HomeMy WebLinkAboutRESOURCE RECOVERY FARM, SAND & GRAVEL MINE - PDP/FDP - 17-99 - CORRESPONDENCE - RESPONSE TO APPLICANT'Project: 99416
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due to the potential layering and lensic nature of the soils at this site, we are
recommending a maximum slope of the pit walls of 3:1 (3 horizontal to 1 vertical)..
Therefore at a maximum assumed depth of mining of 25 feet, a minimum setback of 75
feet from any man made structure will afford adequate protection from the risk of slope
instability.
One of the items for which setback protection will be necessary is a sewer line.
Obviously, this line. is buried. The depth of the line is not known at this time.
However, the critical offset is the sewer line and not necessarily the ground surface
above the line.
Thank you for consulting with us on this phase of the project. If you have any
questions concerning this report, please do not hesitate to contact us.
Sincerely,
i3` 'G T/F'%. SCOTT, COX AND ASSOCIATES, INC.
-
s/�,1 By
t.
M. Edwar Gl ssgow, PE
Profession'a+G eologist
SCOTT, COX&ASSOCIATES, INC.
M. Edward Glassgow, IV, P.E.
May 11,1999
Tom Shoemaker
Natural Resources Director
City of Fort Collins
% Mike Hart
Hart Environmental
P. O. 1303
Boulder, Colorado 80306
Ladies and Gentlemen:
consulting engineers • surveyors
William H. Mitzelfeld, P.E.
Project: 99416
At your request, Ed Glassgow of Scott, Cox & Associates, Inc., has reviewed the
available documentation and the Colorado Mined Land Reclamation Board's
regulations regarding mining and man made structures and have conducted a site
inspection at the City of Fort Collins Resource Recovery farm site, south of Prospect
Road in Fort Collins, Colorado. The purpose of this was to allow us to provide this
letter, which addresses our recommendation for offset of the bottom of the pit from
existing facilities (roadways, buildings and utility lines).
The available documentation (a total of 13 borings completed at scattered locations
across the site) shows that the soils at this site consist of an average of approximately 6.9
feet of overburden (consisting of topsoil, sand, gravel, silt and clay, sometimes
intermixed) atop an average of approximately 11.0 feet of sand and gravel. The
groundwater table, at the time of the investigation ranged from depths of 3 to 11 feet
below the existing ground surface. Bedrock was encountered at an average elevation of
approximately depth of approximately 17.9 feet. It is anticipated, from our review of
the boring logs, that the pit depth could range from 15 to about 25 feet.
The soils encountered at this site have varying angles of repose. The soils that will
control the stability of the slope are anticipated to be the sand and gravel. Based on our
experience in this area, reference geotechnical engineering literature ("Fundamentals of
Geotechnical Analysis" by I. S. Dunn, L. R. Anderson and F. W. Kiefer) an anticipated
conservative angle of internal friction for this material is approximately 26°. This is
based on the assumption that dewatering will keep the face of the pit moderately dry
during the period of time that the pit is active. This corresponds to a slope of just over
2:1. It has been our experience that the risk of instability at that slope is low. However,
1530 55th Street • Boulder, Colorado 80303 • (303) 444-3051 • Fax: (303) 444-3387
Issue 2. Mine dewatering discharge to Boxelder Creek
LaFarge's existing NPDES permit for the Company's East Rigden sand and
gravel mine authorizes groundwater discharges into the Poudre River. It is our
understanding that the relocation of the discharge point to the Boxelder Creek
involves a minor revision to the existing NPDES permit and that LaFarge would
be amenable to such a relocation while mining the RRF parcel.
Issue 3. Post reclamation elevations of the ground along the wastewater
lines.
The reclamation of the RRF parcel does not contemplate any changes in the
existing ground elevations within the District's sewer line easements.
I trust that this letter has adequately addressed the District's concerns as expressed in
your August 18"' letter to Mr. Hart. If you have any questions or would like further
explanation or clarification regarding the proposed sand and gravel mining at the RRF,
please do not hesitate to call me.
Sincerely,,
Tom Shoemaker
Natural Resources Director
Encl.
Cc: Mike Hart
Steve Olt
Kaia Anderson
Comm 'ty Planning and Environmenta-
Natural Resources Department
City of Fort Collins
October 13, 1999
Mr. Ravi Srivastava, Ph.D., P.E.
District Engineer
Boxelder Sanitation District
P.O. Box 1518
Fort Collins, CO 80522
�rvices y ,' recycled paper
RE: Proposed Sand and Gravel Extraction at the Resource Recovery Farm
Dear Ravi:
I am writing this letter in response to your August 18, 1999, letter to Mr. Michael J. Hart,
consultant to the City of Fort Collins (City), regarding the City's proposal to mine sand
and gravel from the west side of the Resource Recovery Farm (RRF).
The following responses are intended to address the order and substance of the three
primary issues raised in your letter.
Issue 1. Proximity of mining to the District's sewer lines, stockpiling of
overburden and erection of structures on the sewer line alignments, and
access to the sewer lines by the District's maintenance crews.
All mining will be conducted outside of the District's sewer line easements.
Excavation setbacks from the edge of the District's easements will be based in
part on the recommendations of Scott, Cox & Associates, Inc. our consulting
engineers in this matter. I have enclosed a copy of their written recommendations
regarding slope stability and setbacks.
Stockpiles of topsoil, overburden , and sand and gravel will not be placed over the
District's wastewater lines. It is possible that some overburden from Stage 3 will
be trucked from the site. No permanent structures will be placed on the sewer line
alignments; however, field conveyors will be used to transport the sand and gravel
from the RRF. At some point, they will have to cross over the sewer line that
runs along the west side of the area proposed to be mined.
Finally, in order to ensure that mining will not impact any of the District's lines,
we will work with you and the mining operator to locate and stake all of the lines
in the vicinity of the proposed mining area prior to the start of any mining
activities.
281 N. College Ave. • PO. Box 580 • Fort Collins, CO 80522-0580 • (970) 221-6600 • FAX (970) 224-6177