2006-04-12_REVISION - M1983067 (5)

Home Browse Search

Mr. Ken Lambrecht Apri14, 2006 Page 2 The soil profile analyzed consisted of 0.5 feet to 4.5 feet of overburden clays overlying approximately 15 to 60 feet of sand and gravel on top of claystone bedrock. Because the depth to bedrock varies significantly across the site, three initial different cross-sections were modeled. Cross-sections were analyzed with bedrock depths of 13.5, 40, and 62 feet. DMG ANALYSIS. HIGHWALL SETBACK TO PROPERTYLINE The Division of Minerals and Geology (DMG) requires that gravel pit highwalls within 200 feet of significant manmade permanent structures (such as utilities) be investigated for slope stability. Furthermore, the DMG requires that the gravel pii highwalls be modeled as vertical unless it can be demonstrated that the slope angle of the highwall will be controlled during mining. Additionally, the DMG requires that the analysis be performed using the DMG parameters for sites where laboratory testing is absent. These parameters are given below on Table 1: Table 1: DMG Model Parameters Soil Type Effective FriMion Angle ' (de rees) Effective Cohesion c' (psfl Moist Unit Weight y"' (pm Overburden 28 50 114 Gravel 35 0 13D WeatheredBedtock 14 0 124 Bedrock 28 100 124 Co acted Cla Liner 24 100 119 ;parameters for weathered bedrock are much more conservative than those typically encountered in project area and, it is our opinion, that they are not representative of conditions at the site. it the property line setback analysis, we analyzed two cross-sections (a vertical highwall section and a 1 highwall section) assuming a 60-foot bedrock depth. The vertical section corresponds to a steep ghwall mining scenario where the highwall inclination is approximately 0.5:1. The 3:1 corresponds to fining the highwall at the final reclamation slope of 3:1. 2 below summarizes the results of our analysis. This analysis is relevant to the east side of the ty where the slung wall has been constructed with a 30-foot setback from existing utilities or rights-of--way. Typical required factors of safety for temporary mine slopes and reservoir ilanents are provided for comparison. Output graphics for each of the analyses are attached. Table 2: HiQhwall Setback to Slurry Wall DMG Analysis Results Setback Calculated Typical Required Model Distance ({eet) Factor o f Factor ajSafety Sa DMG analysis steep 37 1.00 1.0 hi hwall DMG analysis 3: ] 16 2 02 1.0 hi hwal] .