Tunnels passing through swelling rocks are subjected to stability problems as moisture changes affect the material properties of the rocks. The drying out process during construction may cause local contraction stresses in the rocks near the tunnel periphery, and subsequent swelling after exposure to water may further degrade the rock material as cracks and fissures may occur. Conventional swelling tests may underestimate the swelling behaviour of rocks in cases where they are subjected to moisture fluctuations, as in hydropower tunnels.
This study presents the results of tests on volcanic rocks related to a hydropower project in the Philippines. The mineralogical composition was determined through X-Ray diffraction and the swelling behaviour studied with free and oedometer swelling tests. Oedometer swelling tests under cyclic wetting and drying were performed on intact rock specimens. The results show that the swelling behaviour can be attributed to the laumontite content. This mineral, present in some volcanic rocks, is not usually associated with rock swelling. The effect of the increasing stress with cyclic hydrationdehydration occurs after deformation of the rock is allowed. Expansion and shrinking during the wetting and drying phases deteriorate the rock structure, leading to further swelling behaviour. The well-known relationship between swelling strain and stress for clay rocks might not be valid in the case of cyclic wetting and drying, since the structure of the rock changes with increasing cycles.
Tunnels in weak rock mass are often subjected to stability issues as rock mass from the tunnel periphery deforms and might collapse due to de-stressing, due to unfavourable rock mechanical properties and swelling (Selen et al. 2019). Rock swelling expresses itself either by a volume expansion or by inducing pressure on the surroundings. Swelling and disintegration can result in rapid changes from rock-like to soil-like materials upon exposure to water, leading to numerous stability problems to engineering constructions (Rincon et al. 2016).