A Unified Effective Stress Concept for Variably-Saturated Soil
Event details
| Date | 09.12.2010 |
| Hour | 12:15 |
| Speaker | Prof. Ning Lu, Colorado School of Mines |
| Location |
GC A331
|
| Category | Conferences - Seminars |
A unified effective stress concept based on the suction stress characteristic curve (SSCC) for variably-saturated soil is discussed. Particle-scale equilibrium analyses are employed to distinguish three types of soil interparticle forces: (1) active forces transmitted through the soil grains (Terzaghi's); (2) active forces at or near interparticle contacts (physicochemical); and (3) passive, or counterbalancing, forces at or near interparticle contacts (Born's and steric). It is proposed that the second type of forces, which includes physicochemical forces, cementation forces, surface tension, and the force arising from negative pore-water pressure, can be conceptually combined into a macroscopic stress called suction stress. Suction stress characteristically depends on degree of saturation, or soil suction, thus paralleling well-established concept of the soil-water characteristic curve in soil physics. The existence and behavior of the SSCC are experimentally validated by considering unsaturated shear strength and volumetric behavior data for a variety of soil types in the literature. The characteristics and practical determination of the SSCC are demonstrated. A closed form equation for predicting the suction stress for all soils is found. A case study of shallow landslide initiation
induced by heavy rainfalls illustrates that variation in suction stress can well reconcile the spatial and temporal characteristics of the event. Suction stress provides a potentially simple and practical means to describe the state of stress in unsaturated soil.
Ning Lu is professor of engineering at Colorado School of Mines and the director of the joint CSM/USGS Geotechnical Research Laboratory. He obtained a PhD degree in engineering science from the Johns Hopkins University in 1991. Prior to joining Colorado School of Mines in 1997, he worked at Disposal Safety Inc., at the US Geological Survey, and as an assistant professor at Louisiana State University. His primary research interests are to seek common threads among soil physical phenomena including fluid flow, chemical flow, heat transfer, stress, and deformation, and to build bridges from atomic-scale potentials to particle-scale forces and engineering-scale stresses in soil. He is the senior author of the text book "Unsaturated Soil Mechanics" (John Wiley and Sons), recipient ASCE's Normal Medal (2007) and J. James Croes Medal (2010). He will be the Shimizu Visiting Professor at Stanford University in 2011.
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