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Lined Pit Guide 8 September 2000 CLAY BACKFILL PIT LINERS Applicants seeking the lower bond amount available under the regulated construction option for a reclamation plan that includes a clay lined reservoir must provide a design for the liner. The intent of the design can be effectively conveyed through plan and section views of the pit perimeters where a liner will be installed. The liner cross section should illustrate the dimensions and shape of the liner fill to be installed, the location, size, and shape of the keyway, and the dimensions of liner cover fill that will protect the liner soil from desiccation and erosion. The design should address the following elements that may affect the performance of the liner. Liner Fill Mixture • Fines -The liner fill should contain at least 20 percent fines defined as the percentage, on a dry weight basis, of material passing the No. 200 sieve. • Plasticity Index -The soil should have a plasticity index of at least 10 percent, although some soils with a slightly lower index maybe suitable. Soils with plasticity indices less than about 10 percent have very little clay and usually will not produce the necessary low permeability. Soils with plasticity indices greater than 30 to 40 percent aze difficult to work with, as they form hazd chunks when dry and sticky clods when wet. • Percentage of Gravel -The percentage of gravel (material retained on the No. 4 sieve) should in general not exceed 10 percent. Larger percentages are acceptable if it can be demonstrated that the design standazd for permeability can be achieved and that segregation of gravel into pockets that contain little or no fines will not occw during installation. • Stones and Rocks - No rocks larger than 2 inches should be present in the liner material. If two or more materials will be blended to prepare the liner fill, a description of the mixing process should be included in the design documents,- For example, an Operator may choose to prepaze the liner fill by mixing 50 percent weathered shale ripped from the floor of the pit with 50 percent overbwden. The mixing may be accomplished by a number of methods such as windrowing of the materials and multiple passes of mixing equipment such as a road reclaimer. YVater Content The molding water content at which the maximum dry unit weight is observed for a given compactive energy is termed the optimum water content. Soils compacted wet of the optimum water content tend to have lower permeability. This is because a soil that is slightly wet of optimum will more readily mold into a homogeneous mass that is free of clods. For liner fill, moistwe content slightly wet of optimum should be specified. For example: Material represented by the samples tested having a water more than 1 percent dry of the optimum water content, or more than 3 percent wet of the optimum water content will be rejected and shall be removed or reworked until the water content is between these limits.