2007-08-07_REVISION - M2003016 - Laserfiche WebLink

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1.0 PROJECT OVERVIEW 1.1 Introduction This Construction Report documents the construction activities completed for the Heit Pit Underdrain for the Heit Pit Aggregate Operation in Weld County, Permit M-2003-016. The Heit Pit groundwater impact due to the proposed slurry wall was presented and historic groundwater elevations and fluxuations were determined as part of Technical Revision #2 (TR-2), dated September 14, 2005, by Wright Water Engineers, Inc. Based on conversations with staff members at the Division of Reclamation and Mine Safety (DRMS), an underdrain design was developed, presented and approved in Technical Revision #3 (TR-3), dated March 9, 2007 by Civil Resources, LLC. This Construction Report discusses the overall project concept, observed and reported construction steps, and changes from the approved TR-3. 1.2 Project Summary The underdrain construction was completed to mitigate the potential "mounding" of groundwater against the recently constructed slurry wall. The Heit Pit area prevailing groundwater flow direction is north-northeast toward the South Platte River. Locally, the groundwater levels and flow directions are also impacted by: 1) the groundwater "shadow effect" from the slurry walled Koenig Pit immediately to the south of Weld County Road (WCR) 20 and the proposed Heit Pit, and 2) the Little Dry Creek drainage which roughly parallels the west side of the Heit Pit at distances ranging from 230 feet to 450 feet to the low flow channel. The slurry wall around the Heit Pit could cause "mounding" of groundwater on the upstream side of the proposed slurry wall as described in TR-2. The designed underdrain system minimizes upstream mounding to within an acceptable range of two foot within historic levels. The underdrain design installs a perforated pipe with no slope within the overlapping zones of the permitted two foot rise of average groundwater elevation for the growing and non-growing seasons. The capacity of each underdrain was sized to convey the calculated aquifer flow currently being conveyed through the Heit Pit. Due to the pit configuration and existing topographic features, underdrain systems are designed to flow to Little Dry Creek. The underdrain daylights roughly 400 feet north of the southwest corner of the slurry wall, onto a riprap pad and surface flows across the existing broad drainage to the low flow channel of Little Dry Creek. The construction of the project was completed by Hall-Irvin. The project included approximately S00 feet of six inch diameter perforated pipe, 400 feet of eight inch diameter perforated pipe, filter fabric around each pipe run, three standpipe cleanouts with protective surface casing, riprap outfall protection, and crushed aggregate for pipe bedding and cover. Excess trenching cut material was spoiled on-site. The completed construction was modified from the approved TR-3 as discussed with yourself via phone on July 2, 2007. Those changes are discussed in Section 3.0. 2.0 UNDERDRAIN CONSTRUCTION Construction of the underdrain began at the southwest corner of the piton the south pipe. Due to the prevalence of very sandy soil and groundwater elevations above the pipe invert, a twenty foot trench box was used to shore the trench sides during installation. Multiple submersible pumps were used fo minimize the amount of standing water in the exposed trench. The contractor placed the filter sock covered perforated pipe to specified elevation using a construction level from the multiple construction hubs and stakes placed by their surveyor. Pipe was bedded and covered in imported granular material. Vertical cleanouts were installed per plans. The South six inch diameter perforated pipe was connected to the west side eight inch pipe via a 90 degree sweep followed by an eight inch to six inch coupler and vertical cleanout standpipe. Construction of the eight inch west side drain followed the same procedure as the south side pipe with trench boxing, pumping, and pipe bedding/cover. Both trenches were backfilled with trenching spoils and compacted by track and wheel rolling with on-site heavy equipment. Construction Report- Heit Pil Underdrain - 1 - August, 2007