Seminar - Ning Lu - Effective Stress in Unsaturated Soils
Wednesday, May 5, 2004, 1.10 - 1.50 pm
Civil Engineering Lecture Room 3
The suitability of effective stress concept in unsaturated soils is examined from two perspectives: forces at inter-particle scales and shear strength at macroscopic scales. A particle-scale microscopic force analysis is employed to identify two types of inter-particle forces; forces transmit through the soil grains, and forces counter-balanced at or near inter-particle contacts. Only the first type of forces has been captured in Terzaghi's effective stress for saturated soils (such as pore water pressure and total stress) and Bishop's effective stress for unsaturated soils (such as net normal stress and c(ua-uw)). The second type of forces are important to shear strength and stress-strain behavior of soils. This notion is particularly pronounced for unsaturated soils since the macroscopic pore pressure in saturated state disintegrated into macroscopic inter-particle forces, suggesting that the use of the macroscopic stress of matric suction as an independent stress state variable is inadequate for unsaturated soils. The absence of the second type of forces in saturated effective stress can be reconciled by the classical elastic-plastic stress-strain formalism, suggesting that the second type of forces in unsaturated soils should be captured either in failure criterions (and post-failure stress-strain constitutive laws) or in the definition of effective stress in unsaturated soils. The second type of forces is conceptualized into a macroscopic stress called suction stress, which characteristically depends on a soil's wetness. The importance of suction stress, its characteristic nature, and its macroscopic determination are demonstrated using a broad array of unsaturated shear strength experimental data in the literature. The experimental data, ranging from sand to clay, demonstrate that both Mohr-Coulomb failure and the critical state theory can well be represented by the suction stress characteristic curve. Either the effective stress or shear strength theory for unsaturated soils should include suction stress in addition to net normal stress.
Bio-sketch: Dr. Ning Lu is Professor of Engineering at Colorado School of Mines, the United States. His recent research focus is on unsaturated soil mechanics and fundamental behavior of expansive soils. He has published numerous technical papers including over 35 journal papers in geotechnical engineering. He is the co-author of the textbook "Unsaturated Soil Mechanics" recently published by John Wiley and Sons. Dr. Lu is an Editorial Board Member for the American Society of Civil Engineers' Journal of Geotechnical and Geoenvironmental Engineering. Dr. Lu is currently taking an academic sabbatical at CSIRO Petroleum in Perth, WA.