2018-05-22_PERMIT FILE - M2018027 (4)

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Mr. David W. Hunt May 13, 2018 Page 4 approximately 69 feet from a buried telephone line.A high phreatic groundwater surface was included in the model outside of the slurry wall. This is the section where potentially weak soils are closest to a structure. To be conservative,we modeled overburden in the upper half of the mine slope. ► Section 3: This section is on the west side of the cell and considers the tallest highwall at the mine. In this section the highwall is approximately 95 feet from the slurry wall and the nearest structure is a gas line locate about 17 feet from the slurry wall or approximately 115 feet from the mine limit. ► Section 4: This section is on the northwest side of the main cell and considers an oil field burner structure. The slurry wall is approximately 78 feet from the mine limit and the structure is approximately 115 feet from the mine limit. MATERIAL PROPERTIES The material index and engineering strengths assumed in this slope stability report are discussed below. Overburden The strength properties for the insitu silty to clayey sand overburden were based on field testing data and on our engineering judgment;the following parameters have been used to model the overburden. Dry Unit Moist Unit Saturated Unit Cohesion C'psf Friction Angle 0'° Weight(pco Weight(pco Weight c 103 114 126 50 29 Alluvial Sand and Gravel The sand and gravel is generally a fine to medium-grained sand overlying a fine to coarse grained sand that is typically medium dense and locally gravelly. The alluvial sand and gravel was modeled as follows: Dry Unit Moist Unit Saturated Unit Cohesion C'psf Friction Angle 0'° Weight(pco Weight(pco Weight c 119 129 130 0 35 Bedrock Bedrock below the alluvium is claystone, sandstone and interlaminated to interbedded claystone and sandstone bedrock. Sandstone is typically stronger than claystone. Claystone is generally a weak bedrock. To be conservative, we modeled the bedrock as claystone. For the claystone bedrock,two potential strength conditions were considered. These strength conditions are referred to as: 1) peak strength, and 2) residual strength. Peak strength is the maximum shear strength the claystone bedrock exhibits. The shear strength is made up of both cohesion (diagenetic bonding)and internal friction. Under short-term conditions for unsheared claystone, peak strength governs behavior. If a sheared surface or sheared zone is present within claystone as a result of faulting, slippage between beds due to folding, past shrink-swell behavior,stress relief,weathering,or from a landslide,the cohesion along the sheared surface is reduced to zero, and the angle of internal friction is decreased,due to alignment of clay minerals parallel to the shear plane. Under these conditions a claystone exhibits its lowest strength known as residual strength. Residual strength bedrock occurs in discrete zones, parallel with the sheared surface or zone, whereas fully softened strength occurs over a broader area(not used in this modeling). Based on data from other recent projects and engineering judgment,the residual strength claystone was modeled in a two-foot thick layer overlying the peak strength bedrock as follows: