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Module 1: Concept of Suction Stress

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Suction Stress - Overview
In 2006, Ning Lu and William Likos proposed the concept of suction stress in determining the effective stresses in unsaturated soils. This concept of Suction Stress established three types of forces. These forces are: Type I: They consists of active skeletal forces Type II: They are active local forces
Type III: They are passive particle to particle contact forces. Generally, the type I and type III forces together called body forces, and type II force is called surface force. The characteristic of representing the variation in suction stress with respect to moisture content or matric suction is called Suction Stress Characteristic Curve (SSCC). Suction Stress - Suction Stress Characteristic Curve The advantage of suction stress is that it is valid for any given soil and all different forces are considered. From a routine suction-controlled test, effective stress for a given unsaturated soil can be determined. With effective stress values, one can compute the mean effective stress values.
Estimation of Suction Stress Characteristic Curve. Suction stress consumes energy that is required for the change in energy of soil water due to several physical-chemical factors and capillary effects. The Soil Water Characteristics Curve (SWCC) of soils can be directly utilised to determine its Suction Stress Characteristic Curve. From the Suction Stress Characteristic Curve, the effective stress of the soil can be obtained. Lu (et al) proposed two approaches for estimating the Suction Stress Characteristic Curve of soils. The two approaches are: Semi-quantitative approach: This approach is used when Soil-Water Characteristic Curve data is not known. When only the shear strength data from the Suction-Controlled Direct Shear Test and Suction Control Triaxial Tests are known, this approach could be used to derive the effective stress variation within the soil mass.
Quantitative Approach: This approach is used when Soil-Water Characteristic Curve data is known.