Stability analysis of shallow undrained tunnel heading using finite element limit analysis

This dissertation investigated the undrained stability of shallow tunnel heading problems subjected to varying loading conditions by performing a two-dimensional plane strain analysis. Failure due to the blowout mechanism was highlighted as a major focus area, due to the lack of previous research on the topic. Finite element limit analysis (FELA), employed through the geotechnical software analysis package, Optum G2, was used to determine lower and upper bound factor of safety (FoS) values for a range of various scenarios. The factor of safety values were calculated using the gravity multiplier method (GMM) and the strength reduction method (SRM). These methods were directly compared and the strength reduction method was found to be the most suitable method for analysing scenarios with either a surcharge or internal tunnel pressure applied. The results obtained in this study were validated by comparing a sample to results published by Augarde, Lyamin and Sloan (2003). This comparison found a very good level of agreement. The factor of safety is a function of three dimensionless parameters; the pressure ratio (PR), strength ratio (SR) and depth ratio (DR). The relationship between the factor of safety and these parameters was investigated. A number of plots and displacement vector fields were created to better assist in understanding these relationships and the specific failure mechanism related to each scenario. This process reinforced the need to not only design tunnels for failure due to collapse but to also check for failure due to blowout. The stability of tunnels has historically been expressed in the form of a stability number, similar to the approach adopted by Taylor (1937). This dissertation presents results by applying the factor of safety approach, allowing for direct and clear interpretation of results and any practical implications. The research culminated in the development of a variety of tunnel heading stability design charts. These design charts have been designed for use by practicing engineers in the preliminary stages of tunnel design. A number of select examples are provided to outline some of the potential uses of the design charts. One particularly useful practical application of the design charts is the ability to determine a safe operating range for the pressure that can be applied to the tunnel excavation face by a tunnel boring machine during construction