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• groundwater and surface runoff away from the failed area. Ideally, the trench drain should be excavated <br />down into the underlying bedrock materials a minimum of 2 feet. Based on the subsurface information <br />obtained from the test holes, this would require the drain to be constructed to depths ranging from 20 to 40 <br />feet beneath the existing ground surface. The drain should consist of a six-inch diameter perforated PVC <br />pipe that is placed at the bottom of the trench and be sloped to daylight down to the southwest. The trench <br />should be backfilled back to within 2 feet of the finished ground surface with free draining gravels. The <br />surface of the drain should be constructed with compacted clays and to a V-ditch configuration so that the <br />surface runoff can be diverted away from the downslope azeas. <br />It should be noted that there is a risk of further slope failures occurring during the construction of the above <br />systems due to the excavation of relatively steep and unreinforced slopes during the construction of the <br />drain and removal of the unstable soils from the keyway area constructed at the base of the earthen buttress <br />fill. <br />Another alternative would be to construct a driven steel H-pile or a straight shaft drilled concrete pier <br />retaining wall through the failed areas. This type of wall system would consist of the installation of either <br />steel H-piles or concrete piers placed at relatively close spacing and that will penetrate the underlying <br />bedrock materials as least twice the height of the overburden soils being retained. However, due to the <br />vaziable depth to the underlying bedrock materials, site constraints and access conditions, we do not <br />believe that this type of system will be economically feasible for this project. <br />Regardless of the slope failure mitigation measure used, we strongly recommend that the groundwater and <br />slope conditions at the site be further monitored this fall and next spring and summer so that alternatives <br />for a feasible drainage and stabilization plan can be evaluated. <br />Limitations <br />The recommendations given in this report are based on the soil and groundwater conditions encountered in <br />the test holes and test pits, and those exposed during this investigation. The test holes and pits were widely <br />spaced to determine the general subsurface profile in the failed areas at the subject site. We believe that <br />this information gives a high degree of reliability for anticipating the performance of the <br />reconstructed stabilized slope; however, our recommendations aze professional opinions and cannot control <br />nature, nor can they assure the soils or groundwater profiles beneath or adjacent to those observed. We do <br />not guarantee the performance of the stabilized/reconstructed slopes in any respect, only that our <br />engineering work and judgments meet or exceed the standard of care of our profession at the time they <br />. were rendered. <br />Job Number: 07-7600 NWCC, Iuc. page 8 <br />