2009-06-10_ENFORCEMENT - M1983176

Home Browse Search

DEERE & AULT MEMORANDUM TO: Mr. Dan Hunt f FROM: Christoph Goss PhD, PE/Don Deere PE, PG DATE: June 10, 2009 4"" RE: Speer Pit Short Term Drain 'f 4-/f2 174 This memo is in response to your request of June 5, 2009 to provide a short term drainage solution at the Speer Pit for submittal to the Colorado Division of Reclamation, Mining, and Safety (DRMS) on June 10, 2009. We understand that DRMS has directed you to mitigate the potential impact on local groundwater levels due to prior partial backfilling of the gravel pit with inert fill. We further understand that you require both a short and long term plan to draw groundwater down in the vicinity of the pit. The overall long term design concept is to convey water from the upgradient side of the inert fill to the South Platte River via gravity. Our short term recommendation is to install a temporary drain trench or pipe in the southeast comer of the property as shown on the attached Figure 1. This drain would run from the east property line (20' offset), bifurcate the existing area of fill, and discharge into the existing pit bottom that has not been backfilled. The east half of the drain would run through un-mined sand and gravel, the remainder through the inert fill. The drain would pick up groundwater in the un-mined sand and gravel and convey it into the pit bottom, thereby locally drawing down the groundwater table. We understand that you will pump it from the pit bottom to the South Platte River using your existing dewatering system. As shown on Figure 1, the drain can either be an open trench or buried pipe. Given the temporary nature of this drain, pipe materials should be left to the contractor and may be recycled from other projects. The critical feature is that the pipe be slotted or perforated and encased in coarse grained material along its entire run within the un-mined sand & gravel. If the trench option is chosen, the trench will likely require periodic maintenance to clean out local. sloughs and slope failures. If the groundwater velocity in the sand and gravel is high, causing the material to wash out from the trench walls, the trench should be backfilled to the top of the groundwater table with clean cobbles. The trench invert in the fill should have a layer of onsite dirty cobble ("bones") to limit erosion. We recommend that the east end of the drain be installed at elevation 5012'. Based on observations by Asphalt Specialties in March 2009 at monitoring well MW-3, the groundwater table in this area 600 S. Aitport Road, Building A, Suite 205 Longmont, CO 80503 Phone: 303-651-1468 • Fax: 303-651-1469