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HomeMy WebLinkAboutEngineer Report of Rockfall Condition 08.23.17H.PVKUMAR 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado August 23,2017 Kristen Kellogg and Randy Hill 246 V/est Capital Court New Castle, Colorado 81647 kj kel logg6 @ comcaut. net Project No. 17-7-626 Subject:Review of Rockfall Condition, l,ot 78, Spring Ridge Reserve PUD, Phase 4, Garfield County, Colorado Dear Kristen and Randy: As requested, the undersigned representative of H-P Kumar observed the hillside above the proposed residence at the subject site on August t8,2A17, for potential rockfall impacts to the proposed construction. HP Geotech (now H-P Kumar) previously conducted a subsoil study for design of foundations at the site and presented our findings in a report dated March 25,20L6, Project No. 116 0614. Proposed Construction: We understand the proposed residence will be a one or two story, wood frame structure, possibly with a walk-out basement level. The building will be located in the middle to front portion of the building envelope, generally between the boring locations as shown on Figure I of the referenced subsoil study report. Foundation excavations on the up- slope side of the lot of up to about 10 feet may be necessary to accommodate a basement level. Project Site: The subject lot slopes up to the northeast at about 20 to 25 percent across the building pad portion of the lot. The building pad area is bordered to the northeast by an abandonqd, southeast to northwest trending irigation ditch and small berm bordering the ditch on the nphill side. Above the ditch, the terain generally slopes up at about 20 to25 percent with localized slopes of up to about 40 percent near the crest of the slope. The slope is about 350 feet in length from the abandoned irrigation ditch to the crest. The uphill berm above the ditch is generally about 2 to 3 feet in height. A low-relief swale trends southwest down from near the crest of the slope toward the southern edge of the lot. Surficial soils on the hillside above the proposed residence consist of relatively shallow colluvium, comprised of silty, clayey sand with gravel, cobbles and small boulders, overlying sandstone bedrock of the Maroon Formation. Scattered, mostly flat (tabular) shaped rock fragments, typically up to about I foot in size, are Geotechnical Engineering I Engineering Geology Mateñals Testing I Environmentral Kristen Kellog and Randy Hill August 23,20L7 Page2 exposed on the hillside above the residence. An area of cobbles, larger boulders and the remnants of a bedrock outcrop occurs near the near the crest of the ridge, with boulders ranging from 1 foot to several feet in size. The boulders are generally tabular in shape, and typically partially embedded in the overburden soil. Drainage on the slope above the proposed residence appears to be by sheet flow. Conclusions and Recommendations: Based on our observations and professional experience, we conclude that the rockfall hazardto the proposed construction is low and rockfall mitigation measlrres are not warranted. Although low in probability, down-slope rock movement, if it occurs, will likely originate from the steeper, rocky zone located near the crest of the slope. Cobbles and boulders in the rocky zone are typically tabular in shape and not prone to down-slope movement. Scattered cobbles and boulders with sub-angular to sub-rounded shapes were also observed, and will be more prone to movement down-slope. Storm and seasonal events, resulting in erosion of surficial soils, may cause instability of rock fragments on the hillside. The berm on the uphill side of the iffigation ditch, as well as the ditch, may provide some measure of mitigation for smaller rocks moving downslope, but the level of mitigation is indeterminate based on visual observations alone. If the owner is concerned with the potential for down-slope rock movement, we recommend periodic observation by the owner of the slope to identify potential rocks prone to movement, and we should be contacted if further investigation is desired at that time. If alterations to the slope above the residence are planned in the future, we should be consulted to review the planned alterations for potential rockfall hazard. If you have any questions or need further assistance, please call our office. Sincerely, H.P+ KUMAR James A. Parker, P.8., P Rev. by: DEH JAP/kac Garfield County Community Development - Dave Argo (dargo@garfield-county.com)cc: I t 47?81 0 A, IIt vltZ H-P*KTJMAR Project No. 17-7-626