Collapsible deformation evaluation of loess under tunnels tested by in situ sand well immersion experiments

Abstract

There has been much new traffic engineering construction on the loess plateau, where tunnels can be damaged by collapses based on the stability of the tunnel. Although there are currently two methods, including the laboratory test and the in situ large-scale pit immersion test, to evaluate loess collapsibility, the laboratory test method is inaccurate, and the in situ large-scale pit immersion test method is time-consuming and costly. An in situ sand well water seepage saturated loess test method was proposed for evaluating the collapsible deformation of loess strata. Four immersion tests of sand wells with different depths were carried out in a loess site on the Baoji-Lanzhou high-speed railway. Although loess strata around all sand wells at the same site are typically self-weight collapsible, the self-weight collapsible settlements of sand wells are significantly different. The settlement of the ground surface and well bottom around sand wells was measured by sand well seeping water saturated loess. Combined with the water content measurements of loess layers below and above the sand well bottom, the collapsible deformation characteristics of loess strata around sand wells were analysed to determine the self-weight collapsibility coefficient in different depth loess layers. Fissures in the ground subsidence caused by the settlement difference appeared around the sand wells. Due to the unevenness of the soil layer in the same loess site, the loess collapsibility was obviously different in adjacent sand well sites. With reference to the correction factor of self-weight collapsible deformation in the Long-xi region in China, the rationality of sand well immersion tests was preliminarily demonstrated by comparing the collapse deformation measured by the sand well tests with the value calculated by the laboratory test. The new method provides a simple and feasible means by which to quickly evaluate site and foundation collapsibility in loess tunnel engineering, which is beneficial for perfecting loess collapsibility evaluation methods and proposing measures of foundation treatment in loess tunnels.