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4 0 SOIL ARCHING In 1936, Karl Terzaghi published his seminal work on soil arching. The University of Florida performed an extensive series of experiments further defining the mechanics of soil arching in 1961. At the Waterways Experiment Station(WES), the U.S. Army Corps of Engineers(USAGE)performed a comprehensive set of large scale arching experiments in 1965. Each study supports and refines the work and conclusions carried forward from previous experiments. A brief, simplified overview of some of the conclusions found in these reports follows: • When the bearing pressure below a soil is removed(the large scale experiments utilized a trapdoor configuration),the soil will arch over a cavity centered at the midpoint of the yielding zone. • The pressure over the yielding zone(trap door)will be less than the average vertical stress due to the weight of the soil layer. • The pressure immediately outside the perimeter of the yielding zone will be greater than the average vertical stress due to the weight of the soil layer. • This arching condition is permanent. • Above a given height(the apex of the arch), all loads whether they be from the weight of soil or fi=n a surcharge will arch around the cavity and have no impact on the pressures measured immediately above the yielding zone. • 3-dimensional arching(yielding zone in the shape of a circle)is more efficient than 2-dimensional arching(yielding zone in the shape of an infinitely long trench). • The pressure over the yielding zone is equal to the weight of the mass within the arch, if the height of the soil(and waste)exceeds the apex of the arch. • The dimensions of the cavity formed by the arch are dependent on the soil strength. • The approximate shape of the arch is triangular rather than spherical. As a result, a yielding zone in the shape of a circle would produce an arch in the shape of a cone rather than a half sphere. The subsidence model provides the geometry necessary to analyze the arching phenomenon. The data collected in the experiments discussed above allows one to predict the normal force transferred to the geosynthetic reinforcing material within the landfill liner system. (3)