The characteristics of expansive soil, such as overconsolidation, multi fissure and shrinkage cracking due to water expansion and water loss, are called hidden disasters by the geotechnical engineering community. Its damage mechanism has become an important technical problem that the underground space engineering must face and solve. The expansive soil is mainly distributed in the eastern region in Chengdu. According to the investigation, the soil collapse between the supporting piles often occurs in the construction of the deep foundation pit of the subway station in the eastern region, causing safety accidents such as personal injury or equipment damage. In this paper, taking the stability of soil between piles of deep foundation pit excavation and support piles of the East extension line station of phase II of Chengdu rail transit line 17 as the research object, the instability characteristics of soil between piles of deep foundation pit excavation and support piles of railway stations on expansive land in Chengdu area are systematically studied by means of field investigation, field measurement analysis, instability characteristics analysis and calculation and derivation of soil stability between piles, and the characteristics and causes of soil instability between piles of deep foundation pit excavation and support piles of railway stations on expansive land are proposed, and put forward prevention measures and suggestions.

The structure of sandy cobble soil is discrete, the particle size distribution is asymmetrical, and it has typical particle dispersion peculiarities. The macroscopic continuum mechanics theory method cannot accurately access the instability process and failure mode of sandy cobble surrounding rock of tunnel, which often induce instability collapse of surrounding rock of tunnel face, resulting in ground subsidence, deformation and collapse, which poses a serious threat to engineering safety. In this research, the particle discrete element theory and numerical simulation technology are utilized to conduct a fine analysis of the stability of the tunnel sandy cobble surrounding rock from the meson level. The paper focuses on the longitudinal instability failure mode of the tunnel face and failure characteristics of circumferential surrounding rock during tunnel excavation. The consequence display that after the excavation of the tunnel in sandy cobble stratum by the bench method, in the longitudinal space of the tunnel, the soil in front of the tunnel face is first destroyed, and the closer the distance is to the tunnel vault, the greater the soil deformation is, which is easy to cause surface subsidence. The stress relaxation area gradually spreads from the front of the working face to the upper right, which eventually provokes the instability of the excavation face. Correspondingly, the intrusion degree of surrounding rock and the height of vault collapse arch increase with the increment of water content in sandy cobble stratum. Meanwhile, intrusion size, collapse arch height, width and initial support effect of surrounding rock with different water content are calculated. The research results provide important reference and guidance for the design, construction and maintenance of sandy cobble stratum tunnel engineering.