Journal of Geotechnical and Geoenvironmental Engineering

Three commercial organoclays were evaluated as media for a variably permeable reactive barrier (VPRB) to manage coal-tar creosote, a non-aqueous-phase liquid (NAPL) consisting primarily of polynuclear aromatic hydrocarbons (PAHs). Organoclays and organoclay-sand mixtures having at least 25% organoclay had NAPL conductivities of less than ${10}^{-10}\mathrm{m}/\mathrm{s}$, whereas hydraulic conductivities to water were on the order of $0.001\mathrm{m}/\mathrm{s}$. Water migration was negligible in organoclays solvated with NAPL, but PAHs were released to the aqueous phase when the solvated clay contacted deionized water. Sorption isotherms for aqueous-phase PAHs are convex upward within the range of concentrations of environmental interest, and greater sorption occurs when the aqueous phase contains multiple PAHs. In column tests, breakthrough occurred for only naphthalene (lowest $K_{ow}$) and for only one organoclay (lowest organic carbon content) for tests on 100% organoclay and organoclay-sand mixtures conducted for 208–276 pore volumes of flow (PVF). Breakthrough predicted with solute transport analyses conducted using batch isotherm parameters as input occurred later than observed in the column test because isotherm models fit to the batch test data overpredicted sorption at low concentrations. The findings indicate that organoclays should be effective media for VPRBs to manage creosote NAPL and associated PAHs dissolved in groundwater. They block NAPL advection, readily permit the flow of water with dissolved PAHs, and sorb dissolved PAHs from water flowing through the clays.

The fluid bulk stiffness of a soil is very sensitive to the presence of gas, and a small volume of bubbles can significantly affect the pore pressure response to loading, including Skempton’s $B$ parameter, $P$-wave velocity, and liquefaction resistance. Biologically mediated processes can lead to the production of gases in soils; nitrogen is particularly advantageous because it is not a greenhouse gas, it is not combustible, and it has low solubility in water. Sands, silts, and clayey sands inoculated with Paracoccus denitrificans were monitored to assess the effects of nutrient availability, fines content, and pressure-diffusion on the evolution of nitrogen gas generation and bulk stiffness. Results show clear evidence of biogas bubble formation, earlier gas generation and entrapment in specimens with higher fines content, and a strong correlation between biogas volume and $P$-wave velocity. The volume of gas is correlated with specific surface, suggesting that biogas bubble formation develops as heterogeneous nucleation and that it is directly linked to the availability of nucleation sites on mineral surfaces, which in turn also affect the degree of attainable supersaturation. Results support the viability of biogenic gas generation as a tool to increase the liquefaction resistance of soils subjected to cyclic loading.

Swelling clays, also known as expansive clays, are encountered extensively all over the world. These clays are problematic for geotechnical engineering applications because of distress caused to structures and infrastructure as a result of swelling and swelling pressure. These clays are also often used in geoenvironmental engineering applications because of their effective barrier properties as liner materials, e.g., in landfills, ponds, and cutoff trenches. In this study, we report the effect of dielectric constant of fluid on the permeability and consolidation characteristics of Na-montmorillonite swelling clay to investigate the role of clay-fluid molecular interactions on the macroscale properties of the clay. A new “porous rigid wall, flexible wall” permeability device specifically designed for swelling clays that allows for accurate measurement of permeability, swelling pressure and consolidation characteristics, and evaluation of microstructure of swelling clays with fluids with different polarities is used in this study. Results show that clay-fluid molecular interactions have a tremendous effect on the permeability and consolidation characteristics of swelling clay and its microstructure.

The use of neutron imaging is demonstrated for visualizing and quantifying water distribution in partially saturated granular porous media. Because of the unique difference in the total neutron cross sections of water, sand, and air, a significant contrast for the three phases is observed in a neutron transmission image, and a quantitative analysis provides detailed information on the arrangement and distribution of particles, voids, and water. The experiments in this study are performed at the Neutron Imaging Facility (NIF) at the National Institute of Standard and Technology (NIST). An amorphous silicon flat panel detector was used in this research with a spatial resolution of approximately 250 μm ($127\mu \mathrm{m}/\mathrm{pixel}$). The effect of particle morphology on water distribution in compacted granular columns is investigated by using round and angular silica sand. Silica sand specimens with different bulk gravimetric water contents (0%, 6%, 9%, and 12%) are studied for evaluating the water phase-distribution spatially for compacted sand specimens in an aluminum cylinder.

This paper presents the results and analysis of experimental investigations of compressible Chicago clays with regard to the evolution of stiffness anisotropy. The experimental program was conducted on high-quality block samples obtained from the excavation for the Block 37 project in Chicago. The specimens were consolidated to in situ stresses via a recompression technique and then were subjected to directional stress probes. On-specimen LVDTs and an internal load cell with high accuracy were used for stress-strain response of the specimen. The stiffness and stiffness anisotropy ratio at very small strains were obtained synchronously from bender element tests during the consolidation and stress probes. The structural change as anisotropy evolves within a specimen is discussed. The results of the experimental program showed that compressible Chicago clay is an initially cross-anisotropic material under the in situ stresses. This anisotropy changes at the onset of irrecoverable deformation at stresses defined by previously established $Y_2$ yield surfaces.

Uncertainty is inherent in geotechnical design. In regard to estimating the stability state of dip slopes, most of the uncertainty lies in the geologic model assumed and the geotechnical parameters used in the evaluation. Biplanar (or active–passive) sliding in dip slopes occurs along a slope-parallel sliding surface with toe breakout occurring at the base of the failure. Internal shearing is required to facilitate kinematic release. All three of these release surfaces work together for the slope to fail, but with different degrees of importance depending on the dip-slope inclination. Increased efficiency and value with respect to the site investigation resources can be gained by working toward minimizing the uncertainty of those parameters that have the greatest bearing on the outcome of the slope stability analysis. This can be done quickly and inexpensively by performing scoping calculations facilitated by the use of Spearman rank correlation coefficients. This paper demonstrates that for shallow-dipping dip slopes, stability is primarily dictated by the shear strength of the slope-parallel sliding surface, and therefore, efforts should be focused on constraining the shear strength of this surface. For steep dip slopes, the shear strength related to the toe breakout and internal shear release surfaces becomes dominant, and therefore, the rock-mass shear strength and that for any adversely dipping persistent discontinuities should be the focus of the geotechnical investigation.

Suction embedded plate anchors (SEPLAs) allow for accurate positioning, thus providing an attractive alternative to traditional drag embedment anchors. This paper presents an analytical model for predicting the behavior of SEPLAs during the keying process, with a specific focus on predicting the loss of embedment depth as the anchor rotates from its initial vertical position to its target orientation perpendicular to the direction of loading. The soil is idealized as an incompressible, rigid-plastic material obeying an associated-flow rule. A generalized plastic limit analysis is employed to estimate the trajectories and corresponding capacities of SEPLAs under different loading conditions. The effects of soil resistance on the shank and anchor interaction with the anchor chain are also considered in the model. The SEPLA design commonly features a hinged flap; the effect of the flap is examined theoretically by comparing the solutions for the SEPLA with and without flap. Predicted solutions are shown in reasonably good agreement with some published numerical and experimental results, supporting the validity of the model.

Grout piles are used to reinforce base soil to increase the base stability of excavations in soft clay and to reduce excavation-induced ground movements. To propose a model for the base undrained stability of deep excavation in clay reinforced with grout piles, this paper presents an anisotropic strength criterion for clay reinforced with grout piles and the upper-bound bearing capacity theory for excavation. The proposed model is capable of determining the factor of safety against base heave for an excavation with or without ground improvement for various-strength anisotropy ratios of clay and clay reinforced with grout piles. The suitability of the model is verified with five field excavation cases. Among them, four were with no ground improvement, and one was with ground improvement. Generally, the larger the ground improvement rate, the more the material strength of a grout pile can be mobilized. The factor of safety also becomes larger when the ground improvement rate is higher. However, the factor of safety becomes insensitive to the undrained shear strength of a grout pile when the improvement ratio is less than 10%. Results from the model found that the increment of factor of safety against base heave and the increment of improvement ratio is generally $1:2$. Finally, because the proposed method does not consider the influence of displacement at the retaining wall bottom and the geometry of excavation, it can be used only in excavations with the width and depth of ground improvement zone larger than $1.41D_p$ and $1D_p$ ($D_p$ is depth of the retaining wall minus depth of excavation), respectively.

The strength of soil-nails depends on the behavior of the soil-cement grout interface at saturated and unsaturated conditions. Nowadays, pressure grouted soil-nails are considered to provide better interface strength than gravity grouted soil-nails. Soil-nail pullout tests have limitations to control some boundary conditions. To overcome these limitations, direct shear tests can be used to determine the actual soil-cement grout interface behavior. In the present study, a series of interface direct shear tests are performed between compacted completely decomposed granite (CDG) soil and cement grout at both saturated and unsaturated conditions under different grouting pressures. The interface shear stress increases with matric suction for different grouting pressures, and a strain softening behavior is observed for different suctions except at saturated conditions. A dilative behavior is obvious for interface as the suction value is increased from a saturated condition. However, the dilation values of soil-cement interface for different grouting pressures are less than that of CDG soil under different suctions. The interface shear strength increases with grouting pressure at saturated conditions, whereas, a downward trend is obvious as the suction value is increased from saturated conditions. The rate of increase of shear strength with matric suction is greater for CDG soil compared to soil-cement interface. Interface shear strengths for different grouting pressures are greater than CDG soil at a lower suction range but become less than CDG soil at a higher suction range.

One particularly complex phenomenon is the episodic, tidally driven variation of navigable depth level as a result of fluid mud settlement. This paper presents results from dynamic cone penetration testing with pore pressure measurement (CPTU) as a nonacoustical, direct device to support surveying and management of these areas. The new technique is modular and uses a disk configuration for fluid mud detection. Both disk resistance and pore pressure measurements accurately identify suspended matter concentrations of $90\mathrm{g}/\mathrm{L}$ or more, and the transition from fluid mud to consolidating mud once concentrations exceed $150\mathrm{g}/\mathrm{L}$. Hence, the procedure attests the potential for rapid, reliable assessment of a fluid mud layer and concurrent characterization of the underlying consolidated sediment by monitoring the pore pressure and strength changes during penetration.

A type of prefailure instability which occurs under fully drained conditions is studied in this paper. It is observed experimentally that when a specimen is sheared along a drained stress path involving a decrease in effective mean stress, it becomes unstable after the stress path crosses an instability line. The instability occurring under drained conditions is different from the type observed under undrained conditions. The differences and similarities between the two types of instability are elaborated. The practical implication of the study in analyzing the failure mechanisms of granular soil slope is also discussed.

It is essential to characterize the uncertainties of soil shear strengths, including friction angles of sands, for geotechnical reliability-based design. In particular, it is of practical interest to update the uncertainties in friction angles on the basis of all available information by correlation, given the considerable cost of a typical site investigation. Although it is relatively simple to update uncertainties by correlation when the information is one dimensional (1D) or univariate, it is quite challenging to update uncertainties by using multivariate information through multiple correlations. This study proposes a systematic way of constructing multivariate correlations on friction angles of reconstituted clean sands. A set of simplified equations are obtained through Bayesian analysis for the purpose of updating uncertainties. The inputs to the equations are the results of field and laboratory tests, and the outputs are the updated mean values and standard deviations of the friction angles. Test databases are used to demonstrate the consistency of the proposed simplified equations. Results show that uncertainties in friction angles can be effectively reduced by incorporating multivariate information. Given that reliability-based design can justify more economical design with updated uncertainties, the proposed equations essentially link the value of more and/or better tests directly to final design savings.

Filtration behavior of subballast materials subjected to different cyclic stress pulse shapes at frequencies of 5 Hz and 10 Hz was investigated in the laboratory. Clayey silt was used as a base soil while graded crushed basalt was used as a filter. Rectangular, sinusoidal, and triangular cyclic stress pulse shapes having a stress range of 70–140 kPa were investigated for a piping ratio of 9. Turbidity, permeability, and change of pore water pressure within base soil were recorded over time as the number of stress pulse cycles increased. Results show that triangular and sinusoidal cyclic stress pulse shapes have a similar effect on the filtration behavior while the rectangular cyclic pulse shape had a slightly higher effect than triangular and sinusoidal stress pulse shapes. Based on this study, a piping ratio of 9 was found to be satisfactory for all cyclic stress pulse shapes at 5 Hz.

Reinforced masonry block retaining walls are comprised of a narrow column of stacked blocks at their exposed end. This column is placed on a nonstructural leveling pad to facilitate the placement of facing units. Theoretically, this column can generate very large toe resistance to sliding. A recent publication indicates that an accepted design methodology implicitly counts on this resistance in assessing the reinforcement load. Although not calculated in this design, it unconditionally considers that over 60% of the resultant horizontal force in a 12-m-high wall is carried by the toe, which is made up of 0.3-m-deep blocks. This paper elucidates this issue by explicitly identifying the magnitude of toe resistance and critically reviews whether such high resistance is universally suitable for design. It shows that high toe resistance may not be feasible for most foundation soils. The high impact of toe resistance on the reinforcement force poses a design dilemma as to the reliability of this resistance, even if attainable. Practically, the leveling pad is not intended to serve as a critical structural element and thus should not be relied on for maintaining the toe resistance in long-term design. Economically, ignoring the toe resistance has little impact on the overall cost.