Journal of Geotechnical and Geoenvironmental Engineering

Unsaturated soils in engineered geostructures like embankments or retaining walls may experience seismic compression during earthquakes due to particle rearrangement associated with large-strain cyclic shearing. Although previous studies have investigated volume changes during drained cyclic shearing of unsaturated soils, undrained cyclic shearing presents a more complex situation. During undrained cyclic shearing, changes in total volume, matric suction, degree of saturation, effective stress, shear modulus, and damping ratio may occur that have complex coupling effects that affect seismic compression. To better understand these coupling effects, this study performed a series of undrained cyclic simple shear tests on unsaturated sand specimens. Contractile volumetric strain after 200 cycles in these tests was found to vary nonlinearly with degree of saturation. The largest volumetric contraction occurred at a degree of saturation of 0.4 and coincided with the largest decrease in mean effective stress. The sand specimens followed a wetting-path scanning curve during shearing, with small changes in matric suction. The decrease in mean effective stress during cyclic shearing for all specimens followed the same linear relation with the accumulation of volumetric strain.

One-dimensional ground response analyses (GRAs) are often used with an expectation that they provide unbiased estimates of site effects. Under that hypothesis, epistemic uncertainty in site response arises mainly from uncertain soil properties. This approach has dominated practice for projects where site-specific site response is estimated for use in probabilistic seismic hazard analyses. We extend the uncertainty framework to also consider modeling errors (i.e., inability of GRAs to model site response for some sites). We quantify this epistemic uncertainty using vertical array data in which the downhole motion is input to GRAs to predict ground motions at the surface. Residuals (i.e., difference between observed and predicted ground motion intensity measures in natural log units) are partitioned into between- and within-site components. After correcting for overprediction bias near the site period, we quantify epistemic uncertainty using between-site standard deviation, which ranges from 0.2 to 0.35 using California data. A Japan data set analyzed in the literature provides similar results. This dispersion is only modestly smaller than the site-to-site variability from ergodic models for active tectonic regions, which limits the apparent benefits of site-specific GRAs. Dispersion results are not appreciably affected by varying damping models, although a model informed by site-specific observations minimizes bias relative to alternative models based on geotechnical laboratory tests and seismological crustal attenuation studies.

This study presents an analytical formulation to predict the generation and dissipation of thermal pore pressures and volumetric strains in low permeability soils. The developed relations facilitate mimicking the transient nature of the thermal loads applied to energy geostructures overcoming the commonly adopted assumptions of fully drained or undrained conditions. This theoretical model relies on coupling Darcy’s law with relations predicting the thermal volumetric strains in soils. The developed relations were validated against experimental results under different conditions (e.g., heating rates, confining stresses, and stress states), showing good agreement between the predicted and the experimental results. The developed relations were then used to explain why the maximum thermal pore pressures at the bottom of the tested specimens were observed before the specimen temperature reaches its maximum. Finally, a sensitivity analysis was performed to identify the significance of various parameters to the thermal consolidation process.

Methane hydrate within a soil pore space may significantly modify the mechanical behavior of the sediment. Previous studies have shown that the influence of hydrate on methane-hydrate-bearing sediments (MHBS) is dilative through pore-scale kinematic constraining, which in turn affects mechanical properties such as strength and stiffness. This paper presents a new analytical–empirical approach for evaluating kinematic aspects of MHBS. The suggested approach incorporates explicit experimental kinematics data of the host soil together with an analytical amplification factor. Based on known stress-dilatancy expressions, this analytical factor is found with a strong relation to stress ratios of MHBS to host soil during mechanical testing. A stress ratio model is suggested, which is developed based on a thorough study of various stress ratios from test results of both artificial and natural methane-hydrate-bearing sediments from different works. Using the suggested approach, mechanical models may consist of fewer model parameters, which may increase the model efficiency and yield a deeper understanding of fundamental mechanical characteristics of MHBS.

Poisson’s ratio is an elastic property needed as input in a wide range of geotechnical engineering applications. Despite experimental evidence, there is currently no model in the literature to explicitly account for the effect of saturation on the Poisson’s ratio. This paper presents the concept of Poisson’s ratio characteristic curve (PRCC), which establishes a relationship between Poisson’s ratio and degree of saturation (or matric suction). The PRCC concept is developed based upon the observation that variations of Poisson’s ratio are mainly dominated by water retention mechanisms. A sigmoidal function is employed to describe the PRCC with two fitting parameters, funicular degree of saturation and pore fluid continuity, and both are related to the soil water retention curve (SWRC). The functional form is calibrated and validated against Poisson’s ratios calculated from measured compressive and shear wave velocities from experimental data sets for 22 different soils from the literature. Further, a set of laboratory tests is performed to measure wave velocities using bender elements and determine the Poisson’s ratio of Bonny silt at different suctions. The PRCC fitting parameters are shown to be linearly correlated with the SWRC fitting parameters. To illustrate the PRCC application, three sets of laboratory-measured data of at-rest earth pressure coefficient for different unsaturated soils are collected from the literature and compared against the predicted values using the PRCC model and those using constant Poisson’s ratio values. The values using the PRCC model closely match the measured values, whereas using a constant Poisson’s ratio can significantly underpredict or overpredict the at-rest earth pressure coefficient. The proposed model can readily be incorporated into analytical and numerical models, leading to more accurate assessments of unsaturated soil behavior.

We used waveforms from the 2018 $M_{\mathrm{W}}7.1$ Anchorage earthquake and six selected aftershocks, ranging from $M_{\mathrm{W}}4.2$ to 5.7, to quantify site properties including shear-wave velocity profile, predominant frequencies, borehole amplification, soil-damping ratio, and shear modulus at Delaney Park in downtown Anchorage, Alaska. The waveforms were recorded by surface and six borehole (up to 61-m depth) three-component accelerometers. The deconvolution of the waveforms at various borehole depths on horizontal sensors with respect to the corresponding waveform at the surface provides incident and reflected traveling waves within the soil column. The shear-wave velocities determined from these events are consistent, and generally agree well with the in situ measurements. The borehole amplification based on surface-to-borehole traditional standard spectral ratio (SSR) and surface-to-borehole response spectral ratio (RSR) and cross-spectral ratio (c-SSR) were also evaluated. Based on c-SSR, we computed the borehole amplification as 4.8 at 1.35 Hz (0.74 s), close to the predominant frequency of the soil column.

Offshore geoengineering requires reliable sediment parameters for analysis and design. This study proposes a robust framework for effective stress-dependent geotechnical and geoacoustic properties for seabed analysis based on geophysical models, new experimental data, and extensive data sets compiled from published studies that cover a wide range of marine sediments and depths. First, effective stress-dependent porosity versus depth profiles are computed using compaction models that are valid for a wide stress range. Then, P- and S-wave velocity data are analyzed in the context of effective stress-controlled density, shear stiffness, and bulk modulus within a Hertzian-Biot-Gassmann framework. Finally, this study selects six distinct “reference sediments” that range from clean sands to high-plasticity clays and assigns self-consistent compaction and shear stiffness properties using well-known correlations reported in the literature in terms of specific surface, plasticity, and grain characteristics. Results show that robust physical models for compaction and stiffness adequately predict depth-dependent geotechnical and geoacoustic properties according to sediment type. The asymptotic void ratio at low effective stress $e_L$ determines the sediment density ${\rho }_o$ at the sediment–water boundary. New experimental studies show that the characteristic asymptotic sediment density ${\rho }_o$ at very low effective stress ${\sigma }_z^{\prime }\to 0$ controls the high-frequency acoustic reflection used for bathymetric imaging. The proposed analysis of geoacoustic data can be used to obtain first-order estimates of seafloor sediment properties and to produce sediment-type seafloor maps.

The significant damage observed during recent earthquakes resulting from liquefaction of shallow saturated soil deposits beneath structures has illustrated the need for further research in the area of liquefaction-induced ground movement effects. This study used the shake table facility at the University of California, San Diego to evaluate the liquefaction-induced settlement of a shallow foundation founded on top of liquefiable ground conditions. To study the seismic performance of a shallow rigid foundation, two large-scale shake table tests were conducted using different input motions with varying peak accelerations. The experimental model comprised three soil layers and included a shallow foundation seated over an unsaturated crust layer underlain by saturated loose and dense layers. The model ground was based on similar subsurface ground conditions observed in recent earthquakes in New Zealand, Japan, and Turkey. The seismic response of the model foundation and the soil was captured through intensive instrumentation. The main purpose of this study was to better understand the contributing mechanisms in liquefaction-induced settlement of buildings during strong shaking. Results from this series of tests were used to explore different liquefaction mitigation countermeasures; this study served as a baseline for two follow-on shake table tests which are not discussed in this paper. Detailed discussions of the excess pore-water pressure generation and dissipation, and its effect on the contributing mechanisms of liquefaction-induced settlement are presented, along with the application of standardized cumulative absolute velocity as an intensity measure to estimate the amount of liquefaction-induced settlement. The flow velocity calculation due to hydraulic transient gradient indicated an upward flow in the loose layer, which explains the observed sand ejecta. Measured and estimated foundation settlements were compared using simplified procedures. The observed foundation settlement generally was higher than the estimated settlement. This series of large-scale shake table tests provides a unique benchmark for calibration of numerical models, and simplified procedures to reliably estimate liquefaction-induced building settlements. Future mitigation tests can be evaluated using the results of this baseline experimental study.

The 73-m-tall ($240\text{-}\mathrm{ft}$) reinforced soil slope (RSS) that supported Runway 5 at Yeager Airport in Charleston, West Virginia failed catastrophically on March 12, 2015 after a service period of about $8\mathrm{years}$. In the context of the slope’s stability, this case history describes the RSS design, construction, and performance up to the failure, as well as the postfailure investigation. The postfailure investigation revealed a fill–rock interface region composed of degraded sedimentary rock below the RSS. Shear strength characterization included 150-mm-diameter ($6\text{-}\mathrm{in.}$) consolidated drained triaxial tests on the coarse-grained fill (more than 50% gravel and cobble sized) and fully softened and residual shear strength tests on fine-grained soil from the fill–rock interface region. Limit equilibrium analysis reasonably explained failure by considering changes in shear strength in the sedimentary rock. Although the lowest calculated factor safety was behind the RSS, a local minimum of the calculated factor of safety corresponded closely with the observed failure surface.