2018-11-21_REVISION - M2004044 ‎(22)‎

• Permit No: M2004044
• IBM Index Class Name: Revision
• Doc Date: 11/21/2018
• Doc Name Note: Part 5 of 5
• Doc Name: Request For Amendment To Permit
• From: Aggregate Industries-WCR
• To: DRMS
• Type & Sequence: AM1
• Email Name: JLE
• Email Name: MAC
• Media Type: D
• Archive: No

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TETRA TECH
MEMO
To: Joel Bolduc, Aggregate Industries, Inc.
cc: Connie Davis, Aggregate Industries, Inc.
From: Michelle Franke, El, Tetra Tech; Justin Stoeber, PE, Tetra Tech
Date: November 9, 2018
Subject: Updated Tucson South Proposed Gravel Mine Slope Stability Analysis;
Tetra Tech Job No. 200-23514-18004
This memo presents the results of the updated stability modeling for the proposed Tucson South Resource
aggregate mine (Tucson South) based on the updated factors of safety accepted by the Division of Reclamation,
Mining and Safety Minerals Program (DRMS) on May 16, 20181. The memo describes the methods and results of
an analysis conducted to evaluate the minimum distance between the proposed mine limits and surrounding
critical structures to avoid damage to the structures per the Mine Land Reclamation Board (MLRB) Construction
Materials Rules 6.3.12(b) and 6.4.19(b) and the memorandum from the DRMS dated May 16. 2018.
1.0 SITE LAYOUT
The proposed site is approximately 250 acres, located a half-mile from the City of Brighton, Colorado. The site
lies within the south half of Section 1 and the northwest quarter of Section 12 in Township 1 South, Range 67,
West of the 6th Principal Meridian. Land use around the site is aggregate mining, residential, and agricultural.
Tucson Street separates the proposed East and West Pits of the mine. Colorado Highway 7 (E 1601h Avenue)
separates the proposed East and West Pits from the proposed Southwest Pit. The South Platte River forms the
eastern border for the proposed East Pit and the Brighton Ditch forms the western border of the proposed West
Pit.
Figure 1 shows the proposed mine boundaries and slurry wall alignments. The proposed slurry wall will be
constructed around the perimeter of the East and West Pits. The slurry wall will cross Tucson Street in two
places:
1)Along the northern boundary of the East and West Pits
2)Along the southern boundary of the East and West Pits. No slurry wall will be constructed around the
Southwest Pit. The Southwest Pit will be mined via wet mining.
2.0 GEOLOGY
The site is in the alluvial valley of the South Platte River. Geologic mapping indicates that the mine is located on
an alluvial terrace corresponding with the Piney Creek and Post Piney Creek alluvium2. The bedrock underlying
the alluvium may be the Arapahoe Formation, which consists of claystone and sandstones.
A total of 130 boreholes have been drilled on the site. Drilling logs indicate the general subsurface profile consists
of an average of three feet (one to ten feet) of silty sand overburden, overlying an average of 23 feet (five to 46
feet) of sand and gravel, overlying a claystone bedrock. A mud lens is present in the area, mainly in the West and
Southwest Pits of the proposed mine.
Cazier, T. (2018, May 16). Re: Factors of Safety for Slope Stability/Geotechnical Analyses Associated with
Mining Operations. Denver, CO: Division of Reclamation, Mining and Safety Minerals Program.
2 Trimble, E. D., & Machette, N. M. (1979). Geologic Map of the Greater Denver Area, Front Range Urban
Corridor, Colorado. USGS Map 1-856-H, Version 1.1.
Tetra Tech
1900 South Sunset Street, Suite 1-E, Longmont, CO 80501
Tel [phone] Fax [phone] www.tetratech.com