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HomeMy WebLinkAboutObservation of Excavation 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email: kaglenwood@kumarusa.com www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado May 13, 2024 Stewart Custom Builders Attn: Duane Stewart 382 Meadowood Road Glenwood Springs, CO 81601 duane@stewartcustombuilders.com Project No. 24-7-251 Subject: Observation of Excavation, Proposed Residence, Lot 83, Ironbridge, 392 River Bank Lane, Garfield County, Colorado Dear Duane: As requested, a representative of Kumar & Associates observed the excavation at the subject site on April 19, 24 and 25, 2024 to evaluate the soils exposed for foundation support. The findings of our observations and recommendations for the foundation support are presented in this report. The services were performed in general accordance with our agreement for professional engineering services to Stewart Custom Builders dated April 18, 2024. A subsoil study was previously conducted by Yeh and Associates for the lot and the results were presented in a report dated August 19, 2006. The proposed residence will be a two-story wood-frame residence over a partial walkout basement level with attached garage at the main level. The residence has been designed to be supported on spread footings sized for an allowable bearing pressure of 3,000 psf. At the time of our visit to the site on April 19, the foundation excavation was partially complete and had been cut in one level from 6 to 7 feet below the adjacent ground surface. The soils exposed in the bottom of the excavation consisted of dense, silty sandy gravel and cobbles in the northern area of the basement excavation and silty sand in the southern part of the excavation, both river alluvium. Two pits were excavated in the crawlspace area to expose the soils at design footing grade. The soils consisted of stiff sandy clay. Results of swell-consolidation testing performed on samples taken from the site, shown on Figures 1 and 2, indicate the soils are compressible under conditions of loading and wetting. The exposed soils are not considered suitable for support of the proposed residence as designed and we recommended sub-excavation to expose the underlying gravel soils and backfilling the sub-excavated depth with compacted structural fill consisting of CDOT Class 6 base course. No free water was encountered in the excavation and the soils were slightly moist to moist. At the time of our visit to the site on April 24, the foundation excavation was cut in two levels up to 6 feet below the adjacent ground surface. The soils exposed in the foundation excavation were similar to those previous encountered and we recommended continuing to sub-excavate to expose the underlying gravel soils. Stewart Custom Builders May 13, 2024 Page 2 Kumar & Associates, Inc. ® Project No. 24-7-251 At the time of our visit to the site on April 25, the foundation excavation was cut in approximately three levels up to 11 feet below the adjacent ground surface. The subsoils encountered in the excavation consisted primarily of silty sandy gravel. One area of the garage footer excavation was cut to 11 feet deep and exposed very moist to wet, silty sand. We probed the sand and encountered gravelly soils approximately 12 inches below the excavation depth. We recommended the sand soils be compacted and the sub-excavated depth be backfilled with compacted structural fill consisting of an imported granular material such as CDOT Class 6 base course. We performed compaction testing and observations during the backfilling process. The results of our compaction testing will be presented under a separate cover. Considering the conditions exposed in the excavation and the nature of the proposed construction, spread footings placed on the undisturbed natural soil and compacted structural fill designed for an allowable soil bearing pressure of 3,000 psf can be used for support of the proposed residence. Based on our observations and properly compacted structural fill, post- construction settlement of the foundation should be less than 1 inch. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed soils in footing areas should be removed or compacted. 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 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 on-site granular 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. A perimeter foundation drain is not needed around the garage or crawlspace areas of the main level. Structural fill placed within floor slab areas can consist of the on-site 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 of the building. Landscape that requires regular heavy irrigation, such as sod, and sprinkler heads should not be located within 10 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 Kumar & Associates Kumar & Associates TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 24-7-251 SAMPLE LOCATION NATURAL MOISTURE CONTENT NATURAL DRY DENSITY GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH SOIL OR BEDROCK TYPE IN BOTTOM OF EXCAVATION GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%) (%) (pcf) (%) (%) (PSF) Middle South Basement Grade 9.0 93 Silty Sand Southwest Basement Grade 6.3 87 25 Silty Sand North Crawlspace Grade 13.5 105 Sandy Clay