HomeMy WebLinkAboutLetter from Colorado Geological survey 9.06.05STATE OF COLORADO COLORADO GEOLOGICAL SURVEY Department of Natural Resources 1313 Sherman Street, Roam 715 Denver, Colorado 80203 Phone (303) 866-261 1 FAY. (303) 866-2461 September
6, 2005 b 0<; GA-06-0002 RECElVE1.r S1" NWV., Sec. 17, T7S, R95W SEP 1 2 2005 Mr. Richard Wheeler :1u1RFIElD CCUilTY Garfield County Building and Planning DepaRlNe1{t PLAI~NING 109 8t
Street, Suite 20 I Glenwood Springs, CO 81601 RE: Geologic Hazards Review of Battlement Mesa Resub. of Parcels 5-1 aDd 5-2 Dear Mr. Wheeler, DEPARTMENT OF NATURAL RESOURCES Bill Owens
Governor RlJssell George Executive Director Vincent I.lotthews Division Director and Stote Geologist Thank you for the land use application referraL At your request and in accordance
to Senate Bill 35 (1972) this office has reviewed the materials submitted by your office. Pertinent to this review, we examined the following for this submittal: Section 4.60(E) Evidence
Regarding Geologic Hazards of the application, the Preliminary Geotechnical Study by HP Geotech dated June 13,2005 (Job No. 105 469) and earlier geotechnical reports by Chen and Associates
dating from 1982, and the drainage report by SGM dated June 2005. The CGS conducted a site investigation of the parcels on September 1, 2005. Please consider our comments as you consider
this land use application. The land parcels lie on an ancient mid-Pleistocene debris flow fan that was deposited onto the south side of the ancestral Colorado River valley. Battlement
Mesa lies on this large debris fan that has been further modified by later downcutting of the Colorado River valley, erosion of tributary ephemeral steam channels, and deposition of
more recent low-density surficial soils derived from older colluvial and eolian (windblown) soils. We generally concur with the geologic restraints statement in the application and the
content of the HP Geotech and earlier Chen and Associates reports. The thin mantle of recent clay soil above the debris-flow gravel is prone to hydrocompaction. Swell/consolidation test
results in the geotech reports shows this soil hazard to be moderate and widespread in the parcels. This collapse-susceptible soil is of varying thickness, from none to IS feet as shown
in the borings. Site-specific foundation designs are required, which must include additional subsurface soil investigations within the proposed building footprint. While the recent HP
Geotech report mentions the porous nature of the so ils in their bore log explanation and has a short section on moisture sensitive soils on page 6, further discussion on these types
of soils in the preliminary design recommendations sections ofthe report is lacking. Ground settlement can affect foundations, fl oating floor slabs, and sidewalks and pavements placed
on a su itable
thickness of moderately collapsible soi ls that becomes wetted. What is essential and not really mentioned in the preliminary design recommendations is that hydrocompactive soi ls must
remain as dry as possible. If not, for those locations where a thicker collapsible soil column remains below the foundation excavation, they should be considered susceptible to ground
settlement if those soils become saturated in the future. Where those hydrocompactive soi ls are thin, considerations to overexcavatelrecompact or deepen footings should be considered
to better insure foundation performance for the long-term. Certain residential concepts to prevent undue subsurface wetting should be adhered to: positive grades established away from
foundation walls so water will not pool against them; roofs equipped with gutters and downspouts extenders so that runoff is immediately conveyed away from the structure; and restrictions
in landscaping design and irrigation to prevent irrigation lines and sprinklers from being installed near foundation walls or slabs, or allowed to spray against them. We concur that
debris flow flooding is very low risk. Battlement Creek and stonn flows in Monument Gulch would need to jump two established drainage channels to reach the incised drainageway on the
n0l1heast side ofthis property. Our understanding is that additional offset from the northeast drainage (Studt Gulch) bank is also proposed for slope stability concems. In closing, we
find no geologic hazard or condition that would preclude the re-subdivision of this property, and provided site-specific foundations investigations are conducted that address the hydrocompactive
soi l hazard, the CGS has no further concems. If you have any questions please contact this office at (303) 866-3551 or e-mai l: jonathan.white@state.co .us Sincerely, onathan L. White
Senior Engineering Geologist