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Mr. Mike Owens AEE Project No. 6J77-0OOOtb Sinclair Pipeline Easement Stability Study November a, 1996 Del Camino Pit Pape 2 Weld County, CO A comment about rapid draw down is in order. This refers to the development of hydrostatic uplift pressure in a soil slope when the water level is taken down faster than the ability of the material to drain or dissipate pore water pressure. For a granular soil such as is being mined on this site, we expect fairly rapid dissipation of pore water pressure due to the highly permeable nature of the material. For the temporary condition where a steep, un-reclaimed pit slope is allowed to flood (i.e. tum off the dewatering pumps), and then the pit is pumped out, we would expel the slope to have first raveled to near the angle of repose, somewhere between 1:1 to 1.5H:1V, due to seepage forces and the high ground water exit gradient. Because the permeability of sand and gravel is fairly high, and about 22 feet of hydrostatic head would have to be dealt with to again make the pit dry, it would be reasonable to expect that the perimeter dewatering system will be in operation concurrent with the pumping out of the pit. Therefore, it is unlikely that rapid draw down would occur because drained sand and gravel would be well back into the embankment as the pit is pumped out. Recognizing DMG's comments and the realities of sand and gravel mining, we recommend that mining be done with temporary pit slopes no steeper than 0.5H:1V, and more shallow if pit conditions necessitate. In the unsaturated condition, analysis of this configuration gives a factor of safety of 2.54 for a failure surface extending from the mining limit 30 feet back to the easement line. Considering an average of 3 feet of positive pore pressure along the saturated length of the same failure surface, a factor of safety of 1.36 was generated. This is considered to be adequate. In this analysis, we have considered the pipeline easement as essentially inviolable. The easement line leaves a minimum of 20 feet and a maximum of 40 feet horizontally to the pipeline location. This is with the understanding that should slumping and raveling into the pit occur from gravity, flooding or future dewatering, the failure surface will daylight within the zone between the mining limit (set bads) and the pipeline easement and can be relatively easily repaired. If it must be demonstrated that absolutely no encroachment into the area behind the mining limit can occur, then even temporary pit slopes should be cut no steeper than 1.3H:1V, which is the approximate long-term angle of repose. However, this does limit the recovery of a valuable, permitted resource for little, if any benefit. As with any slope steeper than the natural angle of repose, it is recommended that reclamation to plan contours be done as soon as practicable. It is our opinion that the above recommendations will not result in slope failures that extend within the pipeline easement.