Journal of Geotechnical and Geoenvironmental Engineering

An evaluation of scale effects, stress, waste segregation, and waste decomposition on the immediate compression behavior of municipal solid waste is presented. Laboratory experiments were conducted in 64-, 100-, and 305-mm-diameter compression cells. A field-scale experiment [Deer Track Bioreactor Experiment (DTBE)] was conducted on waste of the same composition and material properties. A methodology is presented for determining the end-of-immediate compression strain (${\varepsilon }_{\text{EOI}}$) that is applicable to both laboratory- and field-scale data. The compression ratio ($C_c^{\prime }$) was comparable between tests conducted in 100- and 305-mm compression cells. Compression tests in 305-mm cells conducted on six wastes (three size-differentiated fresh wastes and three decomposed wastes) yielded $C_c^{\prime }$ ranging from 0.22 to 0.28 in the stress range of 25–100 kPa. A similar $C_c^{\prime }$ (0.23) was determined for the DTBE (20–67 kPa). The variation in $C_c^{\prime }$ is related to the waste compressibility index (WCI), which is a function of waste dry weight water content, dry unit weight, and the percent contribution of biodegradable organic waste (paper/cardboard, food waste, yard waste). A compilation of laboratory data from this study and the literature yielded a predictive relationship for the $C_c^{\prime }$ and WCI. The $C_c^{\prime }$ can be estimated within ±0.087 for a given WCI using this relationship.

The use of biopolymers to improve the engineering properties of soil has received attention in recent years, stimulated by potential cost savings and the low environmental impact of this class of materials. The purpose of this work is to improve the understanding of precisely how biopolymers strengthen soil and to quantify the small-scale mechanical properties of biopolymers for implementation in physics-based numerical models. The authors describe the initial efforts to develop viable methods to form biopolymer bonds between grains of naturally occurring materials and present the results of mechanical properties experiments on these bonds. The subject biopolymer was an exopolysaccharide from Rhizobium tropici (ATCC #49672). The initial experiments indicate that the stiffness of bonds ranged from 1 GPa after approximately 1 h of curing to plateau values as high as 3.8 GPa for extended cure times. For bonds with neck areas in the range of 0.01–0.06 mm², the cohesive tensile strength of the bonds ranged from 16 to 62 MPa, but averaged ≈20 MPa. The associated cohesive failure strains in tension ranged from 0.013 to 0.042. Cyclic loading experiments were conducted to provide information on the mechanical behavior of the biopolymer and to support subsequent constitutive modeling. The results are analyzed and discussed in terms of the underlying viscoelastic behavior, paying particular attention to the variations in stiffness and internal friction as functions of cure time, frequency, and amplitude.

The paper presents a technique to invert two-dimensional (2D) full wavefields using simulated annealing and a finite-difference solution of the 2D elastic wave equation in the time-distance domain. The algorithm generates all possible wave types (body waves, surface waves, etc.) to simulate complex seismic wavefields and for comparison with observed data. Model runs with both synthetic and actual experimental data sets illustrate the capability of the inversion technique. The results from synthetic data demonstrate the potential of characterizing both low- and high-velocity layers in laterally inhomogeneous profiles, and the inversion results from actual data are consistent with the crosshole, standard penetration test N-value, and material log results. Based on the cases presented, the coupling of global optimization with full waveforms is computationally practical; the results presented herein required less than 1 day of computer time on a standard laptop computer.

Despite the increased use of aggregate piers for soil reinforcement, the role of typical construction variables on footing performance remains uncertain. This paper describes a series of full-scale footing load tests conducted to evaluate the effect of aggregate gradation, pier length, and compaction method. Tests were conducted on small (0.76 m) and large (2.74 m) instrumented spread footings supported on single aggregate piers and groups of aggregate piers, respectively. The bearing pressure-displacement response of these tests is presented and the statistical significance of the construction variables is quantified for single piers using a $2^3$ factorial analysis within an ANOVA framework. The statistical significance of the effect of pier length, gradation, and compaction was determined by controlling for the spatial variability in matrix soil strength and stiffness across the test site, which indicated that these variables were largely insignificant at the treatment levels evaluated. Additionally, no appreciable difference in the bearing pressure-displacement performance of the footings on groups of piers constructed with various levels of treatment was observed. Small variations in the observed displacement performance are attributed to sources of variability other than construction variables. The similarity in the performance of the uniformly constructed pier groups, representing production conditions, suggests that variations in the inherently variable matrix soil stiffness and strength control the aggregate pier performance.

Excessive excavation-induced movements are major concerns for most underground construction projects in urban areas. These movements can lead to significant damage in adjacent structures. When average to good workmanship is employed during the installation process of the excavation support systems, the consequent ground movements are most influenced by the support system stiffness. Therefore, choosing the most appropriate stiffness for an excavation support system is crucial to minimizing excavation-related damage to adjacent buildings and utilities. This paper presents a semiempirical design methodology that facilitates the selecting of the excavation support system stiffness in such a way that limits excavation-related ground movement. As part of the proposed design methodology, a new parameter was developed called the relative stiffness ratio. This new parameter relates the strength and stiffness of the soil with the stiffness of the excavation support system and was developed from a comprehensive parametric analysis that incorporated a fully three-dimensional finite-element analysis of a generalized excavation that realistically modeled the excavation geometry, excavation support system configuration, and excavation activities. The performance of the proposed methodology was evaluated using several excavation case histories reported worldwide. The results of the evaluation show that the new relative stiffness ratio performed well in predicting the support system bending stiffness and the actual excavation-induced lateral deformations of the case history support systems.

This paper presents a new method for estimating the base capacity of open-ended steel pipe piles in sand, a difficult problem involving great uncertainty in pile foundation design. The method, referred to as the Hong Kong University (HKU) method, is based on the cone penetration test (CPT), and takes into consideration the mechanisms of annulus and plug resistance mobilization. In this method the annulus resistance is properly linked to the ratio of the pile length to the diameter—a key factor reflecting the influence of pile embedment—whereas the plug resistance is related to the plug length ratio, which reflects the degree of soil plugging in a practical yet rational way. The cone tip resistance is averaged over a zone in the vicinity of the pile base by taking into account the failure mechanism of the piles in sand, the condition of pile embedment (i.e., full or partial embedment), and the effect of soil compressibility. The predictive performance of the new method is assessed against a number of well-documented field tests including two fully instrumented large-diameter offshore piles, and through comparisons with major CPT-based methods in current engineering practice. The assessment indicates that the HKU method has attractive capabilities and advantages that render it a promising option.

This paper presents a procedure to account for the presence of a weaker layer of soil in a beam on nonlinear Winkler foundation analysis of a laterally loaded pile or drilled shaft. Three-dimensional (3D) finite-element (FE) models, considering a single pile embedded in a soil continuum, are used to compute representative p-y curves for various combinations of soil profile and pile diameter. Comparisons between the p-y curves resulting from homogenous and layered soil profiles, in which a weak soil layer is located between two stronger layers, provide the means to identify reductions in the ultimate lateral resistance and initial stiffness of the p-y curves representing the stronger soil owing to the presence of the weak layer. These reductions are characterized in terms of an exponential decay model. Dimensionless parameters are proposed as a means of implementing appropriate reductions for an arbitrary soil profile and pile diameter. Validation of the reduction procedure is conducted through a comparative study, in which the effects of liquefaction-induced lateral spreading on a pile are analyzed using pseudostatic nonlinear Winkler foundation and 3D FE approaches. Use of the reduction procedure is demonstrated with p-y curves defined by existing methods.

Tunneling using the earth pressure balance machines method in clayey soils requires the addition of conditioning chemicals to reduce the torque moment of the machine and to guarantee a safe and economical excavation process. Injection of foams and polymers at the cutter head of the tunnel boring machine with subsequent remolding in the pressure chamber transforms the excavated material into a deformable soil paste with preferably low strength and adhesion. The soil becomes more compressible and can develop viscous properties, such that the pressure and rate dependency of the shear strength of the mixtures are of major importance for the proper design and analysis of the tunneling processes. In this paper, the results of vane shear strength measurements on various conditioned clays and clay-sand mixtures at various shearing velocities and applied pressures are presented and analyzed. The results clearly show the effects of the chemicals used and also point out the differences in their efficiency depending on the clay mineralogy. This allows for an improved understanding of the working mechanisms of the chemicals.

The methylene blue (MB) stain test was used to determine the efficiency of lime modification of clay soils and to define the percentage of lime required to reach the stabilization field. Montmorillonite and kaolinite, mixed in variable proportions with quartz sand, were treated with differing percentages of lime, and the MB test was performed on both untreated and treated soils. Results show that the test can determine whether a soil is prone to efficient modification, follows the evolution of clay minerals in the lime modification field, and it also determines the quantity of lime necessary to enter the stabilization field. X-ray powder diffraction, thermal analyses, and electron microscopy observations of the montmorillonite-richer sample showed its crystal chemical evolution in the modification field. Mineralogical results match the outcome of the MB test, defining the same lime percentage for best modification (4%) and confirming that the test can be used for preliminary and ongoing analyses of lime treatment.

This study focused on landslides resulting from heavy precipitation, such as rainfall from hurricanes. A series of centrifuge model simulations were performed with reference to a landslide that occurred during Typhoon Nabi in Japan in 2005. The procedures of simulation on the slope using a sand-clay soil mixture were illustrated. The rainfall event was simulated by applying precipitation in increments to the slope surface until it exceeded that of the field measurements. The instability was examined using an infinite slope analysis, and the mechanism of rainfall-induced failure was discussed. For this particular study, the results showed that incremental rainfall of less than 200 mm led to local failures, whereas total accumulation of 400 mm resulted in a global slope failure. A reduction in apparent cohesion in the soil combined with an increase in pore pressure because of infiltration was responsible for the slope instability during rainfall.

Traditional slope stability analysis delivers the factor of safety with respect to shear strength, which is the factor by which $\tan \phi \prime \text{and} c\prime$ must be reduced to bring a slope to the point of failure. In the present paper, alternative strategies are considered to include the factors by which destabilizing parameters, such as pore pressures and gravitational loads, must be increased to bring a slope to the point of failure. The approach described gives a more comprehensive insight into the stability of slopes and the sensitivity of failure to different input parameters. Finally, the practical use of the alternative factoring strategies is illustrated through two application examples referring to a slope under different loading combinations (pore-pressure ratio and horizontal seismic coefficient).