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This study presents the use of controlled blasting as a source of seismic energy to obtain the coupled, dynamic, linear-elastic to nonlinear-inelastic response of a plastic silt deposit. Characterization of blast-induced ground motions indicates that the shear strain and corresponding residual excess pore pressures (EPPs) are associated with low-frequency near- and far-field shear waves that are within the range of earthquake frequencies, whereas the effect of high-frequency P-waves is negligible. Three blasting programs were used to develop the initial and pre-strained relationships between shear strain, EPP, and nonlinear shear modulus degradation. The initial threshold shear strain to initiate soil nonlinearity and to trigger generation of residual EPP ranged from 0.002% to 0.003% and 0.008% to 0.012%, respectively, where the latter corresponded to ∼70% of the maximum shear modulus, Gmax. Following pre-straining and dissipation of EPPs within the silt deposit, the shear strain necessary to trigger residual excess pore pressure increased two-fold. Greater excess pore pressures were observed in situ compared to those of intact direct simple shear (DSS) test specimens at a given shear strain amplitude. The reduction of in situ undrained shear strength within the blast-induced EPP field measured using vane shear tests compared favorably with that of DSS test specimens.

Changes in soil fabric following liquefaction have been studied using various in situ methods that often return inconclusive or conflicting observations. The time–rate variation of stiffness, when observed, is usually not evaluated over significant periods of time, limiting investigations about aging in post-liquefaction regain of stiffness. Even more uncommon is the application of geophysical techniques to evaluate changes in shear wave velocity (V S) as a proxy for small-strain stiffness. This study uses controlled blasting to examine long-term post-liquefaction regain of stiffness following multiple blast events. The multi-channel analysis of surface waves (MASW) technique was used to observe changes in V S of aged deposits at a test site in South Carolina. Blast-induced liquefaction of the target liquefiable layer resulted in significant reduction to its initial small-strain stiffness owing to the destruction of the aged soil fabric. The time-rate variation in V S indicated that the initial small-strain stiffness was not re-established over many months following liquefaction. Following a second blast event, the small-strain stiffness reduced again, but recovered more quickly, similar to previously reported observations of young sand deposits. This study provides a significant basis for interpreting in situ body and surface wave measurements of aged and young sand deposits densified using blast liquefaction.

Microbially induced carbonate precipitation (MICP) has been actively investigated as a promising method to improve soil properties. A burning issue impeding its wide application is the severe spatial inhomogeneity of the CaCO3 distribution. Inspiring by the temperature sensitivity of the bacteria activity, a temperature-controlled one-phase MICP method is proposed consisting of two major steps: (1) grouting the specimen with the mixture of cementation and bacteria solutions in a low temperature; (2) inducing CaCO3 precipitation by exposing the specimen to room temperature. A series of experiments are conducted to demonstrate the advantages of the proposed method over the normal two-phase MICP method. Specimens treated with the proposed temperature-controlled method present higher CaCO3 contents with a roughly uniform distribution along the height of the specimen; the strength of those specimens are substantially improved with apparent dilatancy due to the effective bond network formed by the homogeneously distributed CaCO3 precipitation. SEM images indicate that the temperature-controlled method tends to form small crystals distributing uniformly on the grain surface, which may increase the roughness of the grain and the residual stress more effectively.

The geotechnical earthquake engineering profession has struggled with the inherent complexity of the multiphase soil response to cyclic loading owing to the progressive nature of the generation of excess pore pressure (EPP) and degradation of soil stiffness and strength. One approach to improve understanding of the cyclic response and correlate the transition from a two-phase saturated soil to a single, fluid-phase liquefied soil is to treat the soil as a non-Newtonian viscous liquid. However, the work to date suggests that the viscous fluid model approach can only be implemented following the onset of sustained soil liquefaction. This paper presents a unified thixotropic fluid model and framework that effectively links the pre-shear soil fabric and its progressive cyclic response to the onset and maintained state of soil liquefaction. The framework treats the soil fabric as a fluid net-type structure proposed for use with thixotropic fluids, and presents the constitutive state and rate equations describing the deconstruction of the liquefiable soil fabric in response to cyclic loading. The unified framework uses physically meaningful soil parameters that can be obtained from common cyclic laboratory tests to seamlessly link the state-dependent and shear-strain-rate-dependent nature of soils to the generation of EPP, the latter of which is shown to increase in significance as EPP accumulates. The proposed thixotropic-induced excess pore pressure model should prove advantageous for use in forward modelling of the stress–strain rate response of liquefiable soil and generation of EPP.

A thermodynamics-based constitutive model is developed for calcareous sand treated by microbially induced calcite precipitation (MICP) to describe the effects of biocementation and its degr...

The ability of shallow bridge foundations to dissipate earthquake energy via rocking has gained significant attention by bridge engineers owing to recent efforts to quantify the performanc...

Grain crushing is commonly encountered in deep foundation engineering, high rockfill dam engineering, railway engineering, mining engineering, coastal engineering, petroleum engineering, and other geoscience application. Grain crushing is affected by fundamental soil characteristics, such as their mineral strength, grain size and distribution, grain shape, density and specimen size, and also by external factors including stress magnitude and path, loading rate and duration, degree of saturation, temperature and geochemical environment. Crushable material becomes a series of different materials with the change in its grading during grain crushing, resulting in a decrease in strength and dilatancy and an increase in compressibility. Effects of grain crushing on strength, dilatancy, deformation and failure mechanisms have been extensively investigated through laboratory testing, discrete element method (DEM) modelling, Weibull statistics, and constitutive modelling within the framework of the extended crushing-dependent critical state theory or the energy-based theory. Eleven papers summarized in this review article for this special issue addressed the above issues in grain crushing through the advanced testing and modelling. Keywords: Grain crushing, Particle breakage, Strength, Dilatancy, DEM modelling, Constitutive modelling

Crushing of grains can greatly influence the strength, dilatancy, and stress-strain relationship of rockfill materials. The critical state line (CSL) in the void ratio versus mean effective stress plane was extended to the breakage critical state plane (BCSP). A state void-ratio-pressure index that incorporated the effect of grain crushing was proposed according to the BCSP. Rowe’s stress-dilatancy equation was modified by adding the breakage void-ratio-pressure index, which was also incorporated into the formulations of the bounding stress ratio and plastic modulus. A BCSP-based bounding surface plasticity model was proposed to describe the state-dependent stress-strain behaviors and the evolution of grain crushing during shearing process of rockfill materials, and was shown to sufficiently capture the breakage phenomenon. Keywords: Constitutive model, Grain crushing, Critical state, Dilatancy, Strength, State parameter

Microbial-induced calcium carbonate precipitation (MICP) represents a promising approach to improve the geotechnical engineering properties of soils through the precipitation of calcium carbonate (CaCO3) at soil particle contacts and soil particle surfaces. An extensive experimental study was undertaken to investigate the influence of initial relative density on the efficiency of the biocementation process, the reduction of liquefaction susceptibility, and the cyclic response in biocemented calcareous soils. For this purpose, stress-controlled undrained cyclic triaxial shear (CTS) tests were carried out on untreated and MICP-treated calcareous sand specimens for different initial relative densities and magnitudes of biocementation. Improvement in the cyclic response was quantified and compared in terms of excess pore pressure generation, evolution of axial strains, and the number of cycles to liquefaction. The cyclic experiments show that MICP treatment can change the liquefaction failure mechanism from flow failure to cyclic mobility and can significantly change the excess pore pressure generation response of initially loose specimens. Scanning electron microscope (SEM) images indicate the CaCO3 crystals alter the characteristics of the sand particles and confirm the physical change in soil fabric that impacts the dynamic behavior and liquefaction resistance of MICP-treated specimens. Furthermore, the effect of biocementation was contrasted against the effect of relative density alone, and MICP treatment was shown to exhibit greater efficiency in improving the cyclic resistance than densification.

Calcareous sands are known as problematic soils in nature and challenge geotechnical engineers in many practical projects. Microbially induced calcite precipitation (MICP) is an innovative soil improvement technique that uses biomineralisation processes to induce cementation in-situ. The work described in this paper investigates the strength, deformation, and microstructure characteristics of biocemented calcareous sand under different cementation solution to sample volume ratios. A series of laboratory experiments was conducted, including unconfined compressive strength tests, splitting, tensile (i.e., Brazilian) strength tests, and consolidated drained triaxial tests. The results indicate that an exponential function reasonably describes the unconfined compressive strength and splitting tensile strength with increasing cementation solution to sample volume ratios. The tangent modulus at 50% peak strength increases exponentially with an increase in cementation solution to sample volume ratio, whereas it increases linearly with an increase in strength. The strength parameters for this MICP-improved soil, including the peak cohesion and friction angle, are derived to facilitate engineering design. Microstructure analyses are used to illustrate the physical basis for the increase in strength and stiffness with increases in the calcite content, as demonstrated using the cementation solution to sample volume ratio.

Expanded polystyrene composite soil (EPSCS) has been used extensively in various engineering applications. However, the dynamic characteristics of EPSCS are not well understood. In order to understand the dynamic performance of EPSCS, a series of unconfined compression and resonant column tests were carried out to investigate its stiffness and damping characteristics at low strains, with particular focus on the role of expanded polystyrene (EPS) bead content, cement content and confining pressures. First, the unconfined compressive strength (qucs) was quantified using replicate specimens to form a basis for quantifying its static strength and correlation to dynamic properties. Then, the dynamic shear modulus and damping ratio for EPSCS were quantified. It is found that the effects of EPS bead and cement content on qucs and initial shear modulus (G0) are similar. The qucs or G0 of EPSCS increases approximately linearly with the increasing cement content or confining pressure, whereas it decreases with EPS bead content. Empirical models were proposed to predict G0 using the more readily determined qucs. Models used to describe the degradation in normalized shear modulus and damping ratio were calibrated for EPSCS at low strains considering EPS bead content, cement content and confining pressure.

This study presents the use of controlled blasting for the determination of the in situ dynamic response of a sand deposit at a depth of 25 m under effective overburden stresses of approxim...

This study presents a comprehensive investigation into the monotonic, cyclic, and postcyclic response of a medium stiff, plastic, lightly-overconsolidated, alluvial clayey silt deposit. The...

This technical note aims to investigate the effects of biotreatment on the response of a concrete model pile embedded in poorly graded sand through lateral loading tests. The bending moment profiles along pile length were obtained from strain gauges collected during the pile load tests. The pile deflection and soil resistance were derived from a bending moment profile based on beam theory. The ultimate lateral resistance of the pile with biotreatment was 2.6 times as large as that without biotreatment. The bending moment for the pile in biotreated soil is significantly smaller than that for the pile in untreated soil due to the reduction in curvature. The spatial distribution of calcium carbonate generated during the biotreatment was obtained from 1,800 sand samples, which was also validated by the scanning electron microscope images. Biotreatment of sand surrounding a pile is demonstrated to be a promising measure to effectively improve the lateral capacity of the pile. The authors would like to acknowledge the financial support from the National Science Foundation of China (Grant Nos. 41831282, 51922024, and 52078085). Dr. T. Matthew Evans was supported by the US National Science Foundation (Grant No. CMMI-1933355) during this work.

This technical paper investigates the bearing performance of precast concrete piles embedded within calcareous sands with biogrouting at the pile toe. Loading tests of biogrout-improved and...

Observations of the performance of coastal and marine structures in weather events that produce standing and/or progressive waves have pointed to liquefaction of the seabed as a contributin...

The particle breakage and compressibility behavior of sands treated with microbially induced carbonate precipitation (MICP) has been investigated using oedometric compression tests. The aci...

The empirical approach adapting the Stress History and Normalized Soil Engineering Properties (SHANSEP) concept relates the average overconsolidation ratio (OCR) of plastic soils to the ave...

The paper explores the possibility of estimating the horizontal scale of fluctuation (δh) with limited CPTs. The following conditions are desirable: (1) the CPT depth is large; (2) there are more than two CPTs; (3) the CPT separation distances are distinct and preferably less than 2 × δh; and (4) the Whittle-Matérn auto-correlation model is adopted. Keywords: Geotechnical engineering, Site characterization, Spatial variability, Horizontal scale of fluctuation

Liquefaction-induced lateral spreading has caused severe damage and significant financial losses in major earthquakes distributed globally. Groups of piling installed in liquefaction- and lateral spreading-susceptible ground has been proven to be effective in reducing lateral displacements, but further investigation into the soil-structure interactions is required to elucidate the mechanisms for mitigation of displacement. Further, it is hypothesized that cross-, or X-shaped, piling may provide improved restraint on lateral flow deformations due the destructive interference of flow imposed by their cross-section. In this paper, the effectiveness of groups of X-shaped and circular piles to mitigate lateral spreading ground was investigated to improve the understanding of the mechanisms for improvement. Shake table tests were carried out to examine and compare the efficacy and efficiency of X-shaped and circular pile groups. Design parameters including the pile arrangement (square vs. triangular spacing) and orientation (X vs. +) of the X-shaped piling were also taken into consideration. The results demonstrate that the X-shaped pile groups can significantly reduce the lateral displacement and the areal extent of liquefied sand flow as compared to the unimproved and circular pile-improved ground, and that the spacing and orientation play a critical role in the deformation response. These findings will help inform the design of pile-improved ground as well as the design of structural piling adjacent to submerged, liquefiable slopes.

The inherent spatial variability of natural soil deposits should be considered to obtain realistic probabilistically-based estimates of the time for soil consolidation. Although a variety of commercial software packages for solving consolidation analyses within numerical framework exist, they cannot incorporate spatial variability in a robust manner. Therefore, the consideration of the spatial variability is difficult and time consuming. This study proposes two efficient approaches for probabilistic analysis of consolidation considering the spatial variability of coefficient of consolidation (c), as well as a technique for calculating a representative c for combined vertical and radial consolidation. The proposed approaches, which include a first-order reliability (FORM) and stochastic response surface method (SRSM), were applied to examples of 1-D vertical consolidation and combined vertical and radial consolidation problems, and the applicability of the models and spatial variability are identified. As a result, the random field and numerical consolidation analysis were decoupled, and the proposed approaches were able to perform probabilistic consolidation analysis considering spatial variability using existing deterministic finite element codes without any modification. The probability of under-consolidation can be predicted by only 6 and 11 consolidation analysis using the FORM and SRSM approaches, respectively, and the accuracy of these two approaches was shown to be identical to the results of more computationally-expensive, conventional Monte Carlo simulation (MCS) results. It was found that the probabilistic consolidation analyses are sensitive to the spatial variability of c, and the variability of average degree of consolidation increases as the scale of fluctuation increases, which can be effectively identified and treated using either of the proposed approaches.

The use of augered cast-in-place (ACIP) piles for transportation infrastructure requires an appropriate reliability-based design (RBD) procedure. In an effort to improve the accuracy of an existing design model and calibrate appropriate resistance factors, this study presents a significantly revised RBD methodology for estimating the shaft and toe bearing capacity of ACIP piles using a large database consisting of static loading tests in predominately granular soils. The proposed design models are unbiased, as opposed to those currently recommended. Based on the reasonable assumption that a finite lower-bound resistance limit exists, lower-bound design lines are developed for shaft and toe bearing resistance by applying a constant ratio to the proposed design models. Resistance factors are calibrated at the strength or ultimate limit state (ULS) for ACIP piles loaded in compression and tension for two commonly used target probabilities of failure with and without lower-bound limits. For piles loaded in compression, separate resistance factors are calibrated for the proposed shaft and toe bearing resistance models. The inclusion of a lower-bound limit for piles loaded in tension results in a 24%–50% increase in the calibrated resistance factor. For piles loaded in compression, the application of a lower-bound limit results in a 20%–150% increase in the calibrated resistance factor, and represents a significant increase in useable pile capacity. Although the impact of a lower-bound limit on resistance factor calibration is directly dependent on the degree of uncertainty in the distribution of resistance, this effect is outweighed by the type of distribution selected (i.e., normal, lognormal) at more stringent target probabilities of failure due to differences in distribution shape at the location of the lower-bound limit. A companion paper explores the use of the revised ULS model in a reliability-based serviceability limit state design framework.

This study proposes a reliability-based design procedure to evaluate the allowable load for augered cast-in-place (ACIP) piles installed in predominately granular soils based on a prescribed level of reliability at the serviceability limit state. The ultimate limit state (ULS) ACIP pile–specific design model proposed in the companion paper is incorporated into a bivariate hyperbolic load–displacement model capable of describing the variability in the load–displacement relationship for a wide range of pile displacements. Following the approach outlined in the companion paper, distributions with truncated lower-bound capacities are incorporated into the reliability analyses. A lumped load-and-resistance factor is calibrated using a suitable performance function and Monte Carlo simulations. The average and conservative 95% lower-bound prediction intervals for the calibrated load-and-resistance factor resulting from the simulations are provided. Although unaccounted for in past studies, the slenderness ratio is shown to have significant influence on foundation reliability. Because of the low uncertainty in the proposed ULS pile capacity prediction model, the use of a truncated distribution has moderate influence on foundation reliability.

Microbially induced calcite precipitation (MICP) has attracted significant attention as a promising in situ ground reinforcement method, particularly for the improvement of existing structu...

The strength properties of basalt fiber–reinforced biocemented (BFRB) sand specimens with various calcite contents and fiber contents are investigated through a series of unconfined compres...