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HomeMy WebLinkAboutB14-0220 Geotech report.pdf~tech HEPWORTH-PAWLAK GEOTECHNICAL September 29, 2014 Vail Custom Builders Attn: Clinton Amundson P. 0. Box 2809 Edwards, Colorado 81632 ( clintonw{vailcustom.com) ~[Eo~ ~.O~i[E] TOWN OF VAIL i:.i~: l)~l'-945<3454 i J ') 1:.11: ir.::n1!?. ·,ri,.'C,,t:.:l --,.,:tin1 ~ [E[g CEDWCE~ I Arn ,, ., '''J (__. ' ... l. TOWN OF VAIL Job No. 113 368B Subject: Observation of Excavation, Proposed Harmon Residence, Lot 20-1, Bighorn Subdivision, 4284 Columbine Drive, Vail, Colorado Dear Clinton: As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the excavation at the subject site on September 4, 19 and 24, 2014 to evaluate the soils exposed for foundation support as well as the groundwater conditions for underdrain system reconnnendations. The findings of our observations and reco1mnendations are presented in this report. The services were perfonned in general accordance with our agreement for professional engineering services to Vail Custom Builders, dated August 15, 2014. Background Information: We previously observed backhoe pits excavated at the site to detennine groundwater levels and subsoil conditions, and presented our findings in a repmi dated November 13, 2013, Job No. 113 368A. We have been provided a copy of the soil and foundation investigation for the lot prepared by LKP Engineering dated September 13, 2005, Project No. 05152. The LKP repo1i reconunended spread footings designed for an allowable soil bearing pressure of 3,500 psf to supp01i the building. Proposed Construction: The residence was designed to be a two story wood frame structure over a partial basement/partial crawlspace with an attached garage. The building has been designed to be supported on spread footing using an allowable soil bearing of 3,500 psfbased on the recommendations provided in LKP Engineering's previous rep01i. Due to considerable groundwater being encountered in the excavation, the basement level was eliminated. The crawlspace, main level and garage will have slab-on-grade floors. Groundwater Observations and Underdrain Recommendations: On September 4 and 19, we visited the site to observe the groundwater conditions and provide recommendations to dewater the excavation and for a permanent underdrain system. Groundwater was initially encountered near crawlspace elevation and a few above proposed basement floor elevation. After our initial site visit on September 4, we were advised the basement level was eliminated and most of the residence would be over Vail Custom Builders September 29, 2014 Page 2 crawlspace with the exception of the garage and main level floor area on the west side of the residence. At this time, we recommended an underdrain system consisting of a grid of perimeter and connecting interior laterals in the basement/crawlspace areas using 6 inch diameter perforated PVC pipe placed at least 1 Yz feet below finish floor slab grade and surrounded by free draining gravel, and sloped at a minimum Yz% to a suitable outlet. We assumed the drain system would outlet to a sump and pump system. On September 19, we visited the site after groundwater was encountered in a portion of the garage area foundation excavation and recommended the drain system be expanded to include perimeter drains around the garage and main level floor slab areas. We were informed shortly thereafter that gravity outlet of the drain system could be accomplished, which was done, and the drain system was performing well. Excavation Observation: At the time of our September 24 site visit, the foundation excavation which was essentially complete had been cut in two levels from about 1 to 8 feet below the adjacent ground surface. The soils exposed in the bottom of the excavation consisted of relatively dense, silty sandy gravel and cobbles with boulders. The free water encountered in the excavation was observed being adequately removed by the perimeter and underdrain system we had recommended and was being gravity outlet to the northwest. The soils were moist to wet. There shallow depth of free draining gravel observed on the excavation subgrade. Recommendations: The soil conditions exposed in the excavation are consistent with those previously encountered on the site and should be suitable for support of spread footings designed to support the design bearing pressure of3,500 psf. Loose and disturbed soils should be removed in the footing areas to expose firm soils and the water seepage into the excavation should continue to be collected and diverted away from the footing areas and any softened soils and mud removed before concrete placement. A shallow depth of free draining rock over the natural gravels in footing areas is acceptable. Footings should be a minimum width of16 inches for continuous walls and 2 feet for columns. Exterior footings should be provided with adequate soil cover above their bearing elevations for frost protection. Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 10 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for on-site soii excluding topsoil and oversized (plus 8 inch) rocks, as backfill. Structural fill placed within floor slab areas can consist of the on-site sand and gravel soils compacted to at least 95% of standard Proctor density (SPD) at a moisture content near optimum. A minimum of 6 inches of free draining rock should be provided below the slabs. Free- draining granular material used in the underdrain system and below slabs should contain less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum size of 2 inches. Backfill placed around the structure should be compacted to at least 90% SPD (95% in slab areas) and the surface graded to prevent ponding within at least 10 feet of the building. Job No. 113 368B ~tech Vail Custom Builders September 29, 2014 Page 3 Limitations: The recommendations submitted in this letter are based on our observation of the soils exposed within the foundation excavation and the previous subsurface exploration at the site. Variations in the subsurface conditions below the excavation could increase the risk of foundation movement. We should be advised of any variations encountered in the excavation conditions for possible changes to recommendations contained in this letter. Our services do not include detennining the presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is concerned about MOBC, then a professional in this special field of practice should be consulted. If you have any questions or need finiher assistance, please call our office. Sincerely, DAY/ljg Job No. 113 368B ~tech