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TETRA TECH <br />October 27, 2010 <br />Ms. Asune Best Johnson, AICP <br />Lafarge West, Inc <br />Business Paris Circle, Suite 200 <br />Longmont, Colorado 80504 <br />Re: Stability Analysis for Lafarge Iverson Property, Weld County, Colorado; <br />Tetra Tech Job No. 133-23511-10002 <br />Dear Ms. Johnson: <br />This letter addresses Construction Materials Rule 6, Section 4, Subsection 19, Exhibit S - Permanent Man-Made <br />Structures (6.4.19, Exhibit S) for the permit for the above-referenced aggregate mine. <br />The proposed mine is located in the central portion of Section 34, Township 6 North, Range 66 West of the 6'h <br />Principal Meridian. The site is located in Weld County, Colorado. Several aggregate mines are present in the <br />area. Other land uses in the area are predominately agricultural and rural to low density residential. Several <br />structures including the Poudre River Trail recreational path, a communications tower and associated <br />appurtenances, the Boyd Freeman irrigation ditch, gas wells and associated appurtenances, fences, and a single <br />family residence with associated out buildings are within 200 feet of the mine limits of the proposed mine area. <br />The locations of the structures are shown on Figure 1. <br />Regional geologic mapping (see below) indicates mining will be in floodplain alluvial deposits. Borehole data <br />(Attachment B) encountered bedrock at depths ranging from 14 to 29 feet below the surface indicating mining <br />conditions that result in varying highwall heights. <br />GEOLOGIC STRATA AND MINING CONDITIONS <br />Geologic mapping by Hershey and Schneider (1972) indicates the proposed mine site is south of and adjacent to <br />the Cache La Poudre River on the flood plain alluvial deposit known as the Valley Fill alluvium. The deposit is <br />composed of sands, silts, and clays. The site subsurface conditions were investigated through an exploratory <br />drilling and monitoring well installation program conducted by Tetra Tech and Lafarge. <br />The boring data indicates the depth to bedrock along the proposed mine limit ranges from approximately 14 feet in <br />the southeast corner to 29 feet in the northwest corner. Mining of the property will result in highwalls ranging <br />from approximately 14 to 33 feet high. Mine highwall heights will vary with depth to bedrock. Groundwater <br />depths typically averaged 7 to 9 feel below the ground surface. <br />From a geoteclurical standpoint, the sand and gravel will form most of the mine slope. These soils are generally <br />strong and stable, particularly when dewatered. Minor amounts of clay will lie at the ground surface and locally <br />as lenses within the sand and gravel. <br />KNOWN STRUCTURES WITHIN 200 FEET OF MIrVbVG LIMITS <br />Known man-made structures within 200 feet of the mine limits are listed below: <br />• Three Noble Energy gas wells (Labeled Well A, B. and C on Figure 1) <br />• Gas lines associated with the Nobel Energy gas wells <br />• A Noble Energy gas separator