Model Test on the Influence of Surcharge, Unloading and Excavation of Soft Clay Soils on Shield Tunnels

Abstract

In view of the influence of symmetrical surcharge, unloading and excavation of soft clay soils on shield tunnels, the surface settlement, tunnel settlement, vertical additional earth pressure and tunnel additional confining pressure are measured by an indoor model test. The changes of confining pressure, tunnel settlement, surface settlement and vertical earth pressure caused by surcharge, unloading and excavation are studied and analyzed. The results show that, in soft clay, surcharge will cause tunnel settlement, and unloading and excavation will generally cause tunnel uplift. There is hysteresis in surface settlement and tunnel settlement above the shield tunnel caused by surcharge and unloading; the change of additional confining pressure of the shield tunnel caused by surcharge is mainly concentrated in the three directions of 3, 5 and 7 of the radar chart. These points belong to weak points. Unloading and excavation can effectively reduce the additional confining pressure of the tunnel in these three directions; excavation will cause the increase of vertical cumulative additional earth pressure and tunnel confining pressure in local directions. The influence range of surcharge in soft clay is wider than that in sand, but the depth is relatively shallow.

Experimental Study and Theoretical Analysis of the Effect of Surcharge/Unloading at Both Sides of the Ground Surface on an Existing Shield Tunnel at Different Burial Depths

“…This shows that the larger the surcharge above the tunnel, the larger the tunnel settlement and unloading above the tunnel can mitigate the tunnel settlement to a certain extent. This result is different from the results obtained by Xiang et al [25] in soft clay soils, that is, tunnel settlement continues to increase to some extent after unloading, while it decreases very quickly after unloading in sandy soils.…”

“…At present, there are many studies on the effects of ground surcharge on existing underground shield tunnels, which can be mainly divided into four major categories according to different research methods: actual measurement studies [1][2][3][4], theoretical studies [5][6][7][8][9][10][11][12][13], numerical simulation studies [14][15][16][17][18][19][20][21][22], and model test studies [23][24][25][26][27][28][29][30][31][32]. In terms of field measurement studies, Wang et al [1] analyzed the lateral deformation and structural leakage of a subway section in Shanghai due to road fill and provided a specific repair plan and the repair effect.…”

“…In terms of model test research, Liang et al [23] used a 3D-printed refined model tunnel to explore the longitudinal force changes of the model tunnel after surface surcharge/unloading. Wei et al and Xiang et al [24][25][26][27] employed model tests to study the effects of surface surcharge/unloading on tunnel additional-surrounding-pressure, surface settlement, and tunnel deformation in various aspects. Reference [26] focused on the study of the impact of the load directly above on the existing shield tunnel below and reference [27] focused on the study of the impact of different loading positions on both sides on the existing shield tunnel below.…”

Influence of foundation pit excavation on the internal force of adjacent shield segment structure

“…As such, the impact of these excavations on the safe operation of the tunnel is garnering increasing attention. For instance, both Yao et al [3] and Xiang et al [4] utilized model testing and numerical simulations to demonstrate that unloading or excavation can lead to tunnel uplift. It becomes evident that the maximum uplift value is reached when the excavation of the foundation pit reaches its lowest point, and there is a significant reduction in vertical earth pressure throughout the excavation process.…”

Structural displacement and deformation of adjacent shield tunnel induced by foundation pit excavation

“…However, there is a lack of research on lateral unloading. In the aspect of model test, Xiang et al [8] measured the influence of loading, unloading and excavation on tunnel settlement and additional stress in soft clay by indoor model test; Wei et al [9] further delved into the protective impact of the embedded isolation pile system on the pre-existing tunnel during the excavation of the adjacent foundation pit. Chen et al [10] examined the impact of excavation on tunnels with varying overburden depths using centrifugal testing methods.…”