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HomeMy WebLinkAboutExcavation ObservationHEPWORTH - PAWLAK GEOTECHNICAL September 28, 2012 Crystal Springs Builders Attn: Mike Cerise 1044 Main Street Carbondale, Colorado 81623 inike(a?csbldr.net Al. iii 01'C'i-..3ini-c f,rn,. 15 --- Job No. 112 345A Subject: Observation of Excavation, Proposed Basement Under Existing House, 0880 Red Hill Road (CR 107), Garfield County, Colorado Dear Mike: As requested, the undersigned representative of Hepworth-Pawlak Geotechnical, Inc. observed the excavation at the subject site on September 27, 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 Crystal Springs Builders, dated September 27, 2012. The existing 1 % story wood frame house is set into a relatively steep south -facing hillside. The uphill half of the house is supported on a cast -in-place concrete foundation. The downhill half is supported on the north side by the concrete foundation and on the south side by 4 columns with open area underneath. The new basement will include the open area under the overhanging south half of the house and the four columns will be removed. The basement will extend beyond the existing house footprint about 5 feet to the west, 8 feet to the east and about 12 feet to the south. The new basement level will be covered with an exterior deck roof at the main floor level. We understand that the new foundations were designed for an allowable soil bearing pressure of 1,500 psf, Site Observations: At the time of our visit to the site, the foundation excavation had been cut in multiple levels from 2 to 8 feet below the adjacent ground surface. The soils exposed in the bottom of the excavation consisted of large basalt boulders up to I0 feet in size in a whitish, gravelly sandy silt matrix. The basalt boulders were estimated to take up about 80% of the soil volume. Splitting and blasting of the basalt boulders was required in order to excavate them. Holes below footing grade caused by boulder removal were backfilled with compacted imported (3 -inch minus) sandy gravel. The Parker 3M-8411-7119 ® ('i,1 n ddo ' rin..,; 719-6 ' ;-»62 Si!vert NN, in : 070 -468 -1q, -;L) Crystal Springs Builders September 28, 2012 Page 2 previous septic tank was removed from the middle part of the new south footing line and about 2 feet of the compacted sandy gravel was placed in the tank excavation to re- establish footing grade (the footing grade is about 6 feet below natural grade in the old tank area). No free water was encountered in the excavation and the soils were slightly moist. Recommendations: Considering the conditions exposed in the excavation and the nature ofthe proposed construction, spread footings placed on the undisturbed natural soil or compacted granular fill designed for an allowable soil bearing pressure of 1,500 psf should be adequate for support ofthe proposed new basement foundations. Footings should be a minimum width of 16 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 soil as backfill. 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. Structural fill placed within floor slab areas can consist of imported granular soils 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 ofthe building. Landscape that requires regular heavy irrigation, such as sod, and sprinkler heads should not be located within 10 feet ofthe foundation. Subsidence Potential: The site is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. Sinkholes were not observed in the immediate area of the subject lot. Based on our present knowledge ofthe site, it cannot be said for certain that sinkholes will not develop. In our opinion, the risk of ground subsidence at this site is low but the owner should be aware ofthe potential for sinkhole development. The recommendations submitted in this Ietter 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 Job No. 112 345A Crystal Springs Builders September 28, 2012 Page 3 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 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. ap�i�r,,�y„ultkEHHJ!lll1e� pp RE � E. ®se en3/4„ a. o !" O Daniel E. Hardin, a a 0.Rev. by: SLP �,'e" °�Aeamaoe��\�k'`�• ''�i. SIpWAl, to r�,� DEH/ksw cc: KL&A — Attn: Brett McElvain (hnicelvain(i klaa.corn) Job No. 112345A