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  Impacts of Surrounding Ground Stiffness Comprising

  Expansive soils are highly plastic shrink-swell character soils aiding additional stresses around in-service tunnels. The fluctuations in overburden, moisture content, stress state conditions, and surrounding ground deteriorations can induce added pressures on existing tunnels. This study is an effort to evaluate the tunnel’s long-term response subjected to expansive soils by model tests. Bentonite and Toyoura sand were mixed in a weight ratio to model the expansive soil. The free swell tests were performed to comprehend the swelling properties of the mixture. The results show a significant increase in the swelling pressures / swell ratio, owing to the highly expansive bentonite fraction. The sample was additionally employed in a reduced-scale tunnel model setup. The expansive soil layer with varying surrounding ground stiffness was used to investigate the parametric impacts. The model tests revealed a substantial swell pressure near the tunnel invert, implying long-lasting stability issues for tunnels constructed in expansive rocks/soils. The corresponding volumetric changes and node displacements were substantially illustrated by image analysis. The findings suggest that expansive soil in combination with surrounding ground stiffness and moisture fluctuations can seriously influence the sustainability of existing tunnels, causing floor heaving, cracking, or even absolute failure.

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