Journal of Geotechnical and Geoenvironmental Engineering

Despite the availability of numerous methods to predict the load-displacement response of aggregate pier reinforced clay, accurately modeling the nonlinear displacement response remains challenging. Moreover, the uncertainty in the bearing pressure-displacement prediction has not been satisfactorily estimated, preventing the generation of reliability-based design (RBD) procedures. This study proposes a simple RBD procedure for assessing the allowable bearing pressure for aggregate pier reinforced clay in consideration of the desired serviceability limit state. A recently established bearing-capacity model for aggregate pier reinforced clay and its uncertainty is incorporated into a bivariate bearing pressure-displacement model appropriate for a wide range in displacement and calibrated using a high-quality full-scale load-test database. Several copulas were then evaluated for goodness of fit to the measured dependence structure between the coefficients of the selected two-parameter bearing pressure-displacement model. Following the generation of an appropriate performance function, a combined load and resistance factor is calibrated in consideration of the uncertainty in the bearing pressure-displacement model, bearing capacity, applied bearing pressure, allowable displacement, and footing width using Monte Carlo simulations of the respective source distributions. An example is provided to illustrate the application of the proposed procedure to estimate the bearing pressure for an allowable displacement at the desired serviceability limit probability.

The performance of a geosynthetic clay liner (GCL) installed as part of a geocomposite barrier (geomembrane/GCL) to contain a hydrocarbon spill adjacent to the Arctic Ocean was evaluated by examining sacrificial samples exhumed after 1, 4, 6, 7, and 10 years in service. The hydraulic and chemical characteristics of the GCL were most affected by the location within the soil profile relative to the water table (typically about 1.3 m below ground level). The bentonite in the GCL samples exhumed from a depth of 0.0–0.8 m was well hydrated with a dispersed structure. Despite the significant cation exchange that took place between these GCL samples and the surrounding soil (the percentage of the exchangeable sodium decreased from 68% to 10–15%), there was no change in the hydraulic conductivity ($k$) of GCL for tap water or jet fuel. A similar bentonite structure was observed for GCL samples exhumed from 0.8 to 1.3 m below ground level; however, an observed network of horizontal and vertical microcracks ($200–400\hspace{0.17em}\mu \text{m}$ wide) in the bentonite layer was attributed to the formation of ice lenses. As a result, the $k$ of these GCL samples increased by one to two orders of magnitude when permeated by both tap water and jet fuel. The bentonite in GCL samples exhumed from below the water table after 6 and 10 years was flocculated with relatively high free pore space. The $k$ values of these samples increased by one to four orders of magnitude for tap water and jet fuel. Despite this increase in $k$ at some depths, there was no evidence of migration of hydrocarbons through the barrier over the last 10 years, indirectly suggesting that the subsurface geocomposite barrier system is still performing well.

Underground structures located in liquefiable soil deposits are susceptible to floatation following a major earthquake event. Such failure phenomenon generally occurs when the soil liquefies and loses its shear resistance against the uplift force from the buoyancy of the underground structure. Numerical modeling accompanied with centrifuge experiments with shallow circular structures has been carried out to investigate the floatation failure at different buried depths of the structure. The influence of the magnitude of input sinusoidal earthquake shaking was also studied. Both numerical and experimental results showed matching uplift response of the structures and acceleration and pore-pressure measurements in the liquefied soil deposit. A higher uplift displacement of the structure was observed for shallower buried depth, thereby indicating the influence of overlying soil weight against floatation. Results also showed that the structures commenced floatation in the presence of high excess pore pressure, but they ceased when the earthquake shaking stopped. The higher rate of uplift in stronger earthquake shaking further substantiates the dependency of the uplift to the shaking amplitude. A constant rate of uplift of the structure was attained after the soil liquefied, hence postulating a possible limit to shear modulus degradation of the surrounding soil caused by soil-structure interaction. This is inferred by the lower excess pore-pressure generation near the structure. The displacement of liquefied soil around the displaced structure was also confirmed to resemble a global circular flow mechanism from the crown of the structure to its invert as observed in displacement vector plots obtained from numerical analysis and particle image velocimetry (PIV) in centrifuge tests. Further numerical analysis on the performance of buried sewer pipelines in Urayasu City, Chiba Prefecture following the 2011 Great East Japan Earthquake indicated high damage susceptibility of rigid pipelines in the liquefiable soil deposit. These consistencies with field observations clearly demonstrate and pave the prospects of applying numerical and/or experimental analyses for geotechnical problems associated with the floatation of underground structures in liquefiable soils.

By appropriately reducing the size of the footing, rocking behavior caused by seismic loading can occur about the footing base. Recent research has shown how rocking shallow foundations can result in improved seismic performance of bridges. One concern of rocking systems is the potential for significant settlement accumulation in poor soil conditions. This research explores the effectiveness and practicality of using unattached piles to mitigate settlement while still allowing rocking. Two series of centrifuge tests investigated the performance of individual footing-column-mass structural models in these soil conditions on unattached piles. The lateral load connection between the rocking foundation and piles is an important consideration; a variety of possible shear key configurations capable of transferring lateral and dead loads to the piles while allowing separation associated with rocking are discussed. Unique design issues associated with rocking on unattached piles and methods to address these issues are described. Concepts are presented to support the evaluation of the effect of excess pore pressure on the pile capacity. Methods of evaluating effects of pile capacity, number of piles, and arrangement of piles on the residual settlement and base shear coefficient to initiate rocking on piles are also described.

The authors present a method for calculating a mooring line’s tension and angle of inclination at the anchor pad eye to analyze the behavior of a mooring line embedded in sand. They adopted the governing equation from a previous study for clay and reanalyzed the frictional and bearing resistances. The authors performed a number of centrifuge tests at two anchor depths while maintaining relative densities of 76 or 51% in the cylindrical test box to validate the proposed analytical method. The analytical solutions agreed well with the test results and the results of the simplified method. The authors conducted a parametric study to evaluate the effects of various factors on the behavior of an embedded mooring line in sand, including the depth of the attachment point, the internal friction angle of sand, the submerged unit weight of sand, the angle of the mooring line on the seabed, and the self-weight and nominal diameter of the mooring line. The results demonstrated that the proposed analytical method is effective for analyzing the behavior of a mooring line embedded in sand.

Soil bioengineering methods consisting of live poles and brush layers were used to stabilize shallow slides and control erosion on an embankment slope in eastern Ohio. Plant survival rates, slope movement, soil moisture content, and soil suction were measured. Load tests were performed to evaluate resistance of the poles, and results were used to estimate the stability of the repaired slope. Seven years after the repair was completed, vegetation is well established and the slope is stable, which is in agreement with calculated safety factors. This study indicates that soil bioengineering methods can be used for stabilization of shallow failures; however, the method may not be immediately effective, and the time of installation and soil moisture conditions are important for survival of the poles.

The small-strain shear modulus depends on stress in uncemented soils. In effect, the shear-wave velocity, which is often used to calculate shear stiffness, follows a power equation with the mean effective stress in the polarization plane $V_s=\alpha {\left({\sigma }_m^{\prime }/1\hspace{0.17em}\text{kPa}\right)}^{\beta }$, where the $\alpha$ factor is the velocity at 1 kPa, and the $\beta$ exponent captures the velocity sensitivity to the state of stress. The small-strain shear stiffness, or velocity, is a constant-fabric measurement at a given state of stress. However, parameters $\alpha$ and $\beta$ are determined by fitting the power equation to velocity measurements conducted at different effective stress levels, so changes in both contact stiffness and soil fabric are inherently involved. Therefore, the $\alpha$ and $\beta$ parameters should be linked to soil compressibility $C_C$. Compiled experimental results show that the $\alpha$ factor decreases and the $\beta$ exponent increases as soil compressibility $C_C$ increases, and there is a robust inverse relationship between $\alpha$ and $\beta$ for all sediments: $\beta \approx 0.73-0.27\log \left[\alpha /\left(\text{m}/\text{s}\right)\right]$. Velocity data for a jointed rock mass show similar trends, including a power-type stress-dependent velocity and inverse correlation between $\alpha$ and $\beta$; however, the $\alpha \text{-}\beta$ trend for jointed rocks plots above the trend for soils.