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HomeMy WebLinkAboutSubsoil Study for Foundation Design 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 January 6, 2021 S & G Investments Attn: Don Vandevander P.O. Box 1307 Carbondale, Colorado 81623 don.vandevander@gmail.com Project No.20-7-708 Subject: Subsoil Study for Foundation Design, Proposed Metal Building, 1058 County Road 100, West of Old Mid-Continent Facility, Garfield County, Colorado Dear Don: As requested, Kumar & Associates, Inc. performed a subsoil study for design of foundations at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated November 16, 2020. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. We previously conducted a geologic hazards review that included this property and presented our findings in a report dated April 7, 2017, Project 17-7-230. Proposed Construction: The proposed metal building will be about 10,000 square feet, roughly 50 feet by 200 feet, steel frame/metal skin, one story, located on the site in the area of the pits as shown on Figure 1. Ground floor will be slab-on-grade. The area of the proposed building is relatively flat and cut depths are expected to range between about 2 to 3 feet. Foundation loadings for this type of construction are assumed to be moderate and typical of the proposed type of construction. If building conditions or foundation loadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report. Site Conditions: The building site was mostly vacant with parked vehicles, storage containers, tanks and materials mostly on the north and west sides of the proposed building area. The site is relatively flat and appears to have been cut into the hillside on the south side and extensively filled on the north and west sides. There could be up to 12 to 15 feet of fill in the northwest part of the building area. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating six exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are - 2 - Kumar & Associates, Inc. ® Project No. 20-7-708 presented on Figure 2. The natural subsoils, only encountered in Pits 3 and 4 at the east end of the building, consist of medium stiff sandy silt and clay with scattered gravel and cobbles. Results of swell-consolidation testing performed on a relatively undisturbed sample of the sandy silt and clay, presented on Figure 4, indicate low compressibility under existing moisture conditions and light loading and a low collapse potential (settlement under constant load) when wetted. The silt and clay sample was moderately compressible under increased loading after wetting. The shallow soils in Pits 3 and 4 and the entire depth of the other four pits consisted of man- placed fill soils made up of mostly loose to medium dense, clayey to silty sandy gravel with scattered cobbles. Results of a gradation analysis performed on a sample of clayey sand and gravel fill (minus 3-inch fraction) obtained from the site are presented on Figure 5. No free water was observed in the pits at the time of excavation and the soils were slightly moist to moist. Analysis: The fill depth under the proposed building ranges from ½ foot at Pit 3 near the southeast building corner and 6 feet near the northeast building corner to over 7 feet in the rest of the building area. We expect that the fill depth increases under the building area to the north and west. It appears that the fill could be at least 15 feet deep under the northwest part of the proposed building. Assuming at least 2 to 3% settlement of the existing fill under the proposed building, result in expected settlements of about 1½ to 3 inches in the shallower fill areas (up to 7 feet of fill) to 4 to 5 inches in the deeper (up to 15 feet) fill areas to the northwest part of the building. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, spread footings placed on the undisturbed natural soil or existing fill designed for an allowable soil bearing pressure of 1,500 psf could be used for support of the proposed building. The natural clay soils tend to compress after wetting and there could be some post-construction foundation settlement. Based on experience, we expect that footings placed on the loose to medium dense granular fill soils could settle 2 to 3 percent of the fill depth below the footings. As an alternative with a lower risk of settlement, the building could be supported on driven piles, or possibly, helical piers. We should be contacted for recommendations for a deep foundation if desired. Further evaluation of - 3 - Kumar & Associates, Inc. ® Project No. 20-7-708 the existing fill soils by drilling several borings would help to determine depth to bearing soils for pile foundations and to further analyze the potential settlement of shallow spread footing foundations. For preliminary design analysis, footings should be a minimum width of 18 inches for continuous walls and 2 feet for columns. Loose and disturbed soils and existing fill encountered at the foundation bearing level within the excavation should be compacted or removed and the footing bearing level extended down to the denser soils. Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least 36 inches below the exterior grade is typically used in this area. Continuous foundation walls should be heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 14 feet. Foundation walls acting as retaining structures (if any) should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site soil as backfill. Floor Slabs: The on-site soils are suitable to support lightly loaded slab-on-grade construction with a similar long term settlement risk as the foundation. To reduce the effects of some differential movement, floor slabs should be separated from all bearing walls and columns with expansion joints which allow unrestrained vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The requirements for joint spacing and slab reinforcement should be established by the designer based on experience and the intended slab use. A minimum 4 inch layer of ¾-inch road base should be placed beneath slabs for support. All fill materials for support of floor slabs should be compacted to at least 95% of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on- site soils devoid of vegetation, topsoil and oversized rock. Underdrain System: An underdrain should not be needed for the proposed building construction. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the building has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. - 4 - Kumar & Associates, Inc. ® Project No. 20-7-708 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95% of the maximum standard Proctor density in pavement and slab areas and to at least 90% of the maximum standard Proctor density in landscape areas. 3) The ground surface surrounding the exterior of the building should be sloped to drain away from the foundation in all directions. We recommend a minimum slope of 6 inches in the first 10 feet in unpaved areas and a minimum slope of 2½ inches in the first 10 feet in pavement areas. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. Limitations: This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the exploratory pits excavated at the locations indicated on Figure 1 and to the depths shown on Figure 2, the proposed type of construction, and our experience in the area. 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. Our findings include interpolation and extrapolation of the subsurface conditions identified at the exploratory pits and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appear different from those described in this report, we should be notified at once so re-evaluation of the recommendations may be made. This report has been prepared for the exclusive use by our client for design purposes. We are not responsible for technical interpretations by others of our information. As the project evolves, we should provide continued consultation and field services during construction to review and monitor the implementation of our recommendations, and to verify that the recommendations have been appropriately interpreted. Significant design changes may require additional analysis or modifications to the recommendations presented herein. We recommend on-site observation of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 20-7-708 SAMPLE LOCATION NATURAL MOISTURE CONTENT NATURAL DRY DENSITY GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH SOIL TYPE PIT DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%) (ft) (%) (pcf) (%) (%) (psf) 3 2½ 15.1 82 81 Sandy Silt and Clay 4 3 - 4 9.0 25 36 39 33 9 Clayey Sand and Gravel (Fill)