Journal of Geotechnical and Geoenvironmental Engineering

In the conventional design of retaining walls and bridge abutments, the lateral earth pressure due to live load surcharge is estimated by replacing the actual highway loads with a 600 mm layer of backfill. This original recommendation was made several decades ago when the highway truck loads were much lighter. A number of researchers have shown that the pressure exerted on the wall due to live load surcharge is greater near the surface and is diminished nonlinearly throughout the height of the wall. The heavier highway loads and the demonstrated nonlinear earth pressure distribution require a need for a more rational method for obtaining the equivalent height of backfill. This paper discusses theoretical background, an analytical approach to estimation of actual earth pressure, a number of innovative approaches to obtain a simplified pressure distribution, an extensive parametric study, calibration procedures for the traditional method, and recommendations.

In this study, two types of soils with varying soil water potentials were used for evaluating the effect of gravity on water flow through unsaturated soils. Experiments were conducted in both 1- and 0-g environments. Water content distributions were evaluated as a function of distance from the source of water intake and time. The experimental results indicated that the capillary potential and the advective forces due to interfacial tension gradients are overshadowed by the gravitational potential in a 1-g environment. The fast water movement in the 0-g condition is attributed to the capillary potential as well as to the advective forces that developed. In addition, microstructural changes have contributed to water flow in the 0-g condition. Depending on soil type, the magnitude of such an effect (i.e., water movement) varies from three- to four-fold. To analyze the experimental results, a one-dimensional model, based on Darcy’s law and the conservation of mass equations, was developed and solved numerically by the finite difference method. A nondimensional Bond number was extracted from the resulting flow equation and used as a basis for incorporating the gravitational component of the flow process into the formulation. The numerical results compare quite well in some instances with the experimental results. In other cases, significant departures are noted. The departure was attributed to the significant changes in microstructure of soil samples under the 0-g condition. Consequently, the requisite water retention and hydraulic conductivity functions used in the model may not apply in outer space.

In its natural state, loess can be considered as an unstable soil, which develops large deformations when moistened. In Argentina, loess is used in most Geotechnical constructions, including embankments and liners. The interest of this work to evaluate the potential application of electrical conductivity measurements for monitoring the effects introduced by remolding and compaction in the soil. Samples of loess were compacted at varied densities and mixed with electrolytes of different concentrations. Electrical conductivity was measured with a two electrode cell. The effects introduced on the measured conductivity by frequency, degree of saturation, soil density, temperature, and electrolyte type and concentration are addressed. Additionally, hydraulic permeability tests were performed on compacted specimens of loess and the relationship between electrical and hydraulic conductivity was determined. It is concluded here that the ohmic conductivity of compacted specimens depends mainly on the salt concentration in the pore fluid, and volumetric water content. The effect of compaction density was observed to be less significant. The whole behavior of electric conductivity of loess is well described by the Archie’s law.

This paper describes an extension of a new three-dimensional (3D) stability analysis method, including formulation, comparative studies, and examples of new application. The new method uses “two-directional force and moment equilibrium” in the stability analysis of 3D potential failure mass with arbitrary shapes. The use of this new method has resulted in a novel situation wherein the direction of the resultant shear force (or direction of sliding) generated on the potential failure surface can now be calculated instead of the guesswork assumptions that were formerly made. It is also demonstrated that this new method eliminates the labor-intensive work for establishing local coordinate systems performed in conventional 3D analysis. Consequently, this new method facilitates a computer-aided 3D search for the critical failure surfaces in slope areas.

In this paper, intergranular $\left(e_c\right)$ and interfine $\left(e_f\right)$ void ratios and confining stress are used as indices to characterize the stress–strain response of gap graded granular mixes. It was found that at the same global void ratio (e) and confining stress, the collapse potential (fragility) of silty sand increases with an increase in fines content (FC) due to a reduction in intergranular contact between the coarse grains. Beyond a certain threshold fines content $\left({\mathrm{FC}}_{\mathrm{th}}\right)\text{,}$ with further addition of fines, the interfine contact friction becomes significant. The fragility decreases and the soil becomes stronger. The value of ${\mathrm{FC}}_{\mathrm{th}}$ depends on e and the characteristics of fines and coarse grains. At $\mathrm{FC}<{\mathrm{FC}}_{\mathrm{th}}\text{,}$ intergranular contact friction plays the primary role. It is postulated that fines either provide a beneficial secondary cushioning effect or contribute to fragility, depending on the nature of the soil's matrix structure and the magnitude of $e_c\text{.}$ At the same $e_c\text{,}$ the fines that fall within the intergranular voids provide a cushioning effect and slightly reduce the fragility. When fines fall between some of the coarse grains and partially support the coarse grain skeleton, the soil is very fragile. The contribution to fragility is dominant when the soil is very loose in terms of $e_c\text{.}$ At large fines contents $\left(\mathrm{FC}>{\mathrm{FC}}_{\mathrm{th}}\right)\text{,}$ fine grain friction plays a primary role and dispersed coarse grains provide a beneficial, secondary reinforcement effect. At the same $e_f\text{,}$ the collapse potential decreases with an increase in sand content. Beyond a certain limiting fines content, the soil behavior is controlled by $e_f$ only. An intergranular matrix diagram is presented that delineates zones of different behaviors of granular mixes as a guideline to determine the anticipated behavior of gap-graded granular mixes. New equivalent intergranular contact void ratios, $(e_c{)}_{\mathrm{eq}}$ and $(e_f{)}_{\mathrm{eq}}\text{,}$ are introduced to characterize the behavior of such soils, at $\mathrm{FC}<{\mathrm{FC}}_{\mathrm{th}}$ and $\mathrm{FC}>{\mathrm{FC}}_{\mathrm{th}}\text{,}$ respectively.

An analytical solution based on a rod-on-dynamic-Winkler-foundation model is developed for the response of piles in a soil layer subjected to vertical seismic excitation consisting of harmonic compressional waves. Closed-form solutions are derived for: (1) the motion of the pile head; (2) the peak normal strain in the pile, and (3) the group effect between neighboring piles. The solutions are expressed in terms of a dimensionless kinematic response factor $I_v\text{,}$ relating pile-head motion and free-field soil surface motion, a dimensionless strain transmissibility factor $I_{\epsilon }\text{,}$ relating pile and soil peak normal strains, and a pile-to-pile interaction factor α measuring group effects. It is shown that a pile foundation may significantly reduce the vertical seismic excitation transmitted to the base of a structure.

A plasticity constitutive framework for modeling inherently anisotropic sand behavior is presented within a modified form of critical state soil mechanics. A second-order symmetric fabric tensor, $F_{\mathrm{ij}}\text{,}$ describes the material inherent anisotropy, and a scalar-valued anisotropic state variable A is properly defined in terms of a joint invariant of $F_{\mathrm{ij}}$ and the stress tensor. The location of the critical state line in the plane of void ratio and effective mean normal stress is not fixed but depends on A, rendering the soil dilatancy also a function of A. In addition, the plastic modulus is made a function of A. The incorporation of these two modifications in terms of A in an existing stress-ratio bounding surface model, achieves the successful simulation of both the contractive and dilative responses of sand over a wide range of variations in stress and density as shown by experimental data. Of particular significance are the results which exhibit the drastic effect of different principal stress orientations in reference to the material axes of anisotropy.