Journal of Geotechnical and Geoenvironmental Engineering

The performance of landfills during strong earthquake shaking is a matter of considerable concern and deserves to be studied more extensively. This paper investigates the dynamic properties of the OII landfill materials using strong motion recordings, available field measurements, and simplified physical models. Although the earthquakes recorded at the OII landfill generated shear strains as large as 0.08%, the landfill materials behaved as essentially a linear viscoelastic material, showing a negligible reduction in shear modulus with shear strain amplitude. The damping responsible for energy dissipation was found to depend on frequencies between 0.1 and 10 Hz. The physical origins of this frequency-dependent damping are not yet clearly understood. The results of this study are useful in analyzing the dynamic response of landfills and cap systems during small to moderate-size earthquakes. The present analysis is simplified because there were only two recording instruments at the OII landfill. It is recommended that researchers deploy more field instruments at this and other landfills to document their dynamic response during future earthquakes, and develop large-scale laboratory tests to determine landfill material properties under large static and dynamic strains.

As part of predesign studies for closure of the Operating Industries, Inc. (OII) landfill Superfund site, field and laboratory studies were combined with back analyses of strong motion data to characterize the behavior of the OII solid waste when subjected to strong earthquake shaking. Small strain shear modulus values for the solid waste material were established on the basis of field measurements of shear wave velocity and unit weight. Large-diameter (457 mm) cyclic direct simple shear (CyDSS) testing was performed on reconstituted solid waste specimens to investigate the modulus reduction and damping characteristics of solid waste at large strains. Results of two-dimensional finite element back analyses of strong motion data recorded at the site were combined with the results of CyDSS testing to establish solid waste modulus reduction and damping curves over the range of cyclic shear strain required for site closure design. One-dimensional deconvolution of motions recorded on fill at the base of the landfill was an essential step in the two-dimensional finite element back analyses of strong motion data. The resulting modulus reduction and damping curves indicate that OII solid waste is a fairly linear material in the small to intermediate strain range, but that a significant reduction of shear modulus occurs when the cyclic shear strain exceeds approximately 0.1%.

Back-analyses of the recorded seismic response of the Operating Industries, Inc. landfill were performed to estimate strain-dependent shear modulus reduction and material damping relationships for solid waste. Two-dimensional dynamic finite element analyses were performed using the accelerograms from five earthquake events. The sensitivity of the computed results to variations in shear wave velocity, shear modulus reduction, damping, and Poisson's ratio was examined. The recorded and calculated motions at the top of the landfill were compared through an objective statistical analysis of the computed and recorded acceleration response spectra. Statistically significant findings from the analysis indicate that strain-dependent shear modulus reduction and material damping relationships for the solid waste at this site are best characterized by those of a medium plasticity clay.

The structural looseness and the permeability of rock have been found to be strongly correlated, and on-site measurement of permeability is shown to be a promising new method of detecting structural looseness of rock mass and hence the stability of nearby structures. The vacuum permeability test obtains the permeability by suction of air from a sealed test section. It has high sensitivity and fast response to a low permeable medium and is particularly suited for this purpose. The details of the method of measurement, hardware, and the data reduction methods developed so far are described. The loosening to be detected may be due to excavation of various types or to long-term deterioration of the surrounding ground due to natural processes. Practicality of the method is demonstrated by several sample field tests conducted at various sites.

The hypothesis that the substitution of attapulgite clay for bentonite in an otherwise sand-bentonite (S-B) mixture will improve the resistance to change in hydraulic conductivity of the mixture upon permeation with calcium chloride (CaCl₂) solutions relative to permeation with a processed tap water is tested. For mixtures with the same clay soil content, complete substitution of attapulgite clay for bentonite significantly decreases the change in hydraulic conductivity relative to that observed for the S-B mixtures upon permeation with a 0.5 M CaCl₂ solution. In addition, the resistance to change in hydraulic conductivity of the sand-attapulgite clay-bentonite (S-AC-B) mixtures containing 20 clay soil is improved by increasing the percentage of attapulgite clay in the clay soil admixture from 50 to 100, and by decreasing the concentration of the CaCl₂ solution from 0.5 M to 0. The first exposure effect also decreases with increasing amount of attapulgite clay in S-AC-B mixtures containing 20 clay soil. However, the resistance to change in hydraulic conductivity resulting from substituting only half of the bentonite in the S-B mixtures with an equal amount of attapulgite clay is not improved substantially due to the dominating influence of the bentonite portion of the sand-clay mixture.

The response of municipal solid-waste landfills at high levels of earthquake shaking is explored. Results from fully nonlinear dynamic analyses indicate that the seismic response of a landfill, particularly the seismic loading for the cover, can vary significantly due to reasonable variations of waste properties, fill heights, site conditions, and design rock motions. However, for the base sliding case, the maximum seismic loading depends primarily on the amplitude (peak acceleration) and frequency content (mean period) of the design rock motion and the dynamic response characteristics of the landfill represented by its initial fundamental period. The calculated permanent base displacement is largely a function of the k_y/k_max ratio, but it is also influenced significantly by the intensity, frequency content, and duration of the design rock motion and the dynamic response characteristics of the landfill and its foundation. Charts with normalized parameters are presented, which aid in the development of preliminary estimates of the expected seismic loading and displacement. These analytical results are consistent with observations from available case records.

Investigations into pile behavior in dense marine sand have been performed by the Institut Français du Pétrole and Imperial College at Dunkirk, northern France. In the most recent series of tests, strain-gauged, open-ended pipe piles, driven and statically load tested in 1989, were retested in 1994. An 85% increase in shaft capacity took place between six months and five years after installation. The possible causes are evaluated in relation to previous case histories, laboratory soil tests, pile corrosion, and new effective stress analyses developed using smaller, more intensively instrumented piles. The available evidence suggests that a circumferential arching mechanism develops during pile driving that limits the radial stresses acting on the pile shaft. It is concluded that creep leads to a breakdown of these arching stresses, allowing increases in radial stress and hence gains in shaft capacity. Increased dilation due to sand aging may also contribute. Finally, the measured pile shaft capacities are compared to predictions made using existing design methods.

This paper applies numerical limit analyses to evaluate the undrained stability of surface footings on nonhomogeneous and layered clay deposits under the combined effects of vertical, horizontal, and moment (V, H, M) loading. The analyses are able to resolve the true collapse loads to within ±5% by computing rigorous upper and lower bound solutions for these plane strain problems using linear programming methods and finite element discretization. The results focus on the geometry of the three-dimensional failure envelope and illustrate the effects of underbase suction and non-homogeneous undrained strength profiles. The existing empirical bearing capacity factors for inclined, eccentric loading are conservative, often underestimating the exact collapse solutions for footings on homogeneous clay by more than 25%. However, the same correction factors can become completely unreliable when there is a significant undrained strength gradient. The numerically-derived failure envelope for footings on homogeneous clay is described approximately by curve fitting techniques using relatively simple functions that can be used to update the existing bearing capacity factors.