CS5A - Concurrent Session 05A: A Practical Methodology for Static Liquefaction Potential Evaluation and Post Triggering Stability Assessment in Embankment Dams

Abstract Description: Recent Tailings Storage Facility (TSF) and hydro dam failures have led to a paradigm shift in the importance of recognizing credible Potential Failure Modes (PFMs) related to static liquefaction, and the need to incorporate consideration of this PFM into the overall design of the structure. The purpose of this presentation is to discuss a suggested framework for evaluation of static liquefaction susceptible materials, estimate methodologies of peak undrained and post-peak liquefied undrained strengths (Sus), and analysis methodology to help reduce the risk associated with static liquefaction of materials comprising said structures. Designers should perform an evaluation to confirm that dam and embankment designs provide an adequate factor of safety against static liquefaction induced instability. But as simple as that sounds, the process is not always black and white, nor is it consistently identified amongst key stakeholders including Regulators, Consultants and Owners, which is often the case in estimating Sus.

This presentation suggests an approach to identify contractive materials, establish Sus estimates based on various methods used across the dams, mining and traditional geotechnical engineering industries, and perform design analyses in a practical manner commensurate with a level of exploration and analysis consistent with facility type and consequence of failure. This approach begins with screening studies and sensitivity analyses to understand the importance of an estimated Sus estimate for a given structure. Screening studies to identify zones of potentially contractive material may consist of evaluating Cone Penetration Test (CPT) and Standard Penetration Test (SPT) data to visualize and differentiate between relatively strong and weaker layers, often using several methods to estimate in-situ state parameter (ψ). Estimates of Sus discussed include using lower bound estimates from literature based on back calculated values from failures, to more sophisticated methods including; NorSand – CSSM framework using site- and material specific NorSand calibration, using best fit Widget calibration methods, Field Vane Shear Testing (FVST), Steady State Line (SSL) strengths estimated using the steady-state strength approach, and triaxial Consolidated Intact Undrained (CIU) peak undrained strength test divided by a sensitivity value from FVST. The framework provides a basis for key stakeholders to identify an appropriate approach for evaluating static liquefaction and use that information to reduce risk. As a result, stakeholders may collaborate to develop cost effective solutions to address the identified credible failure mode in a consistent fashion, moving the bar from State of the Art to State of the Practice.