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HomeMy WebLinkAboutObservation of Site Conditions 04.18.2016HEPW©RTH-PAWL4K GEOTECHNICAL April 18, 2016 Chris Erickson P. O. Box 3782 Basalt, Colorado 81621 chriseri cksonurt @ gmai Leon n FlETWIrrs1t,-('ilV1,11, GC0101i I' 1, 5020 Comity Road 154 Glenwood Stith0 . Colorado 81601 f'luutc, 97(i 915 7980 Fax 9/9.9-1a-(345'1 cin,til. hppogIli entecli.c.om Job No, 116 110A Subject: Observation of Site Conditions, Lot 13, Cottonwood Hollow, 5000 County Road 113 (Cattle Creek Road), Garfield County, Colorado Dear Chris: As requested, we performed a cursory review of the subject site on April 8, 2016 with respect to possible construction of a residence. The findings of our observations and preliminary recommendations for the foundation and grading designs are presented in this report. The services were performed in general accordance with our agreement for professional engineering services to you, dated April 12, 2016. A single family residence is proposed on the site. The residence would likely retain the existing very steep hillside cut on the north side and daylight to the south and east. Plans for the building are not available at this time and we understand our findings will be considered in the purchase of the property. The lot is located on a moderately steep to steep southerly facing hillside above County Road 113. The building site which is at the westerly building envelope shown on the amended subdivision plat dated October 3, 1994, has been graded relatively flat by cutting into the moderately steep to steep hillside. The cot on the uphill side is about 25 feet high and very steep. There is some fill on the downslope side of the flatly graded area. The cuts at the building site and for the access drive to the building site expose basalt gravel, cobbles and boulders in a sandy silt matrix. There are also basalt cobbles and boulders on the natural ground surface of the slope above the building site. No water seepage was observed from the cut slopes, however, there has been some raveling and erosion of the cuts. The excavation and cuts have been open for some time, possibly on the order of 15 years. Development of the site will be difficult based on geotechnical considerations due to the need to retain the existing very steep cut and the height of the retaining. Potential for hillside instability does not appear to be a concern but the steeply graded cut slopes should be retained or sloped back to a more stable grade. Rockfall from the natural slope above the building site could also be a concern. Provided below are recommendation considered suitable for planning and preliminary design. We should review the plans prior to construction and perform additional analyses as needed. Parker 303-8111..7119 Colorado Springs 71 9-633 •' 562 Silverrhorne 970,1613-1989 Chris Erickson April 18, 2016 Page 2 Lightly loaded spread footings bearing on the granular natural soils should be feasible for foundation support of the building with some risk of settlement, primarily if the bearing (matrix) soils become wetted. An allowable soil bearing pressure on the order of 1,500 psf can be assumed for the footing design. The allowable bearing pressure can be increased by'% for retaining wall toe pressures. Precautions should be taken to prevent wetting of the bearing soils. Continuous foundation walls should be reinforced to span local anomalies and resist appropriate lateral earth pressures when acting as retaining structures. Foundation walls up to 12 feet tall should be designed to resist a lateral earth pressure based an an equivalent fluid unit weight of at least 50 pcf. Foundation walls taller than about 12 feet should be designed to resist a uniform lateral earth pressure of 2511 in psf where H is the retained wall height in feet. The lateral earth pressure values are for the on-site granular soils excluding oversized (plus 6 inch) rocks as backfill. A perimeter foundation drain should be provided to prevent temporary buildup of hydrostatic pressure behind the basement foundation walls and retaining structures and prevent wetting of the below grade building levels. Backfill placed around the structure should be compacted to at least 90% of standard Proctor density at a moisture content near optimum and the surface graded to prevent ponding within at least 10 feet of the building. This may require a swale along the uphill side of the building. Some settlement of deep backfill should be expected. Final graded slopes up to about 8 feet high should be no steeper than Ph horizontal to 1 vertical. Final graded slopes taller than about 8 feet should be no steeper than 2 horizontal to 1 vertical, or be retained to limit the exposed cut height to about 8 feet. The recommendations submitted in this letter are based on our observation of the soils exposed within the existing excavation and our experience in the area, and subsurface exploration. In order to reveal the nature and extent of variations in the subsurface conditions below the excavation, drilling would be required. A formal stability analysis of the hillside was not performed. It is possible the data obtained by subsurface exploration and additional analysis could change the recommendations contained in this letter. We recommend on-site observation of excavations and foundation bearing strath and testing of structural 0[l by a representative of the geotechnical engineer. If you have any questions or need further assistance, please call our office. Sincerely, HEPWORTH — P,AFWLAI Bi3InnliplICAL, INC. • 11. do not include David A. Young, P.1.1/41 tkile6 ../Fer:04., 324.P I 6 ce Rev. by: SLP 1:0,f�r f►41N ,: DAY/ksw--#Jl14t fl%%% Job No. 116 110A