Basal-heave stability of narrow foundation pit in soft soil foundation based on hardening soil with small-strain stiffness (HSS) model

ObjectiveAs urbanization accelerates and utility tunnel projects develop, narrow foundation pits are becoming increasingly prevalent. However, the supporting systems, basal-heave calculation methods, and failure modes of narrow foundation pits differ substantially from those of wider ones, and basal heave is more pronounced in soft soil strata. MethodsTo further investigate the force-deformation behavior, basal-heave stability, and the sensitivity relationships among influencing factors of support structures for narrow foundation pits in soft soil foundations, finite element analysis and field monitoring were conducted using the Wuhan Youyi Avenue rapid reconstruction project as a case study. The hardening soil with small-strain stiffness (HSS) soil constitutive model, and the strength reduction method were employed to study the effects of excavation width, support structure embedment ratio, surcharge loads, and support structure stiffness on the basal-heave stability of narrow foundation pits. ResultsThe results indicated that with higher support stiffness and smaller support spacing, the differences in stress and deformation between the supports and the midpoint between two adjacent supports were minimal. Soil deformation in narrow foundation pits was divided into the soil outside the pit from the ground surface to a certain depth below the surface and the soil at the pit bottom, which induced basal heave and ground surface subsidence outside the pit, respectively. In narrow foundation pits, plastic zones developed at the pit bottom and the base of the structure, with further development, intersecting plastic zones gradually formed and extended into the soil on both sides until the plastic zones coalesced throughout, ultimately leading to failure of the narrow foundation pit. Sensitivity analysis revealed that the embedment ratio and stiffness of the support structure had a significant positive effect on the basal-heave stability of narrow foundation pits, whereas excavation width and surcharge loads exhibited a negative correlation, with the embedment ratio displaying the strongest correlation. ConclusionThese findings provide a scientific basis for the design of support systems and stability assessment of narrow foundation pits in soft soil foundations.