Abstract
The reasonable determination of ultimate bearing capacity is crucial to an optimal design of shallow foundations. Soils surrounding shallow foundations are commonly located above the water table and are thus in an unsaturated state. The intermediate principal stress has an improving effect on the unsaturated soil strength. In this study, the ultimate bearing capacity formulation of strip foundations in unsaturated soils is presented by using Terzaghi’s theory. The unified shear strength equation of unsaturated soils under a plane strain condition is utilized to capture the intermediate principal stress effect. Furthermore, two profiles of matric suction are considered and a hyperbolic function of the friction angle related to matric suction (φb) is adopted to describe strength nonlinearity. The validity of this study is demonstrated by comparing it with model tests and a theoretical solution reported in the literature. Finally, parameter studies are conducted to investigate the effects of intermediate principal stress, matric suction, and base roughness on the ultimate bearing capacity of strip foundations. Besides, the effect of strength nonlinearity is discussed with two methods representing the angleφb.
Highlights
E primary objective of this study is to present a formulation of the ultimate bearing capacity for strip foundations in unsaturated soils under uniformly and linearly distributed suctions. e unified shear strength equation of unsaturated soils and a hyperbolic function of the angle φb are adopted to account for the influences from intermediate principal stress and strength nonlinearity. e validity of the proposed formulation is further verified by comparing with two existing results reported in the literature
II is divided into two segments: when matric suction is not larger than the air-entry valueb, φb is a constant equal to φ′; when matric suction is larger thanb, φb decreases continuously as a hyperbolic function. e expression of φb for Method II is written as φb φ′, when ua − uw ≤ ua − uwb, (3a) φb φ′ −
When b is changed from 0 to 1, qu for the case of uniform suction is increased by 92.7% for0 30 kPa, which is roughly identical to that of linear suction. erefore, the effect of intermediate principal stress should be taken into account to calculate the ultimate bearing capacity of strip foundations in unsaturated soils, and the result without incorporating the intermediate principal stress effect is conservative
Summary
Fan et al [44] applied the unified strength theory to account for the effect of intermediate principal stress and obtained a unified expression of ultimate bearing capacity for strip foundations in saturated soils with equation (4). E unified shear strength equation for unsaturated soils under plane strain conditions as expressed in equation (6) is capable of describing comprehensive effects of matric suction and intermediate principal stress. When b 0, the prediction of equation (23) is distinctly less than the result of this model test It means that the Mohr–Coulomb criterion (i.e., b 0) underestimates actual bearing capacities of unsaturated soils due to neglecting the intermediate principal stress effect.
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