Journal of Geotechnical and Geoenvironmental Engineering

Railway ballast comprises unbounded discrete grains that are often used to form a load-bearing platform for tracks. Ballast degradation as trains pass over the tracks and infiltration of external fines including slurried (pumped) fine subgrade soils are two of the main reasons for ballast fouling. Fouling causes tracks to settle and also reduces the load-bearing capacity, which is associated with a reduction in internal friction and increased lateral spreading of the ballast layer. This paper presents a study of mobilized friction angle, volumetric behavior, and associated evolutions of contact and fabric anisotropy of fouled ballast subjected to monotonic triaxial loading using a series of large-scale triaxial tests and discrete element modeling. Monotonically loaded and drained triaxial tests were carried out on ballast with levels of clay fouling that varied from 10 to 50% void contamination index (VCI) subjected to three confining pressures of 10, 30, and 60 kPa. The results showed that an increase in the level of fouling decreased the mobilized friction angle and increased the ballast dilation. The discrete element method (DEM) was used to study the mobilized friction angle and fabric anisotropy of fresh and fouled ballast by simulating actual large-scale triaxial tests. Irregular shaped grains of ballast were simulated by clumping bonded circular balls with appropriate sizes and positions together. Ballast fouling was approximately simulated in DEM by adding 1-mm particles into the pore spaces of the fresh ballast. The predicted mobilized friction angles and volumetric changes obtained from the DEM simulations agreed well with those measured in the laboratory, indicating that the peak friction angle of fouled ballast and dilation decreased as the degree of fouling increased. The DEM simulations provided an insight into the distribution of contact force chains, contact orientations, and evolution of fabric anisotropy of fresh and fouled ballast that could not be captured in the laboratory. These observations are important for a better understanding of the load-deformation behavior of fouled ballast from the perspective of micromechanics.

Sulfate heaving has had a major impact on civil engineering infrastructure for the past few decades. Past researchers have attributed the provenance of this heaving mechanism to the chemical reactions between soluble sulfates and calcium-based stabilizers, but despite the increase in studies related to sulfate heaving, very little effort has been made to address the variability of sulfate concentrations in the field. The natural deposition of sulfate pockets causes inevitable problems to chemical stabilization techniques. Because of the high variability of sulfate concentrations present in nearby locations, it is possible that engineers may mischaracterize the sulfate concentrations or not able to design appropriate stabilization measures in the field. In this study geostatistical analysis was used to quantify and map sulfate concentrations along a mega water pipeline project located in north Texas. Various soil samples, originating from six different geological formations along the pipeline alignment, were collected. Laboratory tests were performed on these soil samples to determine soluble sulfate concentrations using the modified University of Texas at Arlington (UTA) approach. The inherent variability present in sulfate concentrations was determined and modeled using variogram analysis. The variability models, along with the kriging algorithm, were used to determine the distribution of the sulfate concentrations at unsampled locations along the pipeline alignment. This analysis provided the distribution of soluble sulfate concentrations along the six geological formations and indicated the areas of having low-risk to high risk zones when treated with calcium-based stabilizers. This research highlights the adoptability of geostatistics in modeling and mapping the high variable soluble sulfate concentrations present in native clayey soils.

This paper presents a new simplified liquefaction prediction and assessment method that is capable of considering the influence of the waveforms and durations of earthquakes. The concept of an effective number of waves was introduced to represent the characteristics of the irregularity of waveforms and the different number of load cycles of irregular seismic motions, as well as their influence on the occurrence of liquefaction. A comprehensive set of laboratory liquefaction tests and statistical analyses were performed to study and validate this concept. The validity of the proposed method, which adopts a wave correction coefficient based on the proposed concept, was verified using the case histories of five past major earthquakes, demonstrating that the predictive capability of the new method was improved compared with that of the conventional method for the cases of liquefaction and no liquefaction. A unique feature of the new simplified method is its universality, allowing it to be applied to various types of liquefaction charts, facilitating more-rational liquefaction prediction and assessment worldwide.

Based on an extensive in situ shear-wave velocity ($V_s$) database, this study aims to not only examine the stress-dependent characteristics of in situ $V_s$ from a generalized $V_s$-stress model [$V_s=\alpha \times {({\sigma }_c^{\prime })}^{\beta }$], but also provide valuable reference information on site-specific stress-dependency of $V_s$ for global geotechnical sites. For in situ $V_s$, the site-specific material constants ($\alpha$ and $\beta$) at each site had wider ranges when compared to typical values reported from laboratory tests. This is mainly attributed to different test conditions and inherent in situ site characteristics. The in situ-based site-specific material constants in this study are compared with laboratory-based material constants suggested in previous studies. Evidently, each $\alpha$–$\beta$ trend varies on laboratory tests and in situ tests and a unified relationship is appropriate only when $\beta$ is less than 0.5. Thus, separate relationships between material constants are proposed for laboratory-based and in situ-based measurements. The results imply that the new models should be developed to apply in situ data.

This paper uses the San Luis Dam upstream slide to evaluate the pore-water pressures at failure and progression of the phreatic surface through the fine-grained core for drawdown stability analyses. The hydraulic conductivity and compressibility parameters of saturated and unsaturated soils are calibrated using the reservoir hydrograph and 13 piezometers in order to evaluate the pore-water pressures at failure. The analyses show unsaturated and transient seepage analyses can be used to estimate pore-water pressures during drawdown for various stability analyses and evaluate the progression of the phreatic surface through the fine-grained core. The transient results also indicate the van Genuchten parameter $\alpha$ significantly influences unsaturated soil response during drawdown. Different meshing techniques can produce consistent moisture content profiles in two different seepage software packages, but in situ measurement of moisture content and suction pressure is recommended to develop an unsaturated and transient seepage model.

For concrete dams, cohesive strength often constitutes a significant share of the overall shear strength of partially bonded concrete-rock interfaces. However, cohesive strength is also associated with great uncertainties that may have a significant impact on the assessed stability of the analyzed structure. In this paper, the merits of including cohesion are evaluated using a probability-based approach to analyze the sliding stability of an existing concrete gravity dam. The shear strength properties of the interface are inferred from a limited number of site-specific tests and previous knowledge from similar structures using Bayesian updating. The study shows that the potential gain from cohesive strength is strongly related to the involved uncertainties and identifies the bonding percentage and basic friction angle as the most influential parameters. The importance of testing, both with regard to the specific project and for future projects, is also highlighted.

This paper describes numerical analyses of laboratory thick-walled cylinder (TWC) tests used to study the stability of vertical wellbores in hard clays. The TWC experiments were designed to simulate the drilling process by reducing the internal cavity pressure although maintaining the external radial stress and measuring internal changes in volume within the model wellbore. Radial deformations of the cavity (borehole squeezing) attributable to plastic deformations of the clay represent a critical condition for the design of wells drilled through ductile formations (i.e., very hard soils and poorly lithified rocks). The current analyses simulate coupled flow and deformation in the TWC soil specimens using a finite element (FE) program and the MIT-E3 model to represent the effective stress-strain-strength properties of the reference clay, resedimented Boston blue clay (RBBC). The model is freshly calibrated using results of laboratory triaxial element tests performed at the same stress levels as the TWC experiments (${\sigma }_{\mathrm{vc}}^{\prime }=1–10\mathrm{MPa}$). The numerical simulations are compared with data from experiments performed at different levels of confining stress, rates of depressurization, and wall thickness. The numerical model provides good estimates of the instability measured in the TWC tests and provides an accurate estimate of the undrained shear strength controlling cavity deformations. These validations provide confidence in the application of the model for estimating behavior of prototype (i.e., vertical and deviated) wellbores in similar hard clay formations.

In bender-element (BE) testing, shear-wave velocities are measured using two main methods: time-domain analysis of vibration data from pulse excitation and frequency-domain analysis of vibration from sine-sweep excitation. The frequency-domain analysis can be performed automatically. However, its results show high variability because they are affected by different variables such as the frequency content of the excitation, the resonant frequency of the BE system, and the resolution of the frequency-domain analysis. The reliability of the frequency-domain method is studied in this paper using simplified mathematical models and experimental results from BEs, miniature accelerometers, and resonant-column (RC) tests. The mathematical models are used to understand the effects of different variables in the frequency-domain method. Both laboratory and numerical results show that amplification peaks in the transfer function of the BE system can significantly affect the estimation of the shear-wave velocity. A modified frequency-domain analysis is proposed to enhance the reliability of the method using a high-frequency narrowband excitation outside the resonant peaks of the BE system. Results from the modified frequency-domain analysis tests show less than 10% error in the measurement of the shear-wave velocity with respect to the RC results.

The constitutive behavior of sheared sand is highly influenced by particle morphology, gradation, mineralogy, specimen density, loading condition, stress path, and boundary conditions. The current literature lacks a three-dimensional (3D) systematic experimental study that investigates the influence of particle morphology, confining pressure, and specimen density on the failure mode of sheared sand. In this paper, surface texture, roundness, and sphericity of three uniform sands and glass beads with similar grain size were quantified by using 3D images of particles. In situ nondestructive 3D synchrotron microcomputed tomography (SMT) was used to monitor the deformation of medium-dense and very dense dry sand specimens that were tested under axisymmetric triaxial loading condition at 15 and 400 kPa confining pressures. The particles were identified and tracked in 3D as shearing progressed within the specimens, and maps of incremental particle translation and rotation were developed and used to uncover the relationship between particle morphology, specimen density, and confining pressure on the deformation and failure mode of sheared sand. This paper discusses the relationship between the failure mode and particle morphology, specimen density, and confining pressure.

This paper reports results from a series of model tests undertaken to provide insight into the behavior of a stiffened caisson anchor during installation and monotonic pullout in lightly overconsolidated calcareous silt. The tests were carried out in a beam centrifuge, varying the installation process (jacked in or jacked in followed by suction-assisted installation), and the consolidation period before anchor pullout. The mudline load inclination was also varied to encompass various mooring configurations. The centrifuge data were used to calibrate analytical installation and vertical pullout capacity models, based on conventional bearing and frictional resistance factors but with strain rate-dependent and strain-softened undrained shear strength for the soil. Piezocone-based direct design approaches were also proposed, deriving end-bearing and frictional resistance from cone tip resistance and sleeve friction, respectively. Holding capacity of suction caisson under inclined loading was presented as failure envelopes expressed in terms of dimensionless vertical and horizontal components of caisson net resistance, which agreed well with a plastic limit analysis-based envelope developed for suction caissons in clay. The regain of anchor capacity was found to be in good agreement with predictions based on the cavity expansion framework.

Experiments conducted to quantify potential leakage from holes in two geomembranes [1-mm-thick linear low-density polyethylene (LLDPE) and 2-mm-thick high-density polyethylene (HDPE)] placed on a silty-sand [underliner hydraulic conductivity ($k_{\mathrm{UL}}$), $1.1\times {10}^{-7}\le k_{\mathrm{UL}}\le 1.2\times {10}^{-5}\mathrm{m}/\mathrm{s}$], or pea gravel ($k_{\mathrm{UL}}=1\times {10}^{-2}\mathrm{m}/\mathrm{s}$) underliner (foundation/subgrade) and covered with saturated fine tailings at 65% initial solids content are discussed [tailings hydraulic conductivity ($k_T$), $2.9\times {10}^{-8}\le k_T\le 1.6\times {10}^{\mathrm{-6}}\mathrm{m}/\mathrm{s}$]. Tests results show that the overlying tailings, with a $k_T<k_{\mathrm{UL}}$, filled the geomembrane hole and had a larger effect on flow through the hole than the underliners examined. For the tested conditions, leakage through 10 and 20-mm-diameter holes were essentially the same whereas the leakage through a 1.5-mm-diameter hole was three orders of magnitude lower. Introduction of a $580\mathrm{g}/{\mathrm{m}}^2$ nonwoven needle-punched geotextile (GTX) layer between the geomembrane and tailings increased the flow by approximately 60%. A gap below geomembrane hole attributable to a stone on the underliner filled with tailings, which flowed through the hole. The evidence of fines migration through the hole, from the tailings to the underliner is discussed.

Geosynthetic reinforced soil (GRS) walls and abutments are increasingly used to support transportation infrastructure. A pressing question in their response is the amount of horizontal deflection expected under service loads. This paper presents an evaluation of six methods for predicting the lateral deformation of GRS walls and abutments, namely the FHWA, Geoservice, CTI, Jewell-Milligan, Wu, and Adams methods. Field and laboratory performances of 17 GRS walls and abutments are compared with the predicted results from the six methods. A statistical analysis is then used to evaluate the conservativeness, accuracy, and reliability of these methods in predicting the maximum lateral deformation of GRS walls. The Adams method is the most accurate method for predicting the maximum lateral deformation if the amount of vertical deformation is reasonably known. Among the Geoservice, Jewell-Milligan, and Wu methods, which have the ability to predict the lateral deformation of GRS walls at various elevations where reinforcements are located, the Wu method is the most accurate and reliable method for predicting the lateral deformation of GRS walls.