Journal of Geotechnical and Geoenvironmental Engineering

The development of a numerical model, CST1, for coupled large strain consolidation and solute transport in saturated porous media is presented. The consolidation algorithm is one-dimensional and includes the capabilities of a previous code, CS2, with the addition of time-dependent loading, unload/reload effects, and an externally-applied hydraulic gradient. The solute transport algorithm is two-dimensional and accounts for advection, longitudinal and transverse dispersion, first-order decay reactions, and linear equilibrium sorption. Solute transport is consistent with temporal and spatial variations of porosity and seepage velocity in the consolidating layer. The key to the transport model is the definition of two Lagrangian fields of elements that follow the motions of fluid and solid phases separately. This reduces numerical dispersion and simplifies transport calculations to that of dispersion mass flow between contiguous fluid elements. The effect of relative numerical resolution of fluid and solid elements on the accuracy of sorption/desorption is also discussed. This paper presents the theoretical and numerical development of the CST1 model. A companion paper presents verification checks of CST1 and the results of simulations that illustrate the significance of consolidation-induced solute transport for some interesting numeric examples.

The results of numerical simulations for coupled large strain consolidation and solute transport, obtained using the CST1 model, are presented. CST1 accounts for advection, longitudinal and transverse dispersion, first-order decay reactions, and linear equilibrium sorption. Verification checks of CST1 show excellent agreement with analytical solutions for one-dimensional (1D) transport in rigid porous media, including various Peclet numbers and concentration boundary conditions. Similarly excellent agreement is observed for two-dimensional advection-dispersion transport in rigid media and 1D advection-dispersion transport in compressible media undergoing large strain consolidation. CST1 is then used to investigate consolidation-induced solute transport for a single composite liner system and a confined disposal facility for dredged contaminated sediments. In both cases, solute transport was found to be strongly affected by consolidation-induced advection both during and after the consolidation period. Consolidation has a lasting effect on solute migration because transient advective flows change the distribution of solute mass, which then becomes the initial condition for subsequent transport processes.

This paper presents the results of 150 finite-element simulations conducted to define the effects of excavation geometry, i.e., length, width, and depth of excavation, wall system stiffness, and factor of safety against basal heave on the three-dimensional ground movements caused by excavation through clays. The results of the analyses are represented by the plane strain ratio (PSR), defined as the maximum movement in the center of an excavation wall computed by three-dimensional analyses normalized by that computed by a plane strain simulation. A simple equation for PSR is presented based on excavation geometry, wall system stiffness, and factor of safety against basal heave. This PSR equation reasonably represents trends in results of the 150 simulations as well as those simulations reported in literature. When the excavated length normalized by the excavated depth of an excavation wall is greater than 6, results of plane strain simulations yield the same displacements in the center of that wall as those computed by a three-dimensional simulation.

This paper presents an evaluation of the soil-geogrid interaction, conducted to quantify the contributions of passive and interface shear mechanisms to the overall pullout resistance of geogrids. An experimental testing program was conducted in this investigation using both large-scale and newly developed individual-rib pullout devices. The large-scale pullout tests were conducted using uneasily coated geogrid specimens with and without transverse ribs. On the other hand, the individual-rib pullout tests were conducted using individual longitudinal and transverse ribs. A stress transfer model was implemented to predict the results of large-scale pullout tests using the parameters obtained from the individual-rib pullout tests. Good agreement was obtained between the results of large-scale pullout tests and the predictions obtained using parameters collected from individual-rib tests. For the dense mesh geogrids used in this investigation, the development of passive mechanisms in front of geogrid transverse ribs was found to influence significantly the interface shear mechanisms that develop along longitudinal ribs.

Of all landslides, subaerial flowslides are the most dangerous. According to the literature, flowslides in saturated granular deposits are caused by static liquefaction induced by loss of stability. However, the catastrophic flowslides that occurred on steep slopes in Campania (southern Italy) in 1998, 1999, and 2005 were triggered by rainwater infiltration into shallow deposits of pyroclastic soils, which were initially unsaturated. Starting from the experience collected during the investigations of some of these landslides, the writers consider flowslide evolution as a result of a mechanical chain process characterized by saturation increase due to rainwater infiltration, mechanical degradation, volumetric collapse, and static liquefaction, culminating in complete fluidization of the soil. In the paper this chain process is investigated through wetting tests both in suction controlled triaxial apparatuses and in a well-equipped small scale slope model.

The paper presents the results of a series of field tests performed to study the causes of friction fatigue experienced by displacement piles. Four instrumented model piles were installed at a dense sand test-bed site. The test series was designed to impose different levels of cyclic loading during pile installation. Static and cyclic load tests were subsequently performed to study the differences in the axial capacities developed for ostensibly monotonic and cyclic installations. The test results indicated that the mobilized horizontal effective stress regime that controls pile side friction primarily depends on the in situ sand state, as reflected by the cone penetration test (CPT) $q_c$ resistance. A zone of highly stressed sand that produced a concentration of high shear resistance was mobilized in the vicinity of the pile base. The horizontal effective stress that acted on the pile shaft reduced in response to cyclic loading, with the largest reductions occurring for high-intensity cyclic loading or when the pile had experienced only a few load cycles during installation. Although cyclic loading caused a reduction in the horizontal effective stress that acted on the pile shaft, the elevated stress built up in the vicinity of the pile base during installation remained higher than that remote from the base. The elevated stress in the vicinity of the pile base only dissipated after cyclic tension loading had been applied.

This paper addresses the small-strain undrained shear behavior of clays based on an extensive testing program conducted on normally consolidated resedimented Boston blue clay (RBBC). The uniformity and reproducibility in the behavior of this soil permit a detailed study of the role of consolidation stress level, preshear consolidation stress path, preshear lateral stress ratio $\left(K\right)$ , rate of shear, and duration of laboratory aging on the small-strain $({\epsilon }_a\le 0.1\%)$ nonlinearity in undrained triaxial compression. The small-strain behavior of normally consolidated (NC) RBBC is measured using a LVDT-based device that is capable of resolving on-specimen axial strains smaller than 0.0001% strain over a 10% strain range, thus allowing full description of the small-strain nonlinearity. The testing program relies on a multistage testing approach developed to minimize specimen variability and reduce testing time. The small-strain nonlinearity of NC RBBC is found to be virtually independent of void ratio or stress level, all other testing conditions being equal. $K_0$ -consolidated RBBC presents the smallest linear region and the most rapid stiffness degradation (decrease in Young’s modulus). Increases in $K$ , strain rate and aging all cause a reduction in the small-strain nonlinearity of the soil.

A simple index, the degree of sample disturbance, is proposed in this study to quantitatively evaluate the quality of sensitive clay samples based on the concept of void index proposed by Burland in 1990. The degree of sample disturbance is defined as the ratio of the difference between the in situ void index and the void index of the undisturbed sample tested in the laboratory to the difference between the in situ void index and the void index of the completely remolded clay. All these indices are determined at the same effective overburden stress. Theoretically, the degree of sample disturbance varies from 0% (perfect undisturbed sample) to 100% (completely remolded sample). The proposed index is used in this study to evaluate the sensitive Ariake clay in Japan. Oedometer tests on undisturbed samples of natural Ariake clay obtained from the field using the current sampling practice in Japan show the degree of sample disturbance ranging from approximately 5 to 38%. The clay sample with an in situ void index closer to the intrinsic compression line has a lower degree of sample disturbance. In addition, a series of consolidation and unconfined compression tests were conducted on artificially disturbed samples in the laboratory to demonstrate the change of consolidation yield stress, unconfined compressive strength, and compression index with the degree of sample disturbance. A simple method is proposed in this paper to correct the mechanical parameters measured in a laboratory setting considering the degree of sample disturbance.

Laterally spreading nonliquefied crusts can exert large loads on pile foundations causing major damage to structures. While monotonic load tests of pile caps indicate that full passive resistance may be mobilized by displacements on the order of 1–7% of the pile cap height, dynamic centrifuge model tests show that much larger relative displacements may be required to mobilize the full passive load from a laterally spreading crust onto a pile group. The centrifuge models contained six-pile groups embedded in a gently sloping soil profile with a nonliquefied crust over liquefiable loose sand over dense sand. The nonliquefied crust layer spread downslope on top of the liquefied sand layer, and failed in the passive mode against the pile foundations. The dynamic trace of lateral load versus relative displacement between the “free-field” crust and pile cap is nonlinear and hysteretic, and depends on the cyclic mobility of the underlying liquefiable sand, ground motion characteristics, and cyclic degradation and cracking of the nonliquefied crust. Analytical models are derived to explain a mechanism by which liquefaction of the underlying sand layer causes the soil-to-pile-cap interaction stresses to be distributed through a larger zone of influence in the crust, thereby contributing to the softer load transfer behavior. The analytical models distinguish between structural loading and lateral spreading conditions. Load transfer relations obtained from the two analytical models reasonably envelope the responses observed in the centrifuge tests.

The interaction between soil particles and pore water makes the behavior of saturated and unsaturated soil complex. In this note, upward seepage flow through a granular material was idealized using a one-column particle model. The motion of the individual particles was numerically simulated using the discrete-element method taking the interaction with the fluid into account. The fluid behavior was simulated by the Navier-Stokes equation using the semi-implicit method for pressure-linked equation. This approach has already been applied in powder engineering applications. However, there are very few studies that have used this approach in geotechnical engineering. This note first describes the qualitative and quantitative validation of this method for hydraulic gradients below the critical one by comparing the results with an analytical solution. Then, the ability of the method to simulate the macroscale behavior due to the interaction between particles and pore water at hydraulic gradients exceeding the critical hydraulic gradient is discussed.

The Cooper marl in Charleston, S.C., a deep layer of clayey soils approximately 5– $21\mathrm{m}$ below the ground surface, is generally recognized as nonliquefiable material. Data from field cone penetration tests and laboratory tests of samples taken from the Cooper marl are used to investigate the adequacy of index properties-based criteria for assessing liquefaction susceptibility of clayey soils. In particular, the criterion based on soil behavior type index $\left(I_c\right)$ and that based on Atterberg limits are examined. The results show that the Atterberg limits-based criterion adequately reflected the characteristics of the marl, whereas the $I_c$ -based criterion erroneously identified the marl as being liquefiable. A possible reason for the deficiency of $I_c$ and a modification to overcome this deficiency are presented.

The Berea sandstone used in this study is transversely isotropic with respect to elastic response, with $P$ -wave velocities of $\mathrm{2,160}\mathrm{m}∕\mathrm{s}$ normal to bedding and $\mathrm{2,290}\mathrm{m}∕\mathrm{s}$ parallel to bedding, a variation of only 6%. Triaxial compression and extension tests involving failure by loading and unloading were performed along the two directions of symmetry. With axial stress applied parallel to bedding, the internal friction angle was approximately 55° for compression and extension, indicating no intermediate stress effect for the linear Mohr-Coulomb criterion. However, for axial stress normal to bedding, the friction angle in compression was 50°, whereas in extension it was 44°. This anomalous behavior was attributed to strength anisotropy of the sandstone.