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' Hall-Irwin Corporation Hokestra Slurry Walls October 30, 2014 Page 1 of 8 1 PROJECT OVERVIEW ' The Hokestra Gravel Pit (Hokestra) site is located in the north half of section 2, T2N, R68W and in the south half of section 36, T3N, R68W of the 61h P.M. in unincorporated Weld County, just east of Longmont, CO. The jobsite lies south of the Saint Wain River, east of 1-25, west of CR 93/a and north ' of CR 241/2. Two soil-bentonite backfilled trenches ("slurry walls") and a clay liner were installed from original ground surface into the underlying bedrock. The southern slurry wall enclosed an area that has been partial mined and the slurry wall will function as a groundwater barrier. The northern slurry wall has yet to be mined and will function as a water storage vessel, and may be mined in the future. The clay liner encloses a previously completely mined area and will function as a water storage vessel. The slurry walls and clay liner were designed to be installed completely through the soil into competent, low permeability sedimentary bedrock to significantly reduce groundwater seepage into the pit, thereby ' meeting the seepage standards for a lined gravel pit as defined by the Colorado State Engineer. Performance of the liners was demonstrated using a 90-day liner test as defined by the "State Engineer Guidelines for Lining Criteria for Gravel Pits (August 1999)". On October 7, 2014, the three cells were approved by the state and the approval letters are presented in Appendix A. The slurry walls are composed of a mixed backfill consisting of selected soil blend and bentonite clay. This backfill was designed and constructed to have a maximum permeability of 1.0x10-7 centimeters per second (cm/sec) and functions as a low permeability liner. The north slurry wall is approximately 4,358 ft. long. The wall has a minimum width of 36 in. and extends from/or near ground surface to a depth ranging from 13 ft. to 28 ft., averaging 20 ft. The south slurry wall is approximately 5,908 ft. long. The ' wall has a minimum width of 36 in. and extends from/or near ground surface to a depth ranging from 18 ft. to 27 ft., averaging 22.3 ft. ' The clay liner was installed a minimum of 6 ft. into competent bedrock. Removal of weathered and fractured bedrock was performed before commencing the 6 ft. key excavation. Ground conditions were monitored during excavation, and additional key depth was excavated when directed by the on-site engineer from J&T Consulting out of Ft. Lupton, Colorado (J&T). Martinez Associates of Denver, ' Colorado was retained by Brierley Associates to perform the quality control observations and testing. The clay liner perimeter was approximately 5,400 ft, and it enclosed an approximate surface area of 35 acres. ' Brierley Associates (Brierley) was retained by Hall-Irwin Corporation (Hall-Irwin) to perform design and construction observation and testing for the project. Select features were designed and observed by J&T as shown in the list below. J&T's Construction Summary memorandum will be presented under a separate cover. The features designed for this project were: • North slurry wall (designed by Brierley) ' South slurry wall(designed by Brierley) • Clay liner (designed by J&T) ' • Subsurface drain(designed by J&T) • 5-ft pipe remediation (designed by J&T) Last Chance Subdrain (designed by J&T) ' Slope Embankment (designed by Brierley) • Spillway (designed by J&T) • Pump Station (designed by J&T) ' BRIERLEY ASSOCIATES ' Creating Space Underground