The strength of clay-rich sandstone decreases significantly when in contact with water due to softening effects.This scenario can pose a severe threat to the stability of water diversion tunnels during con-struction and operation periods.To address the issues related to water-induced softening in clay-rich sandstone zones in a water diversion tunnel of Lanzhou Water Supply Project,the microscopic and micromechanical variations of rocks due to increasing water content in two different zones i.e.zones A and B,were determined by various testing methods,such as X-ray diffraction(XRD),scanning electron microscopy(SEM),thin section microscopy,micro-indentation test,sonic velocity test,and slake dura-bility test.The microscopic analysis confirms the presence of montmorillonite mineral which is the dominant problematic geomaterial in engineering application.The integrity and durability of clay-rich sandstone were determined with sonic velocity and slake durability tests to calibrate the results ob-tained by the micro-indentation test.It shows that the elastic modulus and hardness of clay-rich sandstone decrease with the increase of saturation time,up to 144 h,which is more significant and rapid during early stage of saturation.After 144 h of saturation,the elastic modulus decreases by 89％and 97％,and the hardness decreases by 89％and 99％for zones A and B sandstones,respectively.The results of slake durability and sonic velocity indicate that zone A sandstone remains 56.19％durability after 144 h of saturation,while zone B sandstone loses its durability merely after 72 h of saturation.The clay-rich sandstone starts to dissolve in water when the saturation time exceeds 144 h.The significant de-creases in strength and durability of clay-rich sandstone due to water-induced softening are serious threats to tunnel stability.The improvements in the strength of surrounding rock mass by grouting and permeability by installation of drainage galleries can reduce the damage caused by water-induced softening.