ESTIMATION OF SOIL SUCTION UTILIZING TIME-DOMAIN REFLECTOMETRY SENSOR

Abstract

The soil water characteristics curve (SWCC) provides information regarding the hydraulic behavior of unsaturated soil and is frequently used to estimate unsaturated soil properties such as unsaturated shear strength and unsaturated hydraulic conductivity. The current equipment for the measurement of SWCC in the field is limited to a particular suction range, time-consuming, and tedious to operate. In addition, the inaccurate estimation of soil suction leads to errors in soil response during stability analysis of slopes. This study presents a framework for the estimation of soil suction based on calibration with laboratory-measured drying SWCC at a defined moisture content from a time-domain reflectometry (TDR) sensor. The framework employs Newton-Raphson’s numerical technique to derive soil suction from the established fitting parameters and moisture content. The soil consists of fine particles in proportions that exceed 12%. The plasticity index was 6.28, which was on the A-line and classifies the soil as silty sand. Initially, the mathematical model proposed by Fredlund & Xing (1994) and Satyanaga et al. (2013) was modified to derive an expression in terms of suction. Feasibility studies were conducted to identify the appropriate iterative technique using the experimental SWCC. Then, the SWCC equation was differentiated and calibrated with the fitting parameters of the experimental SWCC. Finally, the soil suction was determined based on the moisture content from the TDR sensor. The results of the framework showed unimodal and bimodal SWCCs at the maximum dry density and wet of optimum compaction states, respectively. The pore structure of the soil at the compacted condition yielded both micropores and macropores in the bimodal SWWC. The dual pores were related to the two air-entry values (AEV) in the bimodal SWCC. The unimodal SWCC had a single AEV with fewer macropores and a more homogenous distribution of micropores. Using the estimated SWCCs, stability analysis of a slope-retaining wall system embedded with horizontal drains was performed. Seepage analysis was conducted for a duration of 24 days (12 days of wet periods and 12 days of dry periods). Then, the pore-water pressure variation was incorporated for stability analysis. Five scenarios were considered: no drain, top drain, middle drain, bottom drain, and all drains present. The soil exhibited moderate porosity with a saturated hydraulic conductivity of . The strategic positioning of the horizontal drains was efficient in removing the excess water across the slope and increasing the stability of the slope. The performance of the horizontal drains depended on the ratio of the rainfall intensity to the saturated hydraulic conductivity. The bottom drain configuration proved to be the most effective with the fastest recovery rate after rainfall and a factor of safety of 1.12. Therefore, the proposed framework in this study for the estimation of soil suction from moisture content is recommended.