FE analysis of time-dependent behavior of tunneling in squeezing ground using two different creep models

doi:10.1016/j.tust.2004.09.001

Tunnelling and Underground Space Technology

Volume 20, Issue 3, May 2005, Pages 271-279

https://doi.org/10.1016/j.tust.2004.09.001 Get rights and content

Abstract

Ground movement and contact pressure on the lining of Stillwater Tunnel (Utah, USA) were investigated. Axisymmetirc finite element analysis was used in the analysis. Power law and hyperbolic creep models were used to model ground squeezing and to show the differences in the results between the two models. Creep parameters for the two models were evaluated based on the experimental creep and strength tests that were performed by other investigators on the gouge materials encountered along the tunnel axis through the heavily sheared and fault zones of the Red Pine shale. The results of the analysis which include normalized inward movement at the tunnel crown, normalized radial ground convergence with depth, and lining-ground contact pressure were compared with the results that were measured along the tunnel axis by other investigators. The results of the analysis show that lining pressure and deformation can be predicted well from the use of power law creep model if the delay time before lining erection is considered.

Introduction

Squeezing in tunnels is a time-dependent ground movement around the opening. This behavior is mainly related to the progressive yielding and time-dependent deformation and strength properties of the ground. Terzaghi (1946) described squeezing in rock as the displacement of ground under no volume change conditions. According to Barla (1995), squeezing around the tunnel opening may stop throughout the construction process or it may prolong for considerable amount of time.

Squeezing in rock mainly occurs in marl, argillaceous and phyllitic schist, and materials containing considerable amount of clay or mica such as fault shales. Time dependent deformations should be highly considered when evaluating the stability of the underground opening and designing its support system because considerable amount of deformation and contact pressure may develop with time. Ground deformation and its rate, the extend of the creeping zone around the opening, and the corresponding loads on the support system in squeezing ground depend on many factors such as the rock type, rock mass characteristics, in situ stress conditions, groundwater conditions, excavation and lining erection sequence, relative stiffness between the lining and the ground around the opening, and delay in support installation. Ignoring these factors in a ground that has a potential for squeezing may severely delay the construction program. There are many cases where squeezing occurred during tunneling and caused serious delay in the construction. These include the Stillwater tunnel in USA, the Gottherd tunnel in Switzerland and the Frejus tunnel in France.

It is relatively difficult to estimate ground movement and tunnel support loads in squeezing ground. This is mainly due to the complex behavior of the ground around the opening, even with the increase in our understanding of soil and rock mechanics. Many researchers attempted to evaluate ground squeezing using different approaches. Semple (1973) and Mesri et al. (1981) used the experimental approach to evaluate the soil creep parameters by performing experimental tests on soil samples. Kallhawy (1974), Ghaboussi and Gioda (1977), Ghaboussi et al. (1981), Gioda, 1981, Gioda, 1982, Sulem et al. (1987), Pan and Dong (1991), Gioda and Cividini (1996) used the analytical approach to evaluate ground squeezing by using different creep models. Terzaghi (1946), Deere et al. (1970), Barton et al. (1974), Singh et al. (1992), Goel et al. (1995) used rock mass classification system (empirical approach) to predict squeezing pressure. Other researchers used measured ground deformations around underground openings to evaluate ground squeezing and contact pressures (Phien-wej and Cording, 1991, Panet, 1996, Steiner, 1996).

In this paper, ground deformation and lining pressure in squeezing ground was evaluated using the analytical approach (finite element analysis) combined with the experimental test results. ABAQUS (1997) finite element software was used in the analysis. Power law and hyperbolic creep models were used to evaluate lining pressure and ground convergence with time. Data from experimental creep and shear tests that were performed on the gouge materials encountered through the sheared shale zones along Stillwater Tunnel (Utah, USA) was used to extract the creep parameters. Axisymmetric finite element analysis was used in the analysis. The results of the two creep models were compared with each other. Measured values of lining contact pressure and deformation of the same tunnel were also compared with the results of the analysis.

Section snippets

Analyzed tunnel and geologic conditions

Stillwater Tunnel (Utah, USA) was used in the analysis of this work. The tunnel is 13 km long with 3.5–4 m excavated diameter. According to Phien-wej and Cording (1991), the tunnel passed through zones of sandstone, argillite, and Red Pine shale. Many heavily sheared-zones and faults with high dipping angles encountered the tunnel and these weak zones constitute about 10% of the tunnel length. The faults include zones of moderately to closely jointed shale and silt and gouge materials of silt

Creep models of the analysis

The analysis was performed using non-linear visco-elastic models. The author tried to introduce plastic models in the analysis namely, Cam-clay plasticity model beside the creep models, but the testing data were not enough to fully evaluate the plastic model parameters. Two creep models were used in the analysis of ground squeezing around the tunnel opening, namely, the hyperbolic and the power low creep models. Hyperbolic creep model was used because it includes the effect of soil shear

Loading condition and excavation sequence

Squeezing analysis was performed using ABAQUS finite element analysis software. Fig. 3 shows the analyzed tunnel with the applied loads and boundary conditions. The analysis was performed using what is called in ABAQUS step analysis. Excavation was simulated by deactivating model elements inside the tunnel. Concrete segment lining was introduced by activating the lining elements. Creep deformation was introduced throughout individual steps after each step of excavation and lining erection.

Lining and crown deformation

Fig. 4, Fig. 5 show the normalized crown displacement with the normalized distance along the tunnel axis using power law and hyperbolic creep models, respectively. In these figures, U_a is the crown convergence, a is the tunnel radius, σ_v is the overburden pressure and E_r is the rock mass modulus. Both figures show that the crown inward displacement increases with time at a decreasing rate. The displacement starts to level off at a distance of 2D (D is the tunnel diameter) behind the tunnel face

Conclusions

Finite element analysis of ground movement and contact pressure of Stillwater Tunnel (Utah, USA) using power law and hyperbolic creep models and using the measured strength and creep properties of the sheared shale around the tunnel combined with the measured data of lining deformation and contact pressure led to the following conclusions:

1.
Crown inward displacement increases as the creep time increases with a decreasing rate. Power law and hyperbolic creep models predict the crown displacement

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FE analysis of time-dependent behavior of tunneling in squeezing ground using two different creep models

Abstract

Introduction

Section snippets

Analyzed tunnel and geologic conditions

Creep models of the analysis

Loading condition and excavation sequence

Lining and crown deformation

Conclusions

Engrg. Geol.

Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

Eng. Geol.

Int. J. Rock Mech. Min. Sci. Geomech. Abstr.

Tunn. Undergr. Sp. Technol.

Squeezing rocks in tunnels

ISRM News J.

Engineering classification of rock masses for the design of tunnel support

Rock Mech.

On the time-dependent effects in advancing tunnels

Int. J. Numer. Methods Geomech.

Time-dependent behavior of solution caverns in salt

ASCE