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.

Microbial biopolymers have been introduced as materials for soil treatment and ground improvement purposes because of their ability to enhance soil strength enhancement and to reduce hydraulic conductivity. Several studies in the field of environmental engineering have reported heavy metal adsorption and removal from contaminated water using common biopolymers. In particular, gellan gum biopolymers have drawn significant attention for use in metal ion adsorption. This study aims to investigate the heavy metal adsorption capacity of a gellan gum biopolymer–sand mixture when nickel-contaminated water is pumped upward through a uniform gellan gum–sand mixture column. The main aims of this study are (1) to clarify the Ni²⁺ adsorption phenomenon of gellan gum-treated sand, (2) to assess the Ni²⁺ adsorbability of gellan gum–sand mixtures with different gellan gum content, and (3) to examine the gellan gum–sand filter thickness and flow rate effects on Ni²⁺ adsorption. The results of this experiment demonstrate the effectiveness of gellan gum in terms of Ni²⁺ adsorption and water flow rate control, which are essential criteria of a filter material for contaminated water treatment.

Since the mid-1990s, several empirical unsaturated shear strength equations have been proposed for use in geotechnical engineering practice. Most of the equations have been based on information associated with the measured drying soil-water characteristic curve (SWCC) in combination with the effective shear strength parameters of the saturated soil. There have been few studies, however, undertaken on verifying and comparing the suitability of the proposed unsaturated shear strength equations. This study involves consideration of a wide range of possible drying SWCCs plotted as a relationship between the degree of saturation and suction (saturation soil-water characteristic curve ( S-SWCC)). The study forms the basis for comparing five methodologies that have been proposed for the estimation of the shear strength of unsaturated soils. The SWCCs are defined by the fitting parameters for the Fredlund and Xing (1994) sigmoidal equation. This study supports the use of unsaturated soil shear strength empirical estimation methods using the S-SWCC anchor points of air-entry value and residual suction of the soil.

Biopolymer soil treatment has been introduced as the latest technological innovation in soil stabilization and improvement. Biopolymers with agar, guar, xanthan, casein, gellan and sodium alginate have been commonly studied to improve strength, reduce hydraulic conductivity and prevent erosion of highly permeable soil materials. Among those, xanthan gum is introduced to perform better in terms of lowering permeability. In this study, the effect of xanthan gum on slope stability under rainfall was evaluated via a 2D finite-difference program (FLAC) considering the wetting behavior of xanthan gum-treated soil. In order to obtain input parameters for numerical modelling, wetting soil-water characteristic tests, permeability tests, and direct shear tests were conducted on xanthan gum-treated soils. The numerical results support the application of biopolymers to soil erosion control during rainfall, mainly due to the significant decrease in the infiltration of xanthan gum-treated soil.