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Home My WebLink About1.04 hpGeotech geo-hazards reportHEPWORTH-PAWLAK GEOTECHNICAL October 13, 2014 Robyn Starr 425 Mountain Shadows Drive Glenwood Springs, Colorado 81601 (robynstarr cr,mindspring.com) Subject: Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 email: hpgeo@hpgeorech.com Job No. 114 333A Geologic Hazards Review, Proposed Residence, 425 Mountain Shadows Drive, Garfield County, Colorado Dear Robyn: This report presents the findings of a geologic hazards review for the proposed residence at the subject site. This report summarizes our findings. This study was performed as additional services to our August 15, 2014 professional service agreement with you. 2014. We previously performed a subsoil study for the residence and our findings were presented in a report dated September 17, 2014, Job No. 114 333A. Review Methods: A field reconnaissance of the project site was performed on October 10, 2014 to observe the geology and geomorphology in the vicinity of the proposed residence. In addition we have looked at aerial photographs and reviewed geologic (Kirkham and Others, 1995) and geologic hazard maps (Lincoln-DeVore, 1978) for the Glenwood Springs area. Using this information an assessment was performed to evaluate potential geologic hazards that could impact the proposed construction. This report summarizes the information used in our evaluations, describes our assessments and presents our findings. Proposed Construction: The residence will be a prefabricated log home over a partial basement with an attached site built, basement level garage with a slab -on -grade floor. Grading for the structure will require cut depths up to about 12 feet. Site Conditions: The site is a developed residential lot on the north (uphill) side of Mountain Shadows Drive with an existing single family residence located in the southwest portion of the lot. The proposed building will be located northeast and uphill of the existing residence and located on a moderately steep, south facing slope north of the driveway to the existing residence. There is an elevation difference of about 15 to 18 feet across the building footprint. North of the proposed building area the slope climbs steeply and a small ravine terminates above the project site. Vegetation in the area of the proposed building includes grasses, weeds, small brush and trees. Geologic Hazards: Based on our site visit, previous subsoil study and our review of aerial photographs and published maps, possible geologic hazards to the proposed construction are summarized below. Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 Robyn Starr October 13, 2014 Page 2 Debris Flow Hazard: The project site is mapped and was observed to be located near the terminus of a relatively recent debris flow fan. The fan was mapped as active in 1977 (Lincoln-DeVore, 1978) and rises 8 to 10 feet above the proposed building area approximately 20 to 25 feet north of the proposed building. The proposed building site appears to be located on an older lower debris fan. A drainage originating on the steep slope to the north is incised in the fan near the bottom of the slope and terminates above and slightly east of the proposed construction. From our site visit, the debris fans did not display evidence of recent sizeable flow events such as accumulated fresh debris, mud fans or disturbed vegetation. Relatively mature vegetation consisting of shrubs and oak brush is located in the channel above the proposed construction indicating a lack of recent large debris flow events. In our opinion, a low to possibly moderate hazard to the proposed structure during its service life exists from hyperconcentrated, debris and/or possible clear water flows discharged from the drainage above the proposed construction during extreme storm events. The flows would likely be relatively small and on the order of 1 to 2 feet or less for debris and hyperconcentrated flows and possibly several inches for clear water flows at the location of the proposed structure. Mitigation of the hazard would be difficult by means such as routing the flow away from the structure due to the proximity of the adjacent development. Positive drainage away from the building foundation as discussed in our subsoil study report should be provided. Construction of the proposed residence would not, in our opinion, increase the risk to the adjacent properties provided that site grading does not alter the route of the channel above the proposed construction. Sinkhole Potential: The potential for sinkhole development at the site due to the evaporite bedrock condition is considered low as discussed in our subsoil study report. Rocicfall Hazard: Based on our observations of the area of the proposed construction and the location and degree of slope of the region uphill from the site, a low but not non- existant rockfall hazard exists for the proposed construction during its service life, with the risk similar to that of the adjacent constructed homes. Rock outcrops exposed approximately 500 feet uphill from the proposed building area and on the west side of the uphill drainage appear capable of producing potentially damaging rockfall. The slope from this direction moderates towards the bottom as it approaches the proposed building location and rockfall originating from this area may not be capable of reaching the proposed building area. The slope directly north of the proposed construction on the east side of the uphill drainage terminates closer to the proposed construction but did not, based on our observations, have rock outcrop source areas capable of producing sizeable rockfall objects. Rocks from cobble to small boulder size occur scattered on the slope and might move downhill toward the project site under certain conditions but in our opinion the risk of this occurring is low. Hydro -Compressive Soils: The subsoils at the site are alluvial and/or debris fan deposits and tend to settle when wetted (hydro -compressive). Methods to mitigate the hydro - compressive soils are discussed in our subsoil study report. Job No. 114 333A ~Piech Robyn Starr October 13, 2014 Page 3 Earthquake Risk: The proposed structure could experience moderate levels of earthquake related ground shaking during a reasonable service life. The site location is not located close to potentially active faults that would increase the earthquake risk usually considered for the region. Potential earthquake ground shaking should not be a problem if the residence is designed according to applicable building codes to withstand moderate shaking without significant damage and stronger shaking without collapse. Limitations: This review was conducted according to generally accepted engineering geological and geotechnical engineering principles and practices in this area, at this time. We make no warranty either express or implied. The information presented in this report is based on our field observations, previous subsoil study, aerial photograph interpretations and review of published geology and geologic hazard maps. This report has been prepared exclusively for our client to assess if the proposed construction is exposed to unusual risks related to the project area geology. If you have any questions or need further assistance, please call our office. Sincerely, HEPWORTH — PAWLAK GEOTECHNICAL, INC. Tames A. Parker, P.E., P.G. Reviewed by: a%s;uth4 RE s tie David A. Young, P.E. JAP/ksw REFERENCES: Kirkham, R.M. and Others, 1995. Geologic Map of the Glenwood Springs Quadrangle, Garfield County, Colorado. Colorado Geological Survey Open File Map 95-3. Lincoln-DeVore, 1978. Geologic Hazards of the Glenwood Springs Metropolitan Area, Garfield County, Colorado. Colorado Geological Survey Open File Report 78-10. Job No. 114 333A G tech