The root cause of ground deformation during pipe jacking construction

The fundamental reason for ground deformation during the pipe jacking construction of concrete precast components is the disturbance of the surrounding soil caused by the pipe jacking construction. The ground deformation consists of the following five parts:

(1) Ground deformation before the tunnel boring machine arrives.

When the tunnel boring machine is far away from the measuring point, the soil will be disturbed due to the cutting, mixing and vibration of the cutterhead. There are pores between soil particles (the pores contain water and air). Under the action of disturbance, the water and gas in the pores are discharged, the soil particles move relative to each other, the soil produces a certain amount of compression, and slight settlement occurs on the ground.

(2) Ground deformation when the tunnel boring machine arrives.

When the tunnel boring machine is very close, the soil located directly in front of the tunnel boring machine is squeezed by the thrust of the jack, the cutting shear force of the cutterhead, and the vibration load, and the stress state is very complex.

(3) Ground deformation when the tunnel boring machine passes.

When the tunnel boring machine passes, a shear sliding surface will be formed between the tunnel boring machine shell and the soil layer. Shear stress will be generated in the soil layer near the shear sliding surface, and the shear stress will cause ground deformation. The faster the advancement speed, the greater the shear stress and the greater the displacement of the surrounding soil.

(4) Ground deformation after the tunnel boring machine passes.

In order to reduce frictional resistance, the diameter of subsequent pipe sections is 2-5cm smaller than the diameter of the tunnel boring machine. Therefore, when the tail of the tunnel boring machine passes, the soil around the pipe will move toward the pipe wall to fill the gap around the subsequent pipe sections, which will cause the soil to move. In order to maintain the stability of the soil layer and reduce friction, grouting must be performed around the pipe joints during the construction process, and appropriate grouting pressure must be maintained. Therefore, the displacement of this part of the soil layer is related to factors such as the size of the gap, grouting pressure, and grouting method.

(5) Ground deformation caused by reconsolidation of disturbed soil.

After the soil around the pipe jacking pipe is disturbed by construction, a superstatic pore water pressure zone will be formed. After the tunnel boring machine leaves the area, the excess pore water pressure decreases, the pore water dissipates, and the soil consolidates, causing ground subsidence. This part mainly consolidates and settles. Subsequently, the soil will still creep and secondary consolidation settlement will occur.