2020-06-29_PERMIT FILE - M2020007 (3)

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DEERS (Xr A U L T 600 South Airport Road, Suite A-205 Longmont, Colorado 80503 303-651-1468 schnabel-eng.com I deereault.com MEMORANDUM TO: Jeremy Stanton DATE: June 26, 2020 COMPANY: L.G. Everist, Mountain Division SUBJECT: Response to DRMS Comments on Cannon Lands Slope Stability ADDRESS: 7321 E. 88th Ave., Suite 200, PROJECT L.G. Everist Cannon Lands Henderson, CO 80640 NAME/NO.: DA201041.00 FROM: Susan Rainey, P.E., CC: This memorandum is in response to comments received from the Colorado Division of Reclamation, Mining and Safety (DRMS) on June19, 2020 regarding Ragsdale Reservoirs, DRMS File No. M-2020-007, also known as Cannon Lands. 1. Factor of Safety Selection In the Cannon Lands Slope Stability Technical Memorandum by Deere &Ault dated June 5, 2020, data was presented that made the case that based on general soil index testing properties (gradation, liquid limit and plasticity index), site proximity (one mile away) and similar subsurface geologic profiles', soil and rock strength parameters from the Fort Lupton Pit could be used at Cannon Lands. The case was made that extensive testing at the Fort Lupton Pit, which resulted in the selection of conservative soil and bedrock strength parameters for slope stability analyses, made it appropriate to use the lower Factor of Safety from MLRB Policy No. 30. The comment states that, "the DRMS cannot accept the submitted analyses as demonstrating compliance with MLRB Policy No. 30," and outlines two options; A and B. Both options are to perform a site-specific geotechnical investigation. In response we are providing strength test results of the Laramie bedrock from the Fort Lupton site, and other sites in the front range also situated on the Laramie bedrock. The Cannon Lands site is located within the Laramie Formation. We are providing bedrock testing data because the strength parameters of the bedrock control the slope stability analyses. Often the laboratory strength values for claystone bedrock are much higher than actual empirical values observed in the field. There are three different types of strength of a claystone or stiff fissured clay. These are peak strength, fully softened strength and residual strength (remaining strength after significant shearing displacement). Peak strength is associated with undisturbed claystone. Fully softened strength develops after claystone is disturbed by excavation and wetting/drying cycles. The peak strength degrades over time, years to decades, from this disturbance until fully softened strength is reached, and cohesion is reduced to zero. We chose to use fully softened strength values for the Laramie claystone as a conservative yet realistic representation of bedrock behavior. One method commonly used for obtaining fully softened strength is to take the remolded strength. In 2010 as part of slope design work at the Fort Lupton Pit triaxial strength tests were performed on three samples remolded to approximately 95 ' Based on regional geologic mapping (Colton, 1978), the near surface bedrock in the project area consists of the Upper Cretaceous Age Laramie Formation. The bedrock is overlain by upper Pleistocene and Holocene (Quaternary age) gravel deposits associated with the South Platte River.