Journal of Geotechnical and Geoenvironmental Engineering

This paper discusses military requirements and how these requirements have driven technology. Specifically, these advances in technology have been directly applied to civil engineering and, even more specifically, geotechnical engineering to advance the state of practice as we know it today. Topics include airfield construction, soil dynamics, and rock mechanics. Current gaps in our knowledge are briefly discussed as they relate to both our nation's defense and geotechnical engineering.

The effect of particle size (4-, 6-, and 15-mm nominal sizes) on the rate of clogging of columns of porous media permeated with municipal solid-waste leachate is examined. Clogging is shown to be more localized over a small volume of the porous media near the influent end of the column for smaller particles than for larger particles, where clogging was more uniformly distributed along the column. This is attributed to the greater surface area per unit volume of smaller particles allowing greater biofilm growth per unit volume. This increased the reduction in chemical oxygen demand (COD) and caused greater deposition of inorganic clog material per unit length of column than for larger particles. The distribution of methanogenic bacteria was found to closely correspond to the zones of most severe clogging. The bulk density of clog material is shown to be between 1.6 and 1.8 Mg/m³. The chemical composition of the clog material is essentially independent of particle size, with calcium representing 26% of the dry mass of the clog material and CaCO₃ being the main component of the clog. An examination of the yield of CaCO₃ relative to COD indicates that the carbon in the CaCO₃ represents <4% of the organic carbon represented by the drop in COD. Finally, the data from the column test is used to predict the expected time to clog for an actual landfill and were found to give results consistent with what was observed in the field.

This paper describes the failure of an 8.5-m-high fill slope constructed over weathered claystone and siltstone in Martinez, Contra Costa County, in northern California. The failure occurred about 5 months after construction. Several piezometers and inclinometers were installed to measure the pore pressures and the location of the failure surface(s), respectively. The failure surface was observed just below the interface between the fill and the natural materials. Factors of safety of the failed slope calculated using laboratory-derived effective shear strength parameters were on the order of 1.58–1.95. Reduction factors were necessary to reduce the shear strength values and to obtain reasonable agreements with the field observations. A class “C” prediction of the location failure surface and the factor of safety using the reduced shear strength values indicated results that were consistent with the field observations. This paper highlights the importance of exercising extreme caution when evaluating the stability of slopes in plastic natural clays.

Compression test results of kaolinite with water and nine organic fluids of a wide range of dielectric constants are presented in this paper. As the dielectric constant was increased from approximately 2 in nonpolar fluids to 80 in water, void ratio and compression index of the kaolinite decreased first, reaching a minimum at a dielectric constant of 24 in ethanol, and then increased. The swelling index also increased with the dielectric constant. The effects of the pore fluids are chiefly attributed to van der Waals attractive force. When the attractive force is large, shearing resistance at interparticle contacts is large, enabling soil particles to form an open-flocculated structure with large void ratio and compressibility. Efforts were made to accurately compute the attractive force for the kaolinite in the test fluids. Its variation with the dielectric constant agreed with the variation of compressibility qualitatively and compared with experimental results on stability of kaolinite/fluid suspensions reasonably well. Double-layer repulsive forces were also found to influence the compressibility slightly. Due to an increase in double-layer thickness, the compressibility increased with a decrease in electrolyte valence and concentration. The test results also indicate that the physicochemical effects diminish with an increase in overburden stress. Under an overburden stress of 300 kPa, pore fluid properties had essentially no effect on the compressibility of the kaolinite.

The VERT wall is a new type of top-down retaining wall, which derives its name from the fact that it is vertically reinforced. Typically, three to four rows of 1-m-diameter cemented-soil columns are constructed to the depth of soil to be retained. After curing of the cement, the soil in front of this gravity wall is excavated in one step. To study the viability of this new wall type, a 10-m-high, 40-m-long, and 5.6-m-wide VERT wall was built at the National Geotechnical Experimentation Site at Texas A&M University. The soil is a medium-dense silty sand, and the wall was instrumented for horizontal and vertical movement. One year after excavation the horizontal movement of the wall was 24.6 mm and the settlement was 9.3 mm. These values are very close to the movements of the similar size tieback wall built at the same site. This performance is an indication of the viability of this new wall type.

Nonlinear stress-strain characteristics and shear stiffness-shear strain relationships of sedimentary soils and recompacted sands at small strains have been reported by many researchers. However, research work on the behavior of granitic soils at small strains has not attracted much attention, despite the fact that many countries around the world are underlain by granitic saprolites. In the study reported in this paper, shear stiffness of a natural granitic saprolite from Hong Kong has been investigated in the field, using the self-boring pressuremeter and geophysical techniques such as the suspension P-wave and S-wave logging method, and in the laboratory, using a triaxial apparatus equipped with internal displacement measuring devices. The observed stiffness-strain relationships of the natural granitic saprolite are highly nonlinear at small strains. Shear stiffness decreases significantly as shear strain increases. At very small shear strains (in the order of 0.001%), the elastic shear moduli deduced from the suspension S-wave logging method are generally consistent with the predictions made using an empirical correlation based on standard penetration test N values and also with the results of triaxial tests incorporating local displacement measurements. For shear strains larger than 0.01%, reasonable consistency can be found between the normalized shear stiffness-shear strain relationships obtained using the self-boring pressuremeter and from the triaxial apparatus.

A general constitutive modeling concept called the disturbed state concept (DSC) is developed in this research for the stress-strain and liquefaction behavior of saturated sands. The DSC model is a unified approach and allows hierarchical modeling for options like elastic and elastoplastic responses, microcracking, damage, and softening. The DSC model parameters for saturated Ottawa sand are evaluated using data from multiaxial tests. The model predictions are found to provide satisfactory correlations with the test results. The DSC model with the foregoing parameters is implemented in a nonlinear dynamic finite-element program (DSC-DYN2D). It is used to solve a typical boundary value problem—a shake table test—involving liquefaction behavior. Based on the results, it can be stated that the DSC model is capable of both characterizing the cyclic behavior of saturated sands and identification of liquefaction.

Preliminary results of small-scale model compaction grouting tests performed in a geotechnical centrifuge are presented. The soil response to a one-stage grout bulb injection in dry uniform sand was examined at various prototype depths, and the effects of grout composition on grout bulb development and shape and on soil response are evaluated. Preliminary results indicate that the shape of the injected grout bulb is a function of overburden pressure. For specific grouting conditions, there is a maximum size that the grout bulb will achieve. Observed soil deformations were similar to those observed in models of deep uplift anchors. The addition of either clay or flyash to the grout mix reduced the ability to sustain high injection pressures.