Laboratory investigation of wetting induced deformation in unsaturated expansive soil during shield tunnelling in Yangzhou, China

Unsaturated expansive soils undergo significant swelling, softening, disintegration, and shrinkage upon changes in water content. During shield tunneling through such soils, moisture infiltration from grout or groundwater can induce swelling of the surrounding ground, imposing additional pressure on the segmental lining. Understanding the hydro–mechanical behavior of expansive soil is essential for estimating the swelling pressure and ensuring tunnel stability. This study investigates the wetting induced deformation behavior of unsaturated Xiashu clay through a series of triaxial swelling tests that simulate the stress paths experienced during shield tunneling, including tunneling disturbance, stress release, and wetting induced deformation. Complementary field monitoring and microstructural analyses (SEM) were conducted to elucidate the underlying mechanisms. Triaxial test results show that when the water content is below approximately 21.23%, swelling pressure dominates, leading to radial and axial expansion; above this value, interparticle water films reduce shear strength, causing axial compression and enhanced radial creep. Quantitative analysis shows that at a water content increase from 21.23 to 22.46%, the shear strength decreases from 221 to 148 kPa. Full–process triaxial simulation tests, corroborated by field monitoring, indicate that radial strain development during the wetting phase is the most critical factor affecting segment stability, as continued moisture uptake generates confined swelling pressure against the lining. Microstructural evidence corroborates the transition from interlayer expansion to particle rearrangement and slippage with increasing water content. These findings highlight the importance of considering hydration–induced long–term loading in the design of shield tunnels in expansive soils.