Influence of Steady-state and Transient Flow Conditions on the Bearing Capacity of Shallow Foundations in Unsaturated Soils
Creators: name: Tan, Mengxi
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2024
Subjects: foundation bearing capacity
steady-state flow
transient flow
unsaturated soil
slope
inclined and eccentric loading

description: Shallow foundations are widely used in different types of soils for supporting the loads from the lightly loaded superstructures of various civil infrastructures both on level and sloping ground. Design of shallow foundations in geotechnical engineering practice is widely based on the principles of saturated soil mechanics because they are relatively simple. However, the soil near the ground surface (i.e., vadose zone) in which the shallow foundations are typically placed is in an unsaturated state. The water content variation in unsaturated soils is influenced by hydrological events such as the snow melt, rainfall infiltration, evaporation, and the plant transpiration. Due to this reason, the hydro-mechanical properties (i.e., coefficient of permeability, shear strength and volume change) of unsaturated soils are sensitive to the variation in soil suction associated with water content changes. These properties in turn have a significant impact on the bearing capacity and settlement behavior of the shallow foundations. Therefore, it is rational to investigate shallow foundations’ behavior extending the principles of unsaturated soil mechanics. During the last two decades, there has been a significant interest towards investigating shallow foundations based on unsaturated soil mechanics. Laboratory, field, and model studies highlight that matric suction variation in unsaturated soils has a significant influence on the bearing capacity and settlement behavior of shallow foundations. However, the focus of most of the presently available studies in the literature consider mostly vertical loading conditions on level soil ground. There are limited studies related to the design of shallow foundations on sloping ground and subjected to inclined and eccentric loading conditions. Also, there are only few studies that consider the effect of the steady state and transient flow conditions on the foundation bearing capacity evaluation. Therefore, one of the key objectives of this thesis is directed toward developing rational tools for investigating shallow foundations considering the steady state and transient flow conditions associated with water infiltration and evaporation in unsaturated soils. Comprehensive investigation studies are undertaken to interpret the influence of the steady state and transient flow conditions on the shallow foundations related to: (i) bearing capacity on the sloping ground in different types of soils including expansive soils, and (ii) bearing capacity under the inclined and eccentric loading conditions with homogeneous soil properties and considering spatial variation of soil properties. Succinct details related to investigated studies are summarized below: (1) An analytical method is proposed for quantifying the bearing capacity of the shallow foundations on unsaturated soil slopes considering different rainfall infiltration conditions. The proposed method is a novel tool for considering the simultaneous influence of several parameters that include the flow rates, the infiltration duration, the foundation set-back distance and the ground water table depth on the foundation bearing capacity. (2) Another analytical method is proposed for evaluating the foundation bearing capacity under inclined and eccentric loading considering both the steady state and transient flow conditions. Semi-empirical equations are proposed for describing the failure envelops in vertical and horizontal (V - H) loading space and in the vertical and moment (V - M) loading space. These equations are capable to describe the variation of failure envelops considering the influence of the groundwater table depth variation, internal friction angles, surface flux boundary conditions and different infiltration durations. (3) The influence of infiltration on the combined performance of both the foundation and the slope in cracked expansive soils is evaluated with the aid of a numerical technique. A semi-empirical model that describes the elastic modulus and the matric suction is implemented into the numerical model. Bimodal soil water characteristic curve is used as a tool for understanding the influence of surface cracks in the numerical study in a simplified manner. The influence of the rainfall intensity, rainfall duration, foundation setback distance and foundation loading on the combined performance of foundation and slope were investigated. Results combined with some suggestions for rational design procedures are presented that can be useful for geotechnical engineers in practice applications. (4) Numerical analyses are conducted for shallow foundations under vertical and combined loading subjected to different flow conditions. A numerical code procedure is exclusively developed as a part of this study to: (i) consider the variation of soil properties along with the matric suction fluctuations in the commercial software ABAQUS with the aid of a user developed subroutine USDFLD; (ii) incorporate the spatial variability of soil properties into the finite element model. Comparisons are provided between the numerical study and other methods such as the experimental investigations, the analytical methods, and the semi-empirical equations for bearing capacity failure envelopes. In addition, comparisons are also made between the failure envelopes and the failure mechanisms contour using the model considering soil spatial variability and homogeneous soil properties. The proposed methods in this thesis are simple to use for evaluating bearing capacity of shallow foundations that are subjected to steady state and transient state flow conditions considering two scenarios: (i) foundation on sloping ground (ii) foundation under inclined and eccentric loading. The results from the above studies reveal that it is the relationship between the soil permeability and the rainfall characteristics that mainly control the water infiltration rates. The soil suction and the effective degree of saturation are influenced by the water infiltration rates and have a significant impact on the foundation as well as the slope behavior. More importantly, the investigations undertaken in this thesis contribute towards addressing the research gaps related to the behavior of foundations in unsaturated soils. Various scenarios considered in this thesis include the influence of unsaturated flow have not been considered earlier in the literature. The results of the studies summarized in this thesis are expected to be useful for practicing geotechnical engineers in the optimal design of shallow foundations extending the principles of unsaturated soil mechanics for various soils. Moreover, the proposed methods can be used for interpreting the foundation behavior for their entire life span service. In addition, these methods can be employed to rationally explain the field-measured data and can also be used in the forensics analyses of failed slopes and shallow foundations.