Journal of Geotechnical and Geoenvironmental Engineering

For many applications in geotechnical engineering, an accurate assessment of soil hydraulic conductivity is important to predict settlements and pore water pressure changes. Laboratory test data can inform the permeability parameters adopted in finite-element analyses; however, the anisotropy and evolution of permeability during deformation cannot be easily measured. In this study, the influence of shearing on permeability and its anisotropy in medium-loose liquefiable sands is investigated. The discrete-element method (DEM) is used to simulate monotonic undrained and drained triaxial tests using spherical particles. Finite-volume (FV) simulations using computational fluid dynamics (CFD), and pore network model (PNM) simulations are undertaken to evaluate permeability in three orthogonal directions at different strain levels. The results indicate that shear deformation induces anisotropy in permeability, in both drained and undrained triaxial conditions, and this anisotropy increases with axial strain. Specifically, the results show an increase in permeability in the direction of the major principal stress, whereas a reduction in permeability is observed in the orthogonal plane. Undrained variations are purely driven by fabric, whereas drained changes are strongly influenced by volumetric strain. In both cases, a consistent distortion in both the pores and the constrictions or throats connecting adjacent pores is observed. The small magnitude of the changes suggests that for many, coupled finite-element analyses accounting for the evolution of permeability anisotropy during shear deformation may not be necessary. However, the combination of high-resolution FV simulations with computationally efficient PNM simulations allow to analyze the driving mechanisms of flow in granular materials and to verify the applicability of PNM in medium-loose assemblies. The assumption of constant volume accurately captures the sudden loss of contacts at the onset of liquefaction, but cannot reproduce the permeability changes observed in centrifuges and field tests. Results indicate that the Kozeny–Carman (KC) framework can be used to describe isotropic and shearing permeability changes, if variations in tortuosity and pore shape factor are accounted.

Microbially induced calcite precipitation (MICP) modifies soil behavior and properties through the precipitation of calcium carbonate (${\mathrm{CaCO}}_3$) in the pore space. It has gained prominence as one strategy for biologically induced soil improvement. This study investigates the effect of MICP on hydraulic conductivity reduction and presents permeability reduction models for MICP-treated sands. Four column experiments, each with a different size of poorly graded sand, were subject to low-concentration equimolar MICP treatments while monitoring hydraulic conductivity reduction and precipitated ${\mathrm{CaCO}}_3$ distribution. Multiple MICP treatments produced homogeneous distributions of ${\mathrm{CaCO}}_3$ and caused a gradual reduction in hydraulic conductivity of 50%–90% until a ${\mathrm{CaCO}}_3$ content of $\sim 10\%–15\%$ was achieved. The high-resolution X-ray computed microtomography (CMT) and scanning electron microscopy (SEM) imaging reveals that the pore-scale precipitation behavior changes from a contact-cementing pattern in fine sands to a mixed pattern of contact-cementing and surface-coating precipitation in coarse sands as the grain size increases. The Kozeny–Carman type of permeability models appear to well capture the hydraulic conductivity reduction caused by MICP as a function of volumetric pore fraction of ${\mathrm{CaCO}}_3$. The experimental results presented in this study advance our understanding of the pore-scale ${\mathrm{CaCO}}_3$ precipitation patterns in different sizes of sands and their effect on hydraulic conductivity. Additionally, this study provides unique and reliable hydraulic conductivity data that can be used to develop hydraulic conductivity models for MICP-treated sands.

Regression models are presented to estimate the cyclic resistance of transitional silts as an alternative to semiempirical case history and stress history and normalized engineering parameter-based models using a database of cyclic laboratory tests on intact specimens. Expressions for the plasticity index (PI) dependent semilogarithmic slope, exponent $b$, of the curve describing the variation of cyclic resistance ratio (CRR) and number of loading cycles and the variation of number of equivalent loading cycles with $b$ are developed for subduction zone earthquakes alongside corresponding magnitude scaling factors for use in cyclic failure assessments (i.e., liquefaction triggering and cyclic softening) within the simplified method framework. A PI, overconsolidation ratio, and shear strain-dependent model for the variation of CRR with $N$ is presented to estimate cyclic resistance as a function of cyclic shear strain failure criteria ranging from 1% to 10%. The suite of models can be used together or separately to facilitate assessments of cyclic failure of, or plan cyclic laboratory test programs and/or calibrate constitutive models for, transitional silts in the absence of site-specific laboratory tests.

Many constitutive models have been proposed to describe the mechanical behavior of cemented soil at large strains (above 1%). Less attention has been paid to the highly nonlinear stress–strain behavior at small strains, which are important for accurately analyzing the serviceability of many infrastructures. In this study, a bounding surface model was developed to simulate cemented soil behavior from small to large strains. Some new formulations were proposed to improve the modeling of small-strain behavior, including (1) the elastic shear modulus over a wide range of stress conditions, and (2) the nonlinear degradation of bonding strength ($p_b$) with damage strain (${\epsilon }_d$) in the ${lnp}_b-{\epsilon }_d$ plane. The new model was applied to simulate drained and undrained triaxial tests on cemented soils at different cement contents and confining pressures. Comparisons between the measured and computed results show that the new model can well capture many important aspects of cemented soil behavior, especially the elastic shear modulus at very small strains and stiffness degradation at small strains. Furthermore, the model gives a good simulation of strain softening/hardening and dilatancy/contraction during shearing under various confining pressure and void ratio conditions.

This paper presents finite-element simulations to reinterpret a series of full-model and particle image velocimetry (PIV) (half model) centrifuge tests to study the uplift of skirted foundations. The focus is on the influence of uplift rate on the uplift capacity and the mechanism of breakaway (i.e., the phenomenon of sudden loss of uplift resistance). A backbone curve, quantifying the uplift rate-capacity relationship, is established for uplift based on the numerical and experimental results. The dimensionless velocities $V=6$ and 130 are found to be the boundaries for drained, partially drained, and undrained conditions. The breakaway is found related to ambient water infiltration. In the numerical modeling, the remaining length of the skirt inside soil is used as an indicator for breakaway, i.e., breakaway happens when this length is smaller than a critical value. This hypothesis is demonstrated to be effective by comparing the numerical modeling with the experimental results.

Pore-size distribution (PSD) of soil is a fundamental property that determines flows of water, heat, chemicals, and electricity and controls distributions of stress and deformation. PSD can be intrinsically related to the soil–water retention curve (SWRC), which is a constitutive relationship describing how the soil skeleton retains pore water via adsorption and capillarity. A general framework to quantify soil’s PSD is proposed by first fitting an analytical SWRC model to experimental water retention curves that explicitly separate the SWRC into an adsorptive SWRC and a capillary SWRC. The PSD is then estimated at incremental steps by combining the capillary SWRC and the conventional Young–Laplace equation relating capillary pressure to pore size. Cumulative and density PSD curves for 11 soils covering a wide spectrum of soil types are determined and compared with the PSD functions independently measured using mercury intrusion porosimetry (MIP). Good agreement is observed in the estimated pore size range, distribution pattern, and peak position determined from the two different methods. Differences observed in the methods are attributed to differences inherent in the use of wetting and nonwetting fluids to probe the pore space. The SWRC-based method generally characterizes pore sizes ranging from 100 to 0.02 μm with a major peak located within a pore diameter range of 1–10 μm. As the clay content increases, a secondary peak becomes evident at smaller pore sizes (about 0.01–0.1 μm), where the bimodal pattern reflects the dual-porosity microstructure commonly observed for clayey soils. For soils containing swelling clays, PSD can change substantially with variation in water content; thus, the water-based method is more appropriate than MIP for applications to geotechnical engineering practice.

Seismic displacement at Balboa Boulevard during the 1994 $M_{\mathrm{W}}6.7$ Northridge earthquake ruptured a high-pressure natural gas transmission pipeline, a natural gas distribution line, and two pressurized water trunk lines. Three other natural gas transmission pipelines and an oil transmission pipeline were not damaged. This paper presents the available geologic, geotechnical, and groundwater data at the site and summarizes past research efforts studying the seismic displacement during the Northridge earthquake. Characteristics and engineering properties of the pipelines and the soil–pipeline interface, including data not previously published in the public domain, are investigated and summarized. The pipe strains are estimated using an analytical model that accounts for the soil displacement pattern, nonlinear stress–strain characteristics of the pipe steel, and effects of bends and elbows. The expected pipeline performance compares well with the observed performance.

Biostimulation through the enrichment of indigenous ureolytic bacteria has been proven to be a feasible approach for implementing microbially induced carbonate precipitation (MICP). To apply this method to real-world engineering situations, researchers are focusing on (1) increasing the cementation content to improve the mechanical performance of biocemented sand, and (2) minimizing the negative impact of ammonium, a harmful byproduct. In this study, biochars of different shapes (powdered or flaked) were used as additives in calcareous sand. The effects of biochar on shear strength improvement and ammonium removal were investigated through isotropically consolidated drained (CD) triaxial shear tests, as well as ammonium adsorption and leaching tests, respectively. Experimental results indicated that biochar positively impacted cementation content, particularly for powdered biochar with smaller particle sizes, which enhanced shear strength in CD tests. Microscopic analysis of biochar-amended biocemented sand revealed that biochar can serve as additional nucleation sites for precipitation. Moreover, the contributions of friction, dilatancy, and mobilized cohesion of biocemented sand at peak strength and in the maximum dilatancy state were elucidated. Furthermore, biochar demonstrated significant ammonium removal under both aqueous and sand column conditions. These findings hold practical significance for the stabilization of local calcareous sand, as well as for other coastal cities worldwide.

The recent construction of an underground mass rapid transit (MRT) station in Singapore involved 21 m deep excavations within underconsolidated marine clay. The lateral earth support system comprised 1 m thick diaphragm walls socketed into the underlying Old Alluvium and 4 levels of preloaded cross-lot struts. Deep soil mixing (DSM) and jet grouting piles (JGP) were used to improve up to 15 m thickness of the marine clay formation. Field monitoring data showed that these ground improvement processes caused large outward deflections of the diaphragm wall panels at some locations prior to the excavation and may have caused yielding within the wall panels. In this paper, the impacts of these prior wall deformations on the subsequent performance of the excavation support system are investigated. The measured performance at two indicative cross sections is compared with results from simplified 2D finite element analyses. The analyses simulate the effects of ground improvement through prescribed boundary pressures and represent the yielding of the diaphragm wall panels through zones of reduced bending stiffness. We show that large outward wall deflections and curvature observed during jet grouting at one section contribute to higher inward wall movements and strut loads measured during excavation, while smaller movements (and curvature) prior to excavation at a second similar cross section cause negligible change in the performance of the temporary earth retaining system. The results highlight (1) the importance of controlling ground movements associated with ground modification processes such as jet grouting, (2) the uncertainties in estimating mechanical properties for the improved soil mass, and (3) the need to improve the representation of non-linear, flexural properties ($\mathrm{M}\text{-}\kappa$) of reinforced concrete diaphragm panels.