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HomeMy WebLinkAboutObservation of Excavation 05.28.2025 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 email: kaglenwood@kumarusa.com www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado May 28, 2025 Jesse Graber P.O. Box 233 Woody Creek, Colorado 81656 jessgraber1@gmail.com Project No. 25-7-357 Subject: Observation of Excavation, Proposed Residence and Accessory Dwelling Unit (ADU), 1193 County Road 102, Garfield County, Colorado Dear Jesse: As requested, a representative of Kumar & Associates observed the excavations at the subject site on May 22, 2025, 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 you dated May 21, 2025. A subsoil study was previously performed by CTL Thompson and the results were presented in a report dated January 28, 2025, Project No. GS 06934.A00-120R1. The residence and ADU were recommended to be supported by spread footings placed on 2 feet of compacted structural fill and sized for an allowable bearing pressure of 3,000 psf. It is our understanding the proposed residence and ADU can be supported on spread footings sized for an allowable bearing pressure of 2,000 psf. The proposed residence will be a modular structure over a partial basement level with attached garage. The ADU will be a single-story modular structure over a shallow crawlspace. At the time of our visit to the site, the residence foundation excavation had been cut in 3 levels from about 2½ to 11 feet below the adjacent ground surface. The ADU foundation excavation had been cut in one level from 1½ to 2½ feet below the adjacent ground surface. The soils exposed in the bottom of the excavations consisted of sandy clay in the shallow cut areas and silty sand and gravel in the deeper cut areas. There was considerable loose disturbed soils in some of the proposed footing areas. Results of swell-consolidation testing performed on samples taken from the site, shown on Figures 1 through 3, indicate the soils possess nil to low expansion Jesse Graber 25-7-357 May 28, 2025 Page 2 Kumar & Associates, Inc.® potential when wetted under light loading. No free water was encountered in the excavations and the soils were slightly moist. 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 residence and ADU with settlement/heave potential. 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. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose disturbed soils (considerably observed) in footing areas needs to be 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 well reinforced top and bottom to span local anomalies and better resist differential movement 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 60 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. An impervious membrane, such as 20 mil PVC should be provided below the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. Structural fill placed within floor slab areas should consist of imported granular soils such as ¾-inch road base 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 other Kumar & Associates Kumar & Associates Kumar & Associates