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HomeMy WebLinkAboutHelical Pier Report.pdf H-P KU mAR5020 County Road 154 \. Glenwood Springs, CO 81601 Geotechnical Engineering I Engineering Geology Phone: (970)945-7988 Materials Testing I Environmental Fax: (970)945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Denver(HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado December 27, 2017 Paul Graf 528 Potato Patch Drive Vail, Colorado 81657 Pgraf32 @ gmail.com Project No. 17-7-784 Subject: Observation of Excavation and Review of Helical Pier Installation, Proposed West Deck Addition to Existing Residence, 758 Potato Patch Drive, Vail, Colorado Dear Paul: As requested, a representative of H-P/Kumar observed the excavation at the subject site on October 26, 2017 to evaluate the soils exposed for foundation support. We have also reviewed the helical pier install records for partial foundation support of the deck. The findings of our observations and recommendations are presented in this report. The services were performed in accordance with our agreement for professional engineering services to you dated October 23, 2017. We previously observed the foundation excavation for a new driveway retaining wall at the site and submitted our findings in a report dated November 6, 2017, Project No. 17-7-784. The deck will be attached to the west side of the residence and be supported on isolated columns. The columns were originally designed to be supported entirely on helical piers with working design axial capacities of from 5 to 8 kips for the individual columns. Based on our observations of the soils exposed in the open excavations prior to the helical pier installation, we recommended spread footing pads be used to support most of the columns away from the residence where natural coarse granular soils were exposed at excavation grade. The footing pads were sized using an allowable soil bearing pressure of 2,000 psf. At the time of our October 23 site visit, the foundation excavations for the thirteen column support areas for the deck had each been cut one level from about 21/to 5 feet below the adjacent ground surface. The soils exposed in the bottom of the excavations consisted primarily of relatively dense, silty sand and gravel with cobbles. In the three excavations nearest the residence and at the one at the northwest, existing foundation backfill consisting of generally medium dense, silty sand and gravel with cobbles was exposed at subgrade. Depth of the fill Paul Graf December 27, 2017 Page 2 was not determined but estimated to be down to existing residence footing bearing elevation, about 4 to 5 feet. No free water was encountered in the excavations and the soils were slightly moist to moist. Subsequently, Great Divide Corporation installed helical piers consisting of single helix, 11 square shaft Heli-piles in about half(six) of the excavations nearest the residence. Based on information provided by Great Divide Construction, the pier depths ranged from about 2 to 71/2 feet below the bottoms of the excavations with the deeper piers typically in the fill areas nearest the residence. The piers achieved either torque (4 to 7 pin) shear refusal, or penetration refusal likely on a large cobble or a boulder. At this time, we recommended the piers be used as well as pier caps placed on the piers, with spread footing pads in the natural soil areas designed for an allowable soil bearing pressure of 2,500 psf, and the pile caps/footing pads have a minimum width of 3 feet. Based on the plans dated December 13, 2017 by KRM Consultants, this was done. We believe the piers as installed and the exposed soils at excavation grade in the column excavation areas have adequate capacity to support the 5,000 to 8,000-pound individual axial design column loadings for the deck supports. There could some differential settlement between the deck supports and with respect to the existing residence which should be considered in the design. Prior to the footing pad and/or pier cap construction, all loose disturbed soils should be removed. The bearing soils should be protected against frost and concrete should not be placed on frozen soils. The footing pads and pier caps should be provided with adequate soil cover above their bearing elevations for frost protection. The foundation backfill should be compacted to at least 90% of standard Proctor density (SPD) at a moisture content near optimum (and to at least 95% SPD in pavement areas) and have the surface graded to prevent ponding within at least 10 feet of the new deck and existing building foundations. The recommendations submitted in this letter are based on our observation of the soils exposed within the foundation excavations and the information provided to us regarding the helical pier installation, and do not include subsurface exploration to evaluate the subsurface conditions within the loaded depth of foundation influence. This study is based on the assumption that soils beneath the excavations have equal or better support than those exposed. The risk of foundation H-Pk-KUMAR Project No. 17-7-784 Paul Graf December 27, 2017 Page 3 movement may be greater than indicated in this report because of possible variations in the subsurface conditions. It is possible the data obtained by subsurface exploration could change the recommendations contained in this letter. If you have any questions or need further assistance, please call our office. Sincerely, fo(0 40, H-P:KUM •' o ,3. &loos, 7 r is IZAT David A. Young, P E: , ".gyp y.Z -e7�,s• D14. Crag cc: KRM Consultants—Tim Fiennum(tim@krmconsultants.com) H-P*KUMAR Project No. 17-7-784