TWC Seminar Special, Speaker Ning Lu

Colorado School of Mines: Department of Civil and Environmental Engineering

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 Abstract  Slope-stability analyses are mostly conducted by identifying or assuming a potential failure surface and assessing the factor of safety (FS) of that surface. This approach of assigning a single FS to a potentially unstable slope provides little insight on where the failure initiates or ultimate geometry and location of a landslide rupture surface. A unified effective stress framework for variably saturated porous media and a scalar field of factor of safety are employed to account for effective stress variations under rainfall conditions. The scalar field of FS is based on the concept of the Coulomb stress and the shift in the state of stress towards failure that results from rainfall infiltration. The FS at each point within a hillslope is called the local factor of safety (LFS) and is defined as the ratio of the Coulomb stress at the current state of stress to the Coulomb stress of the potential failure state under the Mohr-Coulomb criterion. Comparative assessment with limit-equilibrium and hybrid finite-element limit-equilibrium methods show that the proposed LFS is consistent with these approaches and yields additional insight into the geometry and location of the potential failure surface and how instability may initiate and evolve with changes in pore-water conditions. Quantitative assessments demonstrate that the LFS has the potential to overcome several major limitations in the classical FS methodologies such as the inherent underestimation of slope instability. Comparison with infinite-slope methods, including a recent extension to variably saturated conditions, shows further enhancement in assessing shallow landslide occurrence using the LFS methodology. The LFS provides a new means to quantify the potential instability zones in hillslopes under variably saturated conditions using stress-field based methods.

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  /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman","serif";} Biographical Sketch Ning Lu is professor of civil and environmental engineering at Colorado School of Mines (CSM) and the director of the joint CSM/USGS Geotechnical Research Laboratory in Golden, CO. He is a recipient of ASCE 2007 Norman Medal and the recipient of ASCE 2010 Croes Medal, and an elected fellow of Geological Society of America and American Society of Civil Engineers. His current research focuses on developing a unified coupled hydro-mechanical framework for variably saturated porous media and applying it to rainfall-induced landslide analysis. He is the senior author of widely used textbook Unsaturated Soil Mechanics (John Wiley and Sons, 2004) and Hillslope Hydrology and Stability, (Cambridge University Press, 2013). He can be reached via ninglu@mines.edu.