2007-10-31_REVISION - M2006080 (2)

Home Browse Search

III. SEEPAGE ANALYSIS The purpose of this section is to document our fmdings with respect to estimated seepage rates resulting from various penetration depths of the proposed slurry wall into bedrock and to compare them to the allowable seepage rates as defined by the State of Colorado. Seepage rates from/into reservoirs are limited by the State of Colorado. Design and performance standards aze calculated for each site based on the height of the slurry wall and the surface area of the bedrock enclosed in the reservoir. The design of the slurry wall must meet the design standard. The perfom7ance standard is three times the design standard and accounts for field conditions encountered during construction. For the current project the calculated design standard seepage rate is 42 gallons per minute {gpm) and the performance standard is 126 gpm. The geology at the site consists of sand and gravel overlying highly permeable fractured shale underlain by relatively impermeable, massive claystone and sandstone. Due to the high- strength, massive nature, and depth of the claystone and sandstone bedrock excavatability was predicted by the Contractor to be time consuming and difficult. Therefore Lyman Henn was requested to analyze seepage rates for a slurry wall surrounding the gravel pits that would partially penetrate the fractured shale and not extend into the massive bedrock. The modeling software Seep/W by Geoslope was used estimate the seepage rates under the proposed slurry wall. Hydrogeologic properties were obtained from Lyman Henn's Draft Design Report, Soil-Bentonite Cutoff Wall for Loveland Ready Mix, dated August 17, 2007. The table below shows the input parameters used for modeling. The values of unit thickness and permeability represent averages derived from the subsurface investigation. Actual values varied across the site but were within a range such that arithmetic averages were considered appropriate to represent overall conditions. Hvdroeeoloeic Pronerties used in Seepage Analysis Geolo is Unit Thickness (ft) Permeabilit (cm/s) Sand & Gravel 21 Infinite Fractured Shale 13 4.70E-04 Massive claystone >15 3.90E-06 Slurry Wall 2.5 (horiz) 1.00E-07 Lyman Henn analyzed several penetmtion depths of the slurry wall into the fractured shale bedrock. A penetration of 10 feet into the bedrock, which would result in an average gap of 3 feet between the bottom of the wall and the massive claystone, was the shallowest penetration modeled. Progressively smaller gaps were modeled to determine what depth would satisfy the design standard seepage rate. Ahead pressure of 21 feet was used, which represents the maximum anticipated differential on either side of the slurry wall. Based on the results of our modeling and analyses, seepage rates exceed the allowable design standard if the slurry wall terminates in the fractured shale. In order to provide sufficient G:~PROIECI'S\I%021-000 Iovelaed Ready Mu Shirty Wall\R. Recordv avd Cortapovdcace~R.l l Repons~Fival Shirty Wall Desigv Repon.doc 4