Journal of Geotechnical and Geoenvironmental Engineering

The effect of geosynthetic layers on moisture distribution and plastic deformation of paved and unpaved road sections is studied using numerical simulations. The geosynthetic layers consisted of, from top to bottom, a transport layer, a geonet, and a nonwoven geotextile (referred to as a geocomposite capillary barrier drain by previous researchers). Two geotextile types were modeled as the transport layer: woven fiberglass and nonwoven polypropylene. The numerical models were verified against published results obtained from a soil-geotextile column. Inclusion of the geosynthetic layers at the interface of the aggregate base course (ABC) and subgrade increased suction in the subgrade and decreased it in the ABC during a simulated rainfall event. The woven fiberglass geotextile led to higher suctions in the ABC compared with the nonwoven polypropylene geotextile. The geosynthetic layers decreased the plastic deformation in both paved and unpaved road sections through combined mechanistic and hydraulic actions. Increasing the thickness of the asphalt and ABC layers decreased the reinforcement effect of the geotextile while increasing its beneficial hydraulic effect in term of the suction level. In sections with a thinner asphalt layer, the woven fiberglass, functioning as a transport layer, decreased the plastic deformation of the profile by up to 20% compared with the profile with the nonwoven polypropylene geotextile. Increasing the thickness of the asphalt layer, however, reduced this difference to approximately 4%. In unpaved sections, the inclusion of the woven fiberglass layer decreased the plastic deformation by approximately 24% more than the profile with nonwoven polypropylene geotextrile, regardless of the aggregate base course thickness used in the analysis.

The paper investigates the influence of backfill soil, foundation soil, and horizontal joint vertical compressibility on the magnitude of vertical loads developed in steel-reinforced soil concrete panel retaining walls at the end of construction. Measurements of toe loads recorded from instrumented field walls are reviewed and demonstrate that vertical toe loads can be much larger than the self-weight of the facing. In extreme cases, these loads can result in panel-to-panel contact leading to concrete spalling at the front of the wall. Vertical loads in excess of panel self-weight have been ascribed to relative movement between the backfill soil and the panels that can develop panel-soil interface shear and downdrag loads at the connections between the panels and the steel-reinforcement elements. A two-dimensional finite-element model is developed to systematically investigate the influence of backfill soil, foundation soil, bearing pad stiffness, and panel-soil interaction on vertical loads in the panel facing. The results show that an appropriately selected number and type of compressible bearing pads can be effective in reducing vertical compression loads in these structures and at the same time ensure an acceptable vertical gap between concrete panels. The parametric analyses have been restricted to a single wall height (16.7 m) and embedment depth of 1.5 m, matching a well-documented field case. However, the observations reported in the paper are applicable to other similar structures. The general numerical approach can be used by engineers to optimize the design of the bearing pads for similar steel-reinforced soil wall structures using available commercial finite-element model packages together with simple constitutive models.

The neutral plane solution has been widely used to estimate downdrag settlements and drag loads mobilized in piles in consolidating soil profiles. Pile settlement is typically assumed equal to soil settlement at the neutral plane depth corresponding to effective stress conditions at the end of consolidation. This paper demonstrates that, in general, pile settlement is not equal to soil settlement at the neutral plane depth; rather, it is the relative velocity between the pile and soil that is zero at the neutral plane depth. A beam on nonlinear Winkler foundation (BNWF) solution, in which the shaft friction capacity is proportional to effective stress, is utilized to demonstrate that pile settlement is not equal to soil settlement at the neutral plane depth, because the neutral plane depth evolves as consolidation progresses. The BNWF solution also shows that pile settlement depends on drainage conditions, with more settlement occurring when consolidation occurs first near the top of the consolidating soil layer, and less settlement occurring when consolidation initiates at the bottom. A modified neutral plane solution that is amenable to hand calculation is formulated to account for the evolution of neutral plane depth on pile settlement. Finally, the proposed BNWF and modified neutral plane solutions are compared with measurements of downdrag settlement from a centrifuge test program. The proposed methods produced more accurate estimates of pile settlement than the traditional neutral plane solution.

To consider the benefit of geosynthetic reinforcement in a mechanistic-empirical pavement design method, the resilient modulus and permanent deformation behavior of a geosynthetic-reinforced unbound granular material (UGM) must be considered. Many researchers conducted repeated-load triaxial (RLT) tests to investigate the resilient and permanent deformation behavior of the geosynthetic-reinforced UGM. However, these tests are difficult to perform, and the results are often interpreted empirically. Thus, implementation of the research results is limited. In this study, an analytical model was developed to predict the resilient modulus and permanent deformation of the geosynthetic-reinforced UGM in RLT tests. The analytical model is compatible with the resilient modulus and permanent deformation models in the current mechanistic-empirical pavement design guide. Both planar and three-dimensional geosynthetics can be analyzed using this model. RLT test results from two published studies were selected to validate the proposed analytical model. In general, the analytical results confirmed and explained the typical test observations from previous studies that geosynthetic reinforcement is more effective in reducing the permanent deformation than increasing the resilient modulus of the UGM sample. A parametric analysis was conducted to investigate the effect of input parameters (i.e., material properties, sample dimensions, and stress level) on the analytical model. The limitations, assumption, and implementation of the model are also discussed.

Internal erosion involves selective loss of fine particles within the matrix of coarse soil particles under seepage flow, which affects the hydraulic and mechanical behavior of the soil. In this research, extensive laboratory internal erosion tests were conducted under complex stress states following three stress paths: isotropic, drained triaxial compression, and triaxial extension stress paths. These tests were designed to investigate the initiation and development of internal erosion and the effect of stress state on critical hydraulic gradients. The entire erosion process can be divided into four phases: stable, initiation, development, and failure. Accordingly, three critical gradients termed as initiation, skeleton-deformation, and failure hydraulic gradients, can be defined. These critical gradients correspond to the onset of erosion of the fine particles filling the large pores of the skeleton, the buckling of the strong force chains formed by the coarse particles, and the soil failure, respectively. The initiation gradient under compression stress conditions generally increases with the shear stress ratio first and then decreases when the stress conditions approach failure. The tests under isotropic stress conditions show the largest initiation and skeleton-deformation gradients at the same porosity.

In current design methods for reinforced soil walls, it has been tacitly assumed that reinforcement strength and reinforcement spacing play an equal role. This fundamental design assumption has led to the use of larger reinforcement spacing (0.3–1.0 m) in conjunction with stronger reinforcement to reduce construction time. Recent studies, however, have clearly indicated that the role of reinforcement spacing is much more significant than that of reinforcement strength. With closely spaced (reinforcement spacing $\le 0.3\hspace{0.17em}\text{m}$) reinforcement, the beneficial effects of geosynthetic inclusion is significantly enhanced, and the load-deformation behavior can be characterized as that of a composite material. A reinforced soil mass with closely spaced geosynthetic reinforcement is referred to as geosynthetic-reinforced soil (GRS). In this study, an analytical model is developed for predicting the ultimate load-carrying capacity and required reinforcement strength of a GRS mass. The model was developed based on a semiempirical equation that reflects the relative roles of reinforcement spacing and reinforcement strength in a GRS mass. Using measured data from field-scale experiments available to date, it is shown that the analytical model is capable of predicting the ultimate load-carrying capacity and required reinforcement strength of a GRS mass with good accuracy.

The load-displacement responses of laterally loaded, single bored piles in weathered granite soil are investigated using field model tests and numerical analyses. Field model tests were conducted to simulate the loading conditions of foundations for offshore structures subjected to large overturning moments with small lateral loads. The results of numerical analyses from existing methods were compared with experimental data to assess their suitability to predict lateral load capacities. These comparisons revealed the following reasons for underestimating the lateral load capacities of piles in weathered granite soil in practice: cohesion component of soil, friction between pile and soil, volume expansion characteristics of soil, and the underestimation of ultimate lateral soil resistances of $p\text{-}y$ curves. Parametric studies were used to determine the influence of the previously described factors on the lateral load capacities of piles and confirmed the importance of the appropriate estimation of these factors and their reflection in the design of engineering practices.

In recent years, the suction stress characteristic curve (SSCC) has been shown to represent a key constitutive relationship for defining effective stress in variably saturated soils. The SSCC can be deduced either from shear strength tests under variably saturated conditions or from soil water retention tests. This paper provides an alternative way to obtain the SSCC by conducting nonfailure $K_0$ consolidation tests. Multiple remolded specimens from two different granitic residual soils, a silty sand (SM) and a clayey sand (SC), were evaluated in suction-controlled triaxial tests under $K_0$ conditions. The results from these tests show that the stress paths for remolded specimens with different initial suction values differ during $K_0$ consolidation when plotted in terms of the net normal stress, but follow a single path when plotted in terms of the effective stress defined using the SSCC. This observation confirms the validity of the effective stress principle before failure. Further, the results of the shear strength tests under different matric suctions show that the failure envelope can be uniquely defined by the effective stress representation, a reconfirmation of the validity of the effective stress principle in describing the shear strength behavior of soils. The results obtained from the $K_0$ consolidation tests also reveal that $K_0$ (or the horizontal to vertical stress ratio) defined by effective stress is invariant to matric suction, whereas $K_0$ defined by the total stress varies greatly with matric suction. These results provide a further confirmation of the validity of the effective stress principle. Because the SSCCs deduced independently in this study from shear strength tests, soil water retention tests, and $K_0$ consolidation tests compare favorably with each other, the SSCC-based effective stress can be used to describe the consolidation and shear strength behavior of unsaturated soils.

Small-strain elastic wave propagation is a constant-fabric phenomenon ideally suited to monitor processes in soils. However, the determination of very small changes in travel time limits our ability to resolve changes in soil stiffness caused by internal processes or changes in boundary conditions. The first-arrival reflects the fastest path between the source and receiver of the propagating wave field; later arrivals in the coda correspond to longer paths after multiple boundary reflections and internal scattering. Therefore, time shifts between the codas of two consecutive signals are longer and easier to detect than between the signals’ first arrivals. Slight changes in coda waves can be determined by cross-correlating time windows, time-stretched signals, or frequency-stretched spectra. Basic coda analysis assumes a homogeneous velocity change throughout the medium, propagation modes (P, S) that are equally affected by the process and the preservation of $V_{\mathrm{P}}/V_{\mathrm{S}}$ ratio during the process. The resolving power of coda wave interferometry is explored in an experimental study conducted with quartzitic sand subjected to loading, creep, and unloading stages. The results reveal that coda wave analysis can be used to detect changes in wave velocity on the order of $\mathrm{\Delta }V/V<0.1\%$ (this corresponds to a stress change smaller than $\mathrm{\Delta }\mathrm{\sigma }\prime /\mathrm{\sigma }\prime \approx 1\%$ in uncemented soils). Such a high velocity resolution permits the study of creep, aging, and diagenetic processes even in relatively short duration tests.

The importance of particle rolling resistance to the mechanical behavior of granular materials is well recognized and has been a topic subject to intensive discrete element method (DEM) investigation over the last two decades. However, little effort has been made to explore the energy input and dissipation behavior under the influence of varying degrees of interparticle rolling resistance, especially in relation to the development of shear band. This paper aims to eliminate this deficiency through a comprehensive two-dimensional DEM study on the relationship between the particle-scale energy dissipation and shear band development. Novel insights into the energy allocation at the small- and large-strain stages, and the development of localized bands of sliding and rolling dissipations, as well as the anisotropy of accumulated sliding and rolling dissipations within the shear band are presented for the first time.

A three-dimensional (3D) kinematically admissible rotational failure mechanism is extended from toe failure to include face failure and base failure for homogeneous slopes in both purely cohesive and frictional/cohesive soils. In the strict framework of limit analysis, an analytical approach is derived to obtain the upper bounds on slope stability and the corresponding type of the critical failure mechanism. Compared with the available results from the finite-element limit-analysis method, the 3D rotational failure mechanisms give the best estimate on the upper bound. A set of stability charts is presented in a wide range of parameters for 3D homogeneous slopes under both static and pseudostatic seismic loading conditions. This set is useful in assessing the safety of the 3D homogeneous slopes in practical applications.

This paper presents an interesting and unique case study of a composite compacted site where the upper section of dynamically compacted material achieved in the first stage of compaction was subsequently removed, reinstated, and recompacted in lifts in the second stage using conventional roller compaction. Dynamic compaction was employed initially in this area because of the need to densify deep fill materials. Although a number of mechanical methods are already available for assessing deep compaction, it has been particularly rare to find a cost-effective method that can be applied to a deep and extensive compacted site. Noninvasive techniques based on measurement of the horizontal-to-vertical spectral ratio (HVSR) of ambient vibrations (microtremors) are proposed in this paper to assist in a pilot appraisal of this area, which occupies a part of a deep and laterally extensive compacted site. First, the key features of the measured HVSR curves were interpreted to give a preliminary insight into the quality of compaction achieved. Second, a trial-and-error forward modeling procedure fitting the theoretical HVSR curve to the measured HVSR curve then allowed the shear-wave velocity ($V_s$) profile conveying the compaction quality of the compacted ground to be inferred. An initial calibration was carried out to match the inferred $V_s$ profile in a relative sense against the cone-penetration-test (CPT) data at a test location. Verification was further made by comparing the inferred $V_s$ profiles against independent CPT, dialatometer-test (DMT), and dry-density data at two other locations. The HVSR technique was then applied to appraise the consistency and quality of compaction at grid points not covered by the localized mechanical and other independent tests.

Finite-element limit analysis is used to identify the critical internal skirt spacing for the undrained failure of shallow skirted foundations under conditions of plane strain based on the criterion that the confined soil plug should ideally displace as a rigid block, such that optimal bearing capacity is realized. General loading (vertical, horizontal, and moment) is considered for foundations with skirt embedments ranging from 5 to 50% of the foundation breadth in soil having either uniform strength or strength proportional to depth. The results explicitly identify the number of internal skirts required to ensure soil plug rigidity under arbitrary combinations of horizontal and moment loading expressed as a function of the normalized skirt embedment and the maximum expected level of vertical loading as a fraction of the ultimate vertical bearing capacity. It is shown that fewer internal skirts are required with increasing normalized foundation embedment, but more internal skirts are required with increasing soil strength heterogeneity. The results also indicate the potential for a significant reduction in capacity if insufficient skirts are provided, such that plastic deformation is permitted to occur within the soil plug.

This paper examines the collapse of an important port structure and analyzes the causes of failure. In particular, a length of newly constructed anchored sheet pile wall exhibited very large displacements at the crest, which clearly indicated that a failure condition had been reached within the retained ground. On removal of the failing soil and inspection of the structure, all the barrel/wedge anchors were found to be devoid of the strands they should have been holding. Observations and testing on such components led to the conclusion that sliding of the wedge system within the barrel had actually been inhibited by two main causes: on one hand, corrosion of the metal surfaces, which is in part predictable in a marine environment and, on the other, the presence of cement mortar which blocked the wedge components in place.

An analysis is presented of a database of 67 tests on 21 clays and silts of undrained shear stress-strain data of fine-grained soils. Normalizations of secant $G$ in terms of initial mean effective stress $p\prime$ (i.e., $G/p\prime$ versus $\text{log}\hspace{0.17em}\gamma$) or undrained shear strength $c_u$ (i.e., $G/c_u$ versus $\text{log}\hspace{0.17em}\gamma$) are shown to be much less successful in reducing the scatter between different clays than the approach that uses the maximum shear modulus, $G_{\text{max}}$, a technique still not universally adopted by geotechnical researchers and constitutive modelers. Analysis of semiempirical expressions for $G_{\text{max}}$ is presented and a simple expression that uses only a void-ratio function and a confining-stress function is proposed. This is shown to be superior to a Hardin-style equation, and the void ratio function is demonstrated as an alternative to an overconsolidation ratio (OCR) function. To derive correlations that offer reliable estimates of secant stiffness at any required magnitude of working strain, secant shear modulus $G$ is normalized with respect to its small-strain value $G_{\text{max}}$, and shear strain $\gamma$ is normalized with respect to a reference strain ${\gamma }_{\text{ref}}$ at which this stiffness has halved. The data are corrected to two standard strain rates to reduce the discrepancy between data obtained from static and cyclic testing. The reference strain ${\gamma }_{\text{ref}}$ is approximated as a function of the plasticity index. A unique normalized shear modulus reduction curve in the shape of a modified hyperbola is fitted to all the available data up to shear strains of the order of 1%. As a result, good estimates can be made of the modulus reduction $G/G_{\text{max}}\pm 30\%$ across all strain levels in approximately 90% of the cases studied. New design charts are proposed to update the commonly used design curves.

In this paper, experiments and discrete element method (DEM) simulations were conducted in parallel to examine the underlying mechanisms of aging effects on the stiffness changes of sand with inherent fabric anisotropy. The true triaxial apparatus, equipped with a bender-element system, was used to monitor the evolution of the small strain shear moduli of sand samples, i.e., $G_{\mathit{xy}}$ (or ${\text{G}}_{\mathit{hh}}$), $G_{\mathit{yz}}$ (or ${\text{G}}_{\mathit{hv}}$), and $G_{\mathit{zx}}$ (or ${\text{G}}_{\mathit{vh}}$), during the aging process. DEM simulations where clumped particles were used to mimic fabric anisotropy replicated the experimental observations closely and provided insights from the micromechanical perspective. The inherent fabric anisotropy gave rise not only to the stiffness anisotropy, i.e., $G_{\mathit{xy}}>G_{\mathit{yz}}\approx G_{\mathit{zx}}$, but also to a higher aging rate for $G_{\mathit{xy}}$ than for $G_{\mathit{yz}}$ (or $G_{\mathit{zx}}$), i.e., $G_{\mathit{xy}}$ increased more than $G_{\mathit{yz}}$ (or $G_{\mathit{zx}}$) during aging, thereby increasing the stiffness anisotropy. As the particle aspect ratio or fabric anisotropy increased, the distribution of contact forces became more inhomogeneous and more large forces, in particular contact tangential forces, appeared in the $x\text{-}$ and $y\text{-directions}$ than in the $z\text{-}\text{direction}$. Larger tangential forces in the $x\text{-}$ and $y\text{-directions}$ can induce a higher sliding creep in the same direction during aging. As a result, a greater force redistribution took place in the $x\text{-}$ and $y\text{-directions}$ than in the $z\text{-}\text{direction}$ during the process of contact force homogenization induced by aging. This in turn created a greater stiffness gain, i.e., a higher aging rate for $G_{\mathit{xy}}$ than for $G_{\mathit{yz}}$ (or $G_{\mathit{zx}}$) because of a larger enhancement of weak forces in the $x\text{-}$ and $y\text{-}\text{directions}$ to strengthen the soil structure.

Three-dimensional (3D) slope stability analyses are not performed routinely because they involve increased effort compared with two-dimensional analyses, and because the latter yield more conservative results. However, in cases of distinct limitations of the width of the failure region (e.g., in the case of excavations), an assessment of safety using 3D stability analyses may be more appropriate. The kinematic approach of limit analysis for the assessment of stability of slopes is presented, based on a recently conceived rigid-rotation 3D mechanism modified to include below-toe failures, which are common for gentle slopes. The presence of pore-water pressure in the slope is included approximately using the scalar parameter $r_u$. The results are presented in terms of the critical height (or dimensionless coefficient $\gamma H/c$), and also in the form of charts and a closed-form approximation formula that allow for direct assessment of the safety factor for slopes with constrained width. Not surprisingly, the safety factor decreases with an increase in the pore-water pressure and with an increased width of the slope (increased failure zone).

This paper presents an analytical solution to determine seismic active earth pressure on a rigid frictionless retaining wall of $c\text{-}\varphi$ soil backfill with an infinite slope, considering horizontal and vertical seismic coefficients. This solution is a generalized explicit expression and was derived based on the Rankine earth pressure theory and the Mohr-Coulomb yield criterion. To verify the derived solution, a special case was analyzed, and its result is identical to that obtained by earlier researchers. By analyzing the distribution of the seismic active earth pressures along the wall depth, a tension crack zone behind the wall was identified, and the seismic active earth pressure coefficient considering the tension crack was obtained. This study also investigated the effects of wall friction, soil friction angle and dimensionless cohesion, backfill slope, and horizontal and vertical seismic coefficients on the seismic active earth pressure coefficients. Design charts for seismic active earth pressure coefficients were developed for the practical use of this solution.

A method for assessing stability of slurry trenches within an inclined ground surface is presented using the Coulomb-type force equilibrium method. The influence of ground surface inclination on trench stability and angle of critical failure surface is investigated via parametric studies on an example trench. As expected, the factor of safety decreases considerably with an increase in the ground surface inclination. It is shown that it is unconservative to neglect ground surface inclination while analyzing trench stability. According to the example, no significant error in the minimum factor of safety is introduced by using the failure surface inclination angle of $45°+\varphi \prime /2$ for trenches with an inclined ground surface. It is also found that differences between the angle of critical failure surface and $45°+\varphi \prime /2$ may exist; therefore, the angle of the critical failure surface needs to be determined instead of using $45°+\varphi \prime /2$ for the design of remedial measures.

This paper presents the results of a field investigation into pile aging in soft clay that was conducted over a period of 10 years. Static load tests were conducted after the excess pore pressure generated by the installation of 6-m-long, driven concrete piles were fully equalized. These tests allowed the time-capacity aging profile to be established. A normalized capacity-time trend established for the case history is seen to be consistent with the response observed from a wider database of pile tests in clay compiled from the literature. A simple reliability-based design example is provided to highlight the positive impact that pile aging could have for industrial practice.

A series of resonant column tests have been performed in the torsional mode of vibration to assess the effect of saturation, starting from the near dry state to the fully saturated state, on the damping ratio of sands corresponding to the threshold strain level. Tests were carried out on three different gradations of sand for various combinations of relative density and effective confining pressure. For fine sands, a certain optimum degree of saturation exists at which the damping ratio minimizes; it is known that a decrease in $S_r$ from a fully saturated state leads to a continuous increase in the matric suction. With an increase in the relative density, the optimum degree of saturation for fine sand increases marginally from 1.38 to 1.49%, but does not show any dependency on the effective confining pressure. In contrast, the minimum values of the damping ratio for medium and coarse sands are found to always correspond to the near dry state. The damping ratio decreases continuously with increases in relative density and effective confining pressure. The threshold strain level has been found to decrease continuously with increases in relative density and effective confining pressure.