Shanghai Rail Transit Pearl Line: Lancun Subway Station (2000-2001)

Lancun Subway Station is an interchange station between a subway system and an elevated light rail transit system. Part of the station is buried underneath the soil for the reception of Tunnel Boring Machine(TBM) and part of the station is planed to be above ground in order to integrate the elevated light rail transit system. The uneven distribution of load in terms of magnitude and directions has the high possibility to induce differential settlement. And for the retaining wall, SMW(Soil Mixing Wall) imbedded with steel sheet piles had never been used in Shanghai’s subway construction before this application. The following pictures show the construction site and the Tunnel Boring Machine reception shaft under construction

The Construction Site	Tunnel Boring Machine Reception Shaft

The interaction between the subway facility and the elevated rail system is quite complicated. First, because the elevated rail system will be constructed two years after the completion of the subway station, the shallow water table condition caused the subway station unable to resist buoyancy force. Second, after the construction of elevated rail system, the huge loading from the train will induce excessive settlement of the subway station. Finally, the distribution of train loading and soil burden is also not even due to the different spans of elevated rail bridges and nonuniform subway structure form. Uneven settlement of the subway station caused by these factors cannot be neglected, as shown in Fig 6. After conducting detailed finite element analysis, I proposed to adopt pile footing as a solution to this problem. According to my proposal, the pile footing has different functions in different phases: it will resist the buoyancy force before the construction of the elevated light rail and carry the huge uneven train loading after the construction of the elevated rail system.

In designing the TBM reception shaft, the shaft under various loading conditions was analyzed by conducting 3-dimensional finite element simulation, as shown in Fig. 7.  The simulation results were used to determine the optimum sequences of excavation, installation and uninstallation of bracing system, the thickness of SMW wall and internal walls.  

3D Finite Model for the Interchange Area	The Displacement
3D Finite Model for the TBM Reception Shaft	The Stress-state of the Shaft

For the SMW method, considering little experience available for its performance, we set up  a real-time field system monitoring the deformation of adjacent surface and buildings, the stress condition of embedded steel sheet piles, the deformation of the retaining structure, the force of bracing system, the earth pressure on the retaining structure. By analyzing the displacement data acquired from this system, I concluded that SMW as a novel retaining structure was effective in keeping displacement acceptable if the excavation depth was less than 16 m. Furthermore, after the steel sheet pile was pulled from the SMW, the cost of SMW method was only 60% percent of other prevail retaining structures like concrete diaphragm wall.

Shanghai Rail Transit Pearl Line: Shanghai Gymnasium Subway Station (2000-2001)

Shanghai Gymnasium Subway Station is the most difficult subway construction project in Shanghai. As shown in Fig. 8, the planed station need to run through the retaining wall of an existing subway station without disrupting the operation of in-service subway system. In the traverse area, the excavation depth exceeded 22m. And along the driving route of Tunnel Boring Machine, the distance between the tunnel and the adjacent high-rising buildings is only 1.5m.

Illustration of Surrounding Construction Environment and Construction Method

In this project, the situation around the intersection part was extremely complicated. On the left side of the existing subway station, an appropriate running through method and an optimum excavation sequence need to be selected to minimize the disturbance to the running train. On the right side of the existing subway station, a cost and risk evaluation need to conducted to compare the two construction plans: one was to build a TBM reception shaft to take the TBM out for reuse. The other was to leave the shell of TBM underground, so no reception shaft was needed.

In order to simulate the various conditions discussed above, I developed several 3-D finite element models to analyze the performance of the selected construction method. Fig.9 shows the model for simulating excavation at the left side of the existing station. The analysis involved modeling of the ground, the existed station and the planed station. The incremental stage-by-stage analysis is employed to simulate the excavation process. And in order to decrease the node number with little influence on computing precision, the excavation area and the existed
station is calculated symmetrically. The different colors show the distribution of the geological layers Fig. 11 shows the accumulated rebound during excavation. In these figures, the meshes for the existed station and the shoring system of the foundation pit were hidden.
Fig. 12 shows the model for simulating a specified running through method in terms of jacking drive. And Fig. 13 shows the settlement of the base slab under possible situations during the jacking driving process, this analysis involved modeling of the existed station and jacking machine. The soil underneath the base slab is simulated by a series of boundary elements. By alternating the stiffness of some boundary elements, I can simulate the different regional soil collapse during construction.

FEA Model Before Excavation	FEA Model After Excavation Completed	Distribution of Heave During Excavation

Running Through Model

Settlement of the Base Slab of the Existing Station during Different Stages (Units: m)

The simulation results revealed a number of very important issues: First, on the left side of the existing subway station, the excavation of the planed subway station would cause significant soil heave, the existing station will suffered from excess torsion effect. Second, on the left side of the existing subway station, not a single running through method can provide acceptable settlement control. However, jacking a steel hollow box underneath the existing subway station can provide minimum settlement. And if the method is combined with freezing soil method, the required control of settlement can be achieved. Third, on the right side of the existing subway station, constructing a reception shaft would be very dangerous and high cost compared with leaving the shell of TBM underground. All these important conclusions from my finite element analysis have been accepted, the proposed methods also have been used in the real construction. Fig.   shows the installation of retaining wall at the left side of the existing subway station. Fig. 14 shows the utilization of freezing soil method.

Installation of Reinforcement for 150 Feet High Concrete Retaining Wall	Adoption of Soil Freezing Methodl at Running Through Section

 At the right side of the existing station, the selected TBM is EPBS. In shield driving, the separate components of the excavation support (borefront, shield jackets, transition zone, shield/tunnel segments) cause changes in the stress situation in the subsoil by which short-term and long-term settlements will develop. And the settlement trough caused by the boring process moves forward together with the TBM as a wave and has a decisive influence on the deformation behavior of adjacent buildings. Because there are lots of buildings adjacent to the driving path, an Interactive Boring Control System (IBCS) was introduced to minimize the impact to the surrounding facilities.  It consists of three parts: virtual tunneling with numerical 3D-model; monitoring actual effects on buildings, ground and in TBM; determination of optimized boring process parameters for next boring section.