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Base Instability Triggered by Hydraulic Uplift of Pit-in-Pit Braced Excavations in Soft Clay Overlying a Confined Aquifer

  • Geotechnical Engineering
  • Published:
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Abstract

The aim of this study is to investigate the coupling effects of re-excavation and hydraulic uplift on base instability of pit-in-pit (PIP) braced excavations. The numerical model of PIP braced excavation in Shanghai soft clay overlying a confined aquifer was established by upper-bound finite element limit analysis (UBFELA) method. The effects of the sensitive design parameters (i.e., the artesian pressure, thickness and undrained shear strength of the aquitard and excavation width of inner pit) on failure mechanisms and upper-bound safety factor (FS) against hydraulic uplift were analyzed. The results show that the value of FS increases with an increase in the thickness and undrained shear strength of the aquitard, but decreases with increasing the artesian pressure and excavation width of inner pit. The failure modes can be typically classified into three categories: circular slip surface in outer pit (M1), hydraulic uplift combined with circular slip surface in entire PIP system (M2), and basal hydraulic uplift in inner pit (M3); then the corresponding critical artesian pressure is determined. Finally, the average value of critical artesian pressure used to distinguish the three types of failure modes is recommended as the design value against hydraulic uplift in the PIP system, and validity is verified by the comparison with the current design methods. The proposed stability design by UBFELA contributes to ensure the serviceability and performance of PIP system.

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Abbreviations

b :

Excavation width of inner pit (m)

B :

Excavation width of outer pit (m)

c :

Cohesion (kPa)

d :

Insertion length of inner wall (m)

D :

Insertion length of outer wall (m)

E :

Modulus of elasticity of diaphragm wall (kPa)

E s :

Modulus of elasticity of steel (kPa)

E ur ref :

Elastic unloading/reloading (kPa)

E 50 ref :

Plastic straining due to primary deviatoric loading (kPa)

h :

Depth of inner pit (m)

H :

Depth of outer pit (m)

H a :

Thickness of aquitard (m)

h i :

Thickness of different strata of soil (m)

h s :

Height of the top of confined aquifer (m)

h w :

Height of hydraulic head (m)

I s :

Inertia moment of steel (m4)

K x :

Horizontal coefficient of permeability (m/d)

K y :

Vertical coefficient of permeability (m/d)

m :

Power exponent related stress level

P :

Artesian pressure (kPa)

P cr :

Critical hydraulic pressure (kPa)

P cr1 :

Initial artesian pressure (kPa)

P cr2 :

Average value of Initial and endmost artesian pressure (kPa)

P cr3 :

Endmost artesian pressure (kPa)

p ref :

Reference pressure (kPa)

R inter :

Strength reduction factor

S u :

Undrained shear strength (kPa)

t :

Thickness of diaphragm wall (m)

W :

Calculation length of diaphragm wall (m)

α :

Improving coefficient of stiffness

γ :

Unit weight of soil (kN/m3)

γ w :

Unit weight of water (kN/m3)

υ :

Poisson’s ratio

ϕ :

Internal friction angle (°)

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Acknowledgements

This study is financially supported by the National Natural Science Foundation of China (Grant No.41572253).

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Authors and Affiliations

  1. College of Civil Engineering, Fuzhou University, Fuzhou, 350116, China

    Fuquan Chen & Guangji Miao

  2. Institute of Geotechnical Engineering, Jiangsu Key Laboratory of Urban Underground Engineering & Environmental Safety, Southeast University, Nanjing, 211189, China

    Fengwen Lai

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  1. Fuquan Chen
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  2. Guangji Miao
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Correspondence to Fengwen Lai.

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Chen, F., Miao, G. & Lai, F. Base Instability Triggered by Hydraulic Uplift of Pit-in-Pit Braced Excavations in Soft Clay Overlying a Confined Aquifer. KSCE J Civ Eng 24, 1717–1730 (2020). https://doi.org/10.1007/s12205-020-1102-2

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  • DOI: https://doi.org/10.1007/s12205-020-1102-2

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