Journal of Geotechnical and Geoenvironmental Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

A significant increase in effective stress can be induced in hydrate-bearing reservoirs when the depressurization method is applied. A series of drained triaxial shear tests were performed on hydrate-bearing sediments with various hydrate saturations to investigate their mechanical characteristics under effective confining pressures of up to 20 MPa. The results show that significant particle crushing of the host sand occurs during shearing under high pressures, and there is no remarkable effect of hydrate saturation on the degree of particle breakage. As the effective confining pressure increases, the stress–strain curves of the hydrate-bearing specimen transformed from strain-softening to strain-hardening. The peak stress ratio and internal friction angle of the sediments gradually decrease and tend to be constant with the increased confining stress, whereas the cohesion in hydrate-bearing sediments exhibits an increasing tendency. The critical state line (CSL) of hydrate-bearing sediments in the $e-\ln p^{\prime }$ space under low-to-high effective confining pressures intersects with the normal consolidation line under the same hydrate saturation. Furthermore, the CSL moves upward and rotates clockwise as the hydrate saturation increases.

Site survey revealed that a liquefiable sand layer was often sandwiched by low-permeable layers with fine particles, which could hinder the dissipation of excess pore water pressure (EPWP) after earthquake excitation. However, the remaining EPWP within the liquefied sand layer may significantly reduce the reliquefaction resistance of the sand during an aftershock of the main earthquake. Many investigations on the liquefaction of stratified sand have been performed in model tests, e.g., 1 g shaking table tests and centrifuge tests as boundary value problems. However, few contributions on this topic from element tests are available in the literature. Investigations on the reliquefaction of sand sandwiched by low-permeable layers are even less available. Therefore, three groups of reliquefaction tests with different reconsolidation ratios, or different dissipation degrees of EPWP, were performed on Toyoura sand using a cyclic triaxial loading apparatus. In each group of tests, two key factors, stress-induced anisotropy formed in the first liquefaction process and the cyclic stress ratio in the second loading, were designed to investigate the reliquefaction resistance of incomplete reconsolidated sand that had already experienced liquefaction in the first loading. The test results indicated that the reliquefaction resistance decreased significantly if residual EPWP existed in the specimens. Furthermore, stress-induced anisotropy has a substantial influence on the reliquefaction resistance of both complete and incomplete reconsolidated sands. In an extreme case, even a small amplitude of aftershock could trigger reliquefaction of sand if the first liquefied sand maintains a high level of stress-induced anisotropy or has a large amount of residual EPWP. Finally, the relations among the cyclic stress ratio, stress-induced anisotropy, and reconsolidation ratio were proposed to predict the reliquefaction resistance of sand sandwiched by low-permeable layers.

The shear modulus ($G$) is the parameter commonly used to describe soil stiffness and to calculate shear deformations at small-to-moderate strains ($\gamma <0.5\%$). The material damping ratio ($D$) is the parameter commonly used along with $G$ to analyze the response of the geotechnical materials to dynamic shearing motions. The following four sets of empirical models are presented in this study: (1) small-strain shear modulus ($G_{\max }$), (2) nonlinear shear modulus ($G/G_{\max }–\log \gamma$) relationships, (3) small-strain material damping ratio ($D_{\min }$), and (4) nonlinear material damping ratio relationships ($D–\log \gamma$). The associated database included all traceable uncemented soil specimens tested in the Soil and Rock Dynamics Laboratory at The University of Texas at Austin using the combined resonant column and torsional shear (RCTS) equipment since the late 1980s. The effects of soil type, index properties, density, confining state, and strain level on the shear modulus and material damping ratio have been quantified through multivariable regression analyses performed in a staged manner. The staged outcomes provide options of models with user-preferred levels of complexity and corresponding accuracy. In conclusion, these empirical models for $G_{\max }$, $G/G_{\max }–\log \gamma$ relationships, $D_{\min }$, and $D–\log \gamma$ relationships perform well in fitting the database and can be applied to predict the shear behavior of uncemented soils at small-to-moderate strains ($\gamma <0.5\%$).

Breach due to overtopping is the most common failure mode of earthen levees. Historic records and future projections consistently show exacerbating patterns in the frequency and severity of floods in several regions, which can increase the probability of levee overtopping. The main objective of this study is twofold: (1) to present a comprehensive data set of levee overtopping events, and (2) to develop a data-driven model for determining the probability of levee breach due to overtopping that can support risk assessment. For this purpose, we first assessed available performance data to develop a refined data set of 185 riverine levee overtopping events within the portfolio of levee systems maintained by the US Army Corps of Engineers. The data set includes several geometric, geotechnical, and hydraulic variables for each overtopping incident. We then employed the data set along with logistic regression to develop, train and validate a model for calculating the probability of levee breach due to overtopping. Among several variables and functional forms examined, levee construction history, overtopping depth, overtopping duration, embankment erosion resistance, and duration of levee hydraulic loading prior to overtopping were found to be statistically significant, thus were included in the proposed model. The model was validated through k-fold cross validation and tested against a separate performance data set aside for validation purposes. The data set presented in this study can be used for identifying key factors controlling overtopping behavior, validation of model results, and providing new insight into the phenomenon of levee overtopping. The proposed model offers a practical yet robust tool for levee risk analysis that can be readily employed in practice.

The severity of surface manifestation of liquefaction is commonly used as a proxy for liquefaction damage potential. As a result, manifestation severity index (MSI) models are more commonly being used in conjunction with simplified stress-based triggering models to predict liquefaction damage potential. This paper assesses the limitations of three existing MSI models and a fourth MSI model that is developed herein. The different models have differing attributes that account for factors influencing the severity of surficial liquefaction manifestations, with the newly proposed model accounting more factors than the others. The efficacies of these MSI models are evaluated using well-documented liquefaction case histories from Canterbury, New Zealand, with the deposits primarily comprising clean to nonplastic silty sands. It is found that the MSI models that explicitly account for the contractive/dilative tendencies of soil did not perform as well as the models that do not account for this tendency, opposite of what would be expected based on the mechanics of liquefaction manifestation. The likely reason for this is the double-counting of the dilative tendencies of medium-dense to dense soils by these MSI models because the liquefaction triggering model, to some extent, inherently accounts for such effects. This implies that development of mechanistically more rigorous MSI models that are used in conjunction with simplified triggering models will not necessarily result in improved liquefaction damage potential predictions and may result in less accurate predictions. This provides the impetus for the development of a new framework that clearly and distinctly separates triggering and manifestation.

A cone penetration test (CPT)-based procedure to estimate liquefaction-induced ejecta severity is developed. The liquefaction ejecta demand parameter ($L_D$) captures the amount of upward seepage pressure that can produce artesian flow due to elevated excess hydraulic head, and the crust layer resistance parameter ($C_R$) captures the strength and thickness of the nonliquefiable crust layer. The procedure is examined using 176 well-documented field case histories consisting of thick, clean sand, and stratified silty soil sites in Christchurch. $L_D$ tends to increase systematically as ejecta severity increases at the thick, clean sand sites, and low $L_D$ values are estimated at stratified soil sites that did not produce ejecta, which resolves the apparent overestimation by other liquefaction indices at stratified soil sites. $C_R$ improves the reliability of the procedure by differentiating the performance of sites with and without a competent crust layer overlying a thick liquefiable layer with a high $L_D$ value. The proposed $L_D–C_R$ liquefaction ejecta severity chart separates cases with severe or extreme ejecta, which have high $L_D$ and low $C_R$ values, from cases with minor or no ejecta, which have low $L_D$ and high $C_R$ values.

This study proposes the capacity cumulative absolute velocity (${\mathrm{CAV}}_c$) as a novel measure for the resistance of granular soils to earthquake-induced liquefaction. The ${\mathrm{CAV}}_c$ is defined as the cumulative absolute velocity (CAV) needed to generate a threshold excess pore pressure ratio ($r_u$) value at a specified depth in a layer of liquefiable soil. A probabilistic model for predicting ${\mathrm{CAV}}_c$ corresponding to $r_u$ values between 0.5 and 1.0 is developed using a database of over 280,000 estimates of ${\mathrm{CAV}}_c$ from one-dimensional (1D), nonlinear, effective stress site-response analyses. These models provide ${\mathrm{CAV}}_c$ as a function of depth, the presence and location of low-permeability interlayers in the soil profile, and soil stiffness (as reflected in the normalized cone tip resistance from cone penetration test results). The standard deviation around the model’s estimates of ${\mathrm{CAV}}_c$ ranges from 0.59 natural log units for an $r_u$ threshold of 1.0 to 0.83 natural log units for an $r_u$ threshold of 0.5. Correlation models are provided for predicting ${\mathrm{CAV}}_c$ throughout the depth of a soil profile, across multiple $r_u$ thresholds, or both. Finally, ${\mathrm{CAV}}_c$ is implemented in a proposed modification of the liquefaction potential index (${\mathrm{LPI}}_{\mathrm{CAV}}$). The new index is validated using data from three earthquakes in Canterbury, New Zealand, and has a slightly improved predictive capability compared to existing indices, while making use of a relatively predictable intensity measure (i.e., CAV of the outcropping rock motion); this intensity measure is also compatible with performance-based methods for predicting liquefaction consequences. Finally, a guide for model implementation and examples of various applications are provided.

A full-scale mechanically stabilized earth (MSE) wall was constructed with inextensible reinforcements, and 23 lateral load tests were conducted on single piles spaced at distances of about 2 to 5 pile diameters ($D$) from the back face of the wall to the center of the pile. Tests were performed on pipe, square, and H-piles. Lateral resistance decreased significantly when piles were located closer than about $4D$ from the wall. $P$-multipliers were developed to account for the reduction in lateral soil resistance for piles near the wall and were approximately 1 for piles located further than about $4D$ from the wall but decreased linearly for smaller distances. They provided a reasonable means for estimating reduced lateral resistance and were not significantly affected by differences in reinforcement length, reinforcing type, or pile shape. Measured tensile force in the reinforcements tended to reach a peak near the pile rather than at the wall face. Equations are developed to predict maximum tensile force in the reinforcements. Tensile force increased with pile head load and decreased with increased transverse distance from the load point.

When performing soil stabilization of soft clays, the effect of temperature on the properties of the stabilized material is usually not considered. In this study, the curing temperature development and its effect on the shear strength and stiffness of lime–cement stabilized Norwegian clays was studied by field measurements, laboratory tests, and numerical analysis. The study includes 18 samples cured under different temperatures and tested by uniaxial unconfined compression (UC) tests. By increasing the curing temperature of the stabilized soil, a significantly faster strength development and higher shear strength values were observed. Monitoring the temperature in the field and introducing the maturity number allowed for the calculation of the maturity of the installed piles. In combination with laboratory results, the use of a maturity number allows to estimate shear strength and the stiffness development of the installed columns. This may allow for a higher utilization of the stabilized material, which further on can lead to reduced amount of binder or a reduction of installed columns, giving reduced costs and carbon dioxide emissions associated with the installation process.

Gas hydrate-bearing sediments (GHBS) are vastly distributed worldwide. Although dissociation of GHBS by depressurization is considered a viable method to produce gas to meet increasing energy demand, it can alter the hydromechanical properties of GHBS by imposing additional stresses on it, which can induce instabilities in the wellbore casing. Despite many short-term gas production trials, the long-term stability and mechanisms of casing–sediment interaction remain unclear. This paper presents a novel centrifuge test using the newly developed energy harvesting chamber to investigate the effects of dissociation on vertical casing–sediment interaction and GHBS response in-flight. Test results reveal that temperature–pressure profiles evolve continuously as hydrate is dissociated, causing extension of dissociation front and inducing changes in effective stress. Dissociation within a perforated interval leads to stress transfer within the formation, resulting in vertical arching. As hydrate dissociation continues, downward movements of soil mobilize negative skin friction. The progressive hydrate dissociation could induce large axial compressive and tensile loads within and above the perforated interval, respectively. The induced axial loads evolve, and the location of neutral planes changes with the progression of hydrate dissociation. During depressurization, the compressive load in the casing exceeds the yield strength of the modeled prototype casing. Although the induced tensile loads are smaller than the yielding strength of the casing, the cement above the perforated interval might crack in tension and jeopardize the well’s integrity. Hence, both induced compressive and tensile axial loads during long-term hydrate dissociation should be carefully considered for safe gas production in GHBS.