Rock squeezing problem in tunnels of Nepal and its prediction

Abstract

Rock squeezing is a common problem in the Nepal Himalayas while tunnelling through low strength rock, fault and shear/weak zone. It reduces the cross-section of a tunnel caused by the in situ stresses, which exceed the rock mass strength. Time of deformation and degree of squeezing generally depends on overburden pressure and non-swelling clay content. Higher the overburden pressure and clay content higher the degree of squeezing. Rock squeezing problems such as inward movement, invert heaving and buckling of walls/crown has been experienced during tunnelling through low strength rock (phyllite, schist, shale, mudstone etc.), shear zones and faults containing considerable amount of non-swelling clay. Based on the squeezing experienced from the different projects of Nepal, larger convergence was recorded in the section where tunnel axis is parallel to the foliation with gently dipping and containing considerable amount of seepage and non-swelling clay. In the Nepal Himalayas, maximum convergence of 30% of the tunnel size is recorded from the Modi Khola Hydroelectric Project while tunneling (Sharma, 2000). Squeezing may stop or continue for a long time. In worse case tunnel can be collapsed. Therefore reshaping and resupporting of the tunnel is time consuming and expensive. Rock squeezing problem delays construction schedule during underground excavation and this led to cost overruns paid by the owner or severe financial strains on the contractors. So it is important to recognise the conditions that are likely to result in rock squeezing. In general all prediction theories are based on overburden depth and rock mass strength/or rock mass characterisation to predict the phenomena. It is found that squeezing ground conditions are greatly influenced by strength, stress condition (overburden), orientation of discontinuity, pore water pressure, excavation methods and stiffness of support but contributions are not the same degree which has been experienced during the construction of the different hydropower project in Nepal. Therefore none of them give accurate results but provide a good indication of potential occurrence of squeezing. In this paper rock squeezing problem in different tunnels of hydropower projects and its prediction are presented. Journal of Nepal Geological Society, 2007, Vol. 36 (Sp. Issue) p.16