What causes the excessive metro tunnel settlement in soft deposits: learned from a detailed case with factor decomposition

Abstract

Widespread tunnel settlement has been observed in the Yangtze River Delta region of China, but its formation mechanism remains unknown. In this study, a settlement trough and its development processes over nearly 7 years were monitored in detail. Notably, this representative case covers the majority of factors that can contribute to tunnel settlement. Records of acting processes of these factors are so comprehensive that factor decomposition was conducted to analyze the quantitative contributions. Specifically, potential factors were classified into internal and external factors, where internal factors (self-consolidation induced by tunnel construction and cyclic loading from traffic) account for 12% of the maximum observed tunnel settlement. Among the external factors, adjacent pile construction and diaphragm wall construction, ground buildings surcharge, adjacent pit excavation, and basement construction accounted for 5%, no more than 10%, no more than 22%, and less than 1% of the maximum observed tunnel settlement, respectively. Adjacent dewatering activities, which accounts for 46 to 67% of settling, were determined to be the dominant factor. Additionally, it was further found that the impact of adjacent dewatering activities on tunnels cannot be reduced by specially designed cutoff diaphragm walls. Wall leakage and the “pit dewatering with adjacent barriers effect” were proved to be potential causing factors. In particular, this effect can be a factor causing significant water head drawdown and subsequent tunnel settlement along metro lines, where high-density buildings tend to develop. These findings were extended to the entire metro line to discuss the mechanism of widespread metro tunnel settlement in this region.

Appendix Estimation of settlement contributions from tower surcharge

This appendix presents the method to roughly estimate the settlement contribution from the towers.

Firstly, as seen in Fig. 16, the tower surcharge applied to the basement level was abstracted as a uniform load p in a semi-infinite half-space. Then, the additional vertical stress σ_zi at the i-th point underneath the tunnel (W₀, L₀, Z_i) can be computed using the Mindlin function (Mindlin 1936) as follows:

$${\sigma }_{zi}=\frac{p}{8\pi \left(1-\mu \right)}{\iint }_{\Omega }\left[\left(1-2\mu \right)\left(\frac{{\beta }_{1}}{{R}_{1}^{3}}-\frac{{\beta }_{1}}{{R}_{2}^{3}}\right)+\frac{3{\beta }_{1}^{3}}{{R}_{1}^{5}}+\frac{3\left(3-4\mu \right){Z}_{i}{\beta }_{2}^{2}-3h{\beta }_{2}\left(5{Z}_{i}-h\right)}{{R}_{2}^{5}}+\frac{30{Z}_{i}h{\beta }_{2}^{3}}{{R}_{2}^{7}}\right],$$

(4)

where ${R}_{1}=\sqrt{{\left({W}_{0}-\xi \right)}^{2}+{\left({L}_{0}-\eta \right)}^{2}+{\left({Z}_{i}-h\right)}^{2}}$, ${R}_{2}=\sqrt{{\left({W}_{0}-\xi \right)}^{2}+{\left({L}_{0}-\eta \right)}^{2}+{\left({Z}_{i}-h\right)}^{2}}$, β₁ = Z_i − h, β₂ = Z_i + h, Ω is the load application area (projected area of the tower), (ξ, η, h) represents the points in Ω, and μ is Poisson’s ratio. The total settlement can be calculated using the layer-wise summation method as follows:

$${S}_{ts}=\sum_{i}\frac{{\sigma }_{zi}+{\sigma }_{z\left(i+1\right)}}{2{E}_{si}},$$

(5)

where S_ts is the tunnel settlement caused by the tower surcharge, and H_i and E_si are the thickness and compression modulus of the i-th sublayer, respectively.

Fig. 16

Schematic diagram of our method for estimation of the settlement caused by tower surcharge: a plan view and b front view

In this case, the surcharge of a tower p can be calculated by multiplying the number of floors by the weight of a single floor, which is estimated conservatively as 15 kPa per floor. The Poisson’s ratio of soft clay μ is 0.36 (Zhang et al. 2013). The compression modulus E_s can be obtained as follows (Mitchell and Soga 2005):

$${E}_{s}={E}_{s1-2}\sqrt{\frac{{\sigma }^{^{\prime}}}{100}},$$

(6)

where E_s1-2 is the compression modulus tested at a consolidation pressure of 100 to 200 kPa listed in Table 1 and σ` (kPa) is the effective stress of the i-th sublayer. The calculation range is within the underneath soil above bedrock.