Journal of Geotechnical and Geoenvironmental Engineering

Dr. Anand Puppala delivered the 21st Ralph B. Peck Award Lecture on February 28, 2020, at the ASCE GeoCongress event held in Minneapolis, Minnesota. This introduction briefly reviews Dr. Puppala’s contributions to geotechnical engineering and discusses some of the highlights of the corresponding paper.

This paper describes key research on expansive soils and the methods employed to characterize them; fallacies in the current characterization of expansive soils are also explained. Novel swell characterization models that account for hydro, chemical, and mechanical behaviors of soils are introduced and used to demonstrate in case studies to improve expansive soil stabilization practices. The first two case studies present the results of expansive soils stabilized by incorporating clay mineralogy and soluble soil sulfate measurements. An innovative design method for successful stabilization of expansive soil is introduced in the first case study, which incorporated both basic clay mineralogy and unsaturated soil behaviors as well as performance-based durability studies. Sulfate soil stabilization works on medium-to-high sulfate soils, including rigorous laboratory and field validation studies, are presented in the second case study. The third case study, which involves a steep earthen embankment built with expansive clayey soils and experiencing recurring surficial slope failures and maintenance issues, is also discussed. Forensic studies explaining the causes of slope failures and their mitigation methods are also included. All case studies reveal the need for detailed data about soil chemistry, including clay mineralogy and sulfate studies, to improve the current field stabilization and infrastructure design on expansive soils. The last section summarizes recent innovations for better health monitoring and management of civil infrastructure built on expansive soils using unmanned aerial vehicle platforms and visualization tools, which will be valuable for validating the application of new materials, designs, and construction processes.

This study compares the differences in calculated relief well performance at the Profit Island vicinity levee (PIVL) near Baton Rouge, Louisiana, between the three-dimensional (3D) USGS MODFLOW-USG groundwater model and the blanket theory. The 3D hydrogeological model was built using indicator kriging with 192 boring logs and calibrated against measured well discharge rates and piezometric heads during the March to May 1997 flood event. The blanket theory indicates that the total heads at 19 relief wells were lower than the ground surface. However, relief well flow rates observed during the 1997 Mississippi River flood imply that the aquifer pressure was sufficient to overcome both the total hydraulic head equivalent to the ground surface and any frictional head losses. The discrepancy is mainly due to the complex geological setting, effects resulting from relief well installation, and the influence of adjacent relief wells that cause observations to diverge from blanket theory assumptions. The 3D MODFLOW model found that all 84 relief wells maintain a factor of safety (FS) above 1.5 during the peak flood stage in 1997 even though several relief wells produced negligible discharge rates. As a result, this case study indicates that 3D seepage modeling is an accountable and precise tool.

Underground structures in liquefiable ground are prone to severe earthquake damage, and the seismic responses of such underground structures can be affected by nearby superstructures. Two centrifuge shaking table tests on a precast underground structure with and without a superstructure above it in mildly inclined liquefiable ground were designed and conducted in this study. The seismic response of the underground structure was analyzed in terms of its acceleration, deformation, internal force, and waterproof performance. The influence of the superstructure on the underground structure seismic response, including the deformations and internal forces, was investigated by comparing the responses of the underground structure in the two tests. It was observed that the joints of the precast underground structure experienced greater deformations than the nonjoint positions and affected the overall seismic response of the underground structure. Among the nonjoint positions, the column and corners of the underground structure experienced relatively large deformations and internal forces. The joint-opening deformations and dynamic bending moments of the precast underground structure were mainly governed by soil shear deformation, and the increased soil lateral forces caused by excess pore pressure (EPP) accumulation increased the bending moment of the side walls. The superstructure could amplify the acceleration of the near-field soil and underground structure, thus intensifying the EPP accumulation and underground structure deformations and internal forces. The detrimental influence of superstructures on underground structures in liquefiable ground merits more attention in seismic design.

This paper presents an investigation of the effect of sediment consolidation on contaminant transport through a sediment-cap system for in situ cappings of subaqueous contaminated sediments. Numerical simulations were conducted using the model CST3 and consider coupled consolidation and contaminant transport for representative layer geometry, material properties, and applied stress conditions. Simulation results indicate that sediment consolidation can have significant effects on contaminant breakthrough time, mass flux, cumulative mass outflow, and concentration distribution within the sediment-cap system. These effects manifest not only during the consolidation process but also long after consolidation has been completed. Traditional advective-dispersive transport analyses fail to consider transient advection induced by sediment consolidation and associated contaminant migration and changes in material properties that can lead to significantly unconservative transport results and adverse environmental impact. For example, simulation results indicate that failure to account for sediment consolidation yields a cumulative contaminant mass outflow that is underestimated by a factor greater than 6,000 within the first 3 years after cap placement and by a factor of 2 at the end of a 100-year simulation period, even though consolidation essentially is completed in the first 1.5 years.

EICP (enzyme induced carbonate precipitation) and MICP (microbially induced carbonate precipitation) treatments were applied through pervious concrete model piles to cement soil around the piles and enhance soil-pile interaction and pile capacity. The behaviors of the pervious concrete piles treated by EICP and MICP when subjected to axial compression loading were compared with each other and with an untreated pervious concrete pile. These tests were performed on 1/10th-scale model piles in the soil-structure interaction (SSI) testing facility at Lehigh University. The piles and surrounding soil were instrumented with strain gauges, bender elements, in-soil null pressure sensors, and a tactile pressure sheet. The responses of the pervious concrete piles and surrounding soil were compared through analysis of shear wave (S-wave) velocities in the treated and untreated soil zones, load transfer along the piles at the ultimate load condition, soil moisture content, calcium carbonate (${\mathrm{CaCO}}_3$) content and ammonium (${\mathrm{NH}}_4^{+}$) concentration in soil, and the characteristics of the precipitated ${\mathrm{CaCO}}_3$ crystals along the soil-pile interface. In addition, comparisons with consolidated drained (CD) triaxial test results were made among sand without treatment and with EICP and MICP treatments. The results presented in this paper demonstrated that both EICP and MICP treatments can create a cemented soil zone surrounding the pervious concrete pile and improve the pile capacity and load transfer under compression loading.

This paper presents a program of centrifuge model tests ($100g$) on the monotonic and cyclic lateral loading behavior of monopiles. The diameter and burial depth of the model pile were 59 and 550 mm, respectively, and the soil used was Fujian sand with a relative density of 70%. During monotonic loading, a linear relation between the lateral soil resistance and the pile deflection was observed, with the slope increasing linearly with soil depth. For the sand sample used in this study, the loading rate had a surprisingly weakening effect on the lateral pile–soil interaction. This was confirmed by the development of positive excess pore pressure (EPP) and the contraction behavior observed in triaxial tests. During cyclic loading, the EPP oscillated at a constant amplitude, without any accumulation with the number of cycles, and the relation of the oscillation amplitude to pile deflection velocity was derived. Implications for practice in the field when assessing the lateral pile–soil interaction of monopiles considering EPP development characteristics were discussed.

The MgO granulated ground blast-furnace slag (GGBS) binder, as a novel binder, has gained increasing attention in recent decades in stabilization/solidification (S/S) to treat contaminated soils. In this study, the physical strength and leaching performance of an MgO-GGBS (MG) binder was examined and compared with (PC) portland cement and PC and fly ash (PF) regarding the S/S treatment of dimethyl phthalates (PAE) contaminated soil after 7-day curing. PC, GGBS, fly ash (FA) and MgO were used as binders and the effect of different MG dosages was discussed. The results show that the order of early-stage strength after 7-day curing was: $\mathrm{PF}<\mathrm{MG}<\mathrm{PC}$, and PAE leachate concentrations was in the order of $\mathrm{MG}<\mathrm{PC}\approx \mathrm{PF}$. The increase in MG dosage can improve the strength of binders and immobilize PAEs, while the increase of initial PAE concentrations ($\mathrm{3,000}–\mathrm{10,000}\mathrm{mg}/\mathrm{kg}$) can reduce the strength and increase the leaching of PAE. Therefore, the MG binder showed a better performance for the S/S of PAE-contaminated soil than PC and PF binders. The relation between leachability and strength was further explored and is expected to provide guidance for the early-stage strength estimation after the S/S of PAE-contaminated clay soil with the MG binder.

An accelerogram-based method is developed and validated for the quick assessment of liquefaction occurrence based on ground motion records. In this method, two frequency-related ground motion indices, termed RL and MIFr, are defined and extracted from accelerograms using signal-processing techniques. RL and MIFr indicate the richness of the low-frequency components and the temporal variation rate of the mean instantaneous frequency in the ground motion records, respectively. A new liquefaction database consisting of ground motion stations with both ground motion records and the corresponding liquefaction observations is compiled. Logistic regression is used to develop a new liquefaction classification model that takes RL and MIFr as inputs and calculates a liquefaction indicator (LQI) that can be used to assess liquefaction occurrence. The performance of the proposed method is evaluated and compared with existing accelerogram-based liquefaction assessment methods using a common database, and the method is further validated using a new liquefaction data set. The proposed method demonstrated superior performance, with an overall accuracy of 92.8% for the common data set. The proposed method has promising potentials for applications in real-time disaster mitigation systems and rapid postearthquake loss estimations.

Engineering practitioners have widely adopted probabilistic methods for estimating liquefaction triggering hazards. Unfortunately, incorrect application of these methods has become commonly accepted in the United States and throughout many parts of the world. In this paper, we describe two specific incorrect applications of probabilistic liquefaction triggering prediction models that are commonly used in engineering practice: (1) improper parametric uncertainty characterization when using the models, and (2) the application of a conservative factor of safety thresholds to deterministic cyclic resistance ratio boundaries. These two practices and their implications are discussed, and illustrative examples and figures are presented. We also discuss the apparent inconsistencies in what constitutes an acceptable risk for liquefaction hazards and other significant geotechnical hazards, such as slope stability. We demonstrate the need to collectively consider the likelihoods of all aspects of liquefaction hazard problems—susceptibility, initiation, and effects—when calculating and considering liquefaction risk. Finally, we point to the concept and framework of performance-based earthquake engineering as a logical and objective path forward for dealing with the challenges raised in this study.

Volume change boundary conditions might dictate significant changes in normal stress and, accordingly, dramatically affect the stress-displacement response of soil-structure interfaces. Using a Norwegian Geotechnical Institute-type direct simple shear (NGI-DSS) apparatus, a series of tests covering a wide range for normal stiffness, initial normal stress, and sand relative density values was performed on interfaces between sand and two different types of geosynthetics. The experimental data presented indicate that the peak and ultimate shear strengths of the dense and medium-dense sand–geosynthetic interfaces increase as the initial normal stress and constant normal stiffness are increased. In contrast, the loose sand–geosynthetic interfaces exhibit a decrease in the peak and residual post-peak shear strengths with increasing normal stiffness because of the sand contraction. A state-dependent elastic-plastic constitutive model is found to capture the influence of various volume change boundary conditions on the mechanical behavior of sand–geosynthetic interfaces.

In urban environments, structures are often built in close proximity to each other. Seismic coupling and structure-soil-structure interaction (SSSI) are known to affect system accelerations, settlement, and tilt with consequential impact on structural damage. Yet, the extent and nature of these interactions are poorly understood, particularly on ground susceptible to liquefaction. In this paper, we use dynamic centrifuge and numerical modeling of isolated and neighboring, similar and dissimilar, shallow-founded structures on layered, level, saturated, and liquefiable soils to provide insight into (1) the mechanisms of SSSI; and (2) the relative influence of key parameters on system performance. Fully-coupled, nonlinear finite-element analyses of the centrifuge experiments indicated that in addition to a comprehensive calibration of the soil constitutive model parameters, use of higher-order elements and a sufficiently large domain size in three dimensions (3D) were critical ingredients to predicting the general trends in system response compared to centrifuge recordings. A limited numerical sensitivity analysis was performed subsequently. The results identified the key parameters affecting SSSI as spacing between the two foundations ($S$) in relation to their width ($W$) as well as the contact stress and geometry of the neighboring foundation-structure system. SSSI slightly decreased the permanent settlement of structures at the expense of a notable increase in their permanent tilt, particularly when $S\le W$. The foundation’s spectral accelerations were observed to amplify at shorter periods when near a taller and heavier building at $S\le W/3$, which was attributed to the added confinement. Increasing the uniform thickness of the critical liquefiable layer increased both adjacent structures’ permanent settlement, while not affecting their residual tilt or SSSI. Importantly, the impact of SSSI on foundation tilt and accelerations remained significant for building spacings exceeding $2.5W$. The results point to the importance of considering the impact of SSSI on structure’s key engineering demand parameters and the urgent need for improved guidelines when assessing and treating liquefaction in urban settings.

The geotechnical design of partially mobile subsea foundations (mudmats) for pipeline/flowline end terminals (PLETs) is presented in this paper. A partially mobile mudmat represents a fit-for-purpose engineering solution that has significant commercial competitiveness. The partially mobile design lies between that of a fully anchored mudmat (which is designed for negligible movements but may be too large, causing installation issues or requiring corner piles to anchor) and a fully mobile mudmat (which moves to fully accommodate the expansion of the connected pipeline, but may suffer excessive settlements that compromise the structural integrity). The partially mobile mudmat is suited to deepwater soft soil conditions. The aim of this work is to help mature this new concept and technology for practical design and to inspire future research to improve the accuracy of predictions. The objective of the paper is to present simple new analytical solutions to predict the long-term accumulated displacements and rotations of a partially mobile mudmat on soft clayey deposits subjected to cyclic loading. The proposed displacement prediction framework combines established elements of consolidation theory, plasticity theory, and critical-state soil mechanics (CSSM). Typical ranges of soil properties pertinent to a partially mobile mudmat are provided for deepwater Gulf of Mexico (GoM) soft clays, and a design analysis example is provided. For these conditions, it is concluded that the dominant displacements of a partially mobile mudmat are caused by primary consolidation and plastic failure. Recommendations for further improvement are provided to inspire further research.

Effective methods for evaluating the bending moment of a pile caused by lateral forces at the pile head are available. However, the effects of axial load variation caused by the overturning moment on pile responses have not been investigated in detail. To investigate the effects of pile settlement caused by axial load variations on the pile behavior under a large earthquake, dynamic centrifuge tests on soil–pile–superstructure models were performed. Centrifuge tests showed that the bending moment of the piles depended on the pile-tip support conditions. The test results were examined via numerical analyses using the monotonic beam on nonlinear Winkler foundation (BNWF) method. When the pile tips were supported by vertical springs and rollers, the bending moments at the pile head resulting from the overturning moment increased with the increase in the horizontal subgrade reaction in the subsurface soil. Our findings contribute toward rational seismic design of pile foundations under the action of a large earthquake.

A soil’s specific surface area (SSA) is commonly determined by two conventional methods: adsorption isotherms using nitrogen gas with interpretation through the Brunauer-Emmet-Teller (BET) equation, and ethylene glycol monomethyl ether (EGME) adsorption. The SSA so obtained may possess several drawbacks: (1) it does not directly reflect the SSA for soil-water interaction, (2) nitrogen sorption applies only to external mineral surface area and does not account for intracrystalline (expandable mineral interlayer) surface area, and (3) the EGME method is labor- and time-intensive. Recently, several water vapor adsorption SSA methods have emerged with improved capability to quantify SSA and directly probe soil-water interactions. Here the authors systematically assess the paradigms of non-water-based methods and water-based methods for their suitability to quantify soil’s SSA through a wide spectrum of soils. Independent adsorption isotherms using water vapor, EGME, and nitrogen gas for a variety of soils are measured and considered together with existing data from the literature to comparatively assess the methods. Among the water-based sorption methods, the augmented BET (A-BET) method compares consistently well with the non-water-based paradigm (EGME method). The A-BET method is superior to the other SSA methods as it can quantify and separate a soil’s external SSA (particle surface) and internal SSA (intracrystalline surface), thus providing a new dimension to understanding the role of soil-water retention in soil classification, swelling, collapsing, multiphysics flow, and mechanical behavior.

Unsaturated hydraulic characteristics of compacted smectite clays are essential in many engineering applications. The knowledge of the accurate soil-water characteristic curve (SWCC) and hydraulic conductivity function (HCF) is vital for simulating the transient water flow through bentonites. HCFs are routinely estimated from SWCCs using the statistical models that consider probabilistic soil pore-size models. Recent studies, however, emphasize a strong influence of adsorption saturation on the wetting of smectite clays. In this work, the hydraulic diffusivities of four different bentonites at different compaction densities were estimated by back-analysis from water imbibition tests. A constant hydraulic diffusivity with water content was found to be a good approximation, and the diffusivity was equivalent to the self-diffusion coefficient of water due to the influence of adsorption forces in bentonites. A novel procedure is proposed based on the identified dominant mechanism to predict the HCFs and diffusivities directly from the wetting SWCCs of compacted bentonites. The proposed approach was used to predict the HCFs, and transient water flows of four Indian bentonites at dry densities of 1, 1.4, and ${1.6\mathrm{Mg}/\mathrm{m}}^3$ and other bentonites from the literature. The proposed approach was superior to the existing statistical methods and provided excellent agreement with the back-estimated conductivity data from the water imbibition tests. On the other hand, the existing statistical models severely underpredicted the conductivities, especially in the higher suction range, due to the dominant influence of adsorption saturation.

A site’s fundamental resonance frequency ($f_0$), or its reciprocal, the fundamental period ($T_0$), is a critical parameter in seismic studies due to its proven ability to aid in predicting local site effects (i.e., frequency-dependent amplification of seismic waves). The horizontal-to-vertical spectral ratio (HVSR) method is a popular nonintrusive technique that can be used to estimate $f_0$ in a time-efficient and cost-effective manner. Although it is becoming more and more common to perform several HVSR measurements to investigate spatial variability in $f_0$ across a site, the measurements often are irregularly spaced due to access and/or budget restrictions. This has the potential of introducing significant bias when attempting to estimate a single, representative $f_0$ across an area of interest. To address this problem, we propose the use of Voronoi tessellations to obtain an unbiased, statistical representation of $f_0$ or $T_0$ from spatially distributed HVSR measurements. After area boundaries are set, Voronoi tessellation yields unique spatial estimates in a relatively simple and fast manner, which makes it appealing for standardization. To accommodate the current state of practice in HVSR processing, two distinct statistical approaches were presented. The choice of which approach to use is governed by whether the $f_0$ values at each HVSR location are reported deterministically (i.e., as single $f_0$ values without variance) or statistically (i.e., as $f_0$ values with associated variance). Three example applications were presented to illustrate potential uses. The first application demonstrated the effectiveness of the adopted approach in correcting for bias introduced by irregular spatial sampling. The second application illustrated how better-informed seismic site classifications can be made using a statistical representation of $T_0$. The third application compared the relative degree of spatial variability in $f_0$ at two downhole array sites to assess the applicability of performing one-dimensional (1D) ground response analyses.

One of the main environmental risks related to the surface storage of reactive mine tailings is acid mine drainage (AMD). AMD generation is often managed by limiting oxygen access to the reactive tailings. To do so, engineered cover systems such as covers with a capillary barrier effect (CCBEs) can be used. Over time, plants may colonize CCBE materials as a result of natural or assisted processes. Plant roots can have an impact on the hydrogeological properties that govern the performance of a CCBE, including saturated hydraulic conductivity ($k_{sat}$) and water retention curves (WRCs). In the present work, flexible wall permeameters and water retention tests were used to characterize the moisture-retaining layer (MRL) of a CCBE colonized by roots. The obtained hydrogeological properties were compared with root colonization measurements using two-dimensional (2D) and three-dimensional (3D) root trait analysis techniques. Results showed that coarser root (diameter $>0.5\mathrm{mm}$) length density had a positive linear correlation with $k_{\mathrm{sat}}({\mathrm{R}}^2=0.90)$ and that, as total root length density (RLD) increased, air entry values dropped (${\mathrm{R}}^2=0.70$). Desorption rates were also shown to decrease with increasing RLD. Although these results suggest that the hydrogeological properties of the studied MRL were not significantly affected by roots seventeen years after construction, they also suggest that, with time, roots could eventually have a negative impact on a CCBE’s performance, and that further research is needed.

This paper presents a novel method for performing equivalent linear analysis that allows for a variation of the stiffness and equivalent viscous damping properties throughout the duration of shaking. The ability to change the dynamic properties throughout the analysis comes from the conversion of the input ground motion into the time-frequency domain using the Stockwell transform whereby different transfer functions can be applied at different times before converting back to the time domain. The equivalent linear Stockwell analysis (ELSA) method provides a fully decoupled approach to modeling the dynamic site response of liquefiable soil deposits that can account for changes in properties due to strain effects and the build-up of excess pore pressure. The simplicity of the method means that only a limited number of dynamic soil properties are required, the same as those used in an equivalent linear analysis, as well as an estimation of the build-up of excess pore pressure and the relative density of the soil. While there are drawbacks of decoupling the estimation of the build-up of excess pore pressure from the dynamic response, this approach means that the effects of liquefaction on ground shaking can be independently assessed using different models for estimating pore pressure. Furthermore, the influence of liquefaction mitigation interventions, or the presence of a building, can present significant modeling challenges for fully-coupled or loosely coupled approaches, whereas they can easily be assessed using simple decoupled tools. Changing the dynamic properties throughout time using the ELSA method provides a rational way to correct equivalent linear analyses for the known drawback of using the same properties throughout the whole duration. Validation studies are presented of the ELSA method against field downhole recordings from the Wildlife Array and fully-coupled nonlinear effective stress analyses.

Several monotonic triaxial consolidated drained (CD) and consolidated undrained (CU) tests were conducted to investigate the effect of biogenic gas formation through microbial-induced denitrification on the mechanical response of poorly graded sand at different degrees of saturation ($S_r$) and varying relative density. The results demonstrated that the effect of desaturation on the mechanical response varied significantly depending on the relative density. For loose sands, the buildup of pore pressure during soil contraction is prevented. Consequently, the undrained strength increases with decreasing degrees of saturation, whereas dense sands indicate the opposite effect in which negative pore pressures resulting from soil dilation are suppressed, reducing the undrained shear strength. Medium dense sands displayed the highest increase in undrained strength, but the increasing effect displayed a peak value at a specific degree of saturation and decreased with a further reduction in saturation. Irrespective of the relative density, a critical degree of saturation exists at which a further decrease in saturation has no additional effect on the undrained response. Furthermore, it was indicated that the critical state line in the $e\text{-}\log p^{\prime }$ plane shifted as a result of biological desaturation, but the shift appeared to be independent of the degree of saturation within the tested range of $S_r$ values from 0.57 to 0.94.

Soft viscoelastic biological products such as biopolymers and biofilms have recently garnered significant interest as alternative biogrout materials for ground improvement because of their nontoxic and biodegradable characteristics. However, the impact of soft gel-like viscoelastic pore fillers on the undrained response of treated soils remains poorly understood. This study involves undrained triaxial compression tests with concurrent shear wave velocity measurements of loose contractive sands treated with soft gelatin. The specimens experience two distinct loading-gelation sequences, either consolidation before gelation (CbG) or confinement after gelation (CaG). Results reveal that the shear wave velocity can be used as an indicator of the effective stress carried by the granular skeleton. The inclusion of the viscoelastic biopolymer hinders the contractive tendency, diminishes postpeak softening, and increases the undrained shear strength of loose contractive sands. These effects become more pronounced for stiffer biopolymers because they provide an enhanced skeletal support against chain buckling and contraction. The presence of biopolymers increases the normalized undrained shear strength from $S_u/{\sigma }_o^{\prime }=\sim 0.1$ to $\sim 1.4$, particularly at low effective confining stress. The biopolymers alter the terminal state in the $p^{\prime }\text{-}q\text{-}e$ space. Therefore, critical states should be reconsidered for biopolymer-treated sands. The confinement-gelation sequence affects the effective stress supported by the granular frame and thus has pronounced effects on the undrained shear strength. This suggests the potential use of viscoelastic pore fillers as an effective treatment of loose sands prone to liquefaction.

The response of a bender transmitter to sinusoidal excitations in air, liquids, and transparent soil is investigated using a novel laser vibrometer technique. The purpose of the study is to investigate how benders vibrate in different media so that the bender–soil interaction can be better understood. This may help in more accurate determination of the travel time of shear waves in soil and in more reliable prediction of the wave velocity. The study is important because, currently, there is no standardization of the bender element (BE) test available because the bender’s behavior is not well understood when it is embedded in soil. BE test results depend on the critical assumption that the bender transducer behaves as a cantilever beam. It is shown for the first time through experimental modal analysis that benders actually vibrate as plates and higher modes of plate vibration play a significant role in bender–soil interaction. The participation of the different modes depends on the confining stresses present in the soil sample. The hydrodynamic interactions of the bender transmitter in different liquids show that the added mass effect of the liquids impact the frequency and, more importantly, the damping of the bender vibration. The changes in vibration mode and damping with a change in confinement have practical implications because the signal-to-noise ratio of bender vibration gets altered by these changes (thereby increasing the error in the estimation of shear waves), which impacts the interpretation of the BE test results. Thus, the study provides important insights into the bender behavior in terms of its displacement, velocity, resonant frequencies, damping ratios, and mode shapes.

Conventional methods of soil stabilization employing materials, such as lime or cement, have considerable environmental penalties due to their high embodied energy. Alternatives such as biopolymers can significantly alleviate this problem. This paper is the first attempt to reveal the basic mechanism of stabilizing sand using bacterial biopolymer by conducting investigations spanning from microscopic to macroscopic scales. Xanthan gum, a bacterial biopolymer, has been microscopically characterized both as a stand-alone binder and with varying proportions of clay reinforcement. Sand columns have been produced using xanthan gum as the binder with varying quantities of clay. The biopolymer stabilized samples were characterized by strength and water absorption. Although xanthan gum was able to bind the sand, exposure to moisture considerably affected its strength. The addition of clay significantly improved the performance by reinforcing the polymer. The mechanism of stabilization has been revealed through advanced microscopic investigations using scanning electron microscopy, nanoindentation, and atomic force microscopy. The study reveals the potential of bacterial polymerization as a means of sustainable soil stabilization.

A force-based limit equilibrium analysis is presented for the stability of a general two-dimensional, two-wedge sliding mass of soil, including a vertical or nonvertical wedge interface. The analysis is conducted using three failure planes and can accommodate variable conditions for wedge geometry, pore pressure, shear strength parameters, reinforcement, applied loads, and pseudostatic seismic coefficients. A constant factor of safety is assumed for each failure plane and reinforcement element, although this assumption can be relaxed through selection of strength parameters. The factor of safety is obtained analytically and requires solution for the roots of a polynomial equation. Verification checks show exact agreement with existing solutions for simplified conditions, including Mononobe–Okabe dynamic active force. Numeric examples are provided to demonstrate the method and illustrate the importance of several parameters for stability of a landfill bottom liner system and reinforced soil retaining wall. The analytical solutions take compact form, provide insight for the two-wedge method, and offer good capability to tailor conditions for applications that can be suitably characterized by wedge failure.

Offshore wind is a key part of the change toward renewable energy sources. Offshore wind farm developments are moving to sites that are characterized by increased water depths and layered soil profiles, e.g., in the North Sea. Suction caisson–based jacked structures are expected to be increasingly used to support wind turbines at such sites. Experience of suction caissons serving as foundations for offshore wind turbines is limited. The mechanisms governing the suction caisson installation in layered soils are poorly understood and no published data of the in-service performance of suction caisson jackets supporting offshore wind turbines in layered soils exists. This gap is addressed here through a series of centrifuge tests investigating the installation and the response of suction caissons in layered soils—sand over clay and clay over sand—subjected to vertical cyclic loading into tension. The results reveal the mechanisms enabling suction caisson installation in layered soils and the complex load transfer mechanisms governing the suction caisson response under vertical cyclic loading. The clay layer plays a key role: in sand over clay, it encapsulates the sand plug, which leads to predominantly undrained behavior. In clay over sand, the uplift of the clay soil plug is critical both in facilitating suction installation and in response to vertical cyclic loading in tension. These findings provide confidence for suction installation and caisson response under vertical cyclic loading into tension in layered soils.

The paper presents observations from a newly compiled database of static load tests comprising 68 instrumented drilled shafts and auger cast-in-place piles in sands, sand mixtures, silt mixtures, and clays at 37 sites around the world. The measured unit shaft friction and base resistance of the database piles are compared with values calculated using well known methods that correlate capacity directly to the cone penetration test (CPT) end resistance. It is shown that the updated Laboratoire Central des Ponts et Chaussées (LCPC) method in 2012 is the best performing of existing CPT-based methods. A new CPT approach is proposed that, similar to other recently published approaches and experimental studies, involves the soil behavior type index ($I_c$) determined in CPTs in the formulation. This approach can be expected to lead to more reliable estimates of pile capacity as it provides an improved fit to the new database while also being consistent with the trends implicit in the LCPC method, which is based on a larger and independent data set.

Optimal whole life design for railways, highways, runways, and metro lines requires an accurate assessment of how their underlying geomaterials respond to large numbers of wheel-loading cycles. This paper presents an experimental study on a natural UK stiff clay with a cyclic triaxial (CT) and hollow cylinder apparatus (CHCA) that imposed $K_0$ and wheel-loading stress conditions. The focus is on Gault clay, a high overconsolidation ratio (OCR) marine clay deposited in the Cretaceous, whose mechanical behavior is significantly anisotropic and in situ $K_0$ values exceed unity. The clay outcrops under sections of most major highways radiating out of London, as well as the HS1 and new HS2 high-speed railways. The experimental investigation explored how the principal stress rotation implicit in wheel loading increases the magnitudes and changes the sign of vertical strain accumulation, as well as accelerating resilient modulus degradation and accentuating stress–strain hysteresis, all of which affect pavement or rail-track serviceability. The clay’s deformation and pore pressure responses are categorized into stable, metastable, and unstable patterns. Comparisons with related studies on low OCR, low $K_0$ soft clay from Wenzhou in southeastern China, confirm the Gault clay’s generally stiffer prefailure behavior and different cyclic response. The stiff clay’s greater brittleness is also emphasized; particle reorientation occurs readily along distinct shear bands, leading to dramatic shear strength reductions that have a major impact on slope and foundation stability and call for appropriate caution in practical design.

Despite increasing awareness that cavitation can be a potential mechanism for drainage of water from soil, comprehensive understanding of its effects on soil water retention (SWR) remains largely elusive. A major obstacle is that pore water pressure is commonly conceptualized and measured as a mean value spatially averaged over the entire representative soil volume, including solid, liquid, and gas phases. Water pressure defined as such does not represent the intermolecular pressure that dictates the cavitation phase change process. Classical theories for predicting water cavitation pressure are critically assessed using recent theory for soil sorptive potential and synthesized to determine cavitation of water in soil. Classical cavitation mechanisms of metastable nucleation that occur on a scale of less than tens of nanometers are unlikely to occur in soil water. Cavitation from entrapped bubbles on rough surfaces of soil particles on a scale of hundreds of nanometers, however, is identified as responsible for soil water cavitation. Specifically, cavitation can occur for soils with pore sizes ranging from ${10}^{-8}$ to ${10}^{-4}\mathrm{m}$ with suction of $0.1–15\mathrm{MPa}$ but is most prominent in soils with pore sizes from ${10}^{-7}$ to ${10}^{-5}\mathrm{m}$ and suction of $0.1–2.2\mathrm{MPa}$ (silty soils). A framework for soil water cavitation is proposed, parametrically analyzed, and experimentally validated against measured SWR data obtained using axis translation (AT) and hygrometer-based methods. Results indicate that AT can artificially suppress cavitation that otherwise may be a viable water drainage mechanism for soils in the natural environment.

A membrane-type vacuum consolidation system, including surcharge loading and prefabricated vertical drains, was applied to rapidly consolidate soft clay beneath a circular embankment located at the National Field Testing Facility (NFTF) at Ballina, New South Wales (NSW), Australia. Most previous studies were devoted to multidrain systems corresponding to an embankment strip loading in two-dimensional (2D) plane strain. So far, no case study has been investigated using vacuum consolidation via prefabricated vertical drains (PVDs) beneath a circular loaded area, where the system conforms to an axisymmetric problem. This paper outlines the site investigation, construction technique, and installation of a suite of instrumentation on this circular embankment. It also describes and discusses consolidation during and after the construction of this embankment in terms of settlement, excess pore water pressure, lateral deformation, and water flow relationships as they pertain to prediction embankment with vertical drains and surcharge only. The case study demonstrates that a loss of vacuum pressure can be prevented using the proposed approach in a membrane system. Treatment of water extracted using the vacuum consolidation technique, especially in acid-sulfate terrain, is also presented.

A theoretical model was developed to determine the seismic passive earth pressure on vertical cantilever retaining walls supporting a two-layered cohesionless backfill with a loose sand layer overlying a dense sand stratum. Seismic inertial forces were considered in the analysis by employing the classical pseudostatic approach. The theoretical formulation was developed based on the method of stress characteristics, which does not require any predefined failure mechanism to initiate the analysis. The charts are presented in terms of passive earth pressure coefficient, which includes the contribution of both the layers. In addition to the generation of the actual failure mechanism, rapid mobilization of stresses at the layer interface and at the wall backface is presented in the form of normalized stress contours. The failure surface and the passive earth pressure distribution behind the wall were found to be nonlinear, with different input parameters. The passive resistance of backfill soil decreased in the presence of the loose sand layer resting on the denser soil stratum.