Elsevier

Journal of Hydrology

Volume 583, April 2020, 124582
Journal of Hydrology

Research papers
Analyses of leakage effect of waterproof curtain during excavation dewatering

https://doi.org/10.1016/j.jhydrol.2020.124582Get rights and content

Highlights

  • The impact on surroundings due to a leakage in the waterproof curtain was analysed.

  • A 3D fluid–solid coupled finite element model was proposed.

  • A user-defined 1D leakage element and a solid leakage element are incorporated.

  • Leakage will lead to both elastic and plastic deformations of the soil.

  • Factors, e.g. hydraulic conductivity, anisotropy, and deformation modulus of soil, are investigated.

Abstract

The waterproof curtain that is used to block groundwater, leaks occasionally during deep excavation and can have a harmful impact on the surroundings. This study adopts a three-dimensional (3D) fluid-solid coupled finite element model (FEM) to analyze the impact due to a leakage in the waterproof curtain during excavation dewatering. In the model, both a user-defined one-dimensional leakage element and a solid leakage element are incorporated to simulate the leakage. The leakage of the curtain in the soil is analysed by monitoring the variation in hydraulic conductivity. The simulated results demonstrate that the leaking rate, leaking volume, groundwater drawdown, and ground settlement are closely related to the hydraulic conductivity of the leakage point. The results also show that the groundwater head outside the pit declines quickly when the leakage occurs, but soil consolidation proceeds slowly. In addition, the characteristics of the soil that is adjacent to the leakage point also have a tangible effect on the groundwater head and ground settlement. The hydraulic conductivity and deformation modulus of the soil has a significant effect on the ground settlement, while the influence of the anisotropy of the soil is relatively low.

Introduction

A large number of underground structures have already been constructed, or are being constructed, in the coastal regions of China in recent years to solve the problems related to a rapid growth in population. As a consequence, numerous excavation pits appear in the coastal region (Tan et al., 2018, Tan et al., 2019, Xu et al., 2014, Xu et al., 2018, Zhang et al., 2018, Zhang et al., 2019). The Quaternary soft deposits are distributed in these regions and the groundwater is characterized by a multi-aquifer-aquitard system with hydraulic connections between the aquifers (Wu et al., 2017, Ren et al., 2018, Atangana Njock et al., 2020.).

The groundwater head in the Quaternary soft deposits is high, and the excavations are usually below the groundwater head. When the excavation is performed in these areas, the groundwater has a great effect on the safety of the excavation pit (Shen et al., 2017, Xu et al., 2019, Zeng et al., 2018, Zeng et al., 2019b, Zeng et al., 2019c). Thus, a combination of large diameter wells and waterproof curtains are often applied to lower the groundwater head inside the excavation pit in the course of the excavation (Pujades et al., 2017, Serrano-Juan et al., 2017, Wang et al., 2017, Wang et al., 2019a, Wang et al., 2019b). However, defects in the waterproof curtain are inevitable because of limitations in construction technology and poor construction quality (Wu et al., 2015a, Wu et al., 2015b, Tan and Lu, 2017). Fig. 1 displays photographs of the leakage in a waterproof curtain above the excavation surface. Fig. 1(a) exhibits the water seepage induced by a slight leakage in the joints between the diaphragm walls in Shanghai. Fig. 1(b) displays quicksand formed because of a hole at the corner joints between the diaphragm walls in Tianjin (Wu et al., 2018a, Wu et al., 2018b). Groundwater from outside the pit flows into the pit through these defects, resulting in a decline of the groundwater head outside the pit and soil deformation. Additionally, serious leakage may result in significant hazards, e.g. ground settlement, ground collapses, building tilting, and pipeline breaking, etc. (Pujades et al., 2012, Pujades et al., 2016, Conway, 2016, Liu et al., 2018, Wu et al., 2019), which will cause high risk of city infrastructures (Lyu et al., 2019, Lyu et al., 2020a, Lyu et al., 2020b).

To prevent construction accidents and potential hazards related to leakage, it is crucial to understand whether there is leakage in the waterproof curtain and the location of the leakage before excavation. At present, geophysical prospection is used to determine the leakage condition of the waterproof curtain. Such prospection methods include ultrasonic detection, the resistivity method, and the temperature tracer method (Paikowsky and Chernauskas, 2003, Rausche, 2004). The hydrogeological method known as the pumping test is also used to determine the condition of the waterproof curtain (Ross and Beijin, 1998; Vilarrasa et al., 2011, Pujades et al., 2012, Wu et al., 2015a). In addition, Zheng et al. (2014a) simulated and investigated the mechanism of underground engineering disasters caused by water and soil erosion through laboratory tests. They performed the particle flow code-computational fluid dynamics (PFC-CFD) to study the variation of the leakage process of sand and water, which discovered a critical width of gap in the leakage process.

When the leakage of the waterproof curtain occurs above the excavation surface, even a slight leakage may have an effect on the surrounding environment. Therefore, it is necessary to analyze the effect of leakage of the waterproof curtain on soils deformation and ground settlement. In general, numerical methods, such as two-step calculation method, partially coupled method and fully coupled method, are applied to predict the ground settlement caused by a decline of groundwater head. For the two-step method, variation of groundwater head is firstly calculated by seepage model, and then the deformation of soil is computed based on the groundwater head (Gambolati and Freeze, 1973). Zhou et al. (2010) predicted the ground settlement resulted from excavation dewatering in Shanghai. The partially coupled method couples the groundwater flow model and settlement model together by some parameters, such as storage coefficient and compressibility coefficient, hydraulic conductivity and pore ratio. This approach is generally based on Terzaghi's consolidation theory. Shen and Xu (2011) used the three-dimensional seepage flow model and one-dimensional soil consolidation model to predict the land subsidence caused by groundwater withdrawal in Shanghai. Wu et al., 2015a, Wu et al., 2015b employed the same method to calculated the ground settlement resulted by excavation dewatering in Hangzhou. The fully coupled method is based on Biot's consolidation theory (Biot, 1941). Gambolati et al. (1974) first used this model to analyze the land subsidence in Italy in the 1970s. Zeng et al., 2018, Zeng et al., 2019a applied a Modified Cam-Clay model to study the surrounding ground deformation caused by the pre-excavation dewatering. Li et al. (2019) employed Duncan-Zhang model and Kozeny-Carman equation to explore the land subsidence induced by both load of high-rise buildings and groundwater withdrawal in Nantong, China. The above researches mainly focused on groundwater withdrawal induced the ground settlement. There are few studies of the effect of leakage of waterproof curtain on ground settlement.

This study evaluates the effect of leakage above the excavation surface by numerical simulation excluding the quicksand hazard. The objectives of this study are as follows: (i) to study the influence of leakage in a waterproof curtain on groundwater head and ground settlement by the fully coupled model; and (ii) to investigate the effect of soil properties on the groundwater head and ground settlement.

Section snippets

Problem description

To understand the effect of leakage in a waterproof curtain, a circular excavation pit is selected, as show in Fig. 2(a). The analysis is conducted based on typical strata in Shanghai that are common in the coastal regions of China (Xu et al., 2014, Wu et al., 2016). There is one phreatic aquifer (labeled as Aq0) and there are two confined aquifers (labeled as AqI and AqII) that are separated by three aquitards (labeled as AdI, AdII, and AdIII). The excavation pit has a diameter of 36 m, and a

Results and analysis

To investigate the effect of the leakage of the waterproof curtain on groundwater seepage and settlement, five cases are calculated by the simulations. Table 3 lists the calculation scheme of the leakage. It is assumed that the waterproof curtain leaks at a depth of 28 m in AqI with a leakage area of 0.00005 m2. The leakage area is so small that a self-defined element is applied. Several numerical results have been obtained from the 3D fluid-solid model by varying the hydraulic conductivity of

Discussion

In this part, the influence of soil properties on the ground settlement around the leakage point is discussed. These properties include the soil permeability, anisotropy, and the deformation modulus. It is assumed that the leakage occurs at a depth of 24.67 to 28 m in the waterproof curtain with a length of 3.23 m and the corresponding area is 10.74 m2. The leakage area is large and therefore a solid element is used to simulate the leakage, which is different from the former simulation with a

Conclusions

This study presented a 3D fluid–solid coupled model to investigate the impact on surroundings due to a leakage in the waterproof curtain over the excavation surface. The following conclusions are reached:

  • (1)

    When a leakage occurs in the waterproof curtain, the seepage path and seepage direction outside of the pit change. Both the leakage rate and leakage volume increase with an increase in the hydraulic conductivity at the leakage point (kl). The relationship between the leakage volume per day and k

CRediT authorship contribution statement

Yong-Xia Wu: Data curation. Shui-Long Shen: Conceptualization, Methodology, Software, Supervision. Hai-Min Lyu: Visualization, Investigation. Annan Zhou: Methodology, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The research work described herein was funded by the National Natural Science Foundation of China (NSFC) (Grant No. 41807235), the Research Funding of Shantou University for New Faculty Member (Grant No. NTF19024-2019), and the Innovative Research Funding of the Science and Technology Commission of Shanghai Municipality (Grant No. 18DZ1201102). These financial supports are gratefully acknowledged.

References (65)

  • S.L. Shen et al.

    Evaluation of hydraulic parameters from pumping tests in multi-aquifers with vertical leakage in Tianjin

    Comput. Geotech.

    (2015)
  • J.H. Shin et al.

    Long-term mechanical and hydraulic interaction and leakage, evaluation of segmented tunnels

    Soils Found.

    (2012)
  • Y. Tan et al.

    Characterization of semi-top-down excavation for subway station in Shanghai soft ground

    Tunn. Undergr. Space Technol.

    (2017)
  • Y. Tan et al.

    Investigation on performance of a large circular pit-in-pit excavation in clay-gravel-cobble mixed strata

    Tunn. Undergr. Space Technol.

    (2018)
  • V. Vilarrasa et al.

    A methodology for characterizing the hydraulic effectiveness of an annular low-permeability barrier

    Eng. Geol.

    (2011)
  • J.X. Wang et al.

    Field experiment and numerical simulation of coupling non-Darcy flow caused by curtain and pumping well in foundation pit dewatering

    J. Hydrol.

    (2017)
  • X.W. Wang et al.

    Evaluation of optimized depth of waterproof curtain to mitigate negative impacts during dewatering

    J. Hydrol.

    (2019)
  • Y.X. Wu et al.

    Characteristics of dewatering induced drawdown curve under blocking effect of retaining wall in aquifer

    J. Hydrol.

    (2016)
  • H.N. Wu et al.

    Soil-tunnel interaction modelling for shield tunnels considering shearing dislocation in longitudinal joints

    Tunn. Underground Space Technol.

    (2018)
  • Y.S. Xu et al.

    Evaluation of the blocking effect of retaining walls on groundwater seepage in aquifers with different insertion depths

    Eng. Geol.

    (2014)
  • Y.S. Xu et al.

    Design of Sponge City: lessons learnt from an ancient drainage system in Ganzhou, China

    J. Hydrol.

    (2018)
  • Y.P. Yao et al.

    A unified constitutive model for both clay and sand with hardening parameter independent on stress path

    Comput. Geotech.

    (2008)
  • C.F. Zeng et al.

    Responses of retaining wall and surrounding ground to pre-excavation dewatering in an alternated multi-aquifer-aquitard system

    J. Hydrol.

    (2018)
  • C.F. Zeng et al.

    Responses of deep soil layers to combined recharge in a leaky aquifer

    Eng. Geol.

    (2019)
  • C.F. Zeng et al.

    Combined recharge: A method to prevent ground settlement induced by redevelopment of recharge wells

    J. Hydrol.

    (2019)
  • C.F. Zeng et al.

    Behaviours of wall and soil during pre-excavation dewatering under different foundation pit widths

    Comput. Geotech.

    (2019)
  • S. Zhang et al.

    Elasto-plastic model of structured marine clay under general loading conditions

    Appl. Ocean Res.

    (2018)
  • G. Zheng et al.

    Test and numerical research on wall deflections induced by pre-excavation dewatering

    Comput. Geotech.

    (2014)
  • G. Zheng et al.

    Experimental study on the artificial recharge of semiconfined aquifers involved in deep excavation engineering

    J. Hydrol.

    (2018)
  • G. Zheng et al.

    Influence of the opening timing of recharge wells on settlement caused by dewatering in excavations

    J. Hydrol.

    (2019)
  • N.Q. Zhou et al.

    Numerical simulation of deep foundation pit dewatering and optimization of controlling land subsidence

    Eng. Geol.

    (2010)
  • J. Bear

    Hydraulics of Groundwater

    (1979)
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