Suction Measurement of Soil Using Salt Solution Technique

Abstract

There are several situation where foundation soil undergoes unsaturated condition, i.e.; the soil is not fully saturated. Due to the unsaturated condition, a negative pore-water pressure develops in soil known as suction. Change in suction (negative pore water pressure) affects the effective stress and shear strength of the soil. The relationship between soil suction and moisture content is commonly known as soil-water characteristics curve. Soil-water characteristics curve is important to understand the unsaturated soil behaviour. The soil suction can be measured using various methods such as pressure plate apparatus, osmotic technique, filter paper technique, Thermal Conductivity Probes, Tensiometer, vapour equilibrium technique, etc. In the present study, vapour equilibrium technique or salt solution technique was used to measure suction of a bentonite and Rourkela local soil. The relationship between suction and water content/degree of saturation was established. The method is economical, simple and provides better accuracy of the result.

Initially saturated soil specimens were kept inside desiccators containing saturated salt solutions of KCl, K2CO3, NaCl, K2SO4 and Mg (NO3)2 creating relative humidity of 84.4, 43.4, 75.4, 97.8, 53.4 respectively. The moisture equilibration of the specimens were took placed inside the desiccators under specified relative humidity. It was observed that moisture equilibration of soil specimen subjected to K2CO3 (Potassium carbonate) solution was faster than other soil specimens subjected to other salt solutions.

Based on known relative humidity and temperature suction was calculated using Kelvin’s equation. Also, moisture equilibration of Rourkela local soil was faster than that of bentonite. At the same suction, the equilibrium water content of Rourkela local soil was found to be much lesser than that of bentonite. In general, it was noted that with an increase in suction, the water content of the soil was found to decrease thereby reducing both void ratio and degree of saturation.