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HomeMy WebLinkAboutObservation of Excavation 06.27.2012Gtech HEPWORTH - PAWLAK GEOTECHNICAL June 27, 2012 Greg Boyd 1253 Kings Row Carbondale, Colorado 81623 greggohrc@gmaiLeo in I le���-orrh-PlwIal< (ieorechnic,il, lnc 5020 (minty Road 15=1 (.5Ien vood tial ine,s, LI,radn 81601 Phone: 970-'-145.7c)Ati Fax: 970-9d 5-,ti=154 email: hpgeu'Nhvgeurech.com Job No. 112 209A Subject: Observation of Excavation, Proposed Addition, Lot 1 Kings Row Subdivision. 1253 Kings Row, Missouri Heights, Garfield County, Colorado Dear Mr Boyd: As requested, the undersigned representative of Hepworth-Pawlak Geotechnical, Inc. observed the excavation at the subject site on June 22, 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 accordance with our agreement for professional engineering services to you, dated June 22, 2012. The proposed 950 square foot addition will be a one-story wood frame structure over partial crawlspace, partial slab -on -grade. Foundations were designed based on an assumed allowable soil bearing pressure of 2,000 psf. Foundation walls acting as retaining walls were designed to resist lateral earth loading by assuming an equivalent fluid unit weight of 50 pcf. The existing house was built in 1978 and is a one-story wood frame structure over a basement level. At the time of our visit to the site, the foundation excavation had been cut in two levels from 1 to 31/2 feet below the adjacent ground surface. The soils exposed in the bottom of the excavation consisted of medium stiff, sandy clay. Results of a swell -consolidation test performed on a sample taken from the site, shown on Figure 1, indicate the soils have low to moderate compressibility under light to moderate loading and have a minor swell potential when wetted under a constant light load. No free water was encountered in the excavation and the soils were slightly moist. The existing basement wall backfill appears to extend out about 3 feet from the basement wall. The footings had been formed and rebar had been doweled into the basement wall. In the northeast portion of the addition, we observed what appeared to be an existing concrete wall below the footing forms Parker 303-841-7119 • Colorado Springs 719-633-7562 • Silverthorne 970-468-1989 Greg Boyd June 27, 2012 Page 2 extending south off the southeast corner of the house. We understand that this is actually a narrow footing for a previous greenhouse located on the south side of the house. We asked the structural engineer to look at this old foundation condition and we understand that the footing can be placed over the old concrete. Considering the conditions exposed in the excavation and the nature of the proposed construction, spread footings placed on the undisturbed natural soil designed for an allowable soil bearing pressure of 2,000 psf can be used for support of the proposed addition. The exposed soils tend to compress or expand when wetted and there could be some post -construction movement of the foundation if the bearing soils become wet. This movement would likely be differential to the existing house. Footings should be a minimum width of 18 inches for continuous walls and 2 feet for columns. Loose and disturbed soils in footing areas should be moistened and compacted or removed and the bearing level extended down to the undisturbed natural soils. 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 soil as backfill. A perimeter foundation drain should be provided to prevent temporary buildup of hydrostatic pressure behind retaining walls taller than about 2 to 3 feet. Structural fill placed within floor slab areas can consist of the on-site soils or a suitable imported material compacted to at least 95% of standard Proctor density at a moisture content near optimum. Backfill placed around the structure should be compacted and the surface graded to prevent ponding within at least 10 feet of the building. Landscape that requires regular heavy irrigation, such as sod, and sprinkler heads should not be located within 6 feet of the foundation. The recommendations submitted in this letter are based on our observation of the soils 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 soils 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 mold or other biological Job No. 112 209A GGE?UPtech Greg Boyd June 27, 2012 Page 3 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 GEOTCHNICAL, INC. Etlllyy,��i f Daniel E. Hardin, P.> 24443 r r �/ DcY s- /% 4; NAI. E N‘e Attachment Figure 1 — Swell -Consolidation Test Results DEH/ksw cc: SGM, Inc — Attn: Nathan Torrez (NathanT[c�sgm-ine.com) Job No. 112 209A GecPtech COMPRESSION - EXPANSION (% ) 1 1 2 3 4 5 Moisture Content = 7.9 Dry Density = 103 pcf Sample of: Sandy Clay From: Footing Grade - South Side of Addition percent Expansion upon wetting 01 1 0 10 APPLIED PRESSURE (ksf ) 100 112 209A Gtech HEPWORTH-PAWLAK GEOTECHNIrCAL SWELL -CONSOLIDATION TEST RESULTS FIGURE 1