User:Ryno2009/Municipal Solid Waste Landfills under Seismic Loading

Introduction
A municipal solid waste landfill must be able to withstand a variety of external and internal threats to its stability over the course of its lifetime. Earthquakes are one possible source of trouble for landfills in seismically active areas across the world.

Problems Associated with Seismic Loading
The tension in a landfill geomembrane rises significantly during an earthquake, as a result of a weak frictional force between the geomembrane and underlying clay liner. This could lead to stretching and possible tears in the liner system. The top of the landfill can also be significantly affected by earthquake loading. Cracking of the cover liner can occur, and methane collection systems can be moved relative to the cover. An extreme example took place at the Chiquita Canyon Landfill, where geomembrane tears were measured up to 23m after the Northridge Earthquake of 1994. Soil liquefaction, which can take place in the soil underlying a landfill during earthquake loading, can lead to a differential displacement of waste.

Properties Governing the Seismic Stability of Fills
Stiffness of the waste in a landfill is one of the most important factors in determining how it will react under seismic loading. Shear wave velocity is the most common way to describe the stiffness of a landfill, and both can be found to increase with depth. As the shear wave velocity and stiffness increases for a waste fill, the maximum equivalent acceleration increases at any given depth. This acceleration value represents the maximum average acceleration acting on the waste above the given depth. Height and unit weight of waste are also primary factors governing the stability of landfills under earthquake loading. Increasing the depth of a waste column, typically will lead to a decrease in the maximum horizontal acceleration felt at the surface of that landfill during an earthquake event. Unit weight of waste is important in the analysis of geomembrane puncture and pipe crushing during an earthquake. Underlying rock formations can be important in determining how a landfill will react to earthquake shaking. Unstable rock such as limestone could yield during an earthquake, leading to a partial collapse of the fill. A major concern in this case would be the potential contamination of water sources that may be located below this landfill. Properties of the earthquake also require consideration during this analysis. Intensity of the earthquake motion felt by a landfill is usually measured by the maximum horizontal acceleration of the fill during the event. The fundamental period of the landfill, defined as four times the landfill waste height divided by the shear wave velocity of the landfill, is an important property which can be compared with the period of incoming earthquake motion. When the period of earthquake motion closely matches the fundamental period of the landfill, significant amplification of ground motions can occur leading to increased displacement in the fill.

Procedures used for Modeling Seismic Stability of Landfills
There are several different available methods for use in analyzing the seismic stability of landfills including mathematical models, laboratory tests, and computer programs. Many mathematical models that are currently being used in the analysis of landfills were initially created to assess the stability of earth embankments. The first common mathematical method used is a pseudostatic slope stability analysis. With this approach, a portion of the landfill is considered to be a sliding mass. A horizontal and static force is applied to the side of this mass, and is used to represent the force from earthquake loading. With this information, a limit equilibrium analysis can be performed to find a factor of safety for the mass sliding. The fact that the earthquake force is modeled as a constant force acting in one direction, represents this models major limitation. In actuality, earthquake loading can change direction and is certainly not constant. A slightly different model can be used to analyze landfills under loading, and is referred to as a seismic permanent deformation analysis. The most common type of permanent deformation analysis is a Newmark type analysis. In the Newmark analysis, a sliding mass of landfill is also considered. This method looks closely at the yield strength of the system, which prevents the mass from sliding. Movement in the system occurs when a driving force on the sliding mass is greater than the yield strength. An interesting assumption is made in this analysis procedure, as the vertical component of ground motions within the landfill is considered to have little effect on the overall displacement in the system. Laboratory simulation of landfill seismic loading is also commonly performed. The first type of test generally used is called a shaking table test. This type of analysis works to explore the strength characteristics of interfaces within the landfills. Of primary concern are the interfaces between soil and geomembrane, as this is usually considered to be a weak point in the system Another test setup is called a dynamic centrifuge test, and works to model the landfill in a scaled down form. Typically in this approach, a small simplified landfill is constructed in a test box. A geomembrane and clay liner are placed at the bottom of a test box. Usually the waste used in this model is synthetically prepared in the laboratory, and consists of a mixture of peat, kaolin clay, and sand. During this testing, earthquake loading is simulated on the sample landfill. A major application of this type of testing is to observe the behavior of geomembranes during ground shaking. To achieve this, load cells can be attached to the test geomembrane prior to testing. Tension in the geomembrane can be measured, and has been found to increase by up to 40% during earthquake loading. A key factor in liner strength involves the rate at which it is loaded. Faster loading rates generally lead to higher resisting strengths within the membrane.

Difficulties Associated with Assessing Seismic Stability of Landfills
There are many challenges that researchers face when assessing the seismic stability of landfills. First, the uniqueness of many landfills makes it difficult to apply test results from one site to another. Also, measuring properties of landfills such as waste unit weight could lead to health concerns, as the handling of waste could be dangerous. Accessing this waste would likely involve a damaging of the cover liner, which could compromise the ability of the system to function properly. Landfills are systems which change over time, so periodic evaluations may be necessary. Finally, finding representative samples for use in testing and analysis is nearly impossible, due to the extreme heterogeneity of waste.

Case Study
The Olympic View Sanitary Landfill, located in Washington state, has been studied to analyze the reactions of landfill covers to earthquake loading. The Nisqually Earthquake, with center located 39 km away from the landfill site, was the source of the ground motions. During shaking, workers present who were standing on the ground next to the fill claimed to feel significant shaking. Those located on top of the fill hardly noticed any motion at all. This works to show the attenuation of the ground motion by the waste fill. The most noticeable deformation that occurred during shaking involved the landfill gas risers. These moved up to 30 mm in some cases, but serious damage was avoided.