Journal of Geotechnical and Geoenvironmental Engineering

Circular skirted offshore foundations on nonhomogeneous soil have been studied numerically, analytically, and physically, with the offshore sediment simulated as a cohesive soil with strength increasing linearly with depth. In the numerical analysis, the h-adaptive FEM is adopted to provide an optimal mesh, in which a strain-superconvergent patch recovery error estimator and mesh refinement with subdivision concept are used. This paper presents two main studies of circular skirted foundations on nonhomogeneous soil, consisting of a bearing-capacity study and a large penetration study. The bearing capacity of the foundations is studied with the degree of nonhomogeneity (kD/s_uo) of soil up to 30, different skirt roughness and skirt depth up to five times the foundation diameter (i.e., D_f/D = 5), using an h-adaptive FEM and extended upper-bound method. The bearing-capacity values and soil flow mechanisms are discussed. In the foundation large penetration study, circular skirted foundations penetrating into normally consolidated and overconsolidated soils are tested physically in the centrifuge and analyzed numerically using the h-adaptive remeshing and interpolation technique with small strain method for soil large deformation analysis. The load-displacement responses from centrifuge test data and finite-element results are compared.

The coupled bridge foundation-superstructure finite-element code FLPIER was employed to predict the lateral response of the single piles and 3 × 3 to 7 × 3 pile groups founded in both loose and medium dense sands. The p-multiplier factors suggested by McVay et al. for laterally loaded pile groups with multiple pile rows were implemented for the predictions. The soil parameters were obtained through a back-analysis procedure based on single pile test results. The latter, as well as the numerical predictions of both the single and group tests, are presented. It was found that the numerical code FLPIER did an excellent job of predicting the response of both the single piles and the 3 × 3 to 7 × 3 pile groups. The latter involved the predictions of lateral load versus lateral deflection of the group, the shears and bending moments developed in the individual piles, and the distributions of the lateral loads in each pile row, which were all in good agreement with the measured results.

A model was developed to simulate mass removal during in situ air sparging. The model captures the physical characteristics of the air plume and employs conventional methods to simulate mass transfer. A parametric study was conducted using the model to assess how chemical properties, the operational method, and media affect mass removal during in situ air sparging. Mass removal is particularly sensitive to Henry's law constant, and the aqueous diffusion coefficient when Henry's law constant is high. Simulations of pulsed and continuous air injection show that pulsed injection can yield greater mass removal than continuous air injection for certain pulse cycles. Mass removal is generally greater at higher injection rates, but the increase in mass removal diminishes as the injection rate increases. Parametric analysis also showed that mass is removed faster when air channels are narrower or more tortuous (i.e., in coarser or more well-graded formations).

Quantitatively directed exploration (QDE) employs a first-order Taylor series expansion to combine sensitivity of a 3D finite-element model (FEM) and uncertainty in geologic data to calculate the variance in project performance, which is employed to direct exploration. This approach is made practical by calculating model sensitivity with direct differentiation of the engineering analysis code, thus producing sensitivity with a single model run rather than multiple runs required by parameter perturbation. Uncertainty in subsurface data is computed through two different extrapolation methods for comparison: kriging and conditional probability (Bayesian updating). Although either of these methods can be employed in QDE, conditional probability is required to quantifiably terminate exploration. The QDE framework is applicable to any subsurface analysis that employs a 3D FEM. A case study illustrates the QDE approach, where settlement is the performance criterion, and layer interface elevations are the uncertain geologic data. Additional boring locations identified by QDE were placed where a combination of model sensitivity and subsurface uncertainty was the greatest, thus directing exploration toward the building footprint and away from existing sampled points.

A series of highly instrumented, large-scale centrifuge models have been tested to investigate the extent of remediation required to control settlement and lateral sliding of soil deposits at a hypothetical bridge site. The baseline model represents a prototype with a 9-m-thick layer of fine sand having a relative density (Dr) of 50%. The sand layer is overlain by clay floodplains with a free face at a river channel. One nearly level floodplain surface supports a bridge abutment. The other floodplain has a 9% slope toward the river. In different models, different amounts of the 50% relative density sand was densified to Dr = 80%. Full depth improvement reduced settlements and lateral sliding of the sand by about a factor of 3. Due to the effects at the clay-sand interface, lateral sliding of the surficial clay deposit was not controlled by densification of the sand. Tests in which the width of the densified zone was only about 75% of the thickness of the loose sand indicated that relatively narrow zones of improvement can control settlement and sliding of the sand. Differences in shear resistance, pore pressures, dilatancy, and energy dissipation in loose and dense sands are presented.

This study presents direct evidence for the influence of the previbration cycles induced in the conventional resonant column tests on the measured dynamic properties of a dry sand. The tests were performed on an energy-injecting virtual-mass resonant column system, under which the material properties can be measured at a very low number of previbration cycles (<20). The test results show that for the sand tested at strain amplitudes beyond the elastic threshold, the influence of previbration is moderate on modulus but very significant on damping, particularly in the low range of the numbers of vibration cycles (<500). This observation is consistent with findings in the literature, which are based on combinations of static and dynamic test data. The effects of the low number of vibration cycles on the normalized modulus reduction and damping curves have also been examined. The results show that the modulus reduction curves are not significantly affected by these previbration cycles, whereas the damping curves in the range exceeding the elastic threshold are.

Systematic studies of pile response during static and dynamic load tests are generally too expensive to conduct in the field, and at model scale may be limited by scaling effects and the ability to obtain accurate stress waves. This paper describes such a study, conducted at model scale in a geotechnical centrifuge, for piles driven into dense sand. Adverse scaling effects were minimized by the use of extremely fine sand (silica flour), and accurate stress waves were obtained using high-frequency data logging, together with a Hopkinson bar arrangement for the measurement of pile-head velocity. The overall aim of the study was to compare dynamic and static test data, for open- and closed-ended piles driven into dense sand, for a range of delivered hammer energies. Open- and close-ended model piles were driven into dense sand and statically load tested at different penetrations, without stopping the centrifuge. Stress-wave data were collected, during continuous driving and from single blows immediately prior to the commencement of static load testing. An assessment of the accuracy of the mobilized soil resistance estimated from dynamic testing, using different levels of analysis, has been made by direct comparison with the static load test data. Particular emphasis has been placed on the performance of the dynamic analyses in light of different driving conditions and delivered hammer energy. From measurements of the delivered hammer energy, the efficiency of the centrifuge pile driving system was also assessed.

The load distribution and deformation of drilled shafts subjected to axial loads were evaluated by a load-transfer approach. The emphasis was on quantifying the load-transfer mechanism at the interface between the shafts and surrounding highly weathered rocks based on a numerical analysis and on small-scale tension load tests performed on nine instrumented piles. An analytical method that takes into account the soil coupling effect was developed using a modified Mindlin's point load solution. Based on the analysis, a single-modified hyperbolic model is proposed for the shear transfer function of drilled shafts in highly weathered rocks. Through comparisons with field case studies, it is found that the prediction by the present approach is in good agreement with the general trend observed by in situ measurements, and thus represents a significant improvement in the prediction of load deflections of drilled shafts.

Some of the deepest structural slurry walls in the world were designed for permanent training walls at the Dam No. 2 Hydropower Project on the Arkansas River. Each of the 152 wall panels had to be optimized for embedment, length, section type, and restraining system. Compatibility for deflection and load transfer of adjacent panels was an integral part in designing individual structural slurry wall panels to behave as a system of panels. Design of the structural slurry walls utilized the computer program RIDO to analyze the deflection, shear, and moment under construction and operation conditions. Structural requirements for the design based on the RIDO analyses were checked using STAAD-III. A third program (SOILSTRUCT) was used to verify the 2D global deformation of the proposed structural slurry wall system. Analyses and design had to be completed in a short 9-month time frame. This paper presents the design methodology, approach, and procedures for optimization of the structural slurry wall system required to make this project a reality.