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Mountain Coal Company, L.L.C. • 5.0 GEOTECHNICALSTUDIES 5.1 Field Investigation Exhibit 51 Lower Refuse Pile A subsurface field investieationwas conducted on the lower refuse pile site. This investigationconsisted of performing nine soil borings. installing five piezometers, excavatingnvo test pits, and performing four in- situ permeability tests. This work was conducted in May and June of 198. The soil boring work consisted of completing nine soil borings with the use of a drilling rig operated. by P8P Drilline under the direct supervision of Rocky Mountain Geotechnical. The soil borings were advanced using either a 4-inch flieh[ auger or a 6-inch hollow stem auger depending on drill hole conditions. Standard Penetration Tests were performed a[ designated intervals using the appropriate ASTM procedures. Bag samples were taken of all soil samples. At several locations, Shelby tubes were pushed into the underlying stratum soils to obtain samples suitable for shear testing. The logs of the nine soil bonne holes are found in Appendix E. The locations of the holes are shown on Fisures LWP-00'? to L WP- 004. Two tests pits were excavated with a small backhoe supplied by MCC and logged by Rocky Mountain Geotechnical. The test pits were created to obtain information on debris flow. Through the use of [he two test pits. six of the various debris flow events, as described in the geology pan of this report, were determined. These holes were later backfilled to the original ground surface. Four.4-inch diameter PVC monitoringvvells were installed in several of the test borings. These monitoring .+ wells were spaced around [he pile so that long term eroundvvater and water quality information could be obtained during construction of the lower refuse pile. A small I-1/? inch PVC piezometer was installed in Test Boring No. SB-4. This piezometer was used solely to determine groundwater elevation. All piezometers were installed by initially slotting a ~-foot section of the 4-inch or I-I/'' inch PVC pipe. The PVC pipe was then inserted into the queer hole and the slotted screen was placed as close to the bedrock surface as possible. The hole was backfilled with available gravel to within several feet of the natural surface. The remainder of the hole was sealed with bentonite pellets to insure that surface water does not enter the piezometer. The logs of these piezometer installationscan be found in Appendix F. The four monitoring wells and one piezometer installed at the time of the investigation showed [he colluvium [o be dry with the exception ofthe monitoring well adjacent to Sylves[erGulch. Sylvester Gulch is an ephemeral stream and it appears that a water mound exists beneath the gulch which dissipates rapidly into the colluvium. The presence of the waste pile will not increase the amount of surface infiltration which has historicallyoccurted (the refuse pile will be non-impounding). Since no "aquifer" was present during the investigation,~ve do not believe that one will develop in the future. The presence of the waste pile and the underdrains are likely to intercept and reduce the amount of infiltration crossing the interface between the waste material and the native colluvium, and, therefore. reduce historical values. Field permeability tests were performed on several of [he piezometers installed during the drilling program. The tests were performed by pumping clear water from a large water tank into the piezometer tube until the piezometer had been completely filled with water or an equilibrium level had been obtained. A Falling head permeabilitytest was then performed. •