Due to the increasing expansion of urban areas, the higher density of surface development and the lack of space for intra-urban transportation, the necessity for underground structures such as tunnels and metro stations is essential. As a result, it is important to recognise, research and investigate soil interaction with tunnel structures. This article, based on G2 station on Line 2 of the metro in Mashhad, Iran, considers the behaviour of surface settlements using numerical simulation methods (Plaxis 2D & 3D) in a hardening soil model against Mohr-Coulomb theory and the continuous field monitoring at the time of station construction, to the use of the concrete arch pre-supporting system (CAPS). The results of the modelling show that CAPS reduces ground surface settlement and enhances ground stability.
INTRODUCTION
The redistribution of stresses caused by tunnel excavation induces movements in the ground and ultimately at the surface. The need to control ground surface settlements in urban areas is widely recognised and new construction methods are continuously developed. Settlements induced by underground excavation may cause serious damage to nearby structures and subsurface utilities.
This article outlines a new pre-supporting method – Concrete Arch Pre-supporting System (CAPS) – that can be very effective for stabilising long-span underground spaces in both shallow and soft ground. CAPS is an innovative system similar to the pre-reinforced rib-and-pile system that was first introduced at the Mount Baker Tunnel, Seattle, US by Bennett and Stillberg. The method has since been widely used in the construction of small-diameter tunnels and underground railway stations in the US, and particularly in Japan.
CAPS construction is primarily based on the traditional Iranian small water tunnel construction method called Quanat, which is regarded as generally faster and less expensive than conventional methods such as forepoling. CAPS is a rib-like underground structure that consists of concrete piles and arch beams constructed around a proposed underground space before its construction. This method has been used in several underground stations, such as at the Tehran Metro (2002), but can be applied similarly to any large-span underground space.
Following pre-support, excavation can be carried out in a variety of ways. Multi-stage excavation and support are used to construct the desired section.
In recent decades, numerical simulation has quickly become the dominant method for solving engineering problems, including stability analysis and predicting systems’ behaviour. Numerical modelling is a useful tool of analysis to establish the stability of underground space in sequential construction and determination of the influence of effective parameters (Delezalova, 2002; Galli et al., 2004; Ercelebi et al., 2005). In this research, the CAPS method and construction sequences, along with the influence of some other parameters, are analysed using numerical methods for one of the Mashhad metro stations.
SITE SPECIFICATIONS AND GEOLOGY
Mashhad is a large city in northeast Iran with a population of over four million. The construction of its metro began in 1999. Line 2 of the ‘Mashhad urban railway’, with a construction cost of US$46m and 13 stations, begins in the north of the city near Kashaf village and extends for 14.5km just south of Javan Square. Construction of Shohada station in the city centre began in 2012 and was completed in 2018 at a cost of US$5 million.
The main part of Line 2 is in the city’s downtown area which is characterised by heavy traffic and congested subsurface utility lines; this makes cut-and-cover unsuitable. The main tunnel of Line 2 is being bored by 9.4m-diameter earth pressure balanced tunnel boring machines (EPB-TBM), but most of the stations must be constructed using non-mechanised underground methods. The construction plan sees the main tunnel bored first and the stations afterwards.
The station under consideration, G2 (station number 7) of Line 2, is located beneath Shohada Square (the city’s main square) and measures 120m long by 17m wide; there is a 26m level difference between the northern and southern ends of the station (Figure 2).
The G2 station layout is incorporated into the design of Shohada Square. As a result of scheme constraints, a portion of it has been implemented as a 40m-long open cut (cut and cover) section and as an 80m-long underground section (Figure 2). The overburden is around 10m and the groundwater level in the station area is located around 20m below the surface and about 6m above the rail level.
In terms of altitude, this station is located at the lowest point of Mashhad, which causes a higher groundwater level compared to other stations on Line 2.
Due to the existence of important population centres such as the municipality and the council building of Mashhad, as well as the existence of urban spaces, the construction of the station’s main structure requires special measures to increase the safety of restraining movement and soil settlement. This is made possible, by using CAPS as the structural support system for the station’s underground area.
GEOTECHNICAL INVESTIGATIONS
A thorough geotechnical site investigation was carried out and geotechnical parameters of natural and improved soil were obtained by Iran-based consulting engineer Sano (2009).
An investigation was undertaken to determine the geotechnical properties and groundwater conditions along Line 2. Nine 40m-deep boreholes were sunk in the vicinity of the station and various tests were carried on the cores extracted. The soil surrounding the station is mostly silty sand (SM) and silty clay (CL).
CONSTRUCTION METHOD
In this location, cut-and-cover would not be suitable because of the disruption to street traffic and its effect on subsurface utilities.
All pre-supporting systems were constructed before the start of excavation of the main underground space. CAPS comprises two side-piles and reinforced concrete arch beams (Figure 1). The system was first introduced in 2002 for the construction of Mellat Station on Line 2 of the Tehran Metro.
CAPS has several advantages when used for large underground spaces, such as helping prevent soil deformation and enhancing the stability of the excavation in soft ground; this results in a large excavation with low overburden. The system reduces ground settlement and thus improves overall stability. Furthermore, CAPS can reduce construction time since the piles and arches can be constructed simultaneously from several faces in a short time.
CAPS construction is preceded by multi-stage excavation and initial support of the main space. After the main excavation is completed, a very light initial supporting system is installed, such as shotcrete and welded wire mesh.
To use this method in subway stations, either the main NATM tunnel or the TBM-bored tunnel should be excavated along with the station first. At this station where the NATM tunnel is excavated by manpower (Figure 3a), first access adits from the sides of the NATM tunnel and three longitudinal adits (Figure 3b) are hand excavated at the specified locations along the length of the station (80m). Then at specified intervals inside longitudinal adits, wells are excavated for side piles (Figure 3c) and after installing reinforcement, they are filled with concrete (Figure 3d). For the next step, an adit is excavated between side piles in a similar arch shape; after installing reinforcement, they are also filled with concrete through the gallery above and connected to the mass that forms an integrated concrete arch frame, after which the overall excavation of the station is carried out. (Figure 3e)
The longitudinal adits are not modelled using numerical analysis. As indicated above, the series of embedded concrete arch frames forming the pre-supporting system act like a rib-shape supporting structure while the main excavation is carried out.
After achieving pre-stabilisation, the main excavation begins at a proper distance. Depending on the ground condition, shape, and size of the CAPS and main underground space, the sequence of the main excavation is determined. Usually, three concrete frames are required to be constructed ahead of the main excavation face. Therefore, by providing some ‘lagging’, the construction of CAPS and the main station can be executed simultaneously.
For G2 Station, the actual proposed CAPS has the following properties. The piles have 1.2m-diameter and a 13m depth, while the dimensions of the horseshoe cross-section of the arch adits is 1×1.5m and the longitudinal distance between every arch frame is 2.5m. The procedure for performing this task is shown schematically in Figure 4.
CONSTRUCTION STAGES OF THE MAIN UNDERGROUND STATION
Excavation and support for the underground space for the station was carried out in several phases. Sprayed concrete (shotcrete) and a layer of wire mesh were laid on the ground between the piles and arches to control stability and reduce displacements. The final lining is reinforced concrete, which is used in the final stages. Figure 5 shows the excavation stages of the main cross-section. First, the crown is excavated and shotcreted (Figure 5A), Then the walls and middle section (Figure 5B), followed by the lower section (Figure 5C). Finally, the reinforced concrete lining of the invert, walls, and crown is executed. In the longitudinal section, each excavation step has a minimum 3m length. The final lining of the base and wall section is installed at a constant 6m distance behind the excavation face, and the final lining of the crown is executed at 12m behind the lower excavation face.
It is assumed that groundwater has been lowered to invert level during construction. This is achieved by dewatering the periphery of the station via side wells and pumping. Consequently, no appreciable seepage occurs during excavation, but with the completion of excavation and final lining with geomembrane and geotextile insulation, the dewatering is halted to allow the water level to reach its natural level and for the water pressure to act on the lining.
The proper distance between the various stages is determined initially during the design phase by numerical modelling of the differing conditions. The appropriate sequence is initially used to start the construction phase. Using field instrumentation, monitoring, and site observation, the construction methods and stages can be modified and optimised.
It is worth mentioning that many operational problems were encountered in the construction of Shohada station, but the expertise of the tunnel engineers involved meant the issues were resolved to allow completion of the works. For instance, to gain access to the east and west entrances, the CAPS piles had to be destroyed during excavation and widening of the main tunnel. This was accomplished by using a cutting wire to reduce the piles into smaller pieces and remove them to the surface using a tower crane.
NUMERICAL MODELLING
For a comprehensive understanding of the system’s behaviour and ground-support interaction, numerical modelling based on the finite element method was used to predict ground settlement, stress distribution, and other ground behaviour. A series of 2D and 3D finite element modelling was used to predict the system’s more realistic behaviour.
Both 2D & 3D-Tunnel code was used to perform these analyses. The construction sequence of the pre-supporting system and main station were modelled and analyses performed step-by-step to simulate exact excavation and construction stages. Construction stages were modelled in 2.5m-long steps with a total of around 32 construction stages. The model has a dimension of 100m width (x-dir.), 50m height (y-dir.) and 70m depth (z-dir.). Along vertical side boundaries, the model cannot move in the x-direction, but is free to move in y-directions and z-directions. Along the bottom boundary, movement in x-, y- and z-directions is fixed.
As explained above, due to dewatering during construction, seepage and consolidation are not considered in the model, thus there is no hydraulic boundary condition. However, after construction of the final lining, dewatering is eliminated; therefore, hydrostatic pressure acts on the final lining up to the natural groundwater level. Concrete elements (i.e., piles, arch beams, shotcrete and linings) were modeled as linear elastic material and a Mohr-Coulomb and hardening soil model was used for soil elements.
Geotechnical parameters obtained from field and laboratory tests are shown in Table 1. Since the main excavation proceeds after the concrete of pre-supporting elements has reached sufficient strength, the elastic modulus used for concrete elements is assumed to be 18GPa for the pre-supporting elements, 12GPa for shotcrete, and 25GPa for the final lining.
Three positions on the constructed section and the ground surface are selected for investigation and comparison of the displacement results. The shape of the maximum ground surface settlement in the cross section is shown in Figure 6. It is clear that the settlement reaches zero at a distance around 1.5D from the station’s centre (D is the width of the station).
Comparison of the results of 2D and 3D of the Mohr- Coulomb and hardening soil model shows 20.3% in the 2D model and 35% in the 3D model; the results of the Mohr-Columb model are less than the values of the hardening soil model. The reason for this could be due to the effect of the accumulation of stresses along the longitudinal length of the tunnel through 3D modelling of the station at the surface settlement. Also, the widespread modelling of the station and the presence of overburden in the 3D model, along with the increase of structural deformation in the middle points of the modelled tunnel, are among these factors.
Surface settlement is also shown in longitudinal section in Figure 7. This shows that surface settlement increases from the starting position, but the rate of increment in the initial position is reduced as excavation progresses further, and the results of the monitoring support this pattern.
To determine the maximum settlement of the tunnel crown in the 2D and 3D models, the values of vertical displacements are calculated and compared with the hardening soil and Mohr-Coulomb theory – the results are shown in Figure 8. Higher values in the 3D model may be the result of stress accumulations and overheads from soil and surface structures along the longitudinal length of the tunnel by the 3D modelling of the settlement in the tunnel crown. Increased structural deformation in the middle of the modelled tunnel may also be effective.
When the 3D model and monitoring results of the longitudinal section of the side drift structure were examined, it was found that the settlements were uncoordinated in the monitoring results, which could have been due to the heterogeneity of the soil around the piles.
CONCLUSION
This article introduces a new pre-support method, the Concrete Arch Pre-supporting System (CAPS) for the construction of large underground spaces. After construction of CAPS, the main underground station is constructed in stages with minimal initial supporting required. Due to the low cost of construction, the CAPS method and the possibility of simultaneous construction of its elements and main station, is found to be a quick and generally economical approach to increase the stability of large-span underground excavations in shallow and weak ground.
CAPS has been used in at least twenty instances underground for the Tehran Metro where the main line has one tunnel and the stations have side platforms. A series of non-linear 2D and 3D finite-element modelling was performed to simulate the construction stages of CAPS at Shohada Station, and to study the effect and influence of CAPS and various parameters. The results of the numerical modelling include the ground settlement and displacements around the underground space. The magnitude of ground settlement and displacement of underground space is closely related to the construction stages of the main underground structure. The 3D model of finite elements has good consistency in the case study with the model measured at ground level. Longitudinal and transverse profiles, related to the displacement of the ground surface due to excavation according to the 3D model of the tunnel, are closer to reality.
It should be noted that these values in the hardening soil behavioural model are 26.2% higher than those values in the Mohr-Coulomb model. Therefore, the behavioural model of hardening soil can be considered more reliable for use in similar projects. Numerical modelling can be used initially to determine an optimum method for the construction stages prior to excavation. After construction begins, the use of field observation, instrumentation and measurements, along with numerical modelling, can result in the determination of more appropriate and practical construction stages.
