Journal of Geotechnical and Geoenvironmental Engineering

Steel grids in the form of bar mat and welded wire mesh are a common reinforcing material in mechanically stabilized earth (MSE) walls. This paper uses a large database of laboratory steel grid pullout tests from multiple sources to evaluate the accuracy of different ultimate pullout capacity models in a consistent statistical framework. Included in the study is a recent empirical-based pullout model for steel grid pullout in frictional soils that is now extended to include cohesive-frictional soils. This model is shown to be more accurate than previous models with default parameters based on analysis of bias statistics. The same calibrated model is shown to be accurate when compared with the results of in situ steel grid pullout tests performed in production walls constructed with cohesive-frictional soils. The new default model is shown to be more accurate than previous models, including the current Federal Highway Administration (FHWA) default model that is most common in North America for design against pullout in MSE walls constructed with frictional (granular) soils. The bias statistics reported for each pullout model in this study are also a prerequisite to compute resistance factors in load and resistance factor design (LRFD) for the pullout limit state using reliability theory–based calibration methods.

The practical application of waste materials such as steel furnace slag (SFS) and coal wash (CW) is becoming more prevalent in many geotechnical projects. While adding rubber crumbs (RCs) from recycled tires into mixtures of SFS and CW not only solves the problem of large stockpiles of waste tires, it also can provide an energy-absorbing medium that will reduce vibration and prevent track degradation. Thus, the engineering insight into the effect that rubber crumbs have on the dynamic behavior of $\mathrm{SFS}+\mathrm{CW}+\mathrm{RC}$ mixtures is in urgent demand. In this study the influence that RC contents and confining pressures have on the deformation, resilient modulus, damping ratio, and shear modulus was investigated by cyclic triaxial tests. Test results reveal that with the inclusion of RC, the axial strain, volumetric strain, damping ratio, and energy-absorbing capacity of the $\mathrm{SFS}+\mathrm{CW}+\mathrm{RC}$ mixture increase, while its resilient modulus and shear modulus decrease. Based on these properties, an amount of 10% RC is recommended as an optimal blended mix to be used as railway subballast. A three-dimensional (3D) empirical model of the relationship between the maximum axial strain, volumetric strain, and resilient modulus with RC contents and the effective confining pressure was developed, and the energy-absorbing capacity of these waste mixtures has also been analyzed for practical purposes based on their comprehensive parameters.

Polyurethane foam adhesive (PFA), a product resulting from exothermic chemical polymerization reactions between the diisocyanate functional group and polyol monomers, can form a closed cellular structure with high strength and stiffness. In the current study, the effectiveness of PFA in improving the strength and ductility of a well-graded gravelly soil was investigated. Using drained triaxial compression tests on the unimproved and PFA-improved soil as the basis for comparison, the peak strength and residual-state strength of the PFA-improved soil increased significantly (from 1.5 to 7 times and from 1.4 to 5.4 times of that of unimproved soil, respectively) with increasing PFA content (from 2 to 8%) at a confining pressure of 100 kPa. For the scenarios of low confining pressure, the PFA additive resulted in a change in volumetric response from contractive to highly dilative. The residual (or pseudo-critical) state lines (RSLs) for PFA-improved soils were observed to be parallel in the $q\sim p^{\prime }$ and $e\sim p^{\prime }$ planes, with an upward translation with increasing PFA content. Strain-dependent reductions in stiffness were observed to become more pronounced with increases in PFA content and confining pressure. In particular, the brittleness index for the PFA-improved soil (ranging from 0.01 to 0.29) was found to be much lower than that for the lime-cemented soil (ranging from 0.03 to 2.36), indicating that the PFA-improved gravel exhibited much better ductility than similar gravelly soils improved with lime due to the ductile nature of the polymer bonds. From the perspective of ductility, strength, and economy, the optimal PFA content ranges from 4 to 6% (with an optimum value of approximately 5%). Given its ductile response and stress-strain-strength characteristics, PFA-improved gravelly soils could offer a promising alternative for application to high rockfill dams or railway embankments.

Tunneling beneath piled structures may compromise the stability and serviceability of the structure. The assessment of potential structure damage is a problem being faced by engineers across the globe. This paper presents the outcomes of a series of geotechnical centrifuge experiments designed to simulate the effect of excavating a tunnel beneath piled structures. The stiffness and weight effects of piled structures are examined independently using aluminum plates of varying stiffness (equivalent beam approach) and the addition of weights supported by aluminum piles. Greenfield displacement patterns and results from pile loading tests are also provided. The variation of structure displacement profiles with plate stiffness, weight, and tunnel volume loss are used to illustrate the main effects of tunnel-pile interaction and the contribution of the superstructure to the global tunnel-pile-structure interaction. Results indicate that piles have a detrimental role in tunnel-structure interaction problems, whereas the superstructure stiffness and weight can, respectively, reduce and increase structure distortions and settlements. Finally, the potential for structural damage is evaluated by comparing structure and greenfield deflection ratios as well as resulting modification factors. The paper presents a unique set of results and insights that provide valuable guidance to engineers working across the ground and structural engineering disciplines.

The stress-dependent curve of shear modulus degradation with increasing shear strain amplitude is a fundamental mechanical property of soils. Although it is well known that the degree of saturation has an important impact on the small strain shear modulus of unsaturated soils, its role on the shear modulus evolution with strain has not been thoroughly investigated. A testing program has revealed strong correlations between two key parameters of the shear modulus degradation curve, the reference strain and the coefficient of curvature, and parameters of the soil water retention curve (SWRC). An SWRC model capable of distinguishing between soil water in the capillary and adsorption regimes was used to correlate the reference strain to the maximum adsorption water content and pore size distribution of a soil, and to correlate the curvature coefficient to the maximum adsorption water content. A hyperbolic equation for the shear modulus reduction curve using these correlations shows good performance in predicting the shear modulus under unsaturated small or finite strain conditions. The new model was validated using the shear modulus reduction curve of independent data sets measured at different shear strains.

Time domain reflectometry (TDR) can be used to measure the dry density of compacted soils, although it is believed that TDR could also be used to monitor the long-term performance of aging geotechnical assets. Understanding the deterioration of aging assets (earth dams, embankments) can be problematic; monitoring the relative condition with time may prove advantageous. In such applications, it would be likely that commercially available TDR probes and multiplexers would be used, and this paper illustrates that the current method does not perform particularly well with these. Therefore, an alternative method has been developed that, when applied to six fine-grained soils (exhibiting a range of plasticities), can deal with the impacts of multiplexers and commercial probes. It is shown that the dry density and gravimetric water content can be predicted with an accuracy of $\pm 5$ and $\pm 2\%$, respectively. The accuracy can also be improved by correcting the TDR parameters for temperature. The new method is robust, relatively independent of the compactive effort and only marginally affected by the presence of multiplexers, making it suitable for field-monitoring applications.

Hydraulic conductivity of soil-bentonite (SB) slurry-trench cutoff walls is highly influenced by the stress state. A model based on force equilibrium is proposed to predict the stresses in SB cutoff walls, considering not only the arching and lateral squeezing effects but also the interaction between SB backfill and surrounding soil by assuming a sliding wedge under limit equilibrium conditions based on Coulomb’s theory. This model is applied to a well-documented SB cutoff wall case, and good agreements are found between the predicted horizontal effective stress and those evaluated from cone penetration test data. The decrease in stresses within the deep portion of SB cutoff walls is successfully simulated. A parametric study shows the horizontal effective stress is sensitive to the properties of the surrounding soil, but insensitive to those of the backfill. General estimation of horizontal stress can be made in practice according to the geometry similarity in stress profiles.

During hydraulic fracturing in shale oil and gas reservoirs, hydration swelling due to shale water imbibition is believed to create secondary fractures that improve permeability, as observed at ambient conditions. In this paper, the influence of hydration swelling on fracture generation and permeabilities in Mancos shale at different stress conditions was investigated. Hydration swelling was characterized by swelling strain tests. Generated fractures were observed by computed tomography (CT) scan tests. The permeability of the shale was measured before and after water imbibition. In shale with isotropic confining pressure increased to 2.0 MPa or more, water imbibition can cause fracture closing instead of propagation. As a result, permeability decreased significantly, up to two orders compared with that before water imbibition. On the other hand, the differential stress required to activate shear-induced fractures was greatly reduced after hydration, up to 50% in shale after water imbibition. For shale cores under higher confining pressure, with decreased hydration swelling magnitude, shear-induced fracture generation was reduced. Water permeability of shale with a shear-induced fracture was nearly 100 times higher than that of shale without a fracture. In other words, water imbibition can enhance the generation of shear-induced fractures under anisotropic stress to improve shale permeability, but it can cause fractures to close in shale under isotropic stress.

This paper examines the long-term durability of two dual wedge welded 1.5-mm-thick high-density polyethylene (HDPE) geomembranes (GMBs). The two GMBs, which had different resins, and the welds initially met relevant standards. The welded GMBs were immersed in a brine solution with $200\mathrm{g}/\mathrm{L}$ total dissolved solids and pH 8.7 at four different temperatures (40, 65, 75, and 85°C) for 3 years. Changes in standard oxidative induction time (Std-OIT) and high-pressure oxidative induction time (HP-OIT) were monitored at (1) the GMB well away from the weld, (2) the heat-affected zone beside the weld, and (3) the welded zone. The Std-OIT and HP-OIT depletion were fastest in the heat-affected zone adjacent to the weld and slowest in the weld itself. Both the Std-OIT and HP-OIT were still depleting, without having reached a residual value, when degradation in physical properties was observed. Brine has a greater effect on time to nominal failure for the peel break strength than for the shear break strength and elongation. Aging of the GMB in the heat-affected zone beside the weld controls the time to nominal failure. The importance of the welds in the consideration of GMB service-life is highlighted.

Compressibility of five different samples of waste foundry sand (WFS) was evaluated through one-dimensional and triaxial compression tests to assess permanent strains under vertical and deviator stresses present in typical highway embankments. Results show that WFS containing a blend of sand and bentonite had higher compressibility than compacted natural sand. Compressibility of WFS is dependent on stress magnitude, time, and bentonite content. All samples of WFS exhibited one-dimensional secondary compression and deviatoric creep response similar to that of compacted natural soils. Compressibility and potential for creep rupture systematically increased with increasing bentonite content. For samples with relatively high bentonite content (12.8%), the maximum deviator stress to prevent creep rupture was reduced to 65% of the material’s compressive strength. One-dimensional compressibility parameters for compacted WFS were characterized as functions of bentonite content. Models were developed to estimate secondary compression ratio, deviatoric creep strain rates, and time to creep rupture for WFS at different stress levels and bentonite contents.

The drainage conditions during cone penetration is a critical factor in the assessment of tailing properties, as the sedimentation process produces deposits that are spatially variable, and often include materials in the intermediate permeability range. This paper provides evaluation for the parameters linking the rate of piezocone penetration to drainage effects in gold tailings. Cone soundings performed at penetration rates from 0.3 to $57\mathrm{mm}/\mathrm{s}$ are compared and results are interpreted in a space that correlates dimensionless velocity, $V$, to piezocone normalized resistance, $Q$, and normalized pore pressure, $U$. A nonlinear poroelastic model conceived to capture the transient flow effects of the soil around an expanding cylinder has assisted in analyzing the measured behavior. Cone data from variable penetration rate soundings from other reported tests are also analyzed within the same framework to demonstrate that significant errors can occur in the interpretation of soil properties for soil conditions represented by normalized velocities, $V_h$, in the range of 0.01–10.0. In tailings, a practical recommendation is to perform tests at different penetration rates in the same location, which allows appropriate nonstandard penetration rates to be selected according to these empirical observations and mathematical models.

This paper reports numerical modeling results investigating the diving potential and optimization of offshore anchors. A simplified two-dimensional plate anchor model and a complex three-dimensional anchor model were investigated in the finite element analyses. Good agreement with established analytical predictions provided validation of the numerical analysis. A major parameter influencing the dive potential for anchors was found to be the position of the padeye relative to the anchor rotation center. Beneficial padeye offset ranges that encourage diving and benefit ultimate holding capacity were identified for two different anchor geometries. These should facilitate optimization of future anchor designs in offshore engineering.

This case study describes the failure of an interstate connecting-ramp embankment during construction and investigates the failure mechanism, performance of the prefabricated vertical drains (PVDs) installed to accelerate consolidation of the weak embankment foundation soils, embankment shear strength parameters, and design slope stability analyses. The weak, fine-grained foundation soil experienced less drainage, and thus less consolidation and strength gain, than expected via the PVDs because of an overestimate of the design horizontal coefficient of consolidation. As a result, the inverse analyses show the failure was caused by lower than expected shear strength of the foundation soils and an overestimate of the compacted embankment shear strength. The compacted embankment fill strength was characterized using an undrained shear strength, i.e., cohesion, without a tension crack, which inflated the calculated factor of safety. Recommendations to estimate embankment shear strength parameters, depth of embankment tension crack, bearing capacity factor of safety for comparison with limit equilibrium values, and horizontal consolidation properties for PVD design for future embankment projects are presented.

The continuous development of chemical additives facilitated the successful application of the earth pressure balanced (EPB) method in numerous tunnel-excavation projects. One major issue for every successful EPB excavation project is the environmentally sound deposition of a large volume of chemically treated soil of soft consistency and with sometimes viscous properties. In particular, the groundwater must not be polluted with substantial amounts of organic compounds. Here the authors present a laboratory study that shows that the percentage of chemicals leaching out of conditioned soil material can vary between 20 and 90% (by weight), depending on the predominant soil type (clay) and the chemical used. The authors found that lime treatment, which is applied to increase the workability of conditioned soil for deposition, also ensured a low chemical-mass output in the seepage water. A comparison is shown of results obtained in a batch leaching test for a solid/liquid ratio of $1:10$ and in a percolation test with a consolidated soil sample with subsequent analysis of the collected eluate with respect to dissolved organic carbon (DOC). In general the authors observed that for the more realistic conditions of the percolation test, the measured maximum concentrations of DOC in the eluate can be higher compared to the concentrations found in the batch leaching tests. The total percentage of washed-out chemicals however was lower in the percolation test, indicating less leachability of the chemicals under those conditions.

Mechanically stabilized earth-retaining walls (MSEWs) have been widely used in construction and transportation projects. Most codes and guidelines require that the soil used as backfill for MSEWs have fines contents lower than 15% to ensure adequate reinforcement pullout capacity, drainage, and soil stiffness. To use higher fines contents, an understanding of the factors influencing the pullout resistance of the reinforcing elements is crucial for an appropriate design. In this paper, a series of laboratory pullout tests has been conducted to investigate the potential of tapered steel straps with a wedge-shaped tail to enhance the pullout resistance in silty sands with up to 60% nonplastic fines. The pullout capacity was strongly influenced by the dilation angle, which was critically dependent on the silt content and the stress level. For all the tested silty sands, the pullout resistance of the tapered straps was higher than the resistance of rough flat straps, and this enhancement in the resistance increased slightly with the rise of the fines content.

Paul-Mohr-Coulomb (PMC) failure criterion provides enhanced representations of pyramidal failure surfaces, with recognizable material parameters, by considering all three principal stresses. PMC exhibits traits of more complex failure criteria through piecewise linear approximations to a curve failure surface, in changing both mean stress and Lode angle. A new least-squares fitting approach is developed to determine three PMC material parameters: two friction angles, one for compression and one for extension, and the theoretical isotropic tensile strength (the vertex of the pyramid). Experimental data from axisymmetric compression and extension tests are used to construct a six-sided pyramidal failure surface, revealing that the friction angle in extension is larger than the friction angle in compression for isotropic geomaterials, a manifestation of the intermediate stress effect. To enhance the description of the failure surface, multiaxial test data are added, and six parameters are determined by fitting two planes independently with four friction angles and two different vertices. The six-parameter PMC model is able to approximately capture the nonlinear nature of the failure surface for both rock and soil.

Understanding the mechanical properties of shaly sandstone is of great importance in reservoir geomechanics. Because of the lack of core data, measurements based on acoustic wave velocities are often conducted. In such measurements, the mechanical properties such as uniaxial compressive strength (UCS) and Young’s modulus ($E$) are related to the porosity of sandstone and shaly sandstone. However, in many instances, the porosity fails to directly correlate to mechanical properties especially when dispersed clays are present. Therefore, in this study, a set of uniaxial compressive tests on a number of different shaly sandstones was performed. The X-ray computed tomography was conducted on the representative samples from each type of sandstone to assess the porosity, rock composition, and texture. Independent compositional analysis was also carried out on all representative samples to link the samples’ composition to their microstructure. Interestingly, it was found that the mechanical properties of these shaly sandstones are well-correlated to the combined clay volume and effective porosity in a multivariate analysis. It was concluded that the univariate analysis solely performed on porosity cannot always correlate well to the mechanical properties of sandstone containing clays. Therefore, a combination of clay volume and effective porosity is more accurately correlated to mechanical properties.