Journal of Geotechnical and Geoenvironmental Engineering

The objective of this study was to evaluate strategies for addressing missing meteorological (MET) data when predicting the hydrology of a water balance cover for a waste-containment system using a variably-saturated flow code. Predicting the hydrology of water balance covers typically requires site-specific daily MET data, which may be only partially available (e.g., dew point temperature ($T_{\text{dew}}$), solar radiation ($R_s$), wind speed, and cloud cover frequently are only partly available). Thus, some of the input data may need to be estimated or surrogate data employed for hydrologic modeling. The influence of replacing missing MET data with estimates on hydrologic predictions was evaluated for a water balance cover in a semiarid climate. Substitution of single or multiple MET variables with long-term averages led to statistically similar predictions of annual percolation relative to percolation predicted using actual data. Replacing all MET variables ($T_{\text{dew}}$, $R_s$, wind speed, and cloud cover) with long-term averages underpredicted percolation by 25% ($0.58\mathrm{mm}/\mathrm{year}$), on average, relative to percolation predicted with actual data. A strategy for estimating MET data via empirical techniques is described that includes estimating (1) $T_{\text{dew}}$ = daily minimum temperature, (2) $R_s$ with the Hargreaves and Samini model, (3) daily wind speed set equal to monthly averages, and (4) cloud cover estimated as a function of solar radiation. This surrogate MET data technique yielded modest overpredictions of annual percolation of 3 and 46% (0.07 and $1.08\mathrm{mm}/\mathrm{year}$) that were statistically similar to percolation predicted using actual data.

A numerical model, EC2, is developed to simulate the consolidation of a soil layer subjected to combined electroosmosis and surcharge preloading. EC2 includes the capabilities of a previous model, EC1, in accounting for large-strain-induced nonlinear changes in the physical and geoelectrical properties that take place on a layer, with the additions of two-dimensional consolidation, coupled loading and electric fields, time-dependent loading and current density, and an external hydraulic gradient. A highlight of the EC2 model is the superposition of multiple streams between small elements, which improves the prediction accuracy of the rate of flow and the degree of consolidation. The consolidation algorithm of EC2 is one-dimensional and conducted using finite-difference and piecewise-linear methods. This paper presents the theoretical and numerical development of the EC2 model. A companion paper presents the validation of the model and the results of simulations that illustrate the optimization of the consolidating processes for some interesting design scenarios.

The results of numerical simulations for electroosmosis–surcharge preloading consolidation, obtained using the EC2 model, are presented. EC2 accounts for hydraulic and electroosmotic flows under the conditions of changing physical and geoelectrical properties of saturated compressible porous media. Verification checks for EC2 show excellent agreement of the pore pressure and degree of consolidation with analytical solutions for one- and two-dimensional small-strain consolidation. Similarly, excellent agreement is attained for two-dimensional large-strain consolidation occurring to a kaolinite layer subjected to an experimental test. The EC2 model is then used to investigate consolidation optimization for soil layers in three example problems. The electroosmosis–surcharge preloading combined method outperformed single-process methods, particularly where hydraulic conductivity is on an order of magnitude of ${10}^{-8}\mathrm{m}/\mathrm{s}$ or lower. Applying voltage in steps optimizes electric power usage, as does cutting off the power supply when the degree of consolidation reaches approximately 80%.

This study presents a unified method for estimating the ultimate bearing capacity of shallow foundations resting on variably saturated soils under a steady flow. The effective stress approach originally employed for predicting the bearing capacity of saturated soils is extended by incorporating a suction stress-based representation. A closed-form equation is used to define the suction stress characteristic curve, capturing changes in the effective stress because of varying soil saturation and matric suction. The proposed method uses the classic effective shear strength parameters and two fitting parameters to represent the soil water characteristic curve. The method can be used for different soil types, various degrees of saturation, and different surface flux boundary conditions. The results from the proposed formulation were compared, and good agreement was observed against three sets of experimental results from model footing and plate load tests. A parametric study was performed, and the ultimate bearing capacity profiles are presented for various surface flux boundary conditions and depths of the water table. It is shown that different flow conditions can significantly affect the ultimate bearing capacity of clay. However, different flow conditions have negligible effects on the ultimate bearing capacity of sand.

Geologic profiles cannot be identified with certainty due to inherent spatial variability and limited site-investigation data. Random field theory is a powerful tool for evaluating the spatial characteristics of soil and rock properties, but may overestimate the site variability if the site investigation data at borehole locations within the site are not considered. This paper aims to propose a method to generate a conditioned geologic random field based on available measurement information. It incorporates both direct and indirect information within the site to constrain the random field. The conditioned random field matches the measured data at the measurement locations, and has much-reduced uncertainty adjacent to the measurement locations due to spatial correlation. The proposed method is applied to evaluate the depth of Grade III rock surface at a construction site in Hong Kong using borehole data. With the proposed method, a more accurate description of the geologic profile can be obtained, which is desired for geotechnical design and construction.

This study focuses on an evaluation of the subsurface ground temperature distribution during operation of a soil-borehole thermal energy storage (SBTES) system. The system consists of an array of five 9 m-deep geothermal heat exchangers, configured as a central heat exchanger surrounded by four other heat exchangers at a radial spacing of 2.5 m. In addition to monitoring the temperature of the fluid entering and exiting each heat exchanger, 5 thermistor strings were embedded in boreholes inside and outside of the array to monitor changes in ground temperature with depth. After 75 days of heat injection at a constant rate of $20\mathrm{W}/\mathrm{m}$, corresponding to 11.5 GJ of thermal energy, the average ground temperature in the array increased by 7°C. However, depending on the storage volume definition, only 2.43–4.86 GJ of thermal energy was stored attributable to heat losses. After a 4-month rest period the heat storage was observed to decrease by 60% owing to further heat losses. The trends in subsurface temperatures during heat injection were consistent with results from a simplified heat injection simulation using the system thermal conductivity estimated from a line source analysis. Although the heat injection rate of $20\mathrm{W}/\mathrm{m}$ is smaller than that expected in actual SBTES systems ($35–50\mathrm{W}/\mathrm{m}$), an energy balance analysis indicates the number of boreholes in the array was too few to effectively concentrate the heat injected within the subsurface. Nonetheless, the results provide an experimental reference point between a single borehole and a larger SBTES system.

The strength and stress-dilatancy of uniform sands has been studied extensively in geotechnical investigations, and practitioners can draw on a wealth of previously reported data for the estimation of their volumetric response. However, the suitability of accepted stress-dilatancy theory and empiricism has not been evaluated for well-graded gravelly soils. Axisymmetric, isotropically consolidated drained compression, and pure shear, plane strain quasi-$K_0$ consolidated drained tests were performed on well-graded Kanaskat gravel using confining pressures ranging over three orders of magnitude to determine its stiffness, strength, and stress-dilatancy response. The plane strain stiffness, strength, and stress-dilatancy of Kanaskat gravel is observed from tests performed using a large cubical true-triaxial device with flexible bladders. The observed response is interpreted with a view of experimental boundary conditions and their impact on the volumetric response. The observed plane strain shear modulus and friction, and dilation angles of well-graded sandy gravel soils commonly used in practice are significantly higher than those measured in the triaxial compression stress path. Existing empirical and modified stress-dilatancy expressions proposed for low confining pressures underestimate the observed dilation response; however, another common empirical approach appears to adequately capture the dilatancy. The data reported herein should help practitioners estimate plane strain behavior of sandy gravel mixtures.

A series of centrifuge tests were performed to investigate the behavior of laterally loaded pile groups in improved and unimproved soft clay. The soil profile consisted of four lightly overconsolidated clay layers overlying a dense layer of sand. The pile groups had a symmetrical layout consisting of $2\times 2$ piles spaced at 3.0 and 7.0 pile diameters (D). After improving the soft clay in situ using simulated cement deep soil mixing (CDSM), pile foundations were driven into the improved ground. Centrifuge tests revealed that CDSM is an effective method to increase the lateral resistance of pile foundations. The lateral resistance of the improved pile group at 7D spacing increased by 157%. Due to pile–soil–pile interactions, the lateral resistance in the 3D pile group increased by only 112%. In both improved and unimproved pile groups with 3D spacing, the leading row of piles carried larger loads and bending moments than the trailing row of piles. No group interaction effects were observed in all pile groups with 7D spacing. At very large deflections, cracks developed and tension failure occurred in the CDSM block of the improved 3D pile group. Values for p-multipliers were back-calculated and incorporated into a computer code to perform a parametric study on improved pile groups with 3D spacing. It was seen that for CDSM block depths greater than 9D, increases in lateral resistance are practically negligible. At deflections less than 8 cm, lateral resistance of the improved pile groups is not sensitive to undrained shear strength of the improved clay representing typical cement contents.

This paper presents a study of the load-deformation behavior of geocell-stabilized subballast subjected to cyclic loading using a novel track process simulation apparatus. The tests were conducted at frequencies varying from 10 to 30 Hz. This frequency range is generally representative of Australian standard gauge trains operating up to $160\mathrm{km}/\mathrm{h}$. The discrete-element method (DEM) was also used to model geocell-reinforced subballast under plane strain conditions. The geocell was modeled by connecting a group of small circular balls together to form the desired geometry and aperture using contact and parallel bonds. Tensile and bending tests were carried out to calibrate the model parameters adopted for simulating the geocell. To model irregularly shaped particles of subballast, clusters of bonded circular balls were used. The simulated load-deformation curves of the geocell-reinforced subballast assembly at varying cyclic load cycles were in good agreement with the experimental observations. The results indicated that the geocell decreased the vertical and lateral deformation of subballast assemblies at any given frequency. Furthermore, the DEM can also provide insight into the distribution of contact force chains, and average contact normal and shear force distributions, which cannot be determined experimentally.

The effect of extensive liquefaction on the liquefaction resistance of heavily preshaken saturated silty sand is studied using a centrifuge experiment. The base of a 6-m homogeneous deposit was subjected to a total of 91 shaking events of different horizontal base accelerations and durations. Three event types were used in alternating patterns: mild preshaking Events A, stronger preshaking Events B, and strong liquefying Events C. The experiment was divided in two stages. In Stage One, reported in a previous publication, 66 preshaking Events A and B, were applied. In Stage Two, which is the focus of this paper, 25 additional shakings were applied which included two Events C in addition to continuing the same pattern of Events A and B. Stage One resulted in a significant increase in liquefaction resistance of the deposit. In Stage Two, extensive liquefaction produced by Events C resulted in a dramatic immediate reduction in liquefaction resistance of the deposit to a level comparable to that before preshaking. These findings are consistent with other reported field observations and laboratory results.

The efficacy of removal and replacement as a mitigation alternative for expansive soil is discussed in view of numerical unsaturated flow/deformation results. Observations with respect to selection of depth and replacement soil type (similar, more, or less permeable than underlying expansive clay) are presented in the context of unsaturated flow and stress/deformation principles. Comparisons are made between various remove and replace options on the basis of extent and degree of wetting (changes in soil suction) and on ground surface total and differential movements. Selection of optimal removal and replacement depth and replacement material type requires consideration of unsaturated flow principles and understanding of the role of both net normal stress and matric suction in unsaturated soil response. Complexities in the unsaturated flow process which impact the efficacy of the remove and replace method are primarily because of the highly nonlinear nature of the unsaturated soil storage function (SWCC) and hydraulic conductivity function. Further, because the unsaturated hydraulic conductivity of soils depends on the value of soil suction, placement water content of the replacement layer has significant effect on performance. The net result is that the impact of various remove and replace options is not always intuitive. The findings presented, although not encompassing of all possible removal and replacement configurations or surface flux conditions, set the stage for application of remove and replace mitigation methods based on fundamental unsaturated soil mechanics principles.

The true triaxial compression test is the three-dimensional test, where the intermediate principal stress is not less than the minor principal stress and not more than the major principal stress. The effect of the intermediate principal stress ratio (i.e., the $b$-value) on particle breakage of rockfill material (RFM) was investigated by conducting a series of true triaxial compression tests under the constant-$p^{\prime }$ and constant-$b$ loading condition. It was observed that the relative breakage extent of RFM increased with an increase in confining pressure but decreased with an increase in $b$-value. The effect of the $b$-value on the particle breakage of RFM was well predicted by an empirical equation.

The 75-μm particle size is used to discriminate between fine and coarse grains. Further analysis of fine grains is typically based on the plasticity chart. Whereas pore-fluid-chemistry-dependent soil response is a salient and distinguishing characteristic of fine grains, pore-fluid chemistry is not addressed in current classification systems. Liquid limits obtained with electrically contrasting pore fluids (deionized water, 2-M NaCl brine, and kerosene) are combined to define the soil “electrical sensitivity.” Liquid limit and electrical sensitivity can be effectively used to classify fine grains according to their fluid-soil response into no-, low-, intermediate-, or high-plasticity fine grains of low, intermediate, or high electrical sensitivity. The proposed methodology benefits from the accumulated experience with liquid limit in the field and addresses the needs of a broader range of geotechnical engineering problems.