HomeMy WebLinkAbout1.17 Geotechnical Letter-7-ribi
TETRA TECH
July 19, 2010
Mr. Sean Frisch
Lafarge West, Inc
10170 Church Ranch Way, Suite 200
Westminster, Colorado 80021
Re: Stability Analysis for Lafarge Cerise Property, Garfield County, Colorado;
Tetra Tech Job No. 133-23511-10003
Dear Mr. Frisch:
This letter has been prepared to address Construction Materials Rule 6, Section 4, Subsection 19, Exhibit
S - Permanent Man -Made Structures (6.4.19, Exhibit S) for the permit amendment for the above -
referenced aggregate mine. This analysis is for the Lafarge Cerise Mine Property.
The proposed mine is located in the southeast t/a of Section 26 and the southwest 1/4 of Section 25,
Township 7 South, Range 88 West of the 6th Principal Meridian. The site is outside of Carbondale,
Colorado in Garfield County. Several aggregate mines are present in the area. Other land uses in the area
are predominately agricultural and rural residential. Several structures including power poles, irrigation
ditches, fences, a gas line and meter, a telephone line, a private bridge, and state and county highways are
within 200 feet of the mine limits of the proposed mine area. The locations of the structures are shown on
Figure 1.
Regional geologic mapping (see below) indicates mining will be in terrace and pediment alluvial deposits.
GEOLOGIC STRATA AND MINING CONDITIONS
Geologic mapping by Ellis and Freeman (1984) indicates the proposed mine site is north of the Roaring
Fork River on an undifferentiated terrace and pediment alluvial deposit Holocene to Pleistocene in age
(1.8 mya to present). The deposit is composed of various amounts of cobbles, gravels, sands, silts, and
clays. The site subsurface conditions were investigated through an exploratory drilling program
conducted by others.
The results of exploratory borings drilled in 2007 indicate that bedrock was not encountered to the
maximum depth explored (119') within the proposed mine limit. Mining in the proposed mine limits
will result in maximum highwalls approximately 100 feet high. Mine highwall configurations will vary
depending on the location within the mine, and will have either a 3:1 (horizontal (H):vertical(V)) slope or
have a benched configuration. Locations of the highwall configurations can be found on Figure 2.
Groundwater was not encountered in the borings, was not observed in onsite piezometers, and is not
anticipated to impact mining operations. Groundwater was modeled at a depth of 90 feet below the
ground surface to represent a conservative scenario.
From a geotechnical standpoint, the sand and gravel will form most of the mine slope. These soils are
generally strong and stable, particularly when no groundwater is present. Minor amounts of clay will lie
at the ground surface and locally as lenses within the sand and gravel.
1900 S. Sunset Street, Suite 1-F Longmont. CO 8050!
Tel 303.772.5282 Fax 303.772.7039 www.tetratech.com
TETRA TECH
KNOWN STRUCTURES WITHIN 200 FEET OF MINING LIMITS
Known man-made structures within 200 feet of the mine limits are listed below:
• Colorado State Highway 82
• Garfield County Road 103 (Crystal Springs Road)
• A buried telephone line
• A buried 10" gas flow line and associated gas meter
• An overhead electric service and associated power poles
• The Basin Ditch
▪ A private bridge spanning the Basin Ditch
• An 18" diameter ilTigation pipe
• Various fences surrounding the property.
STABILITY ANALYSES
Mr. Sean Frisch
July 19, 2010
Page 2
Stability analyses were performed in order to evaluate potential for damage to existing permanent
structures due to mine highwall slope failures. Analyses were performed with the Slope/W 2007 v7.I6
computer program. For each case we analyzed the most critical soil profile(s) based on exploratory
boring data (i.e. the tallest highwall, thickest overburden layer, etc).
The stability analyses were run in accordance with accepted geotechnical standards and DRMS
requirements for temporary pit slopes. The conditions modeled are summarized as follows:
• Two mine highwall scenarios were modeled based on the mining plan
o A 3:1 (H:V) mined highwall
o A 25' high interslope bench configuration with I:1 slopes and 20' wide catch basin.
benches
• Factor of Safety must be greater than 1.0
Groundwater levels on the profiles were input to portray conservative site conditions at an elevation of 90
feet below grade.
Laboratory strength tests were not performed on soils from the site. The soil strength parameters used
were based on typical values for the anticipated soils. The input strength parameters were:
Table 1
Stability Analysis Strength Parameters
Material
Moist Unit
Weight (pcf)
Saturated Unit
Weight (pcf)
Effective
Cohesion
Effective Friction
Angle (degrees)
Overburden material
123
128
0
30
Sand and gravel with
Cobbles
149
155
0
38
Stockpile
120
125
0
28
TETRA TECH
Mr. Sean Frisch
July 19, 2010
Page 3
The scenarios were modeled with a general section including 15 feet of overburden silty sand overlying
sand and gravel with cobbles. The scenarios are shown in Table 2,
Table 2 — Structures within 200 feet of mine limit
Structure
Structure Owner
Structure Location
Colorado State Highway 82
Colorado Department of
Transportation
Located approximately 190 feet south of
the southern mine limit in the southwest
corner of the property. Highway 82 is
located at an elevation equal to the
proposed bottom elevation of the mine.
Garfield County Road 103
(Crystal Springs Road)
Garfield County
Located approximately 150 feet west of
the western mine limit in the
southwestern corner of the property. At
this location, County Road 103 is located
at an elevation 40 feet above the proposed
bottom elevation of the mine.
A buried Telephone Line
Qwest Corporation
Located approximately 180 feet west of
the western mine limit in the in the
southwestern corner of the property. At
this location, the telephone line is located
at an elevation 40 feet above the proposed
bottom elevation of the mine.
A buried 10 -inch diameter gas
flow line
Rocky Mountain Natural Gas
c/o SourceGas, LLC
Located approximately 100 feet south of
the southern mine limit.
A gas meter associated with the
buried 10 -inch gas flow line
Rocky Mountain Natural Gas
c/o SourceGas, LLC
Located approximately 160 southeast of
the southeast corner of the mine limit.
Overhead electric service lines
and associated poles
Holy Cross Energy
Located approximately 100 feet east of
the eastern mine limit, along the eastern
property line.
The Basin Ditch
Basin Ditch Company
Located approximately 140 feet north of
the northern mine limit.
A private bridge spanning the
Basin Ditch
Clifford Cerise Ranch
Company, LLLP
Located approximately 140 feet north of
the northern mine limit.
An 18 -inch diameter irrigation
pipe
Clifford Cerise Ranch
Company, LLLP
Located approximately 95 feet south of
the southern mine limit.
Fence A
Clifford Cerise Ranch
Company, LLLP
The most critical fence location near the
3:1 (H:V) highwall slope, approximately
140 feet north of the north mine limit.
Fence B
Clifford Cerise Ranch
Company, LLLP
The most critical fence location near the
1:1 (H:V) interbench highwall slope,
approximately 10 feet east of the east
mine limit of the silt pond.
1 — The closest fence lines to both the interbench slope and the 3:1 highwall configurations locations were considered the most critical.
Stability analyses were run on randomly generated circles generated through the mine excavation and
highwall, as well as at the nearby permanent structures. The Slope/W program calculated the Factors of
Safety (FOS) using the ordinary, Bishop, Janbu, and Spencer Methods. The Spencer factor of safety
correlates more closely with observations of actual slope failure, providing a realistic and more rigorous
lot TETRA TECH
Table 3
Stability Analysis Results
Mr. Sean Frisch
July 19, 2010
Page 4
Section
Distance to Nearest
Permanent Structure
Highwall
Configuration
Factors of Safety
Colorado State Highway 82
190
Bench
NA'
Garfield County Road 103
(Crystal Springs Road)
150
Bench
4.3
Buried telephone line
180
Bench4.3
Buried 10 -inch diameter gas
flow line
150
Bench
2.0
Gas meter associated with
the buried 10 -inch gas flow
line
160
Bench
2.1
Overhead electric service
lines and associated poles
100
Bench
1.6
Basin Ditch
140
3:1(H:V)
2.6
18" diameter irrigation pipe
95
Bench
1.5
Private bridge spanning the
Basin Ditch
140
3:1(H:V)
2.6
Fence A2
140
3:1(H:V)
2.6
Fence B2
10
Bench
1.4
I — The elevation of CO Highway 82 is at the s3nie elevation as the proposed bottom elevition of the mine. No failure surface could be generated.
2 — The closest fence lines to both the imerbench slope and the 3:1 highwall configurations locations were modeled.
The minimum setbacks are equal to the distances in the table above. All FOS are 1.4 or greater. Mining,
following these minimum setbacks, should not pose a hazard to nearby structures.
LIMITATIONS
Our review is based on regional geologic mapping, present mining plans, exploratory boring data, known
structure locations, and stability analyses using typical soil strength parameters. Should the mining plans
change, subsurface conditions vary or structure locations differ from those portrayed in this letter, we
should be contacted in order to re-evaluate the potential affects on man-made structures.
Please call with any questions or comments that you might have.
Sincerely,
TETRA TECH
eremy Deuto, P.G.
Engineering Geologist
JD:em
Attachments: Figures 1 - 2
Attachment A
is /roc er, P.E.
Vic resident
P:\2351 11133-23511-10003\Deliverables1DRMS1Application Package4Stability Exhibit\Stability Analysis Letter7-19-10.doc
TETRA TECH
REFERENCES
Elllis, M.S. and Freeman, L.V.; 1984; "Geologic Map of the Carbondale 30' X 60' Quadrangle, West -
Central Colorado"; CO.G.S. Coal Investigations Map C -97-A.
- PERRY, ROBERT
BURIED
TELEPHONE
LINE
m
PRIVATE BRIDGE
SPANNING
BASIN ITCH
(_ BASIN
DITCH
42.
•
PERMIT BOUNDARY
ax' /
OVERHEAD
ELECTRIC LINE
. IRRIGATION PIPE
/ — ▪ r
GAS METER
ASSOCIATED
WITH THE BURIED
/6- 10" DIAMETER GAS
FLOW LINE
T
r
CC(CR400 S
r
TATyIGykilx,
82
0 200' 400' 800'
TETRA TECH
www.tetratech.com
LAFARGE WEST, INC.
Project No.: 133-23511-10003
1900 S. SUNSET ST., SUITE 1-F
LONGMONT, CO 80501
PHONE: 303-772-5282 FAX: 303-772-7039
CERISE MINE
SITE PLAN SHOWING PERMANENT
STRUCTURES WITHIN 200' OF MINE LIMITS
Date:
JULY 13, 2010
Designed By:
JD
FIGURE
1
Copyright: Tetra Tech
- PERRY, ROBERT
rs�
F'r �� ♦1
/.;r
4
1.
1:1(H:V) INTERBENCH
SLOPES W/20' CATCH
BASIN HIGHWALL SLOPE
0 200'
400'
SCALE IN FEET
800'
‘ti
TETRA TECH
www.tetratech.com
LAFARGE WEST, INC.
Project No.: 133-23511-10003
1900 S. SUNSET ST., SUITE 1-F
LONGMONT, C080501
PHONE: 303-772-5282 FAX: 303-772-7039
CERISE MINE
HIGHWALL CONFIGURATION
Date:
JULY 13, 2010
Designed By:
JD
FIGURE
2
Copyright: Tetra Tech
ATTACHMENT A
SLOPE WITH CROSS-SECTIONS
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.20
6.15
Cerise Mine
Interbench Slopes
Garfield County Road 103 (Crystal Springs Road)
4.3
•
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Crystal Springs Road
Sand and Gravel wi
-250
-200
-150
-100
-50
0
50 100 150
Distance
200
250
300
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 25h25v highwall slopes, Crystal Springs Road.gsz
350
400
450
500
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.20
6.15
Cerise Mine
Interbench Slopes
Buried telephone line
4.3
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Telephone Line
Sand and Gravel with
-250
-200
-150
-100
-50
0
50 100 150
Distance
200
250
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 25h25v highwall slopes, Telephone.gsz
300
350
400
450
500
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.2
Cerise Mine
Interbench Slopes
Buried 10 -inch diameter gas flow line
2.0
Rocky Mtn. Natural Gas, 10" Gas Line
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Overburden
Sand and Gravel with Cobbles
6.15
-250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500
Distance
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 25h25v highwall slopes, Gas Line.gsz
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.2
Cerise Mine
Interbench Slopes
Gas meter associated with the buried
10 -inch gas flow line
6.15
-250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500
Rocky Mtn. Natural Gas, Gas Meter
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Overburden
Sand and Gravel with Cobbles
41.111110
Distance
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 25h25v highwall slopes, Gas Meter.gsz
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.2
Cerise Mine
Interbench Slopes
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Overhead electric service lines and associated polesName: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Overhead Electrical Line Poles
Overburden
Sand and Gravel with Cobbles
0.1.•__o________6565®6565®6565®v___m____________Cd. ®6565_®_____65___65
6.15
-250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500
Distance
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 25h25v highwall slopes, Overhead Electrical.gsz
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.20
Cerise Mine
3:1 Highwall Slope
Basin Ditch
6.15
-250 -200 -150 -100 -50
0
2.6
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Irrigation Ditch
Overburden
_,4/1/11111111.MMEW
Sand and Gravel with Cobbles
50 100 150
Distance
200 250 300 350 400 450 500
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 3-1 highwall slope_Irrigation Ditch.gsz
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.20
Cerise Mine
3:1 Highwall Slope
Private bridge spanning the Basin Ditch
2.6
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Bridge Over Irrigation Ditch
Overburden
4e1111111.0W
Sand and Gravel with Cobbles
6.15
-250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500
Distance
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 3-1 highwall slope_Bridge over ditch.gsz
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.2
Cerise Mine
Interbench Slopes
18" diameter irrigation pipe
1.5
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Irrigation Pipe
Overburden
Sand and Gravel with Cobbles
6.15
-250 -200 -150 -100 -50 0 50 100 150
200 250 300 350 400 450 500
Distance
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 25h25v highwall slopes, South Irrigation Ditch.gsz
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.20
Cerise Mine
3:1 Highwall Slope
Fence Line A
2.6
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Fence Line
Li
Overburden
Sand and Gravel with Cobbles
6.15
-250 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 450 500
Distance
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 3-1 highwall slope_Critical Fence Line.gsz
Elevation (x 1000)
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.2
Cerise Mine
Interbench Slopes
Fence Line B
1.4
Name: Overburden
Unit Weight: 128
Unit Wt. Above Water Table: 123
Cohesion: 0
Phi: 30
Name: Sand and Gravel with Cobbles
Unit Weight: 155
Unit Wt. Above Water Table: 149
Cohesion: 0
Phi: 38
Fence Line, Critical Location
Overburden
Sand and Gravel with Cobbles
6.15
-250 -200 -150 -100 -50 0 50 100 150
200 250 300 350 400 450 500
Distance
P:\23511\133-23511-10003\SupportDocs\Calcs\Slope Stability\For Stability Exhibit\ Cerise 25h25v highwall slopes, Critical Fence Line.gsz