No preview available
HomeMy WebLinkAboutB12-0245 OBSERVATION OF EXCAVATIONG(5o'I'tech HEPWORTH - PAWLAK GEOTECHNICAL August 15, 2012 Nova Group Attn: Dave Irwin P. O. Box 3342 Vail, Colorado 81658 a -aay5 Hepworth- P.i,.1A Ototeihniial, lnt:. 5010 County Road I i4 Cilem +tk�d Springs, Colorado 81601 Phone: Q70- 945 -79,M Fax: 911.1-945-8454 email: hp, %ro�hpgeotedt.tom Snapoutofit2(d.msn.com Job No. 112 214A Subject: Observation of Excavation, Proposed Kuppinger Residence, Lot 4, Filing 1, Eleni Zneiner Subdivision, 1699 Buffehr Creek Road, Vail, Colorado Dear Dave: As requested, a representative of Hepworth - Pawlak Geotechnical, Inc. observed the excavation at the subject site on July 27 and August 2, 2012 to evaluate the soils exposed for foundation support. The findings of our observations and recommendations for the foundation design are presented in this report. The services were performed in general accordance with our agreement for professional engineering services to Nova Group, dated June 14, 2012. Potential geologic hazard impacts to the project site is beyond the scope of our services. The residence will be a two story structure with the lower levels stepped down the hillside slope and daylighting to the south. Ground floors will be slab -on- grade. The building has been designed to be supported on spread footings using an allowable soil bearing pressure of 2,500 psf. The foundation design was based on a preliminary soils investigation for the subdivision by Koechlein Consulting Engineers dated June 4, 2003, Job No. 03 -056. That report recommended spread footing foundations designed for an allowable soil bearing pressure in the range of 2,000 to 3,000 psf. We have been provided a copy of their report. The site is moderately steeply sloping down to the south. During our July 27 site visit, the foundation excavation was underway with the upper level nearly complete and silty clayey sand soils exposed at subgrade in about the western half and hard sandstone bedrock exposed in about the eastern half of the excavation. At the time of our August 2 site visit, the foundation excavation was essentially complete and had been cut in three levels from about 3 to 24 feet with about a 25 feet high step in grade from the upper level down to the lower level of the excavation. Total elevation difference across the excavation was about 35 feet. The soils exposed in the bottom of the excavation consisted of medium dense, silty clayey sand with gravel and cobbles in about the western ''% and hard to very hard, fractured sandstone bedrock elsewhere. Results of swell - consolidation testing performed on samples taken from the site, shown Parker 303 - 841 -7119 0 Springs 719 - 633- )>()' • �►lverrhorne 970 -468 -1959 Nova Group August 15, 2012 Page 2 loading and wetting with a low hydro - compression potential. The samples were probably partly disturbed due to the rock content. The sandstone was to hard and fractured to obtain undisturbed samples for swell- consolidation testing. No free water was encountered in the excavation and the soils and bedrock were slightly moist to moist. Considering the conditions exposed in the excavation and the nature of the proposed construction, spread footings placed on the undisturbed natural soil and sandstone bedrock designed for an allowable bearing pressure of 2,500 psf can be used for support of the proposed residence and garage. There could be some differential settlement due to the variable bearing conditions and cut depths, and especially at the transition from bedrock to soil bearing areas. Footings should be a minimum width of 18 inches for continuous walls and 2 feet for columns. Loose and disturbed materials and any existing fill in footing areas should be removed and the bearing level extended down to the undisturbed natural granular soils or sandstone bedrock. The bearing soils should then be moistened and compacted. Exterior footings should be provided with adequate soil cover above their bearing elevations for frost protection. Continuous foundation walls should be heavily reinforced top and bottom to span an unsupported length of at least 12 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 the on -site soils and well broken bedrock, excluding topsoil and oversized rocks, as backfill. For retaining walls taller than 15 feet, a uniform lateral earth pressure load of 25H psf where H is the wall height in feet should be used for the design. A perimeter foundation drain should be provided to prevent temporary buildup of hydrostatic pressure behind the basement walls and prevent wetting of the lower level. The underdrain should include at least 4 inches of free draining gravel below the basement floor slab. 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 at a moisture content near optimum. Backfill placed around the structure should be well compacted and the surface graded to prevent ponding within at least 10 feet of the building. Some settlement of deeper backfill areas should be expected even though the backfill is adequately compacted which could result in distress to facilities constructed on the backfill. Landscape that requires regular heavy irrigation, such as sod, and sprinkler heads should not be located within 5 feet of the foundation. The recommendations submitted in this letter are based on our observation of the materials exposed within the foundation excavation 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 materials beneath the footings have equal or better support than those exposed. The risk of foundation movement may be greater than indicated in this report because of possible variations in the subsurface conditions. In order to reveal the nature and extent of variations in the subsurface conditions below the excavation, drilling would be required. It is possible the data obtained by subsurface exploration could change the recommendations contained in this letter. Our services do not include determining the presence, prevention or possibility of Job No. 112 214A Ge&ech Moisture Content 10.3 percent Dry Density 104 pcf Passing No. 200 Sieve 36 percent Sample of: Silty Clayey Sand From: Bottom of NW Portion of Excavation (07-24-12) 0 i ,I, i � �lil!II i, ! I ii i! z 0 Compressi- CO TH upon C/) 2 wetting cc O 3 1 1 i i 1 I. 1 4 j 1 0.1 10 100 1.0 APPLIED PRESSURE (ksf) Moisture Content 9.6 percent Dry Density = 117 pcf Sample of Silty Clayey Sand with Gravel From: Bottom of SW Portion of Excavation (08-02-12) OR z 2 0 Compression U) W upon 3 wetting .72 0 4 5 0.1 10 100 1.0 APPLIED PRESSURE (ksf) — 214A Gg;b� ISWELL- CONSOLIDATION TEST RESULTS FIGURE 1 I _=f]112 NICAL i ,I, i � �lil!II i, ! I ii i! Nova Group August 15, 2012 Page 3 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 further assistance, please call our office. Sincerely, HEPWORTH — PAWLAK gX"LCHNICAL, INC. David A. Young, P:� 32_21$ r Q DAY/ksw ���s`S�ONAi . ge,�� attachments Figure 1, SwelPConsolidation Test Results cc: Structural Design Solutions — Jeff Leonard (sdajeffa vail.net) Ron Gay (rongay2(&gmai1.com) Job No. 112 214A