Abstract
The existence of cavities in shallow ground strata is one of the important causes of urban road collapse under the disturbance of tunnel excavation. Thus, this paper discusses the convergent deformation mode of ellipsoidal cavities. To this end, the convergent deformation of a cavity and the overall displacement of a tunnel were comprehensively examined. A three-dimensional symmetrical calculation model of the soil deformation under the combined action of the tunnel and the cavity was also established. Moreover, three-dimensional formulas for calculating the soil deformation and the surface settlement of the upper part of the tunnel and the cavity were derived. The influence of the different positions of the cavity on the surface settlement of the upper part of the tunnel was also examined. Further, the change in the soil settlement with the direction of the tunnel excavation and the depth of burial of the cavity was analyzed. The results show that the calculated settlement curves are consistent with the ones reported in the related literature. The cavity can also aggravate the surface settlement and deformation of the soil caused by the tunnel excavation. When the cavity is directly above the tunnel, the surface settlement curve is symmetrically distributed. As the position of the cavity changes, the overall settlement curve shifts to the direction of the cavity, showing asymmetry. Additionally, along the x-axis direction of the shielded tunnel, the surface settlement gradually increases to a limit value with a decrease in x and slowly declines to zero as x rises. Finally, along the depth of burial of the cavity, the settlement of the soil continues to enlarge; also, the growth rate of the soil settlement continues to increase further at positions closer to the cavity and the tunnel until it reaches a critical maximum.
Highlights
In recent years, there have been frequent ground collapse accidents caused by the excavation of shallow urban tunnels owing to the existence of a large number of cavities in the shallow space
This paper studies the variation in the soil settlement value with the direction of the tunnel excavation and the depth of burial of the cavity
The calculated surface settlement curve is a normal distribution, and the settlement influence ranges from −20 m to 20 m; the maximum settlement occurs in the center of the tunnel excavation and equals 56.93 mm
Summary
There have been frequent ground collapse accidents caused by the excavation of shallow urban tunnels owing to the existence of a large number of cavities in the shallow space. In Beijing, Chengdu, and other locations where the ground is dominated by silt, silty clay, sand, and pebbles, many stratum cavities are usually found during the construction of subways. The construction of shielded tunnels in the stratum with cavities can give rise to engineering accidents such as cavity collapse and water gushing [1]. Taking an ellipsoidal cavity as an example, due to the influence of the tunnel excavation, its deformation mode is obviously related to the relative position of the tunnel. The surface deformation and the soil deformation caused by the cavity before the tunnel excavation are not within the scope of this paper. On the basis of the random medium theory and the research results of Qi et al [9], the final settlement volume of the upper soil caused by the convergent deformation of the cavity should be equal to the lost volume of the soil
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