Evaluation of earth pressure and performance of shoring systems for Québec sensitive clay: experimental and numerical study

Abstract

Cave-ins are very frequent in the construction industry and present a very serious risk for workers who operate inside trenches or excavations. Studies show that excavations or trenches in Quebec sensitive clay soils are extremely hazardous due to possible severe and sudden loss of soil shear strength. Flexible and semi-flexible supporting structures are used as temporary supports for the vertical sides of excavations. The earth pressure distribution depends on the type of soil, the shoring, and the method of implementation. Generally, no satisfactory consensual theoretical solutions are available to estimate soil pressure for this type of supporting structure with partitions facing land pressures in sensitive clay. Many researchers have suggested theoretical solutions to estimate the earth pressure on a flexible temporary support. A research gap has been observed for trench box shoring at shallow depths of less than 6 m and for soil-structure interactions (SSI). Therefore, the interaction between the soil mass and the support, flexibility in the development of lateral loads, and the movement of structural elements need to be studied for a trench box shoring. This research study focused on the evaluation of earth pressure on retaining structural systems. An Experimental, numerical, and parametric study was carried out to investigate the soil pressure on a temporary flexible trench box shoring in sensitive clay taking into account SSI. In the experimental field test procedures, two trench boxes ‘stacked upon each other’ in conformity with United-States OSHA regulations and placed inside the trench to cover a total depth of excavation of 6 m. Earth pressure was mentioned using total pressure cells (TPC). The field test results were presented in terms of soil pressure distribution along with the depth of excavation with and without a 45 kPa surface surcharge load applied next to the edge of one side of the trench. The Finite-Element (FE) numerical study using PLAXIS-3D (SSI- software) was carried out to reproduce and validate the full-scale experimental in-situ test and investigated the earth pressure on a flexible temporary trench box shield in soft and sensitive clay soil. The excavation trench model was considered as nonlinear and anisotropic clay. Both Mohr- Coulomb (MC) and hardening soil (HS) constitutive soil models were considered for FEanalysis. Different values of the soil-modulus reduction factor (MRF) and of the coefficient of earth pressure at rest (K0) were considered to validate the model. Also, two parametric finite element studies have been carried out using the same computer software to meet the two following specific objectives: (i) to evaluate the effective soil pressure on the steel trench box shield for Till, Dry, Wet and Loose sand and sensitive clay soil; (ii) to assess the effects of the trench box material type (Steel and Aluminum) and geometry on the earth pressure. Furthermore, an additional field experimental case study was carried out to investigate the soil pressure on a trench by vertical aluminum hydraulic shoring with plywood sheeting in soft and sensitive clay trench. The installation, instrumentation, and field test procedures are presented for this type of vertical aluminum hydraulic shoring with plywood sheeting (so-called Speed shore) in conformity with United-States OSHA guidelines and placed inside the trench to cover the total 2.4 m (8 ft.) depth of excavation. Earth pressure was monitored using total pressure cells (TPC). The field test results are presented in terms of soil pressure distribution along the depth of excavation with and without a 30 kPa surface surcharge load next to the edge of one side of the trench. The results obtained reveal the potential use of an existing trench excavation and shoring system for different types of soil and validate its performance with theoretical and experimental results reported in the literature. They also allowed to develop a decision making-guideline that will allow the selection of a shoring system given the Quebec soil classes. This guideline is a very useful tool for practicing engineers in the field.

Date	3 Dec 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Omar Chaallal (Supervisor)