Evaluation of the collapsible deformation of surrounding rock of loess hydraulic tunnel considering ground stress variation

Uneven settlement will occur as a result of the collapsible deformation of the loess strata, and the hydraulic tunnel lining structure will also fail. In this work, laterally confined compression tests were carried out on loess and the double-line method was employed to evaluate the loess collapsibility. The deformation of the surrounding rock of a loess hydraulic tunnel under various ground stresses and its effect on the lining structure was modeled. Three stages were noted in the collapsible deformation of loess. The critical point between the former two stages corresponds to the pre-consolidation pressure of saturated loess and that between the latter two is taken as the structural yield pressure of unsaturated loess. From the relationship between the collapsibility coefficient and vertical stress, the deformation of the tunnel under ground seepage primarily originates from two sources, i.e., the collapsible and compressive deformation. The latter source accounts for the deformation of loess adjacent to the lining when the seepage depth is low, while both sources are included when the bottom of the tunnel invert is infiltrated. The collapsible deformation is lower than that of the original stratum due to the stress relaxation during tunnel excavation. The tensile and compressive stresses of tunnel lining increase linearly with the seepage depth, with the maximum appearing at a position of 20 m away from the midline of the collapse and non-collapse domains. The results will provide a theoretical reference to the design and construction of hydraulic tunnels in collapsible loess stratum.