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Experimental study of tunnels in squeezing ground conditions
Arora, Ketan
Arora, Ketan
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2020
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2022-04-17
Abstract
The presence of squeezing ground conditions often poses significant challenges in predicting tunnel response over time and to the design of an adequate support system to stabilize the tunnel. Over the years, many methodologies have been proposed to predict squeezing in tunnels based on tunnel depth, in situ stress, ground mineralogy, and ground strength and deformation behavior. Most of these methodologies are problem-specific and limited in scope. The study presented in this thesis was focused on improving the understanding of tunnel squeezing via a unique physical model test that simulated tunnel boring machine (TBM). To identify the critical parameters contributing towards squeezing, a case study of four tunnels constructed in squeezing clay-rich rock was carried out. It was established from the case studies that by combining normalized engineering behavior of rocks, Peck’s stability number and Geological Strength Index (GSI), the squeezing potential for the tunnels could be determined. A squeezing number S is suggested to classify ground conditions based on the level of squeezing that the ground may experience in response to tunneling. It was demonstrated that by combining the proposed classification system and an existing classification system for squeezing ground conditions, an accurate estimate of tunnel strain could also be obtained. Following the case studies, a novel physical model test to simulate a tunnel boring machine (TBM) excavation in squeezing ground conditions has been proposed. The physical model included a large true-triaxial cell, a miniature tunnel boring machine (TBM), a laboratory-prepared synthetic test specimen having properties similar to natural mudstone, and instrumentations to monitor deformations around the tunnel boundary during and after the excavation. The true-triaxial cell can apply principal stresses up to 13 MPa on a 300x300x300mm3 cubical rock specimen independently on the three principal planes, and this corresponds to real in-situ stress conditions. Miniature TBM can excavate a tunnel having 48-mm and maximum length 150-mm. The instrumentation and monitoring included embedded strain gauges in the form of multiple point borehole extensometer (MPBEx) and a caliper to monitor deformations around and at the tunnel boundary, respectively. The preliminary testing showed the capability of the test setup in capturing crucial features of tunnel excavation, such as tunnel advance, three-dimensional effects, and highly plastic and ductile time-dependent behavior of the ground. The physical model was used to study the behavior of supported and unsupported tunnels at various isotropic stress levels. The deformation data were obtained from the embedded strain gauges, digital borehole caliper and strain gauges on the support system. The degree of tunnel squeezing was characterized using a classification system based on tunnel radial strain. A model for time-dependent tunnel longitudinal displacement profile (LDP) for unsupported and supported tunnels was proposed using measurements of the tunnel convergence at different times and different stress levels. The LDP parameters for both the cases were compared to account for the influence of the support system. The back-calculation of thrust forces on the support system provided an estimate of the additional effects induced in the support due to squeezing. Finally, based on the observations, a recommendation for the analysis to determine safe and economical support for the tunnel constructed in squeezing ground was proposed.
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