopping work activities.
(4) The highly mechanized D&B excavation method allowed an overall good performance and high flexibility when excavating through difficult geotechnical conditions.
(5) The modern and flexible contract system, which was combined with strong cooperation between the client, contractor, design team, and site supervision team, represented the key factor in the successful completion of the project (in terms of quality, timing, and costs).
Acknowledgement
The authors thank the AlpTransit Gotthard, Ltd. for granting permission to publish the data contained in the article.
Compliance with ethics guidelines
Davide Merlini, Daniele Stocker, Matteo Falanesca, and Roberto Schuerch declare that they have no conflict of interest or financial conflicts to disclose.
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