Journal of Geotechnical and Geoenvironmental Engineering

The present study investigated the effectiveness of two soil-bentonite slurry wall backfills, specifically sand/conventional Na-bentonite (NaB) and sand/hexametaphosphate (SHMP)-amended Ca-bentonite (SHMP-CaB), for the containment of the coal combustion residuals (CCR) impacted groundwater. Several series of laboratory experiments were conducted to determine swell index, liquid limit, slump, hydraulic conductivity, and compressibility of the backfills using distilled water or tap water and simulated CCR impacted groundwater. The results showed that NaB exhibited a higher free swell index as compared with that of SHMP-CaB in both the CCR-impacted groundwater and the distilled water. Both NaB and SHMP-CaB possessed lower liquid limit values with the CCR-impacted groundwater. During the test duration of 96 days, the CCR-impacted groundwater caused 1.39 times increase in the short-term hydraulic conductivity of the sand/NaB backfill, while there was a 0.95–0.91 times decrease in the short-term hydraulic conductivity of the sand/SHMP-CaB backfill. The compression index and rebound index values of the tested backfills prepared with the CCR-impacted groundwater were lower than the backfills prepared with tap water. Scanning electron microscope analysis showed that SHMP promoted the Ca-bentonite to exist in more dispersed particle association with smaller particle size, resulting in the observed results for SHMP-CaB backfill. Additional research is recommended to assess long-term chemical compatibility of the backfills with CCR-impacted groundwater.

Naturally occurring materials such as jute and coir have some favorable engineering characteristics and also degrade over time, so they have increasingly been used in engineering applications in recent years. The efficient way that naturally prefabricated vertical drains made from those materials help accelerate soil consolidation has been shown in previous studies, but they also tend to decompose rapidly in adverse environments, where cellulose-degrading bacteria cause a serious deterioration of their favorable drainage properties. This study presents a laboratory investigation into the biodegradation of prefabricated vertical jute drains in saturated soft soils, where the tensile strength of jute and coir fibers and the discharge capacity of drains decrease in response to different environments. Micro-observation also shows a transformation of the jute fibers and destruction of the drain structure due to biodegradation. DNA extraction and sequencing techniques to determine the microbial properties of these decayed fibers indicate that bacteria such as species of the genera Clostridium and Bacillus can cause rapid decomposition of cellulose-based material (i.e., jute), whereas other organic matter–consuming microbes such as sulfate-reducing bacteria do not directly contribute to the biodegradation of jute. In response, an analytical approach that incorporates various forms of drain discharge capacity over time is proposed to predict soil consolidation. The results indicate there is considerable deviation in dissipating the excess pore pressure when the drain degrades in different ways.

Building upon a previously reported hypothesis suggesting a unique linear relationship between the small strain shear modulus and major principal effective stress at failure, this paper proposes a new empirical equation to estimate the drained shear strength from the measured shear wave velocity. A series of drained triaxial tests on clean sands and natural granular soils are performed to measure the shear strength of granular soils. Shear wave velocities are measured using bender elements installed at the top and bottom caps. Although a correlation between maximum shear modulus calculated from the shear wave velocity and effective stress at failure is observed, it is shown that the correlation can be further improved by including the confining stress. A unique relationship between maximum shear modulus, effective stress at failure, and confining stress is shown to exist for all granular soils tested. The predicted secant friction angles from the proposed empirical relationship exhibit good agreement with the measured values. The applicability of the equation is further validated through measured data on silty sand from a published study. Again, very favorable matches with the measured secant friction angles highlight the wide applicability of the proposed empirical relationship. A possible application of the proposed correlation is calculation of the shear strength from the shear wave velocity profile for strength correction in a seismic analysis.

The behavior of sands exhibiting both unstable and stable response, in their loose deposited state, under axial-torsional shearing involving continuous principal stress rotation is investigated using the hollow cylinder apparatus. This paper examines the parameters affecting the major principal stress direction attained at instability and/or phase transformation during torsional shearing following anisotropic consolidation. It is shown that constant stress ratio ($t/p^{\prime }$) lines, including the instability and phase transformation lines, are associated with the same major principal stress rotation with respect to the vertical within a wide range of initial mean effective stresses along the same consolidation stress ratio, $K_c$. In sands exhibiting instability, smaller principal stress rotations are required for the mobilization of the effective stress ratio at the onset of instability as the initial shear stress level increases ($K_c$ decreases). In sands exhibiting stable response, principal stress rotation at phase transformation increases with increasing dilatancy tendencies. The dependence of the angle of shearing resistance, ${\phi }^{\prime }$, mobilized at instability (IL), phase transformation (PTL), and failure (FL) lines on principal stress rotation and the intermediate stress parameter, $b$, is examined to verify whether the mobilized angle of shearing resistance can be considered as a material property. In continuous rotation tests, contrary to fixed principal stress direction and $b$ tests, the angle of shearing resistance at IL can be considered as material property. However, the angle of shearing resistance at PTL depends on $b$ and the direction of the principal stress. Moreover, phase transformation takes place at lower stress ratios as density increases.

The effect of nonplastic fines on the undrained shear response of sand depends on many variables, including fines content, skeleton void ratio (${\mathit{e}}_{\mathit{s}}$), interfine void ratio (${\mathit{e}}_{\mathit{f}}$), global void ratio (e), confining pressure, and relative density. This paper focuses on applying microbe-induced carbonate precipitation (MICP) to silty sands containing fines up to 35% to assess its effect on the undrained shear response. Sporocarcina pasteurii, an alkalophilic soil bacterium was used to catalyze the chemical reaction and induce calcium carbonate precipitation. Shear-wave velocity ($V_s$) measurements are used to assess improvement, with a target $V_s$ of approximately $400\mathrm{m}/\mathrm{s}$. Undrained monotonic direct simple shear loading was used to compare the undrained shear response of untreated and MICP-treated specimens with similar preshear void ratios of $0.7\pm 0.05$. Decreased excess pore water pressure and flow behavior and increased undrained shear strength were the observed general changes due to MICP. The results presented here indicate that the effect of MICP on silty sands depends on relative density, fines content, and the fabric governing the structure of the soil.

Organo-silane (OS) surface modification chemically alters the surface of silica-based materials such as soils and industrial byproducts, rendering them hydrophobic. Such modification offers the potential to transform porous media with relatively high hydraulic conductivity into a water-repellent barrier to infiltration. This paper reports on experimentally observed relationships between dry density, apparent contact angle, and water entry head for Ottawa sand and a Class-F coal fly ash (CFA). Modification with two different OS products resulted in maximum apparent contact angles of approximately 120° and 140° for sand and CFA, respectively. Both products and materials exhibited an asymptotic relationship such that increasing OS dosages beyond a threshold level yielded diminishing increases in contact angle. The water entry head was found to range from 0 to 14 cm of water and 0 to 542 cm of water for OS-modified sand and OS-modified CFA, respectively. Positive correlations were established between dry density, apparent contact angle, and water entry head. Soil hydrophobicity was found to have a negligible effect on saturated hydraulic conductivity; the governing parameter for infiltration control is water entry head. The Washburn equation was found to provide reasonable estimates of experimentally observed water entry head, especially at large values of apparent contact angle. These results suggest that designing with water repellency is a plausible concept for infiltration control.

Tensile strains from gravel indentations in a 1.5-mm-thick high-density polyethylene geomembrane are reported. Strains were imposed using a single steel probe in test cells that are shown to simulate the mean response of nominal 25- and 50-mm poorly graded angular coarse gravels under vertical pressure. New to the assessment of geotextile protection, the screening tests were conducted at 55°C for 100 h to accelerate creep. Strains were also calculated from the deformed shape of the indentation using a new thin plate method not requiring assumptions that underestimate maximum strain. Additionally, a long-term creep factor recently deduced from time-temperature-superposition was used to account for differences between test and field conditions. Very thick geotextiles were required to limit long-term geomembrane strain to allowable strains of 3 or 6% for 25-mm nominal gravel. None of the geotextiles tested were able to adequately limit strain for the nominal 50-mm gravel.

Insights regarding the response of shallow foundation soils from building sites in the central business district (CBD) of Christchurch, New Zealand, that experienced liquefaction during the 2010–2011 Canterbury earthquake sequence (CES) are provided through an examination of cyclic and monotonic triaxial test data. An emphasis is placed on understanding the liquefaction resistance and large-strain steady-state response of key soils so that case histories of buildings damaged because of soil liquefaction during the CES can be developed. Comparisons of cyclic resistances from cyclic triaxial (CTX) testing and a cone penetration test (CPT)–based triggering correlation showed good agreement. Cyclic testing showed that silty sands and clean sands responded similarly in cyclic loading. However, silts tended to have flatter cyclic resistance curves. Interpretation of large-strain monotonic triaxial testing of reconstituted soil specimens provides information on the steady-state lines of the sandy soils of the CBD. The insights garnered from this laboratory testing program are critical for characterizing the Christchurch soils, as only limited cyclic testing results are available.

I presented the Kenneth L. Lee lecture at the 2016 Queen Mary Seminar, ASCE Los Angeles Geo-Institute Chapter, and publish it herein. The topic was lateral spread problems I have encountered as a consultant. The first issue was how deep to bury a pipeline at stream crossings to mitigate the lateral spread hazard. My answer is twice the bank height (2H) beneath approaches or 1H beneath the channel. For shallow liquefiable layers, a shear zone would form well above and not harm the pipe. For deep liquefiable layers, nonliquefiable soil above the pipe should buttress the channel against lateral spread. The second issue was finding the thinnest liquefiable layer that is susceptible to lateral spread. In the database of Youd et al. database, the thinnest layer is 1.0 m. For case histories used by Zhang et al. to verify their procedure, the minimum thickness is 0.6 m. Extrapolation to thinner layers adds uncertainty and a tendency for overprediction. The third issue was how to apply multiple linear regression (MLR) at a site with insufficient SPT data. CPT data were used to create $\psi$ profiles; we then applied the criterion that $\psi <–0.08$ indicates material too dilative for lateral spread to develop. Summing layer thicknesses with $\psi >–0.08$ yielded estimates of ${\mathrm{T}}_{15}$ for use with MLR.

Predicting the seismic performance of slopes involves an assessment of the expected permanent sliding displacement induced by ground shaking. Often, this analysis uses a deterministic approach that predicts sliding displacements using one level of ground shaking and the best-estimate slope properties (e.g., soil shear strengths). However, this approach does not consider the variability inherent in the prediction of sliding displacement, nor the uncertainties regarding slope properties. A probabilistic framework computes a displacement hazard curve using: (1) the entire ground motion hazard curve from a probabilistic seismic hazard analysis, (2) a model for predicting the dynamic response of the sliding mass, and (3) a model for predicting the sliding displacement of the sliding mass. A logic tree can also be included, which incorporates the uncertainties in the input parameters for the displacement calculation. This probabilistic framework is demonstrated through application to a slope at a site in California. The results of this analysis show that the probabilistic approach predicts displacements larger than the deterministic approach, which is a direct result of accounting for the uncertain inputs into the analysis. Incorporating these uncertainties and variability into an analysis via the probabilistic approach better captures the seismic risk associated with seismic slope movements and provides a mechanism to account for a reduction in seismic risk when uncertainties are reduced through additional data collection.

In recent years, the constructions of pile-supported ballastless track-bed have been developed rapidly in China. It appears important to assess the accumulative settlement of this kind of track-bed after years of operation, especially under unfavorable conditions, such as the rising of water level. In this study, a full-scale physical model simulating the pile-supported ballastless track-bed was established. The soil arching effect was previously developed in this model by draining out water in the water bags among the pile caps. The effects of water level and loading cycle on the accumulative settlement of this model were investigated following four testing procedures: water level rising, cyclic loading at high water level, water level lowering, and cyclic loading at low water level. The results indicated that the total accumulative settlement of the track-bed increased rapidly in the beginning of loading and tended to stabilize as the loading cycle increased at high water level, whereas the value varied slightly when loading at low water level. The distribution of the accumulative settlement inside the subgrade at the end of loading with high water level and at the end of this test both presented parabolic shaped variation trends, with the peak point occurring above the water bag and lower values developing above the pile cap. At the high water level, a modified model was applied to estimate the accumulative settlement of the unsaturated zone above the water level (height of soil arch), and the fitting parameters in this model were precalibrated using the testing data. The estimated results revealed that the deformation of the unsaturated zone above the water level accounted for a minor portion of that of the whole tack-bed. By contrast, the zone below the water level is the dominant factor to influence the accumulative settlement of the whole track-bed. From a practical point of view, a well-performing drainage system should be set up to avoid the rising of water level.

Slopes forming convex corners are often constructed in practice. Observed failures of such corners possess three-dimensional (3D) characteristics. Hence, assessing corner stability requires 3D analysis. This study used a variationally derived 3D mechanism to analyze the stability of convex turning corners. Results of 3D stability analysis are presented in a convenient form of stability charts considering the effects of pore-water pressure and seismicity. These charts allow for a quick assessment of the 3D factor of safety. Stability of a convex corner increases as its angle decreases. When corners are subjected to localized loading conditions, its stability could be more critical than the straight slopes forming the corner. Comparisons between the current method and results from existing analytical and numerical studies are conducted.