Journal of Geotechnical and Geoenvironmental Engineering

A study was conducted at seven sites across the United States to evaluate the field hydrology of final covers with a composite barrier (a geomembrane over a soil barrier or a geosynthetic clay liner) for final closure of landfills. The water balance of each cover was monitored with a large ($10\times 20 \text{m}$) instrumented drainage lysimeter. With one exception, the covers limited the average annual percolation to < 2.8 mm/year (< 0.4% of precipitation). The geomembrane barrier at one site (Marina, California) was likely damaged during construction; percolation at this site averaged 30 mm/year (6.9% of precipitation). The annual percolation through the cover at the wettest site (Cedar Rapids, Iowa) ranged between 0.1 and 6.2 mm/year. The annual percolation at arid and semiarid sites was typically no more than a trace (< 0.1 mm/year). Percolation from all test covers generally was coincident with high water storage in the surface soil layer and lateral flow in the drainage layer on the surface of the geomembrane barrier. Water balance predictions were made with the hydrologic evaluation of landfill performance model using site-specific input. Surface runoff was overpredicted and evapotranspiration underpredicted when as-built soil hydraulic properties were used as input. Better agreement was obtained when in-service soil hydraulic properties were used as input. The lateral flow was consistently overpredicted regardless of the hydraulic properties, and no correspondence existed between the predicted and measured percolations.

A simple procedure for assessing the probability of serviceability failure in a braced excavation involving bootstrapping to characterize the effect of uncertainty in sample statistics is presented. Here, the failure is defined when an excavation system’s response in terms of the maximum wall deflection or ground settlement that exceeds the limiting value specified by the client or in an applicable code. The analysis for the probability of failure (or probability of exceedance) necessitates an evaluation of the means and SDs of critical soil parameters. In geotechnical practice, these means and SDs are often estimated from a very limited data set, which can lead to uncertainty in the derived sample statistics. Thus, in this study bootstrapping is used to characterize the uncertainty or variation of sample statistics and its effect on the failure probability. Through the bootstrapping analysis, the probability of exceedance can be presented as a confidence interval instead of a single, fixed probability. The information gained should enable the engineer to make a more rational assessment of the risk of serviceability failure in a braced excavation. The study points to the potential of the bootstrap method in coping with the problem of having to evaluate failure probability with uncertain sample statistics.

The installation of soil-cement columns causes excess pore-water pressures and movements of surrounding ground, which affect adjacent underground structures. In Shanghai, a triple-shaft deep soil mixing (DSM) method has been proposed and is widely used to minimize the installation effects. However, when this DSM method was used to install soil-cement columns close to a Metro tunnel, unacceptable soil displacement was caused, even at the very beginning. Therefore, it was decided to conduct field tests to investigate the effect of major factors affecting DSM installations and then modify the construction parameters so that the soil displacement caused by DSM construction would not exceed the allowable limit. The field tests consisted of two phases: Phase I tests of single DSM column installations close to the Metro tunnel to modify the construction parameters, and Phase II tests of continuous multiple DSM column installations far from the Metro tunnel to validate the modified construction parameters. Detailed pore-water pressure and soil displacement measurements were conducted during the field tests. Based on the field tests, the traditional DSM method was modified by using a higher water/cement ratio, lower mixing speed, and no injection during withdrawal, and adopting a new installation sequence for continuous construction that started from the farthest row and moved closer to the tunnel. Using the modified construction parameters, the triple-shaft DSM method was successfully applied to the large-scale soil improvement of an underground highway excavation project close to the Metro tunnels. This paper describes the background, the field tests, the modified DSM method, and its successful application.

Colluvial soils are usually coarse and widely graded. Shallow-seated failures occur frequently in colluvial soil deposits during rainfall infiltration. This paper investigates the behavior of coarse, widely graded soils under very low confining pressures of 5–25 kPa encountered in shallow-seated failures. Isotropic consolidation tests, drained triaxial tests, and undrained triaxial tests were conducted on several widely graded soils with different coarse contents but with the same void ratio of 0.62. With increasing coarse content, the soil microstructure changes from a fines-controlled structure to a coarse-controlled structure after a critical coarse content of approximately 70%. Silty sand with gravel with a coarse content close to the critical value exhibits the highest compressibility because of the presence of large interaggregate pores. Even under very low confining pressures, such soil still shows strong contractive behavior during drained loading, and generates large positive pore-water pressures during undrained loading. This explains why shallow-seated failures occur frequently in colluvial soil deposits caused by rainfall infiltration. Soils with lower or higher coarse contents than the critical value may show dilative behavior under the same low confining pressures. The critical state friction angle increases with the coarse content.

Aggregate piers, also known as stone columns, are a commonly used ground improvement technique to stiffen existing soils for the support of structure foundations. This paper presents an evaluation of existing analytical expressions for the bearing capacity of spread footings supported on aggregate pier reinforced clay. The accuracy of these models was investigated using a database of high-quality footing load test data. The existing models were compared using the bias (i.e., the ratio of measured and calculated bearing capacity), and they produced a wide range in predicted bearing capacities. Selected analytical models were empirically modified using the load test database, and this resulted in improved accuracy and reduced variability. Back-calculated aggregate pier bearing capacity and cavity expansion factors are shown to be inversely proportional to undrained shear strength, and therefore to the ultimate confining pressure available at failure. This finding is attributed to the curved failure envelope of the angular aggregate used in pier construction. Additionally, a multiple nonlinear regression model suitable for spread footings resting on aggregate piers under a wide range of pier configurations is presented. The regression model is shown to produce more accurate bearing capacity estimates than existing analytical models.

The modulus degradation curves, $\mathit{G}\left(\mathit{\gamma }\right)/{\mathit{G}}_{\text{max}}$, and the damping ratio, $\mathit{D}\left(\mathit{\gamma }\right)$, of 27 clean quartz sands with specially mixed grain size distribution curves were measured in approximately 280 resonant column tests. For each material, tests with various pressures and densities were performed. Based on the test data it is demonstrated that the shear modulus degradation is larger for higher values of the uniformity coefficient, ${\mathit{C}}_{\mathit{u}}={\mathit{d}}_{60}\text{/}{\mathit{d}}_{10}$, while it is rather independent of the mean grain size, ${\mathit{d}}_{50}$. The observed ${\mathit{C}}_{\mathit{u}}$ dependence of curves $\mathit{G}\left(\mathit{\gamma }\right)\text{/}{\mathit{G}}_{\text{max}}$ is not adequately described by common empirical equations because these equations were developed based on tests on uniform granular materials. To consider the influence of the uniformity coefficient, the paper proposes correlations of the parameters of the common empirical equations with ${\mathit{C}}_{\mathit{u}}$. Good agreement between the prediction of the extended empirical equations and the experimental data collected from the literature is demonstrated. Furthermore, the test data reveal that the curves of damping ratio, $\mathit{D}\left(\mathit{\gamma }\right)$, and the threshold shear strain amplitude indicating the onset of residual deformation accumulation, ${\mathit{\gamma }}_{\mathit{tv}}$, are rather independent of ${\mathit{d}}_{50}$ and ${\mathit{C}}_{\mathit{u}}$. The threshold shear strain amplitude at the onset of modulus degradation, ${\mathit{\gamma }}_{\mathit{tl}}$, slightly decreases with increasing values of ${\mathit{d}}_{50}$ and ${\mathit{C}}_{\mathit{u}}$.

Skirted gravity base foundations and suction caisson foundations are considered as viable alternatives to monopile foundations for offshore wind turbines. While recent work has focused on the monotonic moment-rotation response for shallow foundations, the cyclic response and the accumulation of rotation over the life of the turbine must also be addressed. This paper presents cyclic loading tests where approximately 10,000 cycles, with different loading characteristics, were applied to a model shallow foundation (a caisson) in loose sand. On the basis of these tests, a framework for assessing the accumulated angular rotation because of cycling was developed. The settlement and cyclic stiffness response of the caisson were also assessed. It was found, not unexpectedly, that the accumulated settlement of the caisson increased with the number of cycles and cyclic amplitude. It was also found that a cyclic loading regime intermediate between one-way and full two-way cycling produced the largest rotations. The cyclic stiffness was relatively unaffected by the number of cycles. Using an appropriate scaling technique, the proposed framework was used to predict the long-term accumulated angular rotation, for an example, the field-scale monocaisson structure.

Reliability-based design, such as LRFD, aims at meeting desired probability of failure levels for engineered structures. The present work attempts to contribute to this field by analyzing the influence of spatially variable soil/rock strength on the axial resistance uncertainty of single and multiple shafts in group layouts. This includes spatial variability over the individual shaft surfaces, effects of limited data, random measurement errors, and workmanship. A possible correlation between boring data inside or near the footprint of a foundation and the foundation itself is considered. In a geostatistical approach, spatial averaging (upscaling) and a degenerate case of ordinary kriging are applied to develop variance reduction charts and design equations for a series of foundation group layouts (single, double, triple, and quadruple). For the potential situation of an unknown horizontal correlation range at a site, the worst case scenarios are identified and demonstrated in an example problem. Resulting probabilities of failure are applied to the whole foundation (i.e., group) rather than single objects. It is found that a boring at the center of a group footprint can significantly reduce resistance prediction uncertainty, especially under the worst case scenario for unknown horizontal correlation range. In contrast, independent of the presence of a center boring or not, the uncertainty reduction through additional borings becomes small, once four or five borings are available.

Fiber reinforcement can help to enhance soil strength, stabilize near-surface soil layers, and mitigate the risk of soil liquefaction. Evaluation of the strength of fiber-reinforced soils needs a proper failure criterion. This study presents a three-dimensional failure criterion for fiber-reinforced sand. By assuming that the total strength of the composite is a combination of the shear resistance of the host soil and the reinforcement of fibers, a general anisotropic failure criterion is proposed with special emphasis on the effect of isotropically/anisotropically distributed fibers. An anisotropic variable, defined by the joint invariant of the deviatoric stress tensor and a deviatoric fiber distribution tensor, is introduced in the criterion to quantify the fiber orientation with respect to the strain rate/stress direction at failure. With further consideration of the fiber concentration and other factors such as aspect ratio, the proposed criterion is applied to predicting the failure of fiber-reinforced sand in conventional triaxial compression/extension tests for both isotropically and anisotropically distributed fiber cases. The predictions are in good agreement with the test results available in the literature. The practical significance of using this criterion for such problems as inclined slope stabilization is briefly discussed.

Fiber reinforcement represents a promising alternative in projects involving localized repair of slopes and reinforcement of thin soil veneers, where planar reinforcement (e.g., with geotextiles and geogrids) is difficult to implement. Current design methodologies allow quantification of the shear strength of fiber-soil composites in terms of the parameters that independently characterize the soil matrix and fibers. The shear strength of fiber-reinforced soil is considered to have two components, including the shear strength of the soil matrix and the tension mobilized within the fibers. Triaxial compression tests and fiber pullout tests were conducted to evaluate how the fiber tension is mobilized for varying shear strain levels. The results of this evaluation provide insights into whether the shear strength of fiber-reinforced soil is governed by the peak or residual shear strength of unreinforced soil. A revision to existing design methodology is proposed in which the individual contribution of fibers and soil matrix is quantified based on the strain level. The appropriateness of using the peak or residual shear strength of the unreinforced soil for predicting the equivalent shear strength of fiber-soil composites is discussed based on strain compatibility considerations.

Several models have been suggested to estimate the improvement brought by fibers to the shear strength of fiber-reinforced sands. To date, the effectiveness and reliability of these models have not been the subject of a comprehensive evaluation. The objectives of this paper are to (1) compile the experimental data available in the literature on the behavior of fiber-reinforced sands into a comprehensive state-of-the-art database, (2) quantify the model uncertainty and bias of current strength prediction models for fiber-reinforced sands, and (3) provide insight regarding possible modifications that could be made to the formulation of available models to improve their predictive effectiveness and reduce their model uncertainty. Two models that are considered to present the best available approaches to predicting sand-fiber shear strength were evaluated, namely, the “energy-based” model and the “discrete” model. The energy-based model was found to underestimate the measured friction coefficient on average by about 10%, while the discrete model overestimated the friction coefficient by 6%, with associated coefficients of variation on bias values of 0.20 and 0.17, respectively. With the introduction of minor modifications to these models, the average bias error was eliminated, and the coefficients of variation in the ratio of predicted to measured shear strength (bias) were reduced to 0.17 and 0.12, respectively, for the two models.

This paper presents the development of an oil-operated fixed-piston sampler. This sampler includes a vacuum breaker, which prevents the suction between the sample and piston during disassembly, and an improved version of a complex system used for continuous advancement in mechanical samplers. The improvement makes the new sampler easy to operate, similar to the hydraulic sampler. The oil-operated and hydraulic samplers were used at a site in the Nakdong River Delta. Suction, shear-wave velocity, and consolidation tests were performed on 100-mm-long specimens equally divided by the extruded samples, as well as the seismic flat dilatometer and the piezocone tests in the field. The results show that the new sampler gave better sample quality than the hydraulic-type sampler, indicating better recovery ratio. The main difference in sample quality between the two samplers can be attributed to mechanical destructuration, owing to the stepped advancement and the slight tilt of the hydraulic sampler.

Vacuum is proposed as a means for rescuing soil slopes showing signs of impending failure. Two aspects associated with the proposed method were studied; namely, the theory of airflow through dry soils and the effectiveness of vacuum for enhancing the stability of soil slopes. A model test device was developed, and two series of tests were carried out using this device. One was a series of tests on pore-air pressure distributions in dry sand slopes, and the second series involved dry sand slope stability tests. The results revealed that a vacuum (negative pore-air pressure) even as small as −0.4 kPa significantly increased the stability of the model slopes with dimensions of $0.9\times 0.5\times 0.28 \text{m}$ ($\text{length}\times \text{width}\times \text{height}$). The pore-air pressure distributions in the model slopes were simulated using a finite-element partial differential equation solver, FlexPDE. Fick’s law and mass conservation were used to formulate the airflow through dry soils. Good agreement was achieved between the experiment results and the numerical simulations. A computer routine, called Slope-Air, was developed for slope stability analysis using Bishop’s simplified method and considering the pore-air pressure distributions in the slope. The calculated factors of safety of the model slopes at failure were consistent with the results of the model slope stability tests.

Rapid load pile testing (RLT) techniques such as Statnamic were developed as an alternative to more frequently adopted static and dynamic tests. The existing unloading point method (UPM) for deriving equivalent static load-settlement behavior from rapid load tests seems to be adequate in coarse-grained soils but may result in poor prediction in clays and silts. To address these shortcomings, the UPM has been improved to reflect soil type, and new analysis techniques have been developed. To test the performance of the improved UPM and new analysis techniques, pile tests from two clay sites were analyzed. The first case study site was underlain by very to extremely high-plasticity Quaternary London clay, and the second site was underlain by low- to intermediate-plasticity matrix dominant glacial till. The best predictions of static equivalent load-settlement behavior for very-high-plasticity clay were obtained from a new analysis technique that incorporated a soil-specific rate effect parameter (selected on the basis of the clays’ plasticity index) that varied with pile settlement. In general, the UPM performed better for tests undertaken in the low- to intermediate-plasticity glacial till, as there is greater experience of RLT in these soils. The results of the study suggest that the development of analysis techniques would benefit greatly from tests in a wider variety of soil types. On the basis of the findings of this study, improvements to the UPM and Schmuker techniques are presented, which include pile settlement–dependent variation of the damping and rate effect parameters.

There are several problems when dealing with oil-contaminated soil that affect environmental quality and alter the geotechnical properties of the soil. The influence of oil contamination on the uplift performance of model concrete piles embedded in sand at various densities is investigated. In this study, the model parameters are varied, namely, the thickness and diameter of the contaminated layer, type of oil contamination, pile surface roughness, and pile installation method. Contaminated-sand layers were prepared by mixing the sand with an oil content of 0–3% with respect to dry soil to match the field conditions. The results were then analyzed to study the effect of each parameter. To evaluate the scale effects, two sizes of model piles were tested, and the results were compared. This paper also discussed the effect of oil contamination on the relative ground movement around the pile surface. The results indicated that the uplift resistance was drastically reduced by oil contamination. The maximum reduction in uplift resistance and skin friction factor occurred at low contamination (oil content = 1%). The initial sand density and method of pile installation are significant factors affecting uplift capacity of piles embedded in oil-contaminated sand. The experimental results were used for the development of linear regression equations. These equations relate the uplift capacity ratio for piles embedded in contaminated sand to the aforementioned parameters.

Contamination of soils by polychlorobiphenyls (PCBs) is of environmental concern because of their toxicity, persistence, hydrophobic nature, and slow biodegradation potential. Among the PCB remedial technologies, direct oxidation by persulfate is considered to have great potential to be both simple and rapid. However, to produce faster reaction rates, persulfate is often activated using heat, metal chelates, hydrogen peroxide, or high pH. Furthermore, delivery of persulfate in low permeability clayey soils is difficult. Integrating electrokinetic remediation with persulfate has the potential to overcome such difficulties because the applied electric potential can facilitate the delivery of persulfate in low permeability soils as well as activate oxidizing radicals and simultaneously induce oxidative/reductive reactions directly in the soil. This study investigates the potential for in situ oxidation of PCBs in low permeability soils using persulfate as an oxidant and also evaluates the benefits of integrating oxidation with electrokinetic remediation. Several series of laboratory batch and bench-scale electrokinetic experiments were conducted using kaolin, a representative clayey soil, spiked with 50 mg/kg of 2,2′,3,5′ tetrachlorobiphenyl (PCB 44), a representative PCB. Persulfate oxidation activators [elevated temperature (45°C) and high pH (at the cathode)] were investigated to maximize the PCB degradation. In addition, the effect of oxidant dosage on PCB degradation was investigated. The electrokinetically enhanced temperature-only activated persulfate oxidation test resulted in better PCB 44 remediation (77.9%) than the temperature and high-pH activated persulfate oxidation (76.2%) in a 7-day period. The optimal dosage for effective remediation was 30% Na-persulfate (76.2%) because a 20% concentration of the oxidant yielded a lower rate of degradation (55.2%) of PCB 44. The results are encouraging for the use of electrokinetically enhanced persulfate oxidation for the effective remediation of PCBs in soils.