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Home My WebLink About6.03 Colorado Geological Survey CommentsSTATE OF COLORADO COLORADO GEOLOGICAL SURVEY Department of Natural Resources 1313 Sherman Street, Room 715 Denver, CO 80203 Phone: (303) 866-2611 Fax: (303) 866-2461 April 25, 2006 Mr. Fred Jarman Garfield County Planning 108 8th St Suite 401 Glenwood Springs, CO 81601 Re: Hunt Ranch Sketch Plan CGS Review No. GA -06-0008 Dear Mr. Jarman: 20 T7S R87W COLORADO DEPARTMENT OF NATURAL RESOURCES Bill Owens Governor Russell George Executive Director Vincent Matthews Division Director and State Geologist In response to your request I visited the property and reviewed the material in the referral packet, which included a Sketch Plan application prepared by OTAK (3/13/06). The site consists of 561 acres, of which 212 acres would be divided into 91 residential lots. Geologic conditions. Development at the site will be concentrated mostly in the higher areas, while agricultural production will continue in the lower pastures. The northernmost lots are characterized by basalt boulders and are vegetated by sagebrush and pifion pine. On the southern lots, basalt is not as prominent and thick stands of scrub oak are present. At Sketch Plan stage, the geologic information provided by the applicant was obtained from Garfield County GIS information (Exhibit 8 in the Sketch Plan application). A report by HP Geotech is in progress. The GIS information, which possibly originated from the county 1041 maps, states that there are no slope hazards, soil hazards, or landslide hazards within the limits of the site. However, Exhibit 15, the Slope Analysis and Site Plan, shows a number of lots in the northeast part of the site that have very steep slopes. In particular, lots 24-29 show 30-40 percent slopes adjacent to the roadway. Driveway access, construction, and siting of onsite wastewater systems (OWS) on these lots could be very difficult. I did not observe signs of slope movement other than rock creep, but construction of the access road at the base of the steep slopes in this area could destabilize the hillside; the single track road north of these lots also traverses steep terrain. The cuts, fills and retaining wall requirements for the road alignment should be reviewed by an engineer, and a slope stability analysis in the vicinity of lots 24-29 should be considered. For constraints that are identified, mitigation should be proposed. If the county allows residential construction on lots with 30+ percent grade, these lots should be evaluated for slope stability at a later stage of development, incorporating the home design. Erosion. Disturbance on steep slopes can lead to accelerated sheetwash erosion. Where flows become concentrated, gully erosion can form. An erosion control plan should be prepared for the Hunt Ranch, p. 1 site. Homeowners should monitor gullies that form to prevent general lot deterioration and to prevent any adverse effects to neighboring lots. Debris fans. Mapping of the area by CGS in the 1990s showed debris fans in the area of the lot line between lots 48 and 49 and on lot 46. The drainages are small, but could run during periods of heavy precipitation. It did not appear that rocks had been carried by the drainages, which were not deeply incised. The preliminary observations suggest that no action is required beyond building on high ground on these lots. The debris fans, themselves, could be composed of low density soils that are prone to collapse. The small fan on lots 48/49 could probably be avoided in development. However, the fan on lot 46 covers most of the buildable portion of the lot, as the remaining part is composed of slopes of 40 percent or greater. At Preliminary Plan stage, the drainage report that is prepared for the site should include a discussion of these drainages. Also, the fan on lot 49 should be drilled and sampled to determine the characteristics of the soil. Where determined to be necessary, mitigation should be proposed for drainage and soils problems. Onsite ditches. I did not observe any impact on the hillside from the Needham Ditch. For most of its length, the Needham Ditch could be piped in the right-of-way for the access road. It could be rerouted along the lot line for lots 19 and 20. The Mountain Meadows Ditch bisects lots 74 and 80. Either the ditch should be realigned or the lots should be reconfigured. The alignment for both ditches should be shown as an easement. Foundations. Structures on slopes should be designed to resist lateral earth pressures and avoid lateral shear. Local drainage control will be important to maintaining stability. Excavation could be difficult, locally, and blasting could be required. The material that forms the substrate for foundations should be limited in size (rocks generally less than 4 inches in diameter) to prevent differential compaction. Subsurface soil (as opposed to rock) should be tested for geotechnical characteristics, including swell -consolidation, so that foundations can be designed appropriately. Onsite wastewater systems. The possible constraints for OWS are the steep slope on many of the lots and the shallow bedrock or boulders. In the Sketch Plan application, Sopris Engineering recommends that all OWS be engineered. Sopris has experience in designing systems for steep mountainous terrain, and their recommendations should be followed. In summary, the site characteristics at Hunt Ranch are not unusual for the mountainous parts of the state, but there are conditions that require close attention during development. At this stage of development, the stability of the road alignment in the northwestern part of the site should be investigated. Further analyses, as mentioned above, should be performed at Preliminary Plat and Final Plat stages of the project. Please call me if there are any questions. Yours truly, Celia Greenman Geologist 303-866-2811 celia.greenman@state.co.us Hunt Ranch, p. 2