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I I I I I I I I I I I I I I I I I I I A.3. SOIL AND ROCK DESCRIPTION I. General. The overburden soIls at the dam sItes consist of overburden clays overlyIng a residual clay or weathered shale bedrock. The overburden soIls were deposited and distributed by various depositional and erosIonal processes. The materials at the dam site however ate relatively uniform In dIstrIbution and physical characteristics. The soIls are predominantly sandy, silty clays with low to medIum plllstlcIty. The PIerre Shale Bedrock underIles the overburden soIls and the weatherIng Is typically nonuniform In nature and varies In thickness. The upper weathered zones are typIcally a grey - green, brown, and blue In color and are typIcally Iron stained. The weathered zone ueuaIly consists of shales, sIltstones, and claystones wIth some Interbedded sand ~eams. ThIs Is typical of the PIerre Shale throughout this region. The unweathered PIerre Shale Is typIcally blue grey or a dark gray In color. This strata will consist of shales, siltstones, claystones and some Interbedded sandstones that are often hIghly cemented and very hard. The PIere Shale Formation Is known to extend to a depth of several hundred feet In this area. The existing fill In the two dams consists of a mixture of the overburden clays and sandy clays that were most likely borrowed from the reservoir site close to the dams. Thus the fills are somewhaf. varIable In nature but Ilre on the most part made up of soIls of like characteristics. No continuous sand or gravel materials or layers were found Immediate to the dam site and no apprecIable amounts of these materials were detected In the dams except In e.n Isolated zone below the central portion of the maIn (iam. Some sands with a ~mall amount of gravel were detected In a few of the borIngs but are dIspersed over the site as a result of theIr alluvIal orIgIn. 2. FIne GraIned SoIls. The fIne-grained soils at the site consIst of low plastic to medIum plastic silty clays and silty, sandy clays classified predominantly CL by ASTM D2487. These materials make up the bulk of the overburden solIs, the resIdual soIls, and the dam fills at the sIte. A descrIption of the soils and theIr thIckness can be found In the Chen solIs report In APPENDIX D. 3. PIerre Shale Formation. The shale formations at the sIte consIst of shales, claystones, and sandstones of the PIerre Shale Formation. Some determInation of the properties of the shale formation are Included In Table I In the Chen report. The PIerre Shales are known to be expllnslve but are also dense and have hIgh shear strengths In general. SlIcken sIded surfaces are found In some locations In the Pierre Shale and are known to exhIbit low residual shear strengths. For the heIght of the dam for No. 6 the strengths of the shale would be expected to be more than adequate even In a residual etate. 4. Borrow MaterIals. The prImary source of borrow materIals Is expected to come from wIthIn the reservoIr area. The quantity of clay borrow from the reservoIr area Is several times that requIred for the project. The quantity of avaIlable clay type materIals at the site Is estimated to be In excess of 500,000 cublc yards. PAGK 3 Chlpter V - HO. 6 YeulblUty