Study on time-dependent upheaval deformation mechanisms of red-bed soft rock subgrade of high-speed railways

Currently, the continuous upheaval deformation of high-speed railway subgrade during operation period in the region of red-bed soft rock mass has become a key factor to hinder the development of high-speed railways in China. In order to reveal time-dependent upheaval mechanisms of red-bed rock subgrade, and taking the typical upheaval of a deep cut red-bed soft rock subgrade in Southwest China as the research object, the in-situ engineering geology and hydrogeology conditions and the crustal stresses were surveyed, and the swelling and creep tests of red-bed soft rock mass under different hydraulic conditions were conducted. A layered deformation model of red-bed rock subgrade was built based on the time-dependent swelling and hydro-mechanical coupling characteristics, and the upheaval deformation mechanism and characteristics of the subgrade in short, medium and long terms were systematically analyzed. The results show that the red-bed mudstone inter-bedded with thin sandstone in the upheaval subgrade presents sub-horizontal, that the excavation normal unloading leads to the relaxation of micro-cracks in the shallow rock mass while the deep rock mass keeps intact, and that the horizontal stress of the subgrade increases obviously, resulting in significant structure effect of the subgrade deformation. Under the condition of lateral constraint and axial unconstraint, the time-dependent water-absorption deformation of the red-bed mudstone is related to its lithology and structural characteristics. It is also shown that the red-bed mudstone presents a three-phase creep characteristic under low stress condition and the creep strain ratio increases with decreasing the axial stress. Under the hydro-mechanical coupling condition, the creep property of the red-bed mudstone is more significant, and both the creep upheaval strain and the duration time increase more obviously for large unloading. The creep time lasts longer while the total creep strain decreases under the moisture-mechanical coupling condition. Based on the deformation mechanisms of different layered rock masses, the red-bed soft rock subgrade is divided into atmospheric influence layer(C1), moisture-mechanical coupling deformation layer(C2), hydro-mechanical coupling deformation layer(C3) and hydro-mechanical coupling confining layer(C4). The short-term, medium-term and long-term deformations of the subgrade are mainly contributed by C1, C3 and C4, and C2 respectively. The results can provide theoretical foundation and reference for risk assessment and forecast and the design of engineering control measures of time-dependent upheaval deformation of high-speed railway subgrade in the region of red-bed soft rock mass. © 2020, Science Press. All right reserved.