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.

Recent research on the topic of pile base resistance has demonstrated that the guidelines provided by existing design manuals and codes are rather conservative in the case of clayey soils under undrained conditions. With the aim of investigating this topic more rigorously, a computationally intensive study was carried out to quantify the effect of the soil rigidity index on the response of pile base resistance. A precise assessment of the results led to the formulation of hyperbolic relationships that capture the development of the base resistance as a function of settlement. The relationships were implemented in a beam column analysis and validated against a full axisymmetric numerical analysis and also with respect to field data from a pile test performed using an Osterberg cell near the pile base. The validation process demonstrated the usefulness of the method and recommendations arising with respect to mobilization of the pile base resistance. This study should, therefore, assist in the development of more scientifically based guidelines for pile design that balance safety and economy.

Unsaturated expansive soils annually cause major economic losses and structural damages to foundations and earthen structures, primarily due to exhibiting large amounts of swelling and shrinkage when subjected to variations in water content. If an expansive soil is laterally constrained, lateral swelling pressure can exacerbate the unfavorable characteristics of expansive soil by increasing the lateral earth pressure applied to geotechnical structures. This study presents a model to determine lateral swelling pressure considering transient flow in deformable (swelling) unsaturated soils. The transient profiles of water content and suction versus depth during the wetting process are determined considering a coupled hydromechanical behavior. The interplay between the soil deformation, water content, and hydraulic conductivity is accounted for by linking the soil swelling-shrinkage characteristic curve (SSSCC), the soil water retention curve (SWRC), and the hydraulic conductivity function (HCF). An analytical model for quantifying lateral swelling pressure is then developed using a suction stress-based effective stress, the SWRC, and extended Hook’s law for unsaturated soils. The presented model is built upon the conceptual model that lateral swelling pressure is controlled by infiltration-induced changes in suction stress of a laterally-constrained unsaturated soil. The model offers a generalized framework to determine lateral swelling pressure of expansive soils under different degrees of saturation while accounting for the effect of deformation on water content and suction profiles. The model is validated against results attained from three sets of experimental tests available in the literature. Through the three case studies examined, the proposed model exhibited a reasonable accuracy over a wide range of degrees of saturation from 50% to near fully saturated conditions. The findings of this study can contribute toward more accurate evaluations regarding the performance of geotechnical structures in unsaturated expansive soils.

A large number of bridges and structures in Colorado are supported by drilled caissons and shafts socketed in weak clay-bearing rocks (e.g., Denver blue claystone and Pierre shale). During the construction of caissons using the wet drilling technique, drilling fluid gets in direct contact with the bedrock, potentially causing their slaking and degradation and, thus, undermining the foundation capacity. This paper evaluates the effect of water infiltration on the axial capacity of caissons embedded in claystone. The methodology of this research combines laboratory testing and coupled finite-element analysis for unsaturated porous materials. A series of experiments were conducted to determine the hydraulic characteristics and the strength-suction relations of the claystone samples obtained from three sites near Denver, Colorado. These parameters were then used in numerical studies of wet drilled caissons subjected to different wetting times (i.e., the time interval between the completion of drilling and the placement of concrete). Adopting the coupled unsaturated flow and mechanical analysis in ABAQUS, a workflow was developed for modeling the full process of drilling, fluid infiltration, caisson construction, and in situ axial loadings. The results showed that among the three sites, the immediate caisson capacity reduction was the highest ($\approx 19\%$) for the most permeable and water-sensitive formation. On the other hand, the capacity reduction was found to be negligible ($\approx 0\%$) if the rock was relatively intact with low permeability, regardless of its strength-suction characteristics. Parametric studies also showed that bedrocks with a lower initial degree of saturation experience more rapid and significant strength reduction upon wetting. These results provide the basis to establish practical guidance to evaluate and minimize the impact of wetting time on caisson constructions.

Microbially induced carbonate precipitation (MICP) is an innovative approach to strengthening and improving loose porous media. To advance MICP implementation in various geotechnical applications, the work presented herein investigates the distribution pattern of cementation and quantifies the improved material properties through experimental study. Sand was cemented by MICP via a double wall delivery system under submerged conditions and induced spherical flow in a large-scale box. The assessment on MICP-treated sand was conducted using shear wave velocity, hydraulic conductivity, mass of precipitated mineral, and cone tip resistance measurements. Experimental and numerical results were synthesized to evaluate the distribution of MICP with respect to reaction and transport rates. The shear wave velocity and the cone tip resistance were correlated to identify the level of cementation induced in the treated sand. Results included the general cementation pattern of MICP and the level of cementation as affected by induced seepage velocity and density. The relationship between the measured level of cementation, the shear wave velocity, and the cone tip resistance is presented and discussed.

Centrifuge modeling was used to study the performance of loose sand treated with microbial-induced calcium carbonate precipitation (MICP) to improve liquefaction resistance to triggering and reduce the total and differential settlement of shallow foundations. The MICP-treated sand was surrounded by untreated loose and dense sand zones with initial relative densities of $D_R=40\%$ and 75%, respectively. A simple foundation system applied a contact stress of 270 kPa. The model was subjected to five uniform sinusoidal shaking events with a progressive increase in amplitude per event. Cone penetration resistance, shear-wave velocity, accelerations, excess pore pressures, and settlements were measured. The MICP treatment improved liquefaction resistance of the soil, and the presence of loose and dense sand layers near the MICP-treated sand affected the dynamic response and settlement of the soil and foundation systems. The total settlement and maximum angular distortion of the foundation systems were reduced by up to 57% and 75%, respectively, depending on soil layering and MICP improvement depth. Collectively, the results show that MICP improvement can be effective in reducing building damages induced by earthquake shaking.

This paper presents a probabilistic model for evaluating the liquefaction-triggering hazard in level, layered, and saturated granular soil profiles improved with dense granular columns (DGCs). The model is developed using the results of a comprehensive numerical parametric study, validated with a dynamic centrifuge test, and subsequently tested with the available case histories involving DGCs as a liquefaction countermeasure. The numerical database includes a total of 30,000, three-dimensional (3D), fully coupled, nonlinear, dynamic finite-element simulations with a statistically determined range of layer-, profile-, DGC-, and ground motion–specific input parameters. The criteria for the predicted degree of liquefaction (i.e., full, marginal, and no liquefaction) are based on the peak values of excess pore pressure ratio and shear strain anticipated within each soil layer. A machine learning approach that performs multinomial logistic regression along with variable selection and regularization is used to develop a set of functional forms for estimating the probabilities of full-, marginal-, and no-liquefaction in sites improved with DGCs. The proposed probabilistic model is the first of its kind that explicitly considers variations in the area replacement ratio ($A_r$), stiffness, and drainage capacity of the DGC; the thickness, depth, relative density, and hydraulic conductivity range of each layer; the evolutionary characteristics of ground motions; and the underlying uncertainty in the prediction of pore pressures and shear strains within each layer.

Oversimplifying structural modeling in the analysis of the seismic performance of tall buildings introduces variability and biases in the computed seismic response. Engineering demand parameters (EDPs) from coupled soil–structure interaction (SSI) systems can only be realistically determined when nonlinear constitutive soil and structural behaviors are adopted in the numerical formulation. Large-magnitude earthquakes mobilize soil and structural demands that are beyond the elastic response of both materials. This paper aims at evaluating the influence of the structural modeling assumptions in the computed soil response due to seismic excitations using a direct fully coupled nonlinear SSI approach. Numerical analyses are conducted on elastic and nonlinear inelastic-degrading tall building models supported on a mat foundation-to-soil continuum domain using a multiple-yield surface plane-strain constitutive model. The models were subjected to multiple ground motion scenarios representative of broadband frequency content. Large discrepancies occur between linear and nonlinear inelastic tall buildings in terms of interstory drifts, peak horizontal accelerations, structural displacements, hysteretic energy, foundation rotation, and soil settlements. The results show how using nonlinear inelastic-degrading structural models of tall buildings largely affect the computed seismic response of the entire SSI system relative to linear elastic structural modeling approaches. More realistic responses are obtained using nonlinear degrading building models including SSI effects, because energy distribution and tradeoff among both the supporting soils and structure vary significantly as the seismic demands induce stresses and strains in the building beyond the onset of structural yielding.

Desiccation cracking in clayey soils occurs when they lose moisture, leading to an increase in their compressibility and hydraulic conductivity and hence a significant reduction of soil strength. The prediction of desiccation cracking in soils is challenging due to the lack of insights into the complex coupled hydromechanical process at the grain scale. In this paper, a new hybrid discrete-continuum numerical framework, capable of capturing hydromechanical behavior of soil at both grain-scale and macroscale, is proposed for predicting desiccation cracking in clayey soil. In this framework, a soil layer is represented by an assembly of discrete element method (DEM) particles, where each occupies an equivalent continuum space and carries physical properties governing unsaturated flow. These particles move freely in the computational space following the DEM, and their contact network and the continuum mixture theory are used to model the unsaturated flow. The dependence of particle-to-particle contact behavior on water content is represented by a cohesive-frictional contact model, whose material properties are governed by the water content. In parallel with the theoretical development is a series of experiments on three-dimensional (3D) soil desiccation cracking to determine essential properties and provide data for the validation of mechanical and physical behavior. Very good agreement in both physical behavior (e.g., evolution of water content) and mechanical behavior (e.g., occurrence and development of cracks, and distribution of compressive and tensile strains) demonstrates that the proposed framework is capable of capturing the hydromechanical behavior of soil during desiccation. The capability of the proposed framework facilitates numerical experiments for insights into the hydromechanical behavior of unsaturated soils that have not been possible before.

This paper investigates the dynamic response of soil around a laterally vibrating extended pile shaft and its application to the lateral parallel seismic (PS) integrity testing method. Considering the high pile to soil relative stiffness ratio, a new analytical model is proposed incorporating an extended half-space soil model excited by the laterally vibrating pile. The developed equations of the analytical model are solved by employing the Fourier-Hankel integral transform. This scheme avoids the complicated pile–soil coupling analysis while maintaining high accuracy. The developed model and its analytical solution are then utilized to verify the applicability of the lateral PS method to predict the unknown pile length. The effects of pile and soil properties on the soil dynamic response and technical aspects of the PS test are investigated through a comprehensive parametric study. The obtained results demonstrate that the lateral PS test could be successfully used in practice to overcome the limitations of the low-strain pile integrity test (PIT).

Continuous bidirectional shear wave velocity measurements were performed in the vertical ($V$) and horizontal ($H$) directions to characterize fabric evolution of triaxial soil specimens during shear, wherein, in addition to the vertical direction, two specially designed horizontal bender elements’ housings were mounted on samples using a new measurement technique. The specimens were prepared using moist tamping and water sedimentation methods and then subjected to strain-controlled triaxial compression shear under drained condition. Different sets of stress paths were applied to uncover the evolution of fabric during consolidation and shearing. The magnitudes of the shear wave velocities in different directions highlighted severe soil anisotropy at the critical state. It was found that the shear wave velocities were governed by void ratio, effective stress, and sand fabric. Interpretation of results shows the limited accuracy of the conventional empirical functions to predict the initial shear modulus in the consolidation stage. A fabric function taking into consideration the role of the soil fabric on soil elastic moduli was proposed, and its variations were traced during shear. Variations of the fabric function underline that there exists a unique anisotropic fabric at the critical state.

This paper examines the effects of monotonic and cyclic temperature changes of a model energy pile ($\mathrm{diameter}=25\mathrm{mm}$, $\mathrm{length}=264\mathrm{mm}$) on the variations in temperature and volumetric water content of surrounding unsaturated sand. Water flowed away from the pile during heating to 36°C and toward the pile during cooling to 5°C, causing soil drying and wetting near the pile, respectively. The change in volumetric water content was time-dependent, nonlinear, and slower than the change in soil temperature and continued to evolve after the soil temperature changes stabilized. Cyclic heating/cooling induced lower thermohydraulic changes in the soil than monotonic heating and cooling. The most significant changes in soil temperatures and volumetric water content were closest to the pile at a radial distance of 20 mm from the edge of the pile and reduced with increasing radial distance for all cases. The largest change in the degree of saturation was near the pile and was up to 6% for monotonic heating. Cyclic heating/cooling induced irreversible cyclic hydraulic responses near the pile with consecutive thermal cycles and caused a permanent reduction in the soil volumetric water content. However, these irreversible cyclic effects were dominant at a radius of 20 mm and reduced with increasing radial distance from the energy pile. The change in volumetric water content was time-dependent, indicating that the ratio of heating to cooling times during cyclic heating/cooling will have a significant effect on the reversibility of hydraulic responses under temperature cycles.

This paper includes an investigation of the thermal conductivity of biocemented soils to better understanding the regimes of heat transmission through soils treated by microbially induced calcium carbonate precipitation (MICP). A series of thermal conductivity tests using the transient plane source method (TPS) was performed on biocemented silica sand specimens with different gradations, void ratios, and MICP treatment cycles. The results showed that MICP treatment greatly improved the thermal conductivity of sand specimens. An increase in uniformity coefficient or a decrease in void ratio of the sand resulted in an increase in the thermal conductivity of MICP-treated specimens for a given MICP treatment cycle. The increment of thermal conductivity of MICP-treated specimens with respect to that of untreated specimens was also affected by gradation, void ratio, and content of calcium carbonate. The greatest improvements in thermal conductivity were achieved for sands having an initial degree of saturation between 0.82 and 0.85. An empirical equation was established to predict the thermal conductivity of MICP-treated silica sand with different variables, which may be useful in designing energy piles in biocemented sand layers.

Jack-up leg installation, extraction, and reinstallation near extraction-induced footprints are simulated using the coupled Eulerian-Lagrangian method with a strain-softening soil model. A typical footprint considering soil remolding is summarized based on 29 installation and extraction simulations. The reinstallation behavior of flat-base footings, spudcans, and skirted footings near extraction-induced and idealized conical or cylindrical footprints is compared. The horizontal force and bending moment induced by the footprint are explained in terms of predicted soil flow mechanisms and reaction loads acting on various footing surfaces. Considering soil remolding in a footprint, the results indicate that the soil on the near side of the footprint collapses earlier than that on the far side. The back flow soil on the near side of the footprint leads to obvious negative $H$ loading on the footing leg. Two peaks in the $M$ profile are observed. The first one is due to the footing touching the uneven seabed and the second is due to asynchronous collapse of the near-side and far-side walls of the footprint. The effects of the footing type and offset distance on the reinstallation behavior are discussed. The maximum values of $H$ and $M$ in both the positive and negative ranges during reinstallation are presented. Among the investigated footing types, the skirted footing has the greatest potential for mitigating the footing–footprint interaction problem because it provides the largest vertical resistance but induces the smallest horizontal forces and bending moments.

This work presents a new apparatus to study the cracking behavior and tensile-compressive forces developed in soils subjected to drying-wetting cycles. The device is fully instrumented and based on active principles that enable measuring the forces generated in soils during both drying-triggered shrinkage and wetting-induced expansion. The test setup includes a balance to track the water content changes during drying and wetting and a digital camera used to take pictures at predefined time intervals. An image analysis software was used to learn about the evolution of key characteristics of the crack network (i.e., crack intensity factor and crack aperture). The digital image correlation (DIC) technique was adopted to track the displacement field during soil cracking and healing. The study was based on slurry and compacted specimens made up of a bentonite/kaolin mixture. The developed device produced novel results by continuously tracking the time evolution of the forces induced in soils during drying and wetting cycles. The results show that the tensile force was much higher in the 1st drying (more than three times) than those in subsequent dryings. The maximum compressive force developed during wetting was substantially smaller than the higher tensile force measured during drying. The crack intensity factor (CIF) and crack aperture study indicate that the crack networks tended to be denser as wetting-drying cycles progressed. Note that after each drying and wetting stage, new cracks grew in the soil. Both cracks spacing and aperture decreased after each cycle. The results reveal that the methodology developed in this work can study the soil behavior under drying-wetting cycles, and it can assist in a better understanding of this complex problem.

In this paper, the keying of strip anchors in clay has been studied by employing a large deformation finite-element methodology coupled with a soil constitutive model that takes strain rate and soil remolding into considerations. The influences of soil strength degradation during keying on the loss of embedment and undrained pull-out resistance of anchors have been investigated by varying several factors, such as soil sensitivity, initial embedment depth, loading eccentricity, and shear strength heterogeneity. The methodology is validated with results from experimental and theoretical studies available in the literature. A generalized approach is developed to assess the maximum embedment loss and ultimate pull-out capacity. A combined dimensionless parameter as a function of the initial anchor depth and normalized equivalent soil strength is introduced for the prediction of maximum embedment loss. Ultimate pull-out factors normalized by an equivalent shear strength of soil are presented as a function of the initial embedment for different soil-strength heterogeneity and anchor eccentricity. Simplified expressions are provided for adoption in the routine design practice.

The rigid inclusion technique has been used worldwide to reinforce soft soil for road and railway embankments and deep building foundations. This technique is studied through field tests and physical and numerical models. Most studies focus on embankments or slabs on soils reinforced with rigid inclusions. Previous research based on numerical modeling demonstrated that the load-transfer mechanisms for a rigid slab differ from those for an embankment. Here, a simplified physical model was developed to assess the load-transfer mechanism between the inclusion head and the load-transfer platform (LTP) under a rigid slab. This research focused on the use of an alternative material for the LTP, which is a compacted soil with or without cement. When the reinforced soil contribution is neglected, the LTP weight and applied load are transmitted completely to the inclusion cap (load-transfer efficiency of 100%). As the LTP material stiffness increases, the settlement magnitude decreases considerably. The experiments demonstrated that the applied load was transferred to the inclusion head through an inverted truncated load-transfer cone (LTC) over the inclusion; the external angle depended on the LTP material strength in accordance with Coulomb’s theory. The principal stress state at the base of the LTC also was determined. Equations are proposed to determine LTP thickness and inclusion spacing.

A sanitary landfill with significant sliding indications was studied through field investigations and numerical analyses. A translational deformation mode was discovered, and the leachate level of the landfill reached approximately 0.8 times the height of the waste body, which was extremely adverse for stability. Subsequent analyses revealed that the sliding surface started from the top of the landfill slope, developed along the liner system, and ended at the toe of the slope. To improve the stability of the landfill slope, a vertical drainage system was constructed. The leachate of the drainage wells was extruded into the header pipe by high-pressure air so that the possible clogging of pumps could be prevented. The effect of the new drainage system was verified by the subsequent decrease in leachate level and horizontal displacement rate. This case reveals that the vertical drainage system is an effective control measure for unstable running landfills.

Ordinary portland cement (OPC) has been extensively used for decades to improve the engineering properties of a variety of soils. However, the environmental issues related to the production of OPC have created an urgent need to develop and use alternative binders such as alkali-activated cements (AAC). Thus, this work assesses the mechanical performance of a sandy soil amended with an AAC composed of ground waste glass (GWG), carbide lime (CL), and sodium hydroxide (NaOH). The effect of key factors, such as the presence of a NaOH solution, the dry unit weight, and the amount of binder were evaluated on the unconfined compressive strength ($q_u$), initial shear modulus ($G_0$), and accumulated loss of mass (ALM) of compacted sand-binder specimens cured for 7 days. When analyzing the curves that correlated the mechanical behavior of the blends ($q_u$, $G_0$, and ALM) with the $\eta /B_{iv}$ index, the results show that the alkaline solution has a significant positive influence on the mechanical response of the tested specimens. The improved mechanical performance of the alkali-activated treatments was associated with the formation of a blend of C─ S─ H and (C,N)─ S─ H cementitious compounds coexisting in the same cementitious matrix (alkaline hybrid cement), of heterogeneous structure and composition, and characterized by developing in greater quantity than those produced from a pozzolanic reaction.

International gravity dam design guidelines consider that the different joints of a dam or its foundation (rock–rock, rock–concrete, and concrete–concrete joints) are unbounded and recommend using the Mohr–Coulomb criterion to evaluate their shear strength. By implementing these considerations into the calculations, it appears that an apparent cohesion value ($C_{\mathrm{APP}}$) can be used, even at low normal stress levels. It has been suggested that both the mechanical properties of the material forming the unbounded joint and the type of joint (uniform or composite) may influence the apparent cohesion. An experimental protocol comprising 24 shear tests on natural rock joint replicas was developed in this study. Replicas of the same granite joint were made with three types of mortars, exhibiting very different mechanical properties. These replicas were used to reproduce both rock–rock and concrete–concrete lift joints (uniform joints) and the rock–concrete foundation contact (composite joint). Tests were conducted under normal stresses of 100–1,000 kPa, corresponding to those found in gravity dam foundations. The results showed that the joint shear strength and apparent cohesion were not strongly influenced by the type of joint or joint material strength.