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In this study, the impacts to soil thermal properties during and after biocementation via microbially induced calcite precipitation (MICP) method on silty silica sand specimens with varying fines content (0%, 5%, and 15%) were investigated. Firstly, calcium conversion was measured after each pulse; then, the MICP-treated specimens were tested for cementation uniformity. The evolution of thermal conductivity of silty soils with the MICP treatment was assessed using a thermocouple probe. The results show that thermal conductivity of silty saturated sands increased by 17% for specimens treated to 9.7% CaCO3. The improvement in thermal conductivity was attributed to the formation of calcium carbonate bridges binding the soil grains together. The results suggested that the thermal conductivity of silty soil depends on water content, the number of treatment pulses, and the treatment uniformity through the soil specimen. Presence of fines content in the soil was found to play an important role in the distribution and uniformity of biocementation through the soil specimen. However, no statistically significant difference in the thermal conductivity values of MICP-treated specimens with different fines content was observed (p>0.05). The average calcium carbonate content ranged between 10.7% and 7.2% for the soils with 0% and 15% fines content, respectively. The findings of this research could be used to improve the efficiency of geothermal boreholes and other energy geostructures using MICP by improving thermal conductivity of dry and partially saturated soil. [ABSTRACT FROM AUTHOR]

The effect of spatial variability on the liquefaction prediction and liquefaction-induced ground failure of a soil profile with thinly interbedded layer of sand and clay was examined using simplified procedures and fully coupled two-dimensional (2D) nonlinear dynamic analysis (NDA). The Palinurus Road site in Christchurch, New Zealand, subjected to the 2010 Darfield and 2011 Christchurch events, was selected for the analyses. The liquefaction vulnerability of the site was estimated using one-dimensional (1D) liquefaction vulnerability indices (LVIs), and the results were compared with the results of NDAs and field observations. Spatially correlated random fields conditioned on corrected cone penetration tests (CPTs) were generated based on sand-like and claylike portions of the interbedded soil profile. CPT data were corrected for the thin-layer and transition zone effects. The generated random fields were assigned to the interbedded layer of the stochastic model to examine the effect of spatial variability on the system dynamic response, cross-interaction between layers, and liquefaction-induced diffusion during the evolution of liquefaction and postliquefaction. The potential of sediment ejecta manifestation using the artesian flow potential (AFP) and ejecta potential index (EPI) also was investigated for the stochastic models and compared with those of deterministic models. This study demonstrates the value of advanced 2D NDA modeling with realistic soil spatial variability for understanding inconsistent ejecta predictions from simplified tools. The approach can guide future refinements to assessment procedures and provide insights into key factors controlling manifestation in complex stratified sites. [ABSTRACT FROM AUTHOR]

A series of centrifuge experiments were conducted to identify different internal failure modes of a group of soil-cement (SC) columns (e.g., shearing and tension due to bending) under combined effects of embankment loading and liquefaction-induced lateral spreading. The results were also used to investigate how an increase in the flexural/shear strength of SC columns (using high-strength steel reinforcement bars) can affect their ability to limit settlements or deformations. The centrifuge experiments included two centrifuge tests of liquefiable foundation reinforced by SC columns with and without high-strength steel reinforcement cores during strong earthquake loading. It was found that during and after cracking of the SC columns, shear and tilting failure is the prominent failure mechanism. The test results showed that increasing the flexural capacity of SC columns can reduce the potential for earthquake-induced liquefaction and associated damage. Compared with conventional SC columns, reinforced soil-cement (RSC) columns are more effective in providing confinement for the soil under the embankment during strong shaking events, resulting in lower shaking-induced pore-water pressure and a reduction in the lateral deformation of the soil and embankment. [ABSTRACT FROM AUTHOR]

This paper presents the results of nonlinear deformation analyses of an embankment on a spatially variable, liquefiable foundation soil reinforced with spatially variable soil-cement (SC) wall. The objective is to evaluate the merit of employing stochastic modeling approaches, such as spatially correlated random fields, relative to deterministic analysis with uniform properties for the soil and SC wall. Spatial variability, which is specified by mean, coefficient of variation (COV), and scale of fluctuation (SOF), can significantly influence the seismic performance of soils and embankments. The numerical analysis was first validated using data from a dynamic centrifuge test conducted at the Center for Geotechnical Modeling at University of California, Davis. Analyses were performed for different sets of realization of the foundation soil and SC wall as well as the range of the length and strength of SC wall and peak base acceleration. Simulation results of the stochastic models with spatially Gaussian random field were examined and compared with the results of the experiment with an emphasis on the crest vertical displacement, berm horizontal displacement, and contours of shear strain in the embankment, foundation soil, and SC wall. The results revealed that the representative percentile of unconfined compressive strength (qucs) in the range of 35th–50th percentile needed to be used in a uniform model to estimate the median of crest and berm displacement in spatially variable SC wall-supported embankment. [ABSTRACT FROM AUTHOR]

Two centrifuge model tests were used to examine the effect of soil interlayering on the measured cone-penetration resistance in a layered soil model. The first centrifuge model included four soil profiles: two uniform loose and dense soil profiles and two two-layer sand profiles of loose over dense and dense over loose sands. The other centrifuge model involved a layer of loose or dense sand with varying thickness between overlying and underlying layers of low-plasticity clayey silt. Multiple cone-penetration soundings were performed using cone penetrometers with diameters of 6 and 10 mm. The results showed that the measured tip resistance in layered soils as well as the sensing and development distances depend on sand layer thickness and stiffness contrast between the subsequent soil layers. The results of these experiments were compared with the results obtained from analytical and numerical solutions. The results provide insights on the effect of thin-layer presence on cone tip measurements and an archived data set for evaluating design procedures and numerical analysis methods. [ABSTRACT FROM AUTHOR]

Two-dimensional, nonlinear, finite-difference analyses were used to investigate the stability of an embankment supported by spatially variable soil–cement columns. This study evaluated the improvement/merit of employing stochastic modeling approaches, such as spatially correlated random fields, relative to deterministic analysis, which commonly neglects spatial variability. The spatial variability, which is specified by mean, coefficient of variation (COV) and autocorrelation distance (R), can significantly influence the performance of the treated in situ soils and the overlying structure. Reliability analyses with three types of spatial variable soil–cement columns were conducted using a COV value of 0.6 for the soil–cement strength, and the effects of the autocorrelation distance were evaluated. Gaussian spatially correlated random field models (stochastic models) with 100 realizations for each case were used in a strength-reduction analysis to determine the failure surface and safety factor. Results of the models with uniform material properties were compared with each of the stochastic model realizations. ANOVA was used to analyze the statistical difference between the means of safety factors. The stochastic distributions of safety factors and consequences of spatial variability on the failure mode of columns and embankment, and implications for engineering practice were discussed. [ABSTRACT FROM AUTHOR]

A new framework is developed to extend an existing small-strain shear modulus (Gmax) model to determine Gmax of unsaturated silty soils along different paths of the soil water retention curve (SWRC) including the scanning loops. The suitability of the proposed framework is validated against experimental results of a series of bender-element tests performed in this study and data reported in literature. Measured values of Gmax showed a slight hysteresis in the Gmax measurements along the scanning curves of the SWRC, with lower values along the wetting scanning paths. However, results indicated that the value of Gmax was recovered once the main drying path was reached. Results of this study also indicated that Gmax behavior of the silty specimen along the scanning curves was stress dependent. The model was observed to follow the experimental data along different paths of the SWRC including scanning curves. [ABSTRACT FROM AUTHOR]

A pair of large centrifuge tests was conducted to evaluate the effect of soil-cement grid reinforcement on the seismic response of a deep soft soil profile. The soil profile consisted of a 23-m-thick layer of lightly overconsolidated clay, underlain and overlain by thin layers of dense sand. Each centrifuge model had two separate zones for a total of four different configurations: a zone without reinforcement, a zone with an embedded soil-cement grid that penetrated the lower dense sand layer and had a unit cell area replacement ratio of Ar = 24%, a zone with an embedded grid with Ar = 33%, and a zone with a floating grid in the upper half of the clay layer with Ar = 33%. Models were subjected to 13 shaking events with peak base accelerations ranging from 0.005 to 0.31 g. This paper examines the effect of the soil-cement grids on the global responses of the soil profiles, and the internal interaction between soil-cement grids and their enclosed soils. Nonlinearities in the dynamic responses and interaction mechanisms are examined using (1) time series of accelerations, shaking-induced excess pore-water pressures, and postshaking reconsolidation settlements; (2) response spectra and spectral ratios; (3) back-calculated composite stress-strain responses; (4) analyses of internal stress distributions between the grids and enclosed soils; and (5) dynamic crack detections and posttest crack mapping in the soil-cement grids. The results provide insights on the dynamic performance of soil-cement grids and an archived dataset for evaluating design procedures and numerical analysis methods. [ABSTRACT FROM AUTHOR]

The stability of an in-flight undercut slope with a potential failure plane was investigated under a centrifugal acceleration of 50g. The experimental results confirmed the formation of passive arch action in the undercut slope and were found to be in good agreement with the results of 1g physical model tests showing that the arching-type failure in undercut slopes is a scale-independent phenomenon. The maximum stable undercut span is compared with that in the theory developed by the authors, and the results are discussed. [ABSTRACT FROM AUTHOR]

Four centrifuge tests were performed to investigate the reinforcing mechanisms of soil-cement columns in liquefiable sand. Two unimproved baseline models and two models improved with soil-cement columns were subjected to sine sweep and earthquake base motions of varying intensities to observe acceleration, pore pressure, lateral displacement, and settlement responses. The dynamic records were processed to derive the effective natural frequency of the profiles and to obtain the dynamic stress-strain responses for unimproved and improved soil. It was found that the shear reinforcement mechanisms of columns was not effective in reducing cyclic stress ratios in the treated soil; liquefaction triggering occurred nearly at same time for both unimproved and improved soil cases and the magnitude of the resulting soil settlement was not significantly reduced. When the bases of the columns were free to rotate, the columns rocked within the soil and produced negligible shear stiffening of the soil profile. When the bases of the columns were fixed against rocking, the columns deformed primarily in shear and flexure but the observed increase in natural frequency of the improved profile was much smaller than predicted by assuming shear strain compatibility between the columns and soil. Design equations that account for shear strain incompatibility are shown to give reasonable estimates for the observed natural frequency of the improved soil profile, and these equations suggest that the associated reduction in cyclic shear stress in the improved soil is small. The soil-cement columns did, however, remain intact and would have provided a means for supporting overlying structures even after liquefaction was triggering in the soil. [ABSTRACT FROM AUTHOR]