Journal of Geotechnical and Geoenvironmental Engineering

The effect of soil spatial variability on the undrained vertical uplift capacity of plate anchors has been examined using the three-dimensional random finite-element approach. Multiple direct Monte Carlo simulations (MCS) were performed to quantify statistically how the anchor uplift capacity varied for variable soil conditions. This allowed examination of the effect of the magnitude of the coefficient of variation (COV) of the undrained soil strength and the spatial correlation scales of these variations on the capacity statistics. Based on the direct MCS results, subset simulations (SS) were then performed to examine how deterministic design approaches would fare in terms of achieved anchor reliability. It was shown that standard approaches could lead to overconservative anchor design, particularly when the scale of fluctuation was small. Finally, a design framework is suggested to achieve the target probability of failure, whereby the characteristic strength profile is selected based on the particular statistics of the site data compared to the anchor size.

A series of geotechnical centrifuge tests to investigate the long-term heave behavior of basements in over-consolidated clay was performed, and the profiles of the slab displacement and underslab pressure were measured simultaneously. The same prototypes were simulated using Plaxis 2D with the clay stratum represented by the small-strain hardening soil model. The results were compared, and good agreement was found in terms of the profiles of slab-soil contact pressures. The data indicated that existing semianalytical methods of long-term heave predictions can be improved by assuming a quadratic distribution of slab-soil contact pressure. Example calculations for the relaxation ratio method and the relative stiffness method are presented.

A series of centrifuge shake table tests were carried out to investigate liquefaction resistance of sand deposits with different fines content. The effects of clayey fines on the generation and dissipation of excess pore water pressure, acceleration record, surface settlement, and stress-strain behavior were investigated. It was found that for the same initial relative density, liquefaction resistance increased as fines content increased from 0% to 10% and then decreased with a further increase in fines content from 10% to 20% in the centrifuge tests. For all tests, a reduced acceleration within the soil was observed after a certain number of cycles due to the onset of liquefaction. Dilative tendencies are different for all tests, but the sand with 10% clay test showed the most obvious dilative behavior during the cyclic mobility stage. The total ground surface settlement and the time for a complete dissipation of excess pore water pressure increased with increasing fines content. Shear strains in the sand deposits increased as the clay content increased from 0% to 20%. Moreover, cone penetration tests were performed before and after shaking to study the effect of fines on penetration resistance. At the same depth across all tests, the penetration resistance before shaking decreased with increasing clay content. However, the penetration resistance after shaking increased with increasing clay content from 0% to 10% and then decreased with increasing clay content from 10% to 20%. When compared with the before-shaking profile, the percentage change in penetration resistance after shaking increased with clay content at the same depth.

Agreeing on the nominal resistance (aka. capacity) derived from any load-settlement curve requires the use of an interpretation criterion. Few of the currently available criteria have been originally designed for drilled foundations despite the increasing use of such piles for supporting infrastructure projects. The performance of 16 interpretation criteria used in current geotechnical practice was assessed using a database of 194 load tests conducted on drilled shafts. Their performance was evaluated in terms of: (1) applicability; (2) correlation among each other; and (3) the effect of drilled shaft diameter, length, and soil type. Eight of the 16 methods could not be reliably used. Capacities interpreted from the remaining eight were consistent but resulted in excessive settlement in a few cases. Performance was also evaluated in terms of serviceability. A new criterion is proposed, where the nominal resistance is defined as the load corresponding to the smallest of (1) a settlement equal to the elastic compression of a free-standing column plus 0.75 in. (20 mm); (2) the load at plunging or strain-softening; or (3) settlement corresponding to 5% of the pile diameter, unless modified by the structural engineer of record. The proposed method showed good correlation with several established criteria while including a built-in serviceability safeguard against excessive settlement.

Although there are many methods for assessing vertical stiffness of footings on the ground, simplified solutions to evaluate lateral, rotational, and torsional static stiffness are much more limited, particularly for nonhomogeneous profiles of shear modulus with depth. This paper addresses the topic by introducing a novel “work-equivalent” framework to develop new simplified design methods for estimating the stiffnesses of footings under multiple degrees-of-freedom loading for general nonhomogeneous soils. Furthermore, this framework provides a unified basis to analyze two existing design methods that have diverging results. 3D finite element analyses were carried out to investigate the soil–footing interaction for a range of continuously varying and multilayered nonhomogeneous soils, and to validate the new design approach.

Desiccation cracking in clayey soil, which may lead to soil erosion, geotechnical engineering accidents, or even environmental pollution, is a serious problem nowadays. This research proposed a multifield coupling discrete-element model of clayey soil, in which each element represented a certain volume of soil. Meanwhile, the uneven distribution and transfer of moisture were also achieved. By establishing the relationships between water content and element radius, Young’s modulus, and tensile strength, respectively, the model coupled the moisture field with the stress field. Through a discrete-element simulation of desiccation cracking in a thin clay layer, the gradual development of crack network was successfully reproduced, and the proposed model was validated. The uneven moisture distribution in the numerical specimen indicates that cracks can intensify evaporation by increasing the area of soil-air interface. Layer thickness, evaporation intensity, soil-base interaction, and compressive strength are proved to have significant impacts on crack pattern by influencing the equilibrium between desiccation shrinking and cracking. This research provides a new means to study the mechanism of desiccation cracking under multifield coupling effects.

The role of spatially variable soil stiffness and strength on structural performance has been increasingly recognized with the rapid development of geotechnical reliability-based design tools. Performance criteria linked to the angular distortion between foundations and the probability of exceeding certain limit states as a function of linear-elastic soil property autocorrelation have shed light on the link between foundation soils and potential structural damage. However, the role of soil-structure interaction (SSI), especially considering nonlinearity in soil and structural elements, to redistribute shear and flexure within structures that result from differential foundation movements, has been largely neglected. This study presents a critical examination of the effect of nonlinear SSI to redistribute flexural demands within a steel structure founded in soils exhibiting spatial variability and the corresponding differential settlements through a comparison of uncoupled and coupled Monte Carlo simulations (MCS). This study shows that the differential settlement, $\mathrm{\Delta }s$, computed using coupled (i.e., SSI) analyses are significantly smaller than those derived from the uncoupled analyses for the random field models (RFMs) used to simulate soil spatial variability. The effect of SSI is to substantially reduce the probability of exceeding selected limit state criteria due to differential movements that vary in magnitude with the level of stringency of the limit state criteria; the most significant reduction corresponds to the structural ultimate limit state (ULS). Furthermore, the effect of SSI on structural performance and the critical scale of fluctuation, ${\delta }_{crit}$, becomes more apparent and beneficial as the severity of differential movement increases. Smaller and/or “allowable” angular distortions are governed by the local footing-to-footing distance, whereas the probability of exceeding the ULS (i.e., yielding of beams) is controlled by the entire structure working to redistribute the flexural demands. Limit state severity appears to control the critical soil autocorrelation length for angular distortion, indicating that the role of SSI in the performance of structures in spatially varying soils cannot be ignored.

To estimate the cyclic shear-induced ground settlement of clay layers, cyclic simple shear tests were carried out on three normally consolidated clays with different plasticity. The pore water pressure accumulation during cyclic shear and the settlement at the postcyclic recompression stage were measured in tests with unidirectional and two-directional shearing, with different phase shifts between the shear strains applied in the two orthogonal directions and a wide range of shear strain amplitudes. All tests were performed with 200 cycles. In conclusion, firstly, the discrepancies of the pore water pressure accumulation and postcyclic settlement between unidirectional and two-directional cyclic shear were quantified, and the effects of cyclic shear direction on these properties were then described by the total length of strain path. Secondly, a method for estimating the cyclic shear-induced pore water pressure accumulation and postcyclic settlement of clayey soils with different plasticities was then developed considering the effect of cyclic shear direction.

This study investigated the hydraulic conductivity and microscopic properties of two backfills, conventional sand/bentonite (SB) and sand/xanthan gum (XG)-amended bentonite/sand (XG-SB), for use in vertical cutoff walls for controlling lateral migration of lead-contaminated groundwater. A series of laboratory experiments were conducted on the two backfills to assess slump height, specific gravity, liquid limit, and hydraulic conductivity using distilled water, tap water, and lead nitrate ${[\mathrm{Pb}(\mathrm{NO}}_3{)}_2]$ solutions at different concentrations. The results showed that the moisture content corresponding to the target slump height was higher for XG-amended backfill as compared with unamended backfill. XG amendment slightly decreased specific gravity ($G_s$), but substantially increased liquid limit ($w_L$) of the backfill. An increase in concentration of ${\mathrm{Pb}(\mathrm{NO}}_3{)}_2$ solution increased $G_s$, but decreased $w_L$ for both unamended and XG-amended backfills. The hydraulic conductivity ($k$) of both backfills increased with increasing concentrations of ${\mathrm{Pb}(\mathrm{NO}}_3{)}_2$ solutions. XG-amended backfill hydraulic conductivity ($k$) was found to be less than ${10}^{-9}\mathrm{m}/\mathrm{s}$ regardless of type of permeating liquid, whereas unamended backfill $k$ was found to be higher than ${10}^{-9}\mathrm{m}/\mathrm{s}$ when permeated with ${\mathrm{Pb}(\mathrm{NO}}_3{)}_2$ solutions. Scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry (EDS) analyses indicated that XG hydrogels filled the intergranular pores of XG-amended backfill and formed a thin coating over the bentonite particles and bentonite-coated sand granules. The X-ray diffraction (XRD) results showed no intercalation of XG into montmorillonite platelets. The zeta potential of XG-amended bentonite was higher negative relative to unamended bentonite. The microscopic properties elucidated the mechanisms for superior hydraulic performance of XG-amended backfill. A comprehensive comparison of $k$ for conventional backfill and polymer-amended backfills based on this study and previously published studies revealed that the bentonite content, bentonite type, effective confining stress, and cation concentration of a permeating liquid can significantly affect the $k$ of the backfills.

An innovative yet extremely simple approach to reduce the seismic vulnerability of existing buildings on shallow foundations is to remove the lateral contact between the embedded foundation and the surrounding soil. This paper compares the dynamic response of two identical one-degree-of-freedom reduced-scale model sway frames tested in a geotechnical centrifuge. One of the models was founded on fully embedded shallow strip footings, whereas the foundations of the other were disconnected from the soil laterally. Both models were constructed in dry medium density sand whose strength and small strain shear stiffness profiles were obtained by cone penetration and air hammer tests carried out in flight. Various tools were used to monitor the displacements and accelerations both in the structure and in the soil, including micro-electro-mechanical systems (MEMS) accelerometers, piezoelectric accelerometers, LVDTs, and a high-frequency camera. The experimental results demonstrate that the lateral disconnection of the foundation from the adjacent soil significantly reduces the foundation translational and rotational stiffness, which results in a larger predominant period of the structure and a reduction of the absolute floor accelerations obtained for different sinusoidal input signals. The lateral disconnection also mitigates floor drift and story shear forces. The trade-off is the lower radiative damping observed for the laterally disconnected foundation, which results in a larger number of post-earthquake oscillations of the structure.

Cyclic undrained triaxial tests are commonly used in research and practical design to evaluate the liquefaction resistance of sandy soils. This paper aims to propose a methodology to evaluate liquefaction resistance by considering the variability or uncertainty associated with experimentation, using Bayesian statistics with a Markov chain Monte Carlo technique. In addition to conventional nonhierarchical Bayesian modeling, hierarchical Bayesian modeling is adopted to properly incorporate the factor of variability caused by individual differences (e.g., difference between experimenters) into the liquefaction resistance evaluation. Findings show that the regression curves of the cyclic resistance ratio estimated by a nonhierarchical model for all experimenters’ results in a cooperative triaxial test program are too generic and poorly applicable. In contrast, the curves estimated by a nonhierarchical model for each experimenter’s results sometimes deviate from the overall trend, dragged by the individual characteristics. The hierarchical Bayesian modeling demonstrates that both the overall trend and each experimenter’s individuality can be rationally considered in the regression results (e.g., posterior distributions of model input parameters) by referring to the other experimenters’ results, even though the number of test cases is limited for the focal experimenter. Another advantage of the modeling is that, when a different experimenter newly performs similar laboratory tests, the posterior distribution based on the existing dataset can be used as a prior distribution to estimate model input parameters specific to the experimenter. The proposed methodology may also be used to estimate the variability of liquefaction resistance considering individual differences in laboratory tests that are difficult to quantify, e.g., differences in testing apparatus and specimen size.

Cone penetrometer testing (CPT) is a frequently used soil characterization technique for liquefaction assessment; however, this technique has shortcomings in accurately characterizing very thin soil layers having thicknesses less than two to three times the diameter of the cone. In this study, the material point method (MPM) is used to generate numerical “measured” (or “blurred”) CPT tip resistance ($q^m$) in complex soil profiles. Results show that MPM is capable of accurately simulating $q^m$ in soil profiles with layers as thin as 20 mm, even when using basic constitutive models and simplified drainage conditions. It is further shown that MPM simulations are able to replicate the tendency of the CPT to smear the boundaries between very thin, interbedded soil layers in a way that obscures their true thickness and stiffness (typically referred to as thin-layer, transition-zone, or multiple thin-layer effects). While previous numerical studies of CPT have been performed in profiles composed of two or three layers, this study considers highly interlayered profiles with upwards of 27 soil layers. Difficulties of developing and implementing multiple thin-layer corrections are presented. It is shown that a numerical framework like MPM can generate a larger set of data for the development and validation of multiple thin-layer correction procedures with the aim of improving liquefaction severity predictions in complex soil profiles.

This case study focuses on an International Circuit project in China where unmanned automobiles are to be tested in the future. The subgrade undergoes extensive settlement because of the widely distributed lacustrine deposits with high void ratio and compressibility. Prestressed high-strength concrete (PHC) pile-supported earth platforms are used for ground improvement in this area, but their effectiveness has not yet been confirmed. Associated with this adoption, the negative skin friction (NSF) of the piles and the related neutral plane (NP) position is of importance. Although NSF has been examined in detail, the NP position has not, especially with regard to its time dependence. This paper reports the settlement of the pile-supported soft soil subgrade as obtained from long-term observations of full-scale tests. The NP position is determined according to the layered settlement of the soil and the pile settlement, both of which are obtained from long-term field observations. The results reveal that the NP position has a clear time dependence that develops in stages. The NP is initially elevated with time, and then it gradually approaches a constant value. The existing models fail to predict this time dependence of the NP position in the studied sites. A exponential model is used, the fitting parameters of which have practical implications. The adopted model works well with the studied sites, especially in terms of reflecting the time dependence of the NP position and distinguishing the different stages of its development. This paper enhances the understanding of the NP and provides a reference data set for related engineering practices.