PERMFILE134266

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 70 REFUSE PILE EXPANSION
• Media Type: D
• Archive: No

REFUSE PILE EXPANSION
condition factor of safety is 1.22 during Sub-phase 1 B
along Profile 3.
• 5.5.3.2 Regional Stability of Entire Hillside
coefficient of 0.1 was included in the calculations to
account for a possible earthquake event.
5.5.3.3 Stability of RPE Pond and Highway
133
A stratigraphic cross section alone the steepest
topographic gradient of the RPE was developed to
check the regional slope stability. The cross section
also included a profile representing [he maximum
thickness of the coal refuse which will occur at the
end of Phase IV construction. The stratigraphic
sequence of the RPE area was developed primarily
from li[hologic descriptions from three exploratory
wells drilled in September, 1996, in the vicinity of
the proposed RPE. In addition, six shallow auger
holes, drilled in April, 1996, were used to define the
colluvial and alluvial soil thickness along the slope
base. Figure E-1 shows the locations of the borings
and Appendix E contains the drilling logs. Well
data from the three exploratory holes were projected
along strike to align with the cross section shown on
Figure K-7. Bedding dips toward the north-
northeast at approximately 3.5 degrees. Field
mapping of sandstone outcrops agreed with the
projection of the sandstone stratigraphic layers.
Bedrock lithology includes shale, sandstone, and
coal from the Mesa Verde Formation underlying
varying thicknesses of colluvial and alluvial soils.
Well logs indicate competent, continuous bedding,
with the exception of some minor coal seams
feathered within larger carbonaceous shale layers.
For modeling purposes, the minor coal seams were
made part of the larger, weaker shale layers. Soil
strengths assigned to the strata are based upon
samples taken from the RPE site and are provided in
Table 7. The strengths and densities are based upon
geotechnical testing performed by RockTech
(October, 1996); corresponding geotechnical
analytical results are presented in Appendix K
The stability analysis was performed using the
SLOPE/W (Version 3) software produced by GEO-
SLOPE International Ltd., Calgary, Alberta,
Canada. The most likely catastrophic failure
scenario involving the entire RPE mass would occur
on a slip plane located along the base of the
colluvium and alluvium. Results, provided in
Appendix K, indicate construction of the RPE will
• ;effectively improve the stability of the slope as
shown in Table 8. A seismic acceleration
An additional slope stability analysis evaluated the
current Highway 133 conditions for the road fill
above the culvert draining [he wetland. At present,
minor soil erosion has occurred in this area. The
construction of the proposed RPE sediment control
pond will utilize this slope. The stability analysis
indicated a factor cf safety for the road fill to be 1.91
for present conditions and 1.70 for earthquake
loading factors.
6.0 SURFACE HYDROLOGY
6.1 Introduction
A surface hydrology analysis was performed to size
the sediment pond, sediment ditches, and upland
runoff diversion (clear water) ditches required for
the RPE area during each of the four phases.
Peak discharges and maximum runoff computations
used in the design and size verification of all ditches
and sedimentation ponds were based upon
procedures from the following:
Urban Hvdroloev for Small Watersheds,
Technical Release No. 55, Soil Conservation
Service, U.S. Department of Agriculture,
January, 1975
Procedures for Determining Peak Flows in
Colorado, TR-5> Supplement, Soil
Conservation Service, U.S. Department of
Agriculture, March. 1990
HEC-1 Flood Hvdroeraph Package Computer
Program, U.S. Army Corps of Engineers
Hydrologic Engineering Center, Davis,
California, 1991 Revision
All ditches and diversions were designed to
accommodate the peak discharge from a 100-year,
24-hour, type II distribution, precipitation event
with an adequate freeboard of 0.3 feet. A v-notch
channel design was used for all ditches. Because of
Harding Lawson Associates 11