PERMFILE47061

• Permit No: C1981022
• IBM Index Class Name: Permit File
• Doc Date: 10/10/2003
• Doc Name: II West Coal Refuse Facility Underdrain Design ‎(Aug 2003)‎
• Section_Exhibit Name: Exhibit 2.05-E4 Part 9
• Media Type: D
• Archive: No

• Q = AV~n and A= Q/V„n
Where Q is the required flow through the mck underdrain (cfs)
Vv is average water velocity through rockfil] voids (ft/sec)
n is drain porosity (dependent on the size consist of the rock
material, porosity may vary from approximately 0.8 to 1.0, use
].0 to maximize drain size)
A = (8.6 ft3/sec)/(2.03 ft/sec x 1.0) = 4.2 ftZ
A 4' x 10' underdrain is [he minimum specified size for a sandstone underdrain
for a fill of <1 MM cy (Rule 4.09.2(2)(d)). A 4' high by 10'wide triangular drain
has across-sectional area 20 ftZ. Al] underdrain calculations are based on very
conservative assumptions, so even applying a 2.0 multiplier as a Factor of Safety,
a 4' x 8' triangular underdrain configuration (16 ft2) would be more than adequate
(1.9 x required drain size with 2,0 Factor of Safety) to safely pass the maximum
anticipated infiltration flows. It is, therefore, recommended that underdrain
design, approval, and construction be based on a 4' x 8' triangular configuration
with lII:1V sideslopes.
UNDERDRAIN CONSTRUCTION AND INSPECTION
The underdrain will be constructed of naturally-occurring sandstone rock and cobbles
recovered during foundation preparation activities for the coal refuse facility. These
materials will be separated from soil materials and temporarily stockpiled along the
inside edge of the perimeter drains for use in construction. Underdrain construction will
be initiated following completion of foundation preparation activities, including clearing,
grubbing, soil material removal, and minor grading to establish the triangular drain
configuration. Underdrain construction will involve placement of 12 to 14 ounce/square
yard non-woven geo-textile material along the exposed portion of the underdrain
alignment, placement of suitable rock drain materials on top of the geo-textile to establish
a triangular drain configuration at ]east 4 feet deep and 8 feet wide, and wrapping or
placement of additional geo-textile over the completed rock drain to form an enclosed
drainage envelope. Underdrain configuration is graphically illustrated by Figure 1,
Underdrain Design Configuration. No more than 10 percent of the suitable rock drain
material will be less than 12-inches in diameter and maximum rock size will be 30 inches
in diameter. Geo-textile materials will have a minimum overlap of 12 inches at any joint
and will be anchored with metal stakes,
Underdrain construction will be supervised and monitored by a Registered Professional
Engineer. Monitoring will include grading of the design drainage configuration,
placement and anchoring of geo-textile materials, selection and placement of rock drain
materials, and completion of the geo-textile over-wrap. Underdrain construction
activities will be photo-documented and the inspecting engineer will prepare and submit a
• certified inspection report to the CDMG documenting that underdrain construction was
completed consistent with the approved designs.