Self-weight collapsible characteristics of loess in Xi'an urban area

Based on the results of 12 large test pit immersion tests of loess and indoor collapsibility tests conducted during the construction process of Xi'an Rail Transit project, the self-weight collapsible deformation characteristics of loess in Xi'an urban area are systematically studied. The results show that: (1) Within the urban area of Xi'an, the collapse of each landform unit, from strong to weak, is in the following order: Weibei Loess Plateau > Chanhe Tertiary Terrace > pluvial tableland > loess ridge and depressiona > Shaoling Plateau (Duling Plateau, Shenhe Plateau), and Weihe Tertiary Terrace. The self-weight collapsible loess site accounts for 50% of the total number of test sites, and the maximum depth of the self-weight collapsible bottom limit is 20 m. The loess plateau in the southern part of the city is a non self-weight collapsible site. (2) The deformation of the self-weight collapse occurs mainly in the Q₃ loess layer. Based on the immersion tests, the bottom boundary of self-weight collapse is mostly located in the Q₃ ancient soil layer. Only Q₂ losses in two sets of tests exhibit weak collapse. The uniform correction coefficient provided in the current regulations for the Guanzhong region overestimates the collapsibility of Q₂ losses. (3) Distinguishing between geomorphic units and sedimentary ages of strata, the recommended values for the self-weight collapsible deformation correction coefficients of loess in Xi'an urban area are provided. (4) The measured area of settlement diffusion due to loess immersion and collapse is typically related to the depth and amount of self-weight collapse. It usually does not exceed 1.0 times the radius of the test pit and 1 times the measured collapsible limit depth. This can serve as a boundary for water accumulation around the project. (5) The development process of collapsible deformation of loess mainly includes five stages: rapid deformation, slow deformation, stable deformation, rapid deformation after water interruption, and stable deformation after water interruption. The larger the measured self-weight collapses, the greater the daily settlement rate during the sinking phase. The maximum consolidation settlement after water interruption is 95.1 mm.