Journal of Geotechnical and Geoenvironmental Engineering

The influence of the installation method on the soil flow pattern, resulting external radial total stress changes, and final external shaft friction after consolidation has been investigated for caissons in soft clay by means of centrifuge model tests, large deformation finite-element (FE) analysis, and a simple cavity expansion approach. Both the centrifuge measurements and the FE results show that more soil is forced into the caisson under suction than under jacking. However, the difference in the resulting external radial total stress changes or penetration-induced excess pore-water pressure is much less significant, since the expansion-induced excess pore pressure is smaller for thin-walled caissons than for driven piles. After subsequent consolidation, the influence of the installation method reduces further, and the final shaft friction ratios are close for the two installation methods. Based on the magnitude of heave ratios derived from the centrifuge measurements and the FE analysis, a simple form of cavity expansion approach can reasonably estimate external radial stress changes during installation and after consolidation, and final shaft friction ratios for the caissons. An approach for estimating the external shaft friction ratios for vertical pullout of sealed caissons is proposed.

Uncertainties in the geotechnical design variables and design equations have a significant impact on the safety of cantilever retaining walls. Traditionally, uncertainties in the geotechnical design are addressed by incorporating a conservative factor of safety in the analytical model. In this paper, a risk-based approach is adopted to assess the influence of the geotechnical variable and design equation uncertainties on the design of cantilever retaining walls in sand using the “partial factor of safety on shear strength” approach. A random model factor based on large-scale laboratory test data from the literature has been incorporated into the reliability analyses to quantify the uncertainty in the geotechnical calculation model. Analyses conducted using Monte Carlo simulation show that the same partial factor can have very different levels of risk depending on the degree of uncertainty of the mean value of the soil friction angle. Calibration studies show the partial factor necessary to achieve target probability values of 1 and 0.1%.

A rational analysis of extensible sheet reinforcement subjected to an oblique end force has been presented that properly accounts for complex soil-reinforcement interaction and involves stress-deformation relationship implicitly. The results can be used for internal design of geosynthetic reinforced soil walls against pullout failure and tension failure. The pullout force and the end displacement at pullout for an extensible reinforcement are found to be almost the same as those for an inextensible reinforcement if the ratio of the reinforcement stiffness to the axial pullout capacity $J^{*}$ is greater than 15. With decrease in $J^{*}$ below 15, the maximum strain increases, the pullout failure becomes irrelevant, the tension failure dominates and the maximum allowable oblique force decreases. A minimum stiffness of about 25 times the axial pullout capacity is required to avoid the tension failure before the pullout provided the failure strain is 0.1. The predicted results have been calibrated against the finite-element analysis of pullout tests and detailed back analyses of published test data on model reinforced walls constructed with a wide range of extensible materials. The present analysis gives better predictions of the critical height against the pullout and the tension failure in model reinforced soil walls constructed with extensible reinforcements as compared to that of Rankine’s method.

Current practices are reviewed for the identification, description, and classification of soils containing significant very coarse fractions. The review shows that there are serious shortcomings when these fractions are present. To upgrade current practices, improvements are needed in two areas: (1) the grain-size scale and nomenclature; and (2) the basic system for soil identification. First, an improved grain-size scale is proposed that clarifies the very coarse fractions and other nomenclature. Second, it is shown that current U.S. classification systems can not address the very coarse soils adequately, and they can not be adapted easily to do so. However, a relatively straightforward expansion can be made to the Burmister identification system to incorporate these soils. This Expanded Burmister Soil Identification System is flexible enough to describe and identify soils, ranging from very coarse to very fine, in a complete and consistent manner. Adoption of these two recommendations can improve practice and reduce current ambiguities in dealing with soils containing very coarse fractions.

Of the existing methods for the three-dimensional (3D) limit equilibrium analysis of slopes, none can simultaneously satisfy all six equilibrium equations. Except for Fellenius’ method that satisfies only one condition of moment equilibrium, all these methods could encounter numerical problems in their applications. Based on the global analysis procedure that considers the whole sliding body instead of individual columns as the loaded body, it is shown that the 3D limit equilibrium analysis of slopes simply reduces to the solution of a generalized eigenvalue problem in which the largest real eigenvalue is just the factor of safety (FOS). The proposed solution is rigorous and can accommodate any shape of slip surfaces. Under undrained conditions, the problem has a unique solution and the FOS has an explicit expression. In addition, through transforming the volume integrals over the sliding body into the boundary integrals, the proposed method does not need to partition the sliding body into columns.

A new, easy to implement, solution to the consolidation of multilayered soil based on the spectral method is presented. Combined vertical and radial drainage under instantaneous or single ramp loading is considered, ignoring well resistance. Flow in the vertical direction is based on the average hydraulic gradient at a particular depth which allows smear effects to be included. The excess pore-water pressure profile across all soil layers is described by a single expression calculated with common matrix operations. Average excess pore pressures within or across any number of layers are easily calculated from the single expression. The new model is verified against other solutions from the current literature indicating that the more general spectral method model can replace the separate solutions developed for specific problems.

Flexible-wall permeability tests and rigid-wall consolidation/permeability tests were performed to evaluate the hydraulic conductivity and compressibility of a model soil-bentonite (SB) backfill amended with granular activated carbon (GAC) or powdered activated carbon (PAC). The tests were performed as part of an assessment of enhanced SB backfill with improved attenuation capacity for greater longevity of barrier containment performance. Backfill specimens containing fine sand, 5.8% sodium bentonite, and GAC or PAC (0, 2, 5, and 10% by dry weight) were prepared to target slumps of $125\pm 12.5\mathrm{mm}$ . Hydraulic conductivity $\left(k\right)$ and compressibility of backfill test specimens were measured in consolidometers as a function of effective stress, ${\sigma }^{\prime }$ $(24⩽{\sigma }^{\prime }⩽\mathrm{1,532}\mathrm{kPa})$ , whereas flexible-wall $k$ was measured for backfill specimens consolidated to ${\sigma }^{\prime }=34.5\mathrm{kPa}$ . The results indicate that addition of GAC has little impact on the hydraulic and consolidation properties of the backfill, whereas addition of PAC causes a decrease in $k$ and consolidation coefficient $\left(c_v\right)$ and a slight increase in compression index $\left(C_c\right)$ . Differences in behavior between GAC-amended backfills and PAC-amended backfills are attributed primarily to differences in GAC and PAC particle size.

The paper presents a description of physical properties and a characterization of shear strength behavior of JSC-1A lunar regolith simulant. Specific gravity of solids, particle size analysis, and minimum and maximum index densities were measured according to ASTM standards. A series of axisymmetric triaxial experiments were conducted on JSC-1A specimens at two densities and a wide range of confining pressures. The stress-strain and volume change behavior of JSC-1A is presented and discussed. The results show that the peak friction and dilatancy angles are highly influenced by specimen density and confining pressure. Two models were developed to predict the peak friction and dilatancy angle, as a function of relative density and mean effective stress at the critical state. It was found that JSC-1A closely resembles the morphology and specific gravity of solids of some of Apollo regolith samples reported in the literature. Shear strength properties are presented within the framework of Mohr–Coulomb and friction-dilatancy theories. The effect of reduced moon gravity on strength parameters is also presented and discussed.

Scientific design of a railway track formation requires an understanding of the subgrade behavior and the factors affecting it. These include the effective resilient stiffness during train passage, which is likely to depend on the stress history and the stress state of the ground, and the stress path followed during loading. This study investigates the last of these, by means of a two-dimensional dynamic finite-element analysis. The effects of train speed, acceleration/braking, geometric variation in rail head level, and a single unsupported sleeper are considered. Results indicate that dynamic effects start to become apparent when the train speed is greater than 10% of the Rayleigh wave speed, $v_c$ , of the subgrade. At a train speed of $0.5v_c$ , the shear stresses will be underestimated by 30% in a static analysis, and at train speeds greater than $v_c$ the stresses due to dynamic effects increase dramatically. Train acceleration/braking may increase shear stresses and horizontal displacements in the soil, and hence the requirement for track maintenance at locations where trains routinely brake or accelerate. For heavy haul freight trains, long wavelength variations in rail head level may lead to significantly increased stresses at passing frequencies (defined as the train speed divided by the wavelength of the variation in level) greater than 15, and short wavelength variations at passing frequencies of 60–70. Stress increases adjacent to an unsupported sleeper occur in the ballast and subballast layers, but rapidly become insignificant with increasing depth.

This technical note describes the derivation of an analytical expression for the total active force on the retaining wall for $c\text{-}\varphi$ soil backfill considering both the horizontal and vertical seismic coefficients. The results based on this expression are compared with those obtained from earlier analytical expressions for the active force for $c\text{-}\varphi$ soil backfill under seismic conditions, and found to have a similar trend of variation. The parametric study shows that the inclination of the critical failure plane with the horizontal plane decreases with the increase in values of seismic coefficients; the decrease being more for their higher values. The total active force increases with the increase in value of horizontal seismic coefficient; while it decreases with the increase in value of vertical seismic coefficient except for a very high value of horizontal seismic coefficient. Design charts are presented for various combinations of horizontal and vertical seismic coefficients ( $k_h$ and $k_v$ ), and values of cohesion and angle of shearing resistance for estimating the total active force on the retaining wall for $c\text{-}\varphi$ soil backfill for practical applications.

This paper presents the results of an experimental investigation performed to understand the processes influencing the injection of colloidal silica grout into porous media. Based on the combined analysis of grout injection pressures and the visually observed grout distribution patterns, three major processes, gelation, shear, and viscous fingering, have been identified to occur during grout injection. The results demonstrate the dynamic interplay between grout viscosity variations and the resulting flow instabilities.