Journal of Geotechnical and Geoenvironmental Engineering

Soil mechanics traditionally have focused on studying pure clays and clean sands. However, natural soils and other geomaterials usually do not fall into these two classes. Most geomaterials are a complex matrix of fines and coarser particles, with distinct shapes and mineralogy interacting. This paper studied iron ore tailings over a broad range of pressures and stress histories to investigate the mechanical behavior of this nontextbook geomaterial. A thorough experimental program was conducted, and aspects of the stress–strain response, yielding, hardening, and critical state were evaluated. The state boundary surface of iron ore tailings also was investigated. The results demonstrated that the stress history of tailings affects the tailings’ behavior only at elevated stress levels. The existence of particles with distinct mineralogy in the iron ore tailings (mainly iron oxide and quartz) could have induced different breakage patterns, with an evolving particle morphology being the dominant mechanism up to the pressures achieved.

This paper explores the influence of two thermal loading protocols on the evolutions of stress state and soil consolidation characteristics during subsequent mechanical loading. We employed a specially designed thermal consolidometer to conduct a series of drained heating tests with temperature gradient across a saturated kaolin specimen at different levels of vertical effective stress. The thermal consolidometer accommodated for (1) continuous measurements of excess pore water pressure and temperature at different locations within a specimen; and (2) establishment of a steady nonuniform soil temperature distribution, a condition that often exists in the field. Staged and cyclic thermal loading revealed the influences of vertical effective stress on total and nonrecoverable thermal strain. Continuous measurements of pore pressure and soil volume change traced the evolution of average vertical effective stress with void ratio. While the negative pore pressure measurements at the end of thermal consolidation of normally consolidated clay suggested a pseudo-overconsolidated behavior upon heating at low values of vertical effectives stress, such a tendency diminished with an increase in stress level. A gradual shift in normal consolidation line at ambient temperature suggested continuous hardening of a normally consolidated specimen when subjected to repeated thermal loading cycles in the past.

Pile response in unsaturated soils is an area of interest due to its prevalence in real-world scenarios. Significant differences exist in the lateral behavior of single piles in partially saturated soils due to increased soil stiffness. In this study, a single pile was monotonically loaded laterally to investigate the performance of piles in soils with variable water table levels. Pile head lateral response showed that a mix of unsaturated-saturated soil led to an increase in lateral loads on the pile at a similar displacement compared with the fully saturated or dry soil conditions. The lateral response of the pile was obtained based on the curvature data from the strain gauges. A derivative method was used to generate the shear force and lateral resistance along the pile length based on finite differences at the seven strain gauge locations, thereby eliminating errors associated with curve fitting. Various lateral response plots, such as the normalized $p\text{-}y$ curves, $M\text{-}y$ curves, and maximum bending moment response plots, were used to assess the response to lateral load. Further, the effect of two different loading rates were assessed on the fully saturated and unsaturated-saturated soil, and the range of loading rates evaluated had minimal influence on the pile lateral response. Results indicated that considering the influence of the unsaturated soil could increase lateral resistance under similar lateral displacements. However, there is a risk of inducing large pile bending moments, potentially leading to pile flexural failure if the pile plastic moment is reached.

Over the last 15 years, microbially induced calcite precipitation (MICP) has emerged as a possible solution to mitigate coastal erosion. To date, most MICP soil treatments that have been studied involve column injection using a pump. In recent years, MICP application through surface percolation has gained traction as an alternative technique, but data using this technique are limited. More recently, a new treatment recipe/technique was developed, and this technique was termed bioslurry. Like most MICP studies, research with bioslurry has concentrated on the column injection method, and surface percolation has received very limited attention. This paper discusses the treatment of Florida beach sand by surface percolating bioslurry. Researchers experimented with variations of the bioslurry recipe to optimize erosion resistance, which was assessed using a pocket erodometer combined with physical measurements. In addition, treated specimen morphology was preliminarily examined using X-ray diffraction and scanning electron microscopy. Results showed that erosion resistance was maximized when 15% to 25% of the specimens’ pore volumes were filled with bioslurry and that this erosion resistance may be sufficient to withstand worst-case storm events after only one treatment. In addition, previous researchers always used a relatively long (i.e., $\sim 12\mathrm{h}$) stir time when preparing bioslurry. Results presented here show that it may be possible to produce comparable data with much shorter stir times (i.e., 1 to 2 h).

Records of pile lengths are not available for several hundred high-mast light towers (HMLTs) throughout the state of Minnesota. The foundation systems, typically steel H-piles or concrete-filled steel pipe piles connected to a triangular concrete pile cap, risk overturning in the event of peak wind loadings if the foundation piles are not sufficiently deep to provide the designed uplift capacity. Without prior knowledge of the in situ pile lengths, an expensive tower foundation retrofit or replacement effort would need to be undertaken. However, the development of a nondestructive screening tool to determine the in situ pile length—compared with replacing or retrofitting all towers with unknown foundation geometries—would potentially provide significant cost savings. The main goal of the research is the development of nondestructive evaluation testing techniques for determining in situ pile lengths using steady-state vibration and hammer-impact seismic testing. The foundation testing protocol involves (1) a preliminary site investigation to determine the shallow subsurface geometry and orientation of the foundation pile cap, (2) the use of seismic cone penetrometer (SCP) attached to a cone penetration test rig to capture the induced steady-state and impact waveforms, and (3) data-driven analysis of field testing results to determine the pile lengths.

Design of geosystems built on coarse-grained soils with broader gradations are typically based on methodologies developed for clean sands without explicit consideration of the effects of gradation, potentially leading to uncertainty in performance predictions. This study investigates the effect of changes in gradation on the shear strength, stress-dilatancy behavior, critical state parameters, and fabric evolution of coarse-grained soils using three-dimensional (3D) discrete element method (DEM) simulations. The simulations of monotonic isotropically-consolidated drained and undrained triaxial tests were conducted on specimens with coefficients of uniformity ($C_U$) between 1.9 and 6.4 composed of nonspherical particles following the calibration of parameters against experimental triaxial data. Results are used to evaluate the peak and critical state shear strengths, dilatancy responses, critical state lines, shear-induced pore pressures, and fabric evolution. Notably, an increase in $C_U$ leads to increases in peak shear strength, total dilation, rate of dilation, negative pore pressure magnitude, and rate of pore pressure generation. The results show that the state parameter better captures the effect of gradation than the relative density because the former accounts for the difference between the initial and critical states. The trends in triaxial parameters are compared with established frameworks to highlight the differences in response resulting from variations in $C_U$. The particle-level measurements indicate that gradation affects the packing characteristics and contact force transmission, where broader gradations result in greater interlocking between coarser particles, and the presence of coarser particles increases the anisotropy of the strong force networks. The finer particles provide resistance to buckling within these strong force networks. Additionally, particles smaller than $D_{10}$ are inactive in stress transmission, and the percentage of particles inactive in stress transmission decreases with an increasing $C_U$. The combination of macro- and microresults contributes to understanding the mobilization of stress and its dependency on dilatancy in soils of varying gradation.

Soil biocementation through microbially induced carbonate precipitation (MICP) is a promising technique for improving soil behavior in a nondisruptive manner, particularly for rehabilitation and retrofitting applications. Previous studies characterizing the shear behavior of biocemented soils have concentrated on poorly graded sands, whereas research on well-graded gravelly soils, which are extensively used in shallow geotechnical structures, has been lacking. Mohr–Coulomb strength parameters have been predominately employed to interpret the macromechanical effects of biocementation, but the previously reported findings show significant contradictions. In this study, a well-graded aggregate, representative of commonly used well-graded gravelly soils, was biocemented and subjected to monotonic drained triaxial compression. The test results show remarkable improvements in shear behavior, with the observed changes in stress–strain responses, strength and stiffness development, and stress dilatancy agreeing with those reported for biocemented sands as well as conventional cemented soils. Relatively low cementation levels can effectively rectify the mechanical performance caused by poor compaction to that seen at optimal levels, demonstrating the feasibility and potential of biocementation for improving soils of this type. Detailed analysis of the results reveals the decisive role of cementing bonds and their degradation in causing behavioral changes at different shearing stages. The theories of bonded structure and force-chain evolution are used to explain the preyielding observations, while an analytical approach capable of quantifying the evolution of different strength components is presented for postyielding macromechanical characterization. Conversely to the inference drawn from the strength parameters, the largest improvement is found in the frictional rather than the dilative and cohesive components of strength. Further analysis reveals the commonality of the macromechanical effects of biocementation, density, and confinement, and a unique relationship between macromechanical composition and peak stress ratio emerges.

Abstract

Due to insufficient backfilling and compaction, voids existing around the bottom of a buried pipe can have a negative effect on the mechanical behavior of the pipe. Expanded polystyrene (EPS) has been widely utilized in civil engineering as fill, backfill, moisture cutoff, and impact mitigation materials for falling rocks because of its light weight and workability. In this study, centrifuge model tests were conducted in order to investigate the possibility of pipe failure when voids existed around the bottom of the buried pipe, and the effect of installing EPS blocks as a countermeasure. In the test, a model polyvinyl chloride (PVC) pipe having a diameter of 140 mm was buried in the dry sand and loaded at the ground surface in a centrifugal field of 21G. Cases with voids around the bottom of the buried pipe and with EPS blocks were studied, and the differences in the pipe mechanical behavior under standard backfill conditions were discussed. The loading conditions and the shape of the EPS were varied. The test results showed that the loss of soil around the bottom of the pipe caused a stress concentration by a factor of more than 2.5 compared to the typical burial condition, and could directly cause pipe failure. In addition, it is clear that EPS installation was an effective countermeasure to avoid the risk of insufficient compaction, because it did not significantly affect pipe deformation and strain distribution, although the earth pressure tended to concentrate slightly at the bottom of the pipe.

Practical Applications

Pipelines that are buried in the ground usually need to be backfilled uniformly with soil around the pipe. However, due to difficulties in construction, the backfilling and compaction may be insufficient on the bottom sides of the pipe, resulting in voids or localized low-density areas. In this study, centrifuge model tests were conducted to investigate the behavior of pipes buried in such nonuniform ground and the effectiveness of a countermeasure to install expanded polystyrene foam (EPS) blocks as a foundation instead of backfilling with soil. In the tests, a 150-mm-diameter polyvinyl chloride (PVC) pipe was buried in a container, and the strain of the pipe was measured in detail when a vertical load of up to 140 kPa was applied to the ground surface in a 20G centrifugal force field. As a result, when the soil in the lower part of the pipe is not uniform, a large strain is concentrated at the bottom of the pipe, which may lead to the pipe failure. Then, when EPS was installed, no large strains were measured, indicating that EPS installations are effective as a countermeasure.

Soil nailing has become a common method for improving slope stability due to its cost-effectiveness and easy installation. However, concerns have been arisen regarding the quality and integrity of soil nailing works. This study proposed a novel method for soil nail integrity assessment using an actively heated fiber-optic (AHFO) sensor. First, numerical simulations were conducted to determine the thermal conductivity profiles of soil nails with various defects, measured using AHFO. By establishing the relationship between defects and thermal conductivity profiles, the material and severity of soil nail defects can be estimated. It was found that the thermal conductivity profiles obtained by the AHFO sensor accurately correspond to defect locations, and the entrained material and severity of defects in the section can be identified for defects with a length exceeding 0.1 m. Physical model tests were performed to further validate the feasibility of the proposed quantitative soil nail integrity testing method. This paper also provides recommendations for heating parameters and testing processes to facilitate the implementation of this method by potential users.

For decades, researchers have searched for an unsaturated soil effective stress that performs analogously to effective stress for saturated soils and provides some simplifications over net stress and suction approaches. A recent debate is whether problems identified with unsaturated soil effective stress can be resolved via elastoplastic analyses. Effective stress equations are reviewed and categorized as being based on volume change, shear strength, yield, or degree of saturation. These equations are evaluated for consistency with saturated soil effective stress expectations using the Barcelona basic model (BBM), simplified into an integrated elastoplastic framework using the modified state surface approach (MSSA). Under isotropic conditions, saturated soil constant volume and yield curves are coincident, and the effective stress principle applies. In contrast, unsaturated soil constant volume and yield curves diverge, which is at root to the indefinability of an effective stress for unsaturated soils. Although effective stress for saturated soils has been defined on the basis of volume change, unsaturated soil effective stress is often defined on shear strength or degree of saturation, with attempted extrapolation to more general volume change and yield responses. Effective stress equations, when used for constitutive modeling of unsaturated soils, cannot recover the form of saturated soil effective stress models, nor provide the often-asserted simplifications or analytical economies. It is demonstrated that volume change and/or yield can occur under any constant unsaturated soil effective stress. It is shown that it is impossible to define an effective stress for unsaturated soils that functions analogously to that of saturated soils. An effective stress approach for unsaturated soils is demonstrated to be, at most, a mathematical transformation, which may provide simplifications for some limited usages.

The stability analysis of tunnel faces in multilayered soils presents challenges due to the inherent variability in natural soils. Although the random field finite-element methods offer a reliable approach to address such variability, their heavy computational demands have been a significant drawback. To overcome this limitation, this study presents a novel deep learning–based method for efficient tunnel face stability analysis in layered soils with spatial variability. By combining the merits of convolutional neural networks (CNNs) and U-Net, the proposed method trains surrogate models using a small but sufficient number of random field images to effectively learn high-level features that encompass spatial variabilities, which significantly enhances computational efficiency. In particular, U-Net generates precise displacement field images based on random field images, enabling the discrimination of tunnel face collapse failure modes. To validate the effectiveness of this proposal, a comprehensive case study involving layered soils with spatial variabilities is conducted. The remarkable agreement between the outputs of CNNs and U-Net and the predictions of finite-element simulations underscores the promising potential of using deep-learning models as a surrogate for analyzing the stability of tunnel faces in spatially variable layered soils. Last but not least, the key innovation of this work lies in the pioneering application of U-Net for geotechnical reliability analysis.

The static stiffness of suction caisson foundations is an important engineering factor for offshore wind foundation design. However, existing simplified design models are mainly developed for nonlayered soil conditions, and their accuracy for layered soil conditions is uncertain. This creates a challenge for designing these foundations in offshore wind farm sites, where layered soil conditions are commonplace. To address this, this paper proposes a multifidelity data fusion approach that combines information from different physics-based models of varying accuracy, data sparsity, and computational costs in order to improve the accuracy of stiffness estimations for layered soil conditions. The results indicate that the proposed approach is more accurate than both the simplified design model and a single-fidelity machine learning model, even with limited training data. The proposed method offers a promising data-efficient solution for fast and robust stiffness estimations, which could lead to more cost-effective offshore foundation designs.

Abstract

After a wildfire event, ash is a newly formed surficial soil layer with microscale properties such as roughness, morphology, and chemical composition that may impact how ashes form fabrics in situ and so affect the overall hydrological conditions of a burned area (infiltration capacity, permeability, etc.). To examine the effects of ash microscale properties on macroscale behavior, eight wildfire ash samples from California were characterized physically (specific gravity, specific surface area, particle size, etc.), chemically (elemental composition, organic and inorganic carbon content, etc.), and geotechnically (strength, compaction, saturated hydraulic conductivity, etc.). The tested ashes were found to contain predominantly organic unburned carbons and carbonates derived from the combustion of calcium-oxalate rich fuels in temperatures likely ranging from 300°C to 500°C. Ashes had high specific surface areas because morphologically, particles had highly texturized and porous surfaces. Additional water was necessary to coat the particle surfaces, which led to high liquid limits and compaction optimum moisture contents. Hydraulic conductivity values were within range for silty sands (${10}^{-5}–{10}^{-3}\mathrm{cm}/\mathrm{s}$), and specimens had friction angles near 30°. However, tested ashes consistently demonstrated high void ratios and low bulk densities during testing for strength, hydraulic conductivity, and compaction. These anomalies were attributed to unusual carbonate morphologies; the high interparticle friction of these phases allowed ashes to form looser fabrics than a typical silty sand and contributed to the measured high void ratios, low maximum dry unit weights, and high friction angles. Overall, we hypothesize that the relative amounts of inorganic versus organic constituents in our wildfire ash samples affected how the ashes formed fabrics and so affected their geotechnical properties.

Practical Applications

The role of wildfire ash in the postfire hydrological response of a catchment is not perfectly understood. Ash is a very heterogenous material whose properties are directly related to its formation environment (fuel type and accessibility, fire duration, and fire temperature, to name a few). This newly formed surface soil layer has unusual properties compared to natural soils, including low bulk densities and high porosities (sometimes up to 70% or more) in situ, but there is currently not enough information on ash properties in the literature to fully explain why. This study addresses this gap by providing physical, chemical, and geotechnical information about wildfire ashes. This is one of the first studies to specifically test wildfire ash maximum density, Atterberg limits, and shear strength. It provides geotechnical data to the community as well as information about ash chemical and physical properties. We hope that this study not only supplements the current literature on postwildfire landscapes but also informs researchers, engineers, and policy makers about how the formation environment of ash can influence its engineering behavior, such as strength, compressibility, and permeability.

Soil nails and tieback anchors are used extensively for excavation support and slope stabilization; however, their performance can be complicated by limited pullout capacity or installation challenges. This paper presents the results of field load tests performed on anchorage elements with snakeskin-inspired surfaces that do not require grout and that can reduce the force required for installation. These tests evaluated the effects of the asperity geometry, soil type, and embedment depth on the anchor load transfer behavior and pullout capacity. The tests consisted of jacked installation and pullout loading in sites consisting of dense sand and structured silt. The test results in sand indicate that the installation force and pullout skin friction increase as the asperity height is increased and the asperity length is decreased. The pullout capacity of the snakeskin-inspired anchors in sand was between 1.2 and 4.2 times greater than the capacity of a reference rough anchor. In the structured silt site, disturbance during installation influenced the pullout behavior, resulting in a decrease in anchor capacity as the asperity height was increased. However, the anchor capacity with small asperity heights was greater than that of the reference rough anchor. The snakeskin-inspired anchors mobilized direction-dependent skin friction, resulting in pullout skin friction values that were as much as 3.0 and 4.5 times greater than those generated during installation in the sand and silt sites, respectively, due to mobilized passive resistances during pullout. The results indicate that the snakeskin-inspired anchors can outperform conventional driven anchors in sands. However, the possible effects of installation disturbance should be evaluated carefully in sensitive, structured soils.

A method directly using rainfall records to predict a slope’s potential instability is devised. The method consists of three sequential steps: identifying the critical suction stress (pore water pressure when soil is saturated) profile of a given slope, developing a rainfall intensity–duration threshold curve for the slope, and using rainfall record to determine if the threshold is reached (failure occurs) or not (no failure). It innovatively uses a slope’s strength parameters and slope angle to develop the critical suction stress (tensile) or compressive pore water pressure profile where, at each depth within the slope, the effective stress reaches the failure state. Hydromechanical numerical modeling is then conducted under various rainfall intensities to identify their corresponding duration for slope failure, thus, the rainfall intensity–duration threshold curve of the slope. Two previously well-documented and studied rainfall-induced slope instability cases; one near the town of Edmonds, Washington State, and the other near the village of Rüdlingen in northern Switzerland are used to validate the method. Excellent predictions of the slope failure depth and timing are demonstrated, indicating the effectiveness of the proposed method. Because the suction stress–based rainfall intensity–duration curve is characteristic of a given slope and it can be determined a priori, the method provides a practical way to conduct real-time rainfall monitoring and predict instability for a specific slope, and a pathway to forecast instability of natural slopes in a region.

Full-scale axial load tests were performed on five screw injection piles founded in medium-dense to dense sand at a site in Delft, the Netherlands. Each pile was instrumented with distributed fiber-optic sensors along its full length, giving detailed insights into the shaft and base response under compression loading. The paper focuses on the pile base response and combines the test results with a newly compiled database of instrumented load tests on screw displacement piles in sand. Given the range of screw displacement piles on the market, the influence of different installation methods and pile geometries on the base resistance can be assessed through the database. In summary, the analysis showed that all screw displacement pile types tended to mobilize base capacities similar to bored or nondisplacement piles. Despite high variability in the database, no significant trend with pile geometry, such as length or diameter, was evident.

Rockfill columns, also known as stone columns, were installed in a riverbank for slope stabilization measures. The goal of this fieldwork was to prevent slope instability of the riverbank while protecting an in-service aqueduct buried in the riverbank. At this site, rockfill columns were installed with the aid of steel casings (sleeves), which were later removed with a vibrodriver. Peak particle velocities were determined at select locations to monitor the ground vibrations during installation of rockfill columns and during extraction of the steel casings. Instrumentation and monitoring were implemented because there was uncertainty about the potential for structural damage to the nearby aqueduct due to ground vibrations during the stabilization works. In this case study, numerical modeling, calibrated versus field measurements in the ground and on the aqueduct, was used to simulate the ground vibrations due to the installation of three rockfill columns close to the aqueduct. Once calibrated, the numerical models were used to evaluate the effects of vibrations in terms of particle velocities in the ground, displacements of the aqueduct, and frequency spectra on the aqueduct walls. The numerical results showed that the highest particle velocities on the aqueduct were from the rockfill columns that had the steel casings located in the same soil layer as the aqueduct. Based solely on the response in terms of particle velocity, damage to the aqueduct is unlikely. However, the numerical results also showed that the aqueduct moves slightly, both vertically and laterally due to the vibration generated while removing the steel casings; and the frequency range of the waves in the ground are within the natural frequency of the soil, which could impose additional movement to the aqueduct if allowed to move freely with the soil. Numerical results and field data also show that even pulling the steel casings without vibration generated propagation of waves in the ground.

Field-scale biocemented soil columns were installed in dry, poorly graded sand using enzyme-induced carbonate precipitation (EICP) and tube-à-manchette (TAM) permeation grouting. Column diameters ranged between 0.3 and 1.0 m, and column heights ranged from 1.0 to 2.4 m. Urease enzyme was obtained in the field from jack beans using tap water and a crude extraction process, with and without dehusking the beans. The biocementation treatment solution was injected in three cycles at approximately 24-h intervals. Test results showed that leaving the husks on the jack bean during extraction did not affect either injection or cementation. The plate load test showed an increase in bearing capacity by a factor of 3 and an increase in stiffness by a factor of 3 following treatment. Downhole seismic readings showed an increase in shear wave velocity of at least $100\mathrm{m}/\mathrm{s}$ in the biocemented soil. Measurements on exhumed columns showed that the column dimensions were similar to the target dimensions. Needle penetrometer readings taken during exhumation indicated an unconfined compressive strength of at least 500 kPa. Carbonate content measurements on exhumed specimens varied from more than 4% (by weight) close to the TAM to less than 2% at the design radius, and an increased injection rate provided more uniform carbonate content. This field-scale program demonstrates that EICP can be employed using conventional grouting equipment and a simple enzyme extraction method to create biocemented columns with diameters of 0.3 to 1.0 m and a target strength of at least 500 kPa in 3 days.

Microbial-induced calcium carbonate precipitation (MICP) is an emerging in situ grouting technology for sand ground improvement, slope stability, and subgrade reinforcement, featuring rapid implementation and low energy consumption. The precipitated calcium carbonate crystals can rapidly fill and cement sand particles so as to form a new soil structure that effectively reduces liquefaction sensitivity and dynamic damage. The centrifuge shake table test is an effective method for simulating liquefaction of sandy soil layers under shear wave excitation. Many studies have been conducted on this topic in recent years. However, the study on dynamic response, especially the liquefaction resistance of MICP-cemented sands by centrifuge shake table tests, is rare. In order to investigate the cementation effect of microbial treatment, centrifuge shake table tests were performed on two models, i.e., untreated and MICP cemented sand model. The test results indicated that, compared with untreated sand model, the liquefaction resistance of the MICP model was significantly improved in terms of acceleration response, shear stiffness, stress–strain relationship, and ground surface settlement. This study contributes to a better understanding of the mechanical law in the liquefaction process and enriches the engineering application of microbial grouting treatment of sand foundation prone to liquefaction.

This paper proposes a practical approach for data-efficient metamodeling and real-time modeling of laterally loaded monopiles using physics-informed multifidelity data fusion. The proposed approach fuses information from one-dimensional (1D) beam-column model analysis, three-dimensional (3D) finite element analysis, and field measurements (in order of increasing fidelity) for enhanced accuracy. It uses an interpretable scale factor–based data fusion architecture within a deep learning framework and incorporates physics-based constraints for robust predictions with limited data. The proposed approach is demonstrated for modeling monopile lateral load–displacement behavior using data from a real-world case study. Results show that the approach provides significantly more accurate predictions compared to a single-fidelity metamodel and a widely used multifidelity data fusion model. The model’s interpretability and data efficiency make it suitable for practical applications.