The stability of shallow circular tunnels in soil considering variations in cohesion with depth

Abstract

This paper presents an upper bound investigation of the three dimensional stability of a tunnel face in a deposit of soil whose strength varies with depth. The upper bound theorem of limit analysis incorporating the linear variation of the soil cohesion with depth was used to calculate the pressure at the tunnel face of a closed face excavation. For an open face excavation, the factor of safety against the tunnel face instability was calculated using the strength reduction technique and the upper bound theorem. The results, in terms of the minimum required face pressure, were then compared with other solutions available from the literature for verification, and the numerical results in the form of dimensionless design charts are also presented. In addition, a comparative study between the simplified approaches adopting a singular soil cohesion parameter representing the whole layer instead of considering its actual variation with depth is presented. It was concluded that adopting the mean soil cohesion that does not vary with depth would lead to a conservative design, that is, a higher minimum face pressure being required during construction and a lower factor of safety against face instability. However, adopting the local cohesion obtained from the tunnel face may result in underestimating the required face pressure and may lead to an unsafe design.