Flow (or static) liquefaction is one of the most detrimental forms of ground failure. To determine flow slide potential, the residual strength of liquefied soil is needed. However, this is an extremely difficult parameter to estimate for soil deposits due to spatial variability of soil properties, potential for the formation of water films, the intermixing of soils, and the potential for partial drainage during flow liquefaction. Thus, the current state of practice for estimating the residual strength of liquefied soil (Sr) is via back calculations using case histories. However, the complexity of flow slides makes case histories difficult to interpret, and combined with the limited number of case histories, it inherently implies large uncertainties in the derived empirical relationships. Although such empirical relationships define the current state-of-practice for estimating Sr, laboratory studies and fundamental soil mechanics can provide insights and/or can be used to guide the form of the empirical relationships. For example, one issue that is an active area of debate is whether Sr normalizes by initial vertical effective stress (σ’vo). Olson and Stark (2002) present an empirical relationship estimating residual undrained strength ratio (i.e., Sr/σ’vo) as a function of normalized cone penetration test tip resistance whereas Kramer and Wang (2015) showed that Sr does not scale linearly with σ’vo. Hence, the objective of this study is to develop a more mechanistic understanding of the residual shear strength of liquefied soils based on fundamental soil mechanics and laboratory studies. Specifically, an expression for Sr/σ’vo is derived in terms of the effective angle of internal friction for residual strength, ϕ’r, and Skempton’s pore water pressure coefficient for residual conditions, Ar). Data from published laboratory studies are used to develop correlations for estimating both ϕ’r and Ar). The results show that ϕ’r is relatively constant for a range of sands, and the variability in its value does not significantly affect the computed value of Sr/σ’vo. Additionally, the results show that Ar correlates with the state parameter (ψ) for initial conditions and depends on whether the soil grains crush. Additionally, the value of Ar significantly affects the computed value of Sr/σ’vo. The derived laboratory-data-based ψ - Ar relationship is used in conjunction with an empirical relationship relating ψ and normalized cone penetration test (CPT) tip resistance (Qtn) to develop a relationship relating Sr/σ’vo to Qtn, which is then compared to similar relationships derived from back-analysis of case histories. The comparison shows that the proposed correlation most closely resembles Robertson’s (2010) correlation once adjustments are made to the relationship between Qtn and ψ.

Flow (or static) liquefaction is one of the most detrimental forms of ground failure and is when soil on a sloped ground fails and slides due to either earthquake shaking or some other type of sudden or fatigue loading. To determine the flow slide potential, the strength of the soil after it fails is needed. This parameter is called the "residual strength" and is an extremely difficult parameter to estimate due to the variability and unpredictability of the soil. Thus, the current state of practice for estimating the residual strength of liquefied soil (Sr) is via back calculations of Sr using case histories of past flow slides. However, the complexity of flow slides makes case histories difficult to interpret, and combined with the limited number of case histories inherently implies large uncertainties in the derived empirical relationships. Although such empirical relationships define the current state-of-practice for estimating Sr, laboratory studies and fundamental soil mechanics can provide insights and/or can be used to guide the form of the empirical relationships. Hence, the objective of this study is to develop a better understanding of Sr based on fundamental soil mechanics and laboratory studies on soil specimens. Specifically, the form of the equation for the residual strength normalized by the stress from the ground above (Sr/σ'vo) is derived from laboratory tests to determine the independent variables. Expressions for the independent variables are developed using published laboratory data on nine different sands. A correlation is presented between one independent variable, Skempton's �r parameter, which is a measured value in the lab, and another variable called the state parameter (ψ) that is calculated from the laboratory tests. This comparison results in a surprisingly strong correlation between the two variables and allows �r to be estimated from a common field test, the Cone Penetration Test (CPT). The �r parameter was found to greatly affect Sr/σ'vo while the other independent variable, ϕ'r, which is a measure of the soil's friction, does not affect Sr/σ'vo and is relatively constant for the analyzed sands. The derived ψ - �r relationship is used in conjunction with a relationship between ψ and a variable determined from CPT called the tip resistance (Qtn) to develop a relationship relating Sr/σ'vo to Qtn. This relationship is then compared to similar relationships derived from back-analysis of case histories. The comparison shows that the proposed correlation most closely resembles Robertson's (2010) correlation once adjustments are made to match the assumptions. This correlation is a step forward in accurately determining the residual strength of liquefied soil.

Master of Science