Journal of Geotechnical and Geoenvironmental Engineering

An extensive set of in situ and laboratory test data is presented for a footing load test site east of Houston, Texas, in desiccated Beaumont clay. The in situ test program included standard and cone penetration tests (CPTs), the latter of which was selected for statistical analysis to produce vertical and horizontal random field model parameters for corrected cone tip resistance. Given the relatively high sampling frequency of the cone tip resistance in the vertical direction, the vertical random field model parameters were determined using the modified Bartlett’s test statistic with fitted autocorrelation models subject to a strict fitting criterion. Horizontal random field model parameters were generated by collapsing the two-dimensional distribution of the CPTs to a one-dimensional representation and by using less stringent evaluation of the autocorrelation. The results of this study indicate that Beaumont clay exhibits greater inherent spatial variability than previously reported at other clay sites, which is attributed to the secondary structure of the desiccated clay. A companion paper discusses the performance of a footing load test in the context of spatial and transformation uncertainty.

To advance the use of reliability-based design procedures, it is necessary to evaluate the sources of the design parameter uncertainty including inherent variability, measurement error, and transformation uncertainty. The results of a probabilistic evaluation of undrained footing bearing performance are discussed in the context of an extensive test site characterization described in a companion paper. Kriged cone tip resistance values are transformed into design parameters using a second-moment probabilistic approach and compared with the parameters obtained from the laboratory test analyses on specimens retrieved from the test site. The spatial, measurement, and transformation uncertainty are incorporated into probabilistic finite-element and bearing capacity analyses in which the results are compared against a full-scale load test performed at the test site. The results indicate that the reliable assessment of the spread footing response depends to a large degree on the assumed strength anisotropy and soil layering. A comparison of the probabilistic estimates of the bearing capacity to deterministic estimates developed from the correlations and average design parameters indicate that the consideration of uncertainty results in a desirable prediction accuracy.

Experimental studies have observed that the small-strain shear modulus ($G_{\text{max}}$) of unsaturated soils measured during hydraulic hysteresis has a greater magnitude during imbibition than during drainage when plotted as a function of matric suction. To capture this behavior, a semiempirical model was developed to interpret the impacts of the stress state and hydraulic hysteresis on $G_{\text{max}}$ of low plasticity soils. Different from previous empirical relationships for $G_{\text{max}}$, this model incorporates elastoplastic constitutive relationships, which integrate the effects of mean effective stress and hardening because of either plastic changes in volume or changes in the degree of saturation. The effective stress is defined as the sum of the net normal stress and the product of the effective saturation and matric suction, facilitating integration of the soil-water retention curve parameters into the model. An experimental testing program involving measurement of $G_{\text{max}}$ of compacted silt during hydraulic hysteresis was used to develop data to validate a methodology for model calibration. Specifically, hysteretic trends in $G_{\text{max}}$ were defined for different mean net normal stress values using a fixed-free resonant column device with suction-saturation control. The proposed methodology to define the model parameters includes use of correlations from the literature, as well as experimental measurements of $G_{\text{max}}$ for soils in saturated conditions and during drainage. The model was found to fit the trends in experimental $G_{\text{max}}$ data with suction, degree of saturation, and effective stress during drainage, and provided adequate prediction of the $G_{\text{max}}$ data upon subsequent imbibition.

This paper reports the results of two centrifuge tests that were conducted to evaluate the effectiveness of colloidal silica for liquefaction mitigation. Colloidal silica has been selected as a stabilizer material in soils because of its permanence and ability to increase the strength of soils over time. The centrifuge model geometry was selected to study the effects of lateral spreading in a 4.8-m-thick liquefiable layer overlain by a silty clay sloping toward a central channel. The centrifuge test evaluates the response of untreated loose sands versus loose sands treated with 9, 5, and 4% colloidal silica concentrations (by weight). The models were subjected to a series of peak horizontal base accelerations ranging from 0.007 up to 1.3g (prototype) with a testing centrifugal acceleration of 15g. The results show a reduction in both lateral spreading and settlement in colloidal silica–treated sands versus untreated sands. The shear modulus at low strains was determined from shear wave velocity measurements for the untreated and treated loose sands. The hysteretic response during cyclic loading was also determined for various levels of shaking. The results from the centrifuge tests show an increase in cyclic resistance ratios and a decrease in cyclic shear strains for increasing colloidal silica concentrations.

In geotechnical engineering, recycled tires are used in lightweight fills, backfill materials, highway embankments, soil reinforcement, and soil-retaining walls. The rubber-soil mixture can exhibit a greater capacity for energy absorbency than soil alone under cyclic loading and tends to decrease the stress and shocks transferred into the ground. The shear strength and energy absorbance of the rubber-soil mixture is highly dependent on the size of the rubber fragments and the proportion in the soil. In this paper, soil surface settlement, pipe deflection, and pressure distributed over a pipe placed in a trench and subject to cyclic loading were investigated as a function of chipped and shredded rubber used as backfill. The observed responses show that the shredded rubber-soil mixture used over the pipe is more effective in reducing these three parameters when covered by a soil cap, than when using the same soil alone for the whole of the fill. Also, using the rubber-soil mixture in the whole of backfill without a soil cap delivers a negative influence on the responses. Overall, shredded rubber has a better performance in regards to the pipe responses than the chipped rubber. By using a soil cap over the rubber-soil mixture, the reduction in the subsequent soil settlement and plastic pipe deflection, attenuation of the pipe’s accumulating strains, and finally protection of the buried pipe from fatigue are achievable.

The behavior of displacement piles in uncemented calcareous sand is investigated using field piles instrumented with a sensor that simultaneously records the radial stress and shear stress at specific locations on the pile shafts. These tests are interpreted with the assistance of data from adjacent self-boring pressuremeter tests and from monotonic and cyclic direct shear interface tests performed on reconstituted samples. The existence of extremely low radial stresses on the pile shafts is verified. Although dilation during shear is seen to compensate for such low radial stresses, short-term shaft capacities are much lower than capacities of equivalent piles in siliceous sands. The development of a bonded or welded crust to the pile shaft was seen to be the primary contributor to the setup observed at the test site; this crust forced failure to take place at a sand-sand rather than a sand-steel interface and also gave rise to higher levels of dilation during monotonic loading. The welded sand crust did not, however, give rise to a higher long-term cyclic capacity.

Recent studies have highlighted the beneficial role of foundation uplifting and the potential effectiveness of guiding the plastic hinge into the foundation soil by allowing full mobilization of bearing capacity during strong seismic shaking. With the inertia loading transmitted onto the superstructure being limited by the capacity of the foundation, this concept may provide an alternative method of in-ground seismic isolation: the so-called rocking isolation. Attempting to unravel the effectiveness of this alternative design method, this paper experimentally investigates the nonlinear response of a surface foundation on sand and its effect on the seismic performance of an idealized slender single-degree-of-freedom structure. Using a bridge pier as an illustrative prototype, three foundation design alternatives are considered, representing three levels of design conservatism. Their performance is investigated through static (monotonic and slow-cyclic pushover) loading, and reduced-scale shaking table testing. Rocking isolation may provide a valid alternative for the seismic protection of structures, providing encouraging evidence in favor of the innovative idea of moving foundation design toward a less conservative, even unconventional, treatment.

Geomembranes (GMBs) are widely used as advective barriers in landfill liner systems. When exposed to the sun, GMBs exhibit a network of wrinkles as a result of thermal expansion. These wrinkles disrupt the intimate contact between the GMB and the underlying layer. If a hole is coincident with a GMB wrinkle then the space under the wrinkle has the potential to act as a preferential pathway for flow of contaminants. Thus, the size and shape of GMB wrinkles have implications for leakage rates through the composite liner system. However, wrinkles are only a concern if they persist after placement of backfill, which is currently a subject of debate. In this paper, wrinkles are induced in a 1.5-mm-thick, black high-density polyethylene strip GMB specimen overlying a geosynthetic clay liner using natural solar and laboratory energy sources. Particle image velocimetry techniques are employed to record cross-sectional wrinkle geometry during growth and subsequent backfilling. This cross-sectional geometry is found to follow a Gaussian shape in which the height increases with the temperature and the width remains relatively constant. The resulting relationships between the height and temperature permit an estimation of wrinkle height for a known coefficient of thermal expansion for the GMB and an estimate of wrinkle spacing. For the GMB material and conditions tested, the results of the backfilling experiments indicate that when covered with 230 mm of cool sand (21°C), wrinkles of initial height less than about 20 mm disappear completely, while larger wrinkles remain with a reduced height. Furthermore, wrinkles of 20 mm in height are observed to form with increases in GMB temperature of less than 5°C. With application to the field, these findings indicate that a GMB must be covered at or below its installation temperature to achieve a wrinkle-free installation.

The need for probabilistic slope analysis that takes into account the uncertainty of soil properties has been acknowledged by the geotechnical profession. Traditionally, probabilistic slope analysis involves only single-mode failure that is considered based on the critical slip surface. This may result in underestimating the failure probability. In contrast, system reliability analysis for slopes is deemed more rational. This study aims at improving the existing methods of slope reliability analysis by considering system reliability. A stratified response surface method (stratified RSM) is proposed to describe the performance functions of possible failure modes. The proposed method differs from conventional response-surface–based slope reliability analysis (which constructs a single approximate performance function) by generating a group of (stratified) response surfaces. Based on these stratified response surfaces, system reliability analysis can be efficiently carried out by means of either a first-order reliability method (FORM) or Monte Carlo simulations. The efficient FORM based on the concept of a dispersion ellipsoid in the space of the original variables is used. Application of the proposed approach to probabilistic assessment of slopes is illustrated by case studies, and the results obtained are compared with Monte Carlo simulations.

Analyses of load tests on instrumented drilled shafts in intermediate geomaterials (IGMs) and rock are performed using a hyperbolic model for relative displacement ($\mathrm{\Delta }$)/shaft diameter ($d$)versus side friction ($f_s$) data. Ultimate side friction ($f_{su}$) determined from these analyses for IGMs compares well with those determined from pressuremeter tests. For rock with a rock quality designation (RQD) greater than 50%, values of $f_{su}$ are approximately equal to rock socket unit friction ($q_{SR}$), estimated using 28-day strength ($f_c^{\prime }$) of drilled shaft concrete, but for RQD less than 50%, $f_{su}$ is approximately equal to $q_{SR}$, estimated using uniaxial strength ($q_u$) of rock cores and reduction factor (${\alpha }_E$) to account for jointing in rock. Linear elastic shear modulus ($G$) for IGMs determined from these analyses is approximately equal or slightly less than reload modulus ($G_r$) determined from the pressuremeter test. Rock mass modulus ($E_m$) also matches with values determined from correlations.

An algorithm for determining the intersections between fractures is presented in this paper. The algorithm is implemented in GEOFRAC, a stochastic fracture pattern-modeling program. Previously, GEOFRAC allowed one to subdivide a fracture network into isolated subnetworks to assess connectivity. A new GEOFRAC feature can be used to analyze length and orientation distributions of the fracture intersections. Connectivity parameters such as the number of intersections per unit volume, $C_1$, and the intersection length per unit volume can be calculated. Intersections and connectivity are important because they govern the fluid flow and stability behavior of rock masses. The program is used to model fracture intersections in the Boston area. The simulations show that for fractures in the Boston area, the mean fracture intersection length ranges from 0.5 to 0.7 m. The results also show that the relative frequency of long intersections (>2.0 m) increases with increasing modeling volume size. In contrast to these effects of volume, it appears that intersection orientation is not affected by the volume being considered.

The compression behavior of an artificially cemented soil was analyzed by the adjusted porosity/cement index using a correlation established in the recent literature. It was found that for each value of this index, defined as the ratio of porosity to the volumetric cement content, there was a unique normal compression line (NCL). The NCLs of the cemented specimens for each adjusted porosity/cement index did not converge with the NCL of the uncemented silty sand at large stresses, but reached a line parallel to it. The NCL of the cemented sand plotted further from the NCL of the uncemented sand as the porosity/cement index decreased.