The Critical Width Method For Analyzing Stability Against Basal Heave Of Foundation Pits In Complicated Soil Conditions

In order to improve design against the basal heave failure of foundation pits, a new method which is called critical width method (CWM) for calculating the factor of safety against basal failure was suggested. Further, ground water, unsaturated soil, anisotropy of undrained shear strength and stratification was taken into account in CWM, consequently theoretic solution of minimum factor of safety and critical width was deduced. In addition, parameters of basal failure state were studied, employing finite element method.Considering the state that only a single-sided heave slid surface exists in real cases of failure, assuming sliding mechanisms according to Terzaghi's theory of foundation bearing capacity, undrained shear strength in condition of plane strain basing on Sekiguchi-Ohta constitutive model applied to slip line field theory to deduce anisotropic shear strength on slip line, in combination with Mohr-Coulomb shear strength theory and unsaturated soil shear strength theory, the concept of critical width corresponding to the minimum factor of safety was suggested.Employing ABAQUS finite element software, the plane strain foundation pit model was established, considering effect of surface contact model between wall and soil and stiffness and location of support. Sequentially, the ratio of the depth to the width of the excavations, the thickness of the soft soil layer between the excavation base and hard stratum, the depth of the walls inserted below the excavation base, and the stiffness of the walls around the excavation was respectively changed to describe the failure state curve, with x-coordinate angle of internal friction and y-coordinate cohesion.Parametric analyses were conducted for a comprehensive understanding the problem of basal heave failure of foundation pits. The validity and significances of the critical width method was then demonstrated through comparative analyses of practical cases. The criterion of basal heave failure state was developed for getting accurate parameters of foundation pits. From parametric analysis, the conclusion that the stiffness of walls is the key to basal heave failure and the failure state curve is convex downwards can be drawn. When the angle of internal fraction or the cohesion is higher than the fixed value that hinges on the stiffness of wall, basal heave failure can not occur.