2009-05-12_PERMIT FILE - C1980007

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Client Memorandum Geotechnical and Slope Stability Analyses Sylvester Gulch Facilities Area • March 20, 2009 Page 2 2008 GEOTECHNICAL PROGRAM Test Pits and Sampling During October 2008, four test pits were excavated east of the existing Sylvester Gulch Pond 017 (Figure 1). This potential location was selected for the geotechnical study because the area was previously evaluated and permitted for construction of sludge drying lagoons as part of the original design for the Sylvester Gulch dewatering facility. However, the sludge drying lagoons were never constructed. The test pits were excavated by mine personnel using a small backhoe, to depths ranging from 7 to 11 feet below ground surface (ft bgs), and were logged by personnel from Lambert and Associates. The test pit logs provided by Lambert & Associates contain general descriptions of subsurface materials and pocket penetrometer results. Bulk samples and relatively undisturbed tube samples were obtained from the test pits and transported to Lambert's geotechnical testing laboratory for index, compaction, strength, permeability and consolidation testing. Summary of 2008 Geotechnical Testing Results • The test pits all reported similar subsurface conditions, with approximately 6 to 12 inches of stiff, moist silty to sandy clay with organics underlain by silty to clayey soil containing intermittent sandstone fragments of 3 to 24 inches. Pocket penetrometer results indicated a stiff to very stiff soil consistency throughout the test pits. The soil at the base of each test pit was apparently very stiff to hard. Although some weathered rock fragments were encountered, bedrock was not reached in the test pits. Water was not encountered in any of the test pits. The results of the 2008 testing program are reported in a letter report from Lambert and Associates (December 24, 2008) and are summarized in Table 1. The tests indicated primarily sandy clay to silty clay (CL to SC) soils throughout with in-situ moisture contents varying from 6.3 to 18.8 percent and dry densities ranging from approximately 81 pounds per cubic foot (pcf) to 111 pcf. Some zones of gravelly clay (GC) near the base of the southeast test pit (TP-4) were also tested. The soils generally have a moderately high plasticity with plasticity indexes (PIs) ranging from 12 to 23. A sample of one zone of high-plasticity, or "fat", clay (CH), obtained at a depth of approximately 4 ft bgs in the northeast test pit (TP-2), yielded a PI of 37. Two additional samples obtained from the October 2008 test pits, which were not tested originally, were subsequently tested in 2009 to determine if additional high-plasticity clay might be present (Lambert & Associates, 2009). Those two samples, from TP-1 (2 ft) and TP-3 (3 ft), did not indicate high plasticity, with PI results of 19 and 15, respectively. • Rev. 04109-TR716