Ultimate Bearing Capacity of Strip Foundations in Unsaturated Soils considering the Intermediate Principal Stress Effect

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 angle φb.

Safety Factor of Unsaturated Soil Slopes considering the Intermediate Principal Stress Effect and Different Profiles of Matric Suction

“…where c′ � effective cohesion, c t ′ � unified effective cohesion, c tt � unified total cohesion, φ′ � effective internal friction angle, φ t ′ � unified effective internal friction angle, φ b � friction angle relative to matric suction, φ b t � unified friction angle with matric suction, u a � pore-air pressure, u w � pore-water pressure, u a −u w � matric suction, σ � total normal stress, σ−u a � net normal stress, b � unified strength theory parameter, and 0 ≤ b ≤ 1. e parameter b represents the effect of intermediate principal stress on the strength of unsaturated soils, and it can be calculated by the values of basic mechanical properties (e.g., uniaxial compressive strength, uniaxial tensile strength, and pure shear strength) or comparing the limit loci of equation (1) with true triaxial tests of unsaturated soils on a deviatoric plane. In addition, the friction angle φ b is simply taken as a constant not larger than the effective internal friction angle φ′, and its variation with respect to matric suction can be well addressed by a hyperbolic function similar to that of Zhang et al [29] and Yan et al [30].…”

“…Integrated effects of matric suction and intermediate principal stress on the strength of unsaturated soils are reasonably considered by the unified shear strength equation. Meanwhile, the unified shear strength equation has been successfully applied to typical geotechnical engineering problems [29][30][31][32], such as the Earth pressure of retaining walls, the bearing capacity of strip foundations, and the slope stability with the upper bound theorem of limit analysis. e objective of this study is to present theoretical formulations of safety factor for an infinite unsaturated soil slope under four different profiles of matric suction based on the limit equilibrium method.…”

Bearing Capacity of Shallow Foundations on Unsaturated Silty Soils

“…The majority of shallow foundations are found in areas of unsaturated soil, depending on the depth of the groundwater table. The SS of unsaturated soils is important in the design of shallow foundations because the failure in these foundations is usually caused by exceeding the UBC value [8][9][10][11]. Therefore, research on the behavior of unsaturated soils has been prioritized over the last ten years.…”

“…There are various structural models and modeling techniques that can be used for both approaches, but these models require a variety of soil parameters that necessitate complex and extensive laboratory tests. On the other hand, there are various numerical modeling techniques presented in the literature that are extended by modifying the ESA and the TSA to take into account the effect of MS [11,12,[30][31][32][33][34][35][36][37]. The approaches introduced in the literature are called the Modified Effective Stress Approach (MESA) and the Modified Total Stress Approach (MTSA).…”

Shear strength and yield surface of a partially saturated sandy silt under generalized stress states

“…The stress state in many engineering applications is different from that usually considered in elastoplastic models under axisymmetric stress state conditions. Therefore, constitutive models to be used in the analyses of engineering problems such as foundations, earth dams, and embankments should take into account experimental information that comes from stress-strain tests in a generalized stress state (Nasreddine, 2004;Zerfa and Loret, 2003;Grammatikopoulou et al, 2007;Zhang et al, 2010;Zhang et al, 2016;Yao and Yang, 2017;Yan et al, 2020;Li et al, 2021).…”