Journal of Geotechnical and Geoenvironmental Engineering

Analysis of a slug test to estimate hydraulic conductivity of a vertical cutoff wall is complicated by the high compressibility of the backfill materials and by the proximity of the well intake section to the edge of the cutoff wall. The writers developed two methods to evaluate hydraulic conductivity of the vertical cutoff wall with the aid of a numerical program, Slug_3D. The first method, named the type curve method, expands the general type curve concept frequently used by hydrogeologists to take into account the complex boundary conditions of the vertical cutoff wall. The other method, named the modified linear curve fitting method and modified from the Hvorslev and the Bouwer and Rice methods, provides an easy and practical way of evaluating hydraulic conductivity of the vertical cutoff wall. To illustrate the use of the type curve and the modified linear curve fitting methods in practice, a case study involving two slug tests at a landfill site is presented. The methods are shown to be useful and practical.

This paper addresses practical applications of the type curve method and modified linear curve fitting method described in a companion paper for evaluating the hydraulic conductivity of a vertical cutoff wall. Sensitivity analyses were performed for factors that might influence results from a slug test in a vertical cutoff wall. The factors considered are (1) width of the vertical cutoff wall, (2) eccentricity of a well location, (3) well aspect ratio, and (4) distance from the upper and lower boundaries of the wall to a well intake and discharge section. In addition, the effects of variable hydraulic properties—hydraulic conductivity $\left(k\right)$ and specific storage $\left(S_s\right)$ —on the results of a slug test are evaluated. Also, the effect of initial slug size and difference between a rising and falling head slug test are investigated in connection with the variability of the hydraulic properties. Analyses show that the compressibility of the porous medium has a dominant effect. Various errors can arise from the simplifying assumptions used in slug test analysis, but these errors often tend to offset one another. Performing sequential slug tests can yield progressively lower hydraulic conductivity as a result of consolidation of the barrier wall material.

The Olympic View Sanitary Landfill (OVSL) near Port Orchard, Washington, is a modern solid waste sanitary landfill covered, in part, by a composite cover system. The site was subjected to a free-field peak horizontal ground acceleration on the order of 0.16 g during the 28 February 2001 Moment Magnitude 6.8 Nisqually earthquake. To the knowledge of the writers, this is the first documented case history of a composite landfill cover shaken by strong ground motions. Postearthquake reconnaissance did not find any signs of earthquake-induced permanent displacement of the composite cover system. Accelerograms recorded within 1 km of the facility, the results of site-specific shear wave velocity measurements and laboratory interface shear testing, and these postearthquake observations provide a unique opportunity for a posteriori numerical analysis of this important case history. The seismic performance of the OVSL composite cover system was evaluated using four commonly used methods for seismic design of landfill cover systems. These methods include two simple screening procedures, a more rigorous screening procedure, and a decoupled equivalent-linear site response/Newmark-type permanent deformation analysis using the accelerograms recorded at the nearby strong motion station. The yield accelerations of the composite cover system, required for all four methods, were calculated using the results of construction quality assurance interface shear strength conformance testing. All four methods produce results consistent with the observed performance. However, the two simple screening procedures were significantly more conservative than the other two more rigorous methods.

As a result of recent changes in the requirements involving hurricane extreme events (e.g., wind velocities), the Florida Department of Transportation has moved away from cable-stayed signs, signals, and lights systems to mast arm/pole structures. Unfortunately, the newer systems develop significant lateral and torque loading on their foundations (e.g., drilled shafts). Current design practice for a mast arm/pole foundation is to treat lateral loading and torsion separately (i.e., uncoupled); however, recent field-testing suggests otherwise. This paper reports on the results of 91 centrifuge tests. 54 of the tests were conducted in dry sand and 37, in saturated sands. The tests varied the lateral load to torque ratios, shaft embedment depths, and soil strengths. The experiments revealed that even though the torsional resistances of the shafts were not influenced by lateral load, the shafts’ lateral resistance was significantly impacted by torsion. Reductions in lateral resistance of 50% were recorded for shafts under high torque to lateral load ratios. Using the free earth support assumption and the ultimate soil pressure the soil pressure distribution along the shaft was developed. Using force and moment equilibrium, as well as the applied torque, maximum shear, and moments were computed. The predicted values were found to be within 25% (10% on average, except for the tests in saturated dense sand with polymer slurry) of the experimental results.

The writers introduce a design approach for braced excavations based directly on the data of carefully chosen soil tests, conceived within the framework of plasticity theory, but allowing for strain hardening. Mobilized shear stresses beneath and around braced excavations are found by a stability calculation based on a proposed plastic deformation mechanism. Strains required to mobilize these stresses are deduced from a direct simple shear test on a representative sample taken from a selected location in the plastic zone of influence. These strains are entered into a simple plastic deformation mechanism to predict boundary displacements. Hence, the proposed Mobilizable Strength Design (MSD) method can satisfy both safety and serviceability in a single step of calculation, without the need for finite element analyses. In this method, design parameters can be chosen rationally with regard to the initial state of soil, the stiffness following the appropriate stress path, and the level of acceptable deformations under working conditions. Examples demonstrating the success of the MSD method are given for a variety wall and soil conditions. Comparisons are made both with previously published field studies and with comprehensive nonlinear finite element analyses.

Although pressure grouting beneath the tips of drilled shafts had been used successfully worldwide for close to $4\text{decades}$ , it has remained relatively unused in the United States in part due to the absence of a rational design procedure. Previous international usage relied predominantly upon experience and unpublished proprietary approaches. More recently, research aimed at quantifying the improvement that could be derived from postgrouting drilled shaft tips has resulted in a design methodology. This paper briefly discusses the postgrouting process and outlines the full scale test programs used to identify parameters affecting postgrouting performance. Correlations developed between applied grout pressure and end bearing improvement are presented along with a numerical example illustrating the design procedure.

The creep behavior of Fontainebleau sand grouted with a chemically unstable silicate grout was compared with the creep behavior of the same sand grouted with two chemically stable grouts, a microfine cement-based grout and a mineral-based grout. Unconfined and confined creep experiments were performed to highlight drained creep behavior. The duration of the tests was between 100 and 200 days. Results were compared in terms of creep slope, and creep limit strength. Test results have shown that the method with which sand was prepared plays an important part in creep strains of grouted sand. Creep strains were considerably reduced by applying a confining stress. The creep behavior of sands was modified by the grouting treatment: depending on the nature of the grout, the grouted sand has exhibited creep strains of different degrees. The creep slopes of grouted sand were proportional to the creep slopes of pure grout. Creep characteristics of grouted sand may also be deduced from the creep behavior of pure sand and the creep behavior of pure grout.

This paper examines the possibility of introducing synthetic rubber in constructed earthen systems to improve their vibration attenuation. The results of a series of controlled laboratory tests—aimed to investigate the improvement of low-strain dynamic properties of Ottawa sand by mixing ground rubber of similar size—showed a simultaneous increase in both the shear modulus and the damping ratio of the sand specimens up to an optimum volume proportion of the rubber. Predictions using Hertzian particle contact effects and mechanical damping, resulting from thermoelastic effects between the dissimilar particles, were employed to discern the observed behavior. It is postulated and examined through the available data that the large difference between the elastic modulus of the two materials results in the increase in contact area of the sand and rubber particles and consequently the increase in the stiffness matrix. Similarly, the large difference between the coefficient of thermal expansion of sand and rubber may effectively lead to high thermoelastic enhancement of damping in the sand-rubber mixture.

This paper investigates the reduction of vibration due to the foundation slab both in horizontal and vertical directions. Similar reduction efficiency of the foundation slab was obtained among three-dimensional finite element analyses, field experiments, and the averaging scheme of the rigid slab theory. Then, two formulas fitted from numerical simulations were generated. Using those two simple formulas, one can estimate the reduction efficiency of horizontal and vertical vibrations due to foundation slabs. In summary, a suitable mat foundation can significantly reduce the horizontal vibration transformed from soil. The reduction efficiency depends on the size of the mat foundation over the soil wavelength. Enlarging the thickness of the foundation slab can decrease the vertical vibration transferred from soils. However, it is difficult to reduce vertical low-frequency vibrations for a reasonable slab thickness.

Thermal stress imposed on soil is coupled with the expulsion of pore fluid resulting in thermomechanical fluid flow. This paper describes a coupled flow-deformation model subjected to a constant temperature. Two-dimensional numerical modeling was performed based on the finite element method applied to thermoporoelasticity. The model evolves from application of the principles of irreversible thermodynamics. A heating experiment was designed to measure temperature and thermal deformation at predefined locations in a cylindrical clay disk. Results from the experiment are compared with the simulated results to validate the numerical scheme. Sensitivity analysis of the developed code reaffirmed that the coupling effects are most pronounced for high porosity and low permeability medium.

This paper presents a study of the amplification of SV waves obliquely incident on a surface soil layer overlying rock formation. Special attention is placed on the influence of the saturation states of the soil layer and the bedrock on the amplification in both horizontal and vertical directions as well as on the amplitude ratios between the two directions at the surface, where the vertical and horizontal amplification and the amplitude ratios are expressed as functions of the frequency of incident waves. The analysis indicates that while the influence of the saturation state of the bedrock is insignificant, a change of the saturation state of the soil layer may have a marked impact on the vertical amplification. For typical seismic frequencies, an unsaturated soil layer can generate greater vertical amplification than a saturated layer; it can also cause larger amplitude ratios between vertical and horizontal components at the surface. The analysis further confirms the potential importance of the saturation condition of near-surface soils in site response analysis.

Stability failures are often “back-analyzed” in an attempt to estimate the operative shear strength. In fact, back-analysis is commonly believed to be one of the most reliable ways to estimate soil and/or rock strength. However, this paper provides examples to illustrate specific situations in which back-analysis of failures can lead to misinterpretation of strength. Examples from earth and concrete gravity dams are used, and consideration is given to both two- and three-dimensional idealizations. The cases demonstrate that interpreted strength can be in significant error, and in practically all cases the errors are unconservative. Finally, this paper illustrates that back-analysis is reliable only when the model and all assumptions are reasonable and accurate representations of the real system.