Verification of the Fredlund (2019) Unsaturated Shear Strength Function

Thi Phuong An Tran,Delwyn G Fredlund

doi:10.3390/geosciences11040151

Thi Phuong An Tran, Delwyn G Fredlund

Open Access

https://doi.org/10.3390/geosciences11040151

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Journal: Geosciences	Publication Date: Mar 26, 2021
Citations: 8	License type: CC BY 4.0

Affiliation: Hue University, University of Saskatchewan

Abstract

There has been a proliferation of equations proposed to describe the unsaturated shear strength envelope going back to the 1970s. However, there have been limited studies to verify the suitability of one unsaturated shear strength equation over another. Most proposed shear strength equations have attempted to relate the shear strength of an unsaturated soil to some aspect(s) of the soil–water characteristic curve (SWCC). Estimation procedures have generally focused on using that of air-entry value (AEV) as defined by the drying (or desorption) branch of the degree of saturation SWCC (S-SWCC). This paper studies the suitability of using two “anchor points” (or reference points) along the drying S-SWCC to estimate the unsaturated soil shear strength function. The anchor points referred to are the air-entry value (AEV) of the soil and the “residual suction point” of the soil defined in terms of the S-SWCC. Shear strength conditions associated with both so-called anchor points are used as “boundary conditions” that should be satisfied when estimating the shear strength function for unsaturated soils. Past research laboratory measurements published in the research literature are used as part of the verification process for this study.

Highlights

The shear strength for an unsaturated soil takes the form of a nonlinear mathematical function with respect to soil suction
It is quite widely acceptable in geotechnical engineering practice to use nonlinear permeability and water storage functions based on the measurement of soil–water characteristic curves (SWCCs) [1]
The study of existing data sets shows that there is a relationship between the saturation SWCC (S-SWCC)

Summary

Introduction

The shear strength for an unsaturated soil takes the form of a nonlinear mathematical function with respect to soil suction. A single estimation shear strength equation is needed that is sufficiently accurate and of a reasonable cost to implement in geotechnical engineering practice. It is quite widely acceptable in geotechnical engineering practice to use nonlinear permeability and water storage functions based on the measurement of soil–water characteristic curves (SWCCs) [1]. The friction angle (φ0 ) associated with net normal stress (σ − ua ) was assumed to affect shear strength of the soil in a manner independent of matric suction (ua − uw ), where ua is the pore-air pressure; uw is the pore-water pressure; and σ is the normal total stress on the failure plane.

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