Liquefaction assessment of cohesionless soils in the vicinity of large embankments: Fid-Bau Portal

Liquefaction assessment of cohesionless soils in the vicinity of large embankments

Oka, Lalita G. / Dewoolkar, Mandar M. / Olson, Scott M.

Abstract In a typical seismic dam safety evaluation, standard penetration, cone penetration, Becker penetration, or shear wave velocity (V s) tests are often first conducted near the toe of an earth dam to infer if any liquefiable soil exists in the foundation of the dam footprint. In current practice, a level-ground condition is commonly assumed when normalizing penetration resistance and V s, and may be assumed (particularly in preliminary assessments) in applying the cyclic stress method (with or without the K α correction) to evaluate liquefaction. However, the presence of an earth dam, or any other large embankment or structure, significantly alters the normal and shear stresses in the foundation. This paper identifies and quantifies potential errors in ignoring altered stresses near heavy structures, and presents a methodology to incorporate these effects within the framework of the simplified procedure. Specifically, the effects of these altered stresses (in comparison to the level-ground assumption with and without K α correction) on the: (1) normalization of field measurements such as penetration resistance and V s; (2) cyclic stress ratio (CSR); (3) cyclic resistance ratio (CRR); and (4) factor of safety against liquefaction triggering (FS liq), are evaluated by considering static and dynamic analyses of a generic earthen embankment (60m high) resting on a saturated, cohesionless foundation (30m deep). Our analyses indicated that ignoring the presence of induced static shear stresses can result in potentially unconservative errors in overburden correction factors of 30% to 60% at shallow depth (although this error is greatly muted at depths exceeding about 15m), while errors in CSR potentially can range from about 20% too conservative to 40% unconservative. Potential errors in CRR can approach 50% unconservative at shallow depths, but again, this error is muted at depths exceeding about 15m. Combining these factors, potentially unconservative errors in computing FS liq could exceed 100% at shallow depths (less than 15m to 20m) while at greater depth (exceeding 20m) errors approach 20% on the conservative side.

Highlights ► Altered stresses in the vicinity of heavy structures affect liquefaction assessment. ► Simplified method could be unconservative in the vicinity of heavy structures. ► Alternative method using mean stresses as a basis of analysis is proposed. ► Incorporation of mean stresses is possible in the framework of simplified procedure. ► More research and clear guidelines are needed for K α near heavy structures.