Project Name: Seneca Coal Mine Project Start Date: Late 1985 Project End Date: Reclamation work completed in late 1986, monitoring through 1996 Oversight Agency: Abandoned Mine Land Program (AML), Colorado Division of Mines and Geology (Formerly Colorado Mine Land Reclamation Division.

Preconstruction view from the south showing landslide impact on county road.

Location: Routt County in northwest Colorado, on the east side of County Road 52. Elevations range from 6,600 to 6,800 feet. Latitude: 40° 30' N, Longitude: 107° 15 ' W (approximate) Nearest Town: Hayden, Colorado -- aprproximately 17 miles from Craig, Colorado, and 25 miles from Steamboat Springs, Colorado Estimated Cost: Landslide & No. 2 Coal Flat:   $443,500 Revegetation:   $16,300 Subtotal:   $459,800 Coal Flats 1 & 3 with Revegetation:   $111,100 Engineering/Inspection:   $56,400 Project Total:   $627,300

Mine Location

Reasons for Action: Several final pits in the Seneca No. 1 Surface Coal Mine were abandoned in the 1960's without being reclaimed. In addition, a progressive landslide aggravated by the abandoned mine land conditions was threatening a county road and private property.  An AML-funded project to stabilize the landslide and reclaim the final pits was completed in 1986 for the Colorado Division of Mines and Geology (formerly Colorado Mine Land Reclamation Division). The site was visited after ten growing seasons. The land is stable and the vegetation re-establishment is strong. Volunteer native plant species are now invading the reclaimed sites.

History of Mine: The Seneca No. 1 Mine is an abandoned surface coal mine that was operated from 1964 to 1968. The mine is located in Routt County, on the east side of County Road 52, in northwest Colorado as shown on Figure 1. Elevations at the mine site range from 2,012 m to 2,073 m (6,600 to 6,800 ft). Average annual rainfall is 400 mm (15.5 in.) with approximately 40 percent occurring in the growing season. Prior to construction, the project site had about 40 percent vegetative cover. The vegetation consisted of about 40 percent sagebrush, 40 percent grasses, 15 percent forbs and 5 percent riparian species.

Overburden at the Seneca No. 1 Mine had been moved by dragline and deposited at the angle of repose in ridges approximately 30 m (100 ft) apart and 18 m (60 ft) high. The spoil ridges were broadcast and aerially seeded on a yearly basis as each area was stripped. Before seeding, the tops of the ridges were graded off to a width of approximately 6 to 9 m (20 to 30 ft) as required by reclamation regulations in effect at the time. The regulations did not require that final cuts of a mining unit be filled in or revegetated. At the Seneca No. 1 Mine, there were three final cuts, referred to as coal flats, that were to be reclaimed with abandoned mine land (AML) funds as the result of this project. The surface materials in the coal flats were composed of a mixture of coal and carbonaceous shale and supported virtually no vegetation.

In addition to the coal flats, there was a progressive landslide affecting County Road 52 that was stabilitized and reclaimed as part of the project. The landslide was caused by the instability of spoils and underlying spoils in the vicinity of the No. 2 coal flat, exacerbated by a high ground water table suspected to be related to a nearby stock watering pond upgradient of the landslide. Following are brief descriptions of each coal flat and the landslide area.

Site Layout

• Coal Flat 1: Coal Flat 1 was the northern-most and largest coal flat that was reclaimed. The flat was rectangular in shape and oriented in an east-west direction with dimensions of approximately 335 m by 85 m (1,100 ft by 280 ft). Coal outcroped at the east end. The coal flat sloped to the west at a grade of about 20 percent. At the west end of the flat, the cut narrowed and steepened to about 30 percent as it descended to a pond. One corner of the coal flat contained ground water seeps. The total area involved was approximately 2.2 ha (5.5 ac).
• Coal Flat 2: Coal Flat 2 was situated approximately 800 m (0.5 mi) southwest of Coal Flat 1. It was comprised of two irregularly shaped, longitudinal areas bisected by one of the access roads into the site. The upper area rose at a 20% grade to the northeast of the access road for approximately 260 m (850 ft). The maximum width in the north-south direction was 55 m (180 ft). Coal Flat 2 continued downslope at a 15% grade from the access road in a northwesterly direction for approximately 207 m (680 ft). The width of this area was about 12 m (40 ft). The total area covered by coal and carbonaceous shale was approximately 1.8 ha (4.4 ac).
• A pond had formed at the lower end of Coal Flat 2, fed primarily from surface water runoff from the coal flat. The flat itself contained incised erosion channels that had cut through the coal and carbonaceous shale on the bottom of the flat to bedrock. Another pond, which served to provide water to grazing cattle, was located adjacent to Coal Flat 2 along the site access road.
• Landslide: The landslide that had impacted County Road 52, narrowing it almost to one lane, was located 213 m (700 ft) southwest of where Coal Flat 2 crossed the site access road. There was about 58 m (190 ft) of vertical relief from the crest of the landslide to the crown of County Road 52. The overall slope of the landslide surface was about 43 percent (2.4H:1V); the breadth of the landslide was approximately 107 m (350 ft). The landslide was especially active during annual wet periods, particularly during the spring snow melt season.
• Coal Flat 3: Coal Flat 3 was about 966 m (0.6 mi) south of Coal Flat 2. Here, there were two separate irregularly shaped, coal flats that drained into a common pond. The northern flat was northeast of the pond with dimensions approximately 171 m (560 ft) long and 49 m (160 ft) wide. The southern flat ran upslope southeast from the pond. Its dimensions were about 122 by 18 m (400 by 60 ft). The total coal flat area at this site was about 1.1 ha (2.6 ac).

Pre-Reclamation Photos:

Coal Flat No. 2 Pre-Construction view from west side of disturbance looking uphill toward landslide area.	Coal Flat No. 2 Pre-Construction view from south side of disturbance looking downhill toward landslide area.
Landslide Area Pre-Construction view from the south showing landslide impact on country road.	While trying to install a toe buttress, the landslide moved during construction due to high ground water levels plus wintertime conditions.

Reclamation Objectives: Considering the challenges of steep topography and an active landslide, the overall objective of the reclamation project was to provide geotechnically stable land surfaces capable of supporting a diverse productive plant community that would create an environment suitable for wildlife habitat and cattle grazing.

Design Narrative:

There were two major aspects of the site investigation: the geotechnical study associated with the landslide problem and soils testing to assess the suitability of site materials as plant growth medium. The geotechnical investigation was conducted under emergency funding from the Office of Surface Mining (OSM), since the landslide was actively impacting public and private property.

The geotechnical investigation report included the following recommendations to stabilize the landslide:

• Unload the hill slope
• Flatten the overall slope of the hill
• Recontour and revegetate the slope
• Provide adequate surface drainage
• Construct a toe dike buttress

The soils assessment revealed that the mine spoils consisted of weathered and unweathered sandstone and sandy shales. Revegetation had been initiated in 1965 and continued through 1968, so the surface spoil materials had 15 to 20 years to weather and accumulate organic matter. Revegetation had been successful on these spoils which even then supported a thick cover predominated by western wheatgrass, smooth brome, and alfalfa. Vegetative cover was somewhat less on the south facing slopes. Rooting depth varied from 380 mm to 530 mm and did not appear to be inhibited by any spoil characteristics.

It was decided that these spoils would be used to cover the coal flats and to support a productive vegetative community. The field and laboratory investigation found that the spoil material was unsuitable for this use without amendment. Coincidentally, the spoil contained pockets of large cobble-to-boulder-size rock fragments were undesirable and would require segregation. As riprap sources were scarce, screening of the spoil was identified as a means of solving two problems at once.

Construction Narrative:

The overall project goal for the coal flats was accomplished by covering the exposed coal flat areas with a minimum of 2 feet of borrow material obtained from locations adjacent to each coal flat area. A final cover of plant growth medium at least 6 inches thick (that was typically salvaged from the upper two feet and portions of the borrow areas as discussed above) was applied to the final graded surfaces of the coal flats and borrow areas.

The borrow material was used as a source of compacted fill material and rip rap for drainage field construction and surface runoff channel construction. Surface runoff from a one hundred year design storm was routed through a series of diversion channels lined with rip rap in conjunction with check dams and drop structures in steeper portions of the reclaimed areas. Ground water was controlled through a series of compacted embankments and drainage blankets that typically traversed the breadth of the coal flats. The drainage blankets were daylighted into the surface drainage system. The compacted embankments also served as toe dikes for borrow material that was placed on slopes approaching a 25% grade.

The overall project objective for the landslide in Area 2 was accomplished by degrading the hill top by about 35 feet and recontouring the hillside to slopes of 3:1 to 2.5:1 with intermediate benches. The hilltop excavation and hillside resloping resulted in a stable slope with an estimated factor of safety of about 1.3. Surface and groundwater drainage control and a small toe dike along CR 52 were incorporated with the remedial construction. Topsoil treatment was the same as on the coal flats.

Coal Flat No. 2 Post-Construction view from west side of disturbance looking uphill toward landslide area showing riprap channel.	Coal Flat No. 2 Post-Construction view from south side of disturbance looking downhill toward landslide area. Earthwork and a riprap channel down the centerline of the area are complete; reseeding efforts followed shortly after.
Landslide Area Post-Construction (1986)	View from west side of the county road showing recontoured hillside and Coal Flat No. 2 behind it. Prevailing wind direction enhanced re-establishment of natie species on reclaimed land by transporting seeds from adjacent undisturbed land.

Services/Contractors Used:

The reclamation work was conducted from late 1985 to late 1986 in two separate earthwork contracts and a separate revegetation contract. The earthwork contracts addressed the stabilization of the landslide, placement of plant growth material on the coal flat areas and the construction of rip rap channels and groundwater interception drains. The steepness of the final topography (25 percent slopes in places) necessitated the use of drainage swales and ditches to divide the broad coal flat areas into manageable sub-watersheds. This allowed the concentrated runoff flows to only occur in the rip rapped channels.

Site Status 10 Years After Reclamation

Representatives of the Colorado Division of Mines and Geology visited the site in 1987 and 1990 to monitor revegetation progress. No additional maintenance work was required. The engineer for the project visited the site in July, 1996. The following sites were visited:

• Coal Flat 2
• The Landslide
• Coal Flat No. 3

All three areas had lush stands of vegetation. There was little evidence of soil loss or erosion, even on the steeper slopes of reclaimed Coal Flat 2. Wildlife (deer and marmots) were spotted using the reclaimed area; closer examination by wildlife experts would probably reveal evidence of use by other local species. The landslide area showed no signs of movement. A local resident who had been present during the reclamation projects revealed that a local land developer was considering part of the reclaimed parcel for up-scale home sites.

The most remarkable feature of the three sites was the invasion of sagebrush and native plants, especially on the upper slopes of the landslide area. A cursory examination of the dominant species in the seed mix planted in 1986 revealed no sagebrush species.

A Colorado DMG representative familiar with the site speculated that the native sagebrush seeds would have been present in the original plant growth medium salvaged from the spoil ridge crests. If this were the case, at least some sagebrush would have been observed in the reclaimed Coal Flat 2 area, which also received the same salvaged soil as the ridgetop above the landslide area. Such was not the case. The author speculates that the sagebrush is a native volunteer whose primary seed source was the natural ground upwind of the site. The development of this advantageous situation might be encouraged in some surface mining reclamation plans through the judicious use of native species-bearing “islands” of undisturbed land. Undoubtedly, some mines, tiering off Mother Nature, have employed this reclamation tool either unintentionally (as in the case presented) or within the broader context of a well-conceived reclamation plan.

Site after 10 years of reclamation (1996). View facing southeast looking uphill to Coal Flat No. 2. Stock pond in foreground wass pre-dated construction; water quality is good.	Site after 10 years of reclamation (1996). View from south side of disturbance looking downhill toward landslide area. Vegetation in earthwork and a riprap channel down the centerline are well established -- no signs of erosion on slope.
Site after 10 years of reclamation (1996). View from west side of reclaimed area facing uphill. Again, vegetation in earthwork and a riprap channel down the centerline are well established -- no signs of erosion on slope.	Landslide Area 10 Years Later: Reclamation success and landslide area stability are evident in 1996.
Landslide Area 10 Years Later: View from west side of the county road showing recontoured hillside and Coal Flat No. 2 behind it. Prevailing wind direction enhanced re-establishment of native species on reclaimed land.	Landslide Area in 1996: Sagebrush invasion limited to ridgetop of reclaimed landslide area.
Reclamation success and landslide area stability are evident in 1996.

Was this project successful? Yes

For more information on this project, contact:

James J. Gusek, PE
Golder Associates, Inc
Lakewood, CO

Available Documentation: NA

Submitted by: James J. Gusek, PE

Photos By: James J. Gusek, PE and Neil Eurick, Golder Associates, Inc.

Date: July 2004

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