Stability of unsupported tunnels in clay

Abstract

An overall long-term stability of unsupported shallow tunnels in overconsolidated clays which is directly related to the stand-up time is investigated. A new approach that combines finite element methods and the limit equilibrium theory is used to overcome limitations of current design practice. A more realistic initial stress field, unloading due to excavation, and variation of strength and modulus with depth are used. The pore-pressure change is analysed using a finite element model that incorporates an uncoupled consolidation theory. These pore pressures along with the previously obtained stress field are utilized to predict the variation of stability with time for given soil parameters such as strength and coefficient of earth pressure at rest. The results obtained employing a simple mechanism are presented using non-dimensional quantities. These results relate time, stability of the tunnel, and soil strength. The analysis showed that, under certain circumstances, the initial undrained stability may be of no practical value and may only be used as a starting point for more practical long-term stability. This procedure explains the stand-up time phenomenon in tunnels and may also be used in design as a direct tool for its evaluation. Key words : overconsolidated clay, long-term stability, stand-up time, shallow tunnels, finite elements, limit equilibrium.