Search

• A test of shield driving in the cobble stratum is conducted. • The collapse process of the cobble stratum with the shield driving is revealed. • Characteristics of the ground settlement as the cobble stratum collapse is obtained. • Variation of the earth pressure as the cobble stratum collapse is presented. The cobble stratum is prone to collapse during the shield driving. It is important to understand the collapse behavior of cobble stratum for construction safety of the tunnel. Taking Tsinghuayuan Tunnel in Beijing as the background, a test of shield tunnelling in the cobble stratum is performed using the model shield with the diameter of 280 mm. A total of 5 collapse pits occur on the ground surface during the test process. The settlements at the surface and subsurface are measured to analyze the collapse process of the cobble stratum, as well as the earth pressure in the region above the tunnel crown. The quadratic partial derivative of the settlement and earth pressure are used to judge the collapse moment of the soil. The test results show that the collapse of the cobble stratum develops with a uniform speed from the bottom to up with the shield advancing, and that the collapse region extends to the surface after the cutter head passes through the center of the collapse pit 0.5 D ∼ 1.0 D. Meanwhile, the final earth pressure is much smaller than its initial value only in the region above the tunnel crown, indicating the collapse region concentrating in the position directly above the crown in the cross-section. In addition, the shape of surface settlement trough changes significantly with the collapse development. Development curves of two characterization physical quantities of the surface settlement trough, i.e., the maximum settlement and width coefficient, are respectively close to the " S " shape and the inverse " S " shape. [ABSTRACT FROM AUTHOR]

A model test of shield under-crossing in cobble strata is conducted, based on the self-designed earth pressure balance (EPB) shield platform, to analyse mechanical response of the existing tunnel. The test model includes two existing tunnels, i.e., a horseshoe tunnel and a rectangular tunnel, which are designed by taking the interval tunnel of Beijing metro line 10 as background. The earth pressure acting on the existing tunnel, longitudinal strain of the top plate and base plate of the existing tunnels, and circumferential strain in the central cross-section of the existing tunnels, are measured during the shield under-crossing process. Based on the measured results, variation law of the earth pressure on the existing tunnels and their deformation feature along the longitudinal and circumferential direction during the whole shield advancing process are revealed. The deformation difference between the horseshoe tunnel and rectangular tunnel is found, and the weak positions in the two existing tunnels are pointed out. [ABSTRACT FROM AUTHOR]

In this paper, a numerical method of the rising groundwater level is presented through an elastoplastic constitutive model for the unsaturated soil and a numerical realization method for the rising groundwater level. A model test of the rising groundwater level is conducted to validate the rationality of the presented numerical method. Taking an interval subway tunnel in Beijing as the background, a numerical simulation with a homogeneous soil layer, which contians the existing tunnel, is conducted according to the presented numerical method of the rising groundwater level. Based on the numerical simulation results, the variations of the ground saturation, the ground stress state, and the ground vertical deformation under the condition of the rising groundwater level are studied through a homogeneous soil layer, and the normal contact pressure on the outer surface of the lining, the radial deformation, the bending moment, the hoop thrust, and the shear force of the lining under the condition of the rising groundwater level are analyzed through a homogeneous soil layer with the existing tunnel. [ABSTRACT FROM AUTHOR]