|
These activities are briefly described in the following sections.
<br />Site-specific investigations required for design and permitting of the new ventilation shaft included a cultural
<br />resource survey of potential disturbance areas, soil and foundation characterization as the basis for shaft pad and
<br />. access road design, and completion of a wetland/AVF survey for the shaft pad and access road. The cultural
<br />resource survey was conducted by Metcalf Archaeological Consultants, the engineering soils characterization by
<br />Northwest Colorado Consultants, and the soils/wetland/AVF survey by Habitat Management, as described in Section
<br />2.04.
<br />The perimeter of all areas affected by surface facilities will be clearly marked before the beginning of surface
<br />disturbances. The proposed shaft pad surface area is approximately 280' square (1.8 acres), with a total pad
<br />disturbance area (includes cut and fill slopes) of 2.5 acres. The associated access road corridor is approximately
<br />7,100' long, with a finished top width of 24 feet (road corridor 35 feet wide, 5.7 acres). See Exhibit 25T, 18 Right
<br />Ventilation Shaft Geotechnical Pavement Design, for details on the road, and Map 24-CD1, a new road profile and
<br />cross-section. Approximately 4,400 feet of the access road crosses upland areas, with the remaining 2,700 feet
<br />crossing lowland areas where road construction will involve removal of any large vegetation and stripping and
<br />stockpiling of other vegetation, topsoil, and organic materials.
<br />Given an average topsoil depth of approximately 6 inches in the upland area and a total road disturbance area of
<br />approximately 5.7 acres, approximately 4,600 CY of topsoil will be salvaged from the road corridor, and 2,000 CY
<br />from the 2.5 acre pad area. Prior to initiating topsoil removal activities, stream/wetland buffer zones will be marked
<br />with identifying signs to prevent surface disturbance within buffer zone areas. The topsoil from the road corridor
<br />was initially windrowed downslope beyond the edge of the 35-foot road disturbance area, and topsoil from the pad
<br />area was placed in a stockpile adjacent to the shaft pad and within the 100-foot road corridor, for future reclamation
<br />use. As a result of concerns related to protection of windrowed topsoil from erosion and contamination, a decision
<br />was made to recover topsoil from the windrows and place it in stockpile. The topsoil salvage volumes are indicated
<br />on Table 49A. Natural vegetative materials (mulch) incorporated into the topsoil, and seeding with the topsoil
<br />stockpile stabilization seed mixture identified on page 2.05-121 will stabilize the windrowed and stockpiled topsoil.
<br />• Following topsoil removal, required drainage and sediment control structures will be constructed or installed. These
<br />structures include the upslope road drainage ditch, road crossings of smaller ephemeral drainage channels; discharge
<br />control structure (rock check dam) at the down-gradient limit of the shaft pad, and a temporary diversion ditch
<br />around the up-gradient perimeter of the shaft pad. The drainage from the shaft pad is addressed under a Small Area
<br />Exemption (SAE), as both the associated drainage area (4.3 acres) and the pad disturbance (2.5 acres, includes 0.2
<br />acre ditch disturbance) are relatively small, the pad will be gravel-surfaced, and the remainder of the drainage area is
<br />undisturbed and vegetated. The access road follows variable topography, generally consisting of rolling terrain, and
<br />will be gravel-surfaced, so access road drainage control requirements are minimal. Atypical road drainage ditch
<br />design is provided in Figure 2 of Exhibit 8T, prepared by Water & Earth Technologies, Inc, April 2004. Six (6)
<br />culverts are required to remove the flow from the runoff ditches to limit runoff velocities in the ditches to less than
<br />or equal to 3.75 feet per second (fps). This will assure that the ditches remain stable with no channel scour or
<br />degradation during peak flow events. Breaks in the topsoil windrow will allow runoff to drain. All drainage
<br />calculations and documentation are provided in Exhibit 8T, the SAE demonstration is provided later in this section,
<br />and the drainage structures are shown on Map 24. The SAE demonstration includes ditch sizing for the upland
<br />diversion ditch and road ditch.
<br />Construction of the ventilation shaft access road will involve topsoil recovery and windrowing, installation of
<br />required drainage structures, scarification and re-compaction of surface materials, and placement and compaction of
<br />approximately 8 inches of pit-run gravel and 3 inches of suitable road-base material. The access road surface will be
<br />approximately 24 feet wide and will be graded and crowned to promote effective drainage. Road construction
<br />requires minimal cut and limited fill, so the actual road disturbance area will average approximately 35 feet
<br />wide. Cut slopes will be established at a maximum of 1. SH:1 V and fill slopes will be graded to 2H:1 V or less,
<br />with all disturbed slopes to be stabilized by seeding with the topsoil stockpile stabilization seed mixture. For
<br />. permitting purposes, a road disturbance corridor 100 feet wide has been defined as encompassing all project-
<br />related activities, including road construction and topsoil stockpiling/windrowing.
<br />MR08-229 2.05 - 45.4 08/01/08
<br />
|