2013-07-26_REVISION - M2008078 (4)

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M- 2008 -078 — GP Aggregates, LLC — West Farm Pit — Technical Revision Request No. 2 July 26, 2013 -2- Revision (reduction) of the total mining area As a result of the minor revisions to the phase boundaries discussed above the total mining area has been reduced from 185.52 acres to 185.26 acres, a 0.26 acre reduction. There is no change to the permit area. Revision of mining slope from vertical to 3:1 The mining slope has been revised from vertical to 3:1 with a minimum setback of 25 feet from the edge of the slurry wall. Although a 3:1 mining slope is inherently stable, we have verified that the slope stability of the new mining slope configuration will be stable during rapid drawdown conditions with an acceptable factor of safety for the varying soil conditions. See the attached "West Farm Pit Seepage and Rapid Drawdown /Slope Stability Analyses" by Cesare, Inc. dated March 21, 2013 for the results of our analysis. Also attached is the geotechnical investigation and report showing the borings and laboratory testing done for the basis for the seepage and rapid drawdown /slope stability analysis. Additional solid stem borings were done at approximately 100 foot intervals along the alignment to verify the top of competent bedrock depths. Addition of an interior impermeable slurry wall around revised phases 1 -4 An interior slurry wall has been added around Phases 1 -4. This was done to reduce dewatering volumes when those phases are mined during the dry mining of the sand and gravel within those phases. The slurry wall is currently under construction using a long boom trencher methodology. This methodology employs a trencher to cut through the overburden and sand /gravel into the underlying bedrock to achieve a cutoff of groundwater. The trencher adds bentonite to the trench and continuously mixes the soils for the entire depth of the wall. This produces a highly consistent and homogenized soil mixture throughout the wall. Water is added as the trenching occurs to ensure the mixture stays at a slump between 5 to 7 inches. This facilitates the homogenous mixing of the bentonite into the soils. The following quality control testing is being done on samples taken from varying depths within the slurry wall: Test Type Testing Interval Along Ali nment—F Specification During Construction Slump 75 feet 5 to 7 inches (target only) Unit Weight field 75 feet Info only Unit Weight lab 300 feet Info only Permeability 150 feet 1 x10 -7 cm /s P200 % fines 300 feet Info only Post Construction Hollow Stem Borin 500 feet Visual Homogenous Mix Verification Permeability 500 Feet 1x10 -7 cm /s SEO Leak Test Entire Perimeter Design Standard The wall will be tested to pass the design standard for leakage in the State Engineer Guidelines for Gravel Pit Lining prior to any groundwater being exposed in Phase 1 of mining. See the attached West Farm Pit Slurry Wall and Reservoir Construction Plans for detailed slurry wall alignment and depth information. 305 Denver Avenue, Suite D • Fort Lupton, CO 80621 • Ph: 303 - 857 -6222 • Fax: 303 - 857 -6224