Determination of Undrained Residual Strength of Fine-Grained Soil

ABSTRACT Undrained residual strength, sur, often termed remolded or postcyclic strength, is a critical input into embankment dam numerical deformation analyses. There are multiple methods available to assess sur for fine-grained soils, each with advantages and disadvantages. Field tests, such as the vane shear test and the cone penetration test, can provide reliable in situ measurements of sur. In the laboratory, sur can be estimated by measuring the shear stress mobilized at high strains in monotonic tests such as direct simple shear or triaxial shear. sur is also frequently determined from postcyclic monotonic testing; however, the postcyclic stress–strain curves can be difficult to interpret because of high excess pore water pressure existing at the start of monotonic shear due to the sample being previously subjected to cyclic loading. Such analyses often have a significant amount of uncertainty. The work described here presents two new methods developed to quantify sur through lab testing, namely, analysis of stress paths from postcyclic monotonic tests and iterative strain-controlled cyclic loading. This paper introduces the new approaches and presents results from testing performed on five fine-grained soils from the foundations of embankment dams. Values of sur from the new approaches are compared with those from VST and monotonic and postcyclic monotonic direct simple shear testing. The paper details the new approaches and presents results and conclusions from five fine-grained soils from various sites across the western United States.