2019-01-18_REVISION - M1974086 (16)

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Ralston Quarry Slope Stability The contractor should take pro-active measures to control surface waters during construction and maintain good surface drainage conditions to direct waters away from excavations and into appropriate drainage structures. Temporary slopes should be protected against erosion. SURFACE DRAINAGE Effective surface drainage should be established following remedial construction. The graded area should be observed and maintained, re-establishing effective drainage as necessary so that water is not allowed to pond on or below the slope. In GROUND's opinion, the retention ponds constructed along the toes of the slope in the early 2000's reportedly for water quality purposes were major factors contributing to the subsequent slope failures. Those retention should be removed and the approximate prior surface gradients should be re-established. EMBANKMENT BACK DRAINS As indicated above, the benches excavated for construction of the stability embankments should be provided with back drains at the heels of the excavation. A typical layout of a back drain array is provided on Figure 14. Multiple high points and discharge lines should be constructed, as appropriate. Geotechnical Parameters for Back Drain Design Back drain design should incorporate the parameters below. The actual back drain layout and outlets should be developed by a civil engineer. Typical, cross-section detail of back drains for this project are provided in Figure 15. Each back drain array should be tested by the contractor after installation and after placement and compaction of the overlying backfill to verify that the system functions properly. 1) The back drain systems should consist of perforated, rigid, PVC collection pipe at least 6 inches in diameter, non-perforated, rigid, PVC discharge pipe at least 6 inches in diameter, free-draining gravel, and filter fabric. Job No. 18-3046 GROUND Engineering Consultants, Inc. Page 28