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HomeMy WebLinkAboutSubsoil Study 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 17, 2022 The Home Group, Inc. Attn: Jack Wheeler 183 North 12th Street Carbondale, Colorado 81623 wheeler@thehomegroupinc.com Project No. 21-7-932 Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 57, Spring Ridge Reserve, Hidden Valley Drive, Garfield County, Colorado Mr. Wheeler: 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 December 17, 2021. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The proposed 1 to 2-story structure will be located within the western part of the building envelope as shown on Figure 1 with a detached garage planned to the north. Ground floors will be structural over crawlspace and slab-on-grade. Cut depths are expected to range between about 3 to 14 feet, with the deeper excavations planned on the east side of the lot cut into the hillside. Foundation loadings for this type of construction are assumed to be relatively light 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 residence area was undisturbed natural hillside vegetated with scattered pinon and juniper trees and an understory of scattered grasses, weeds and sage brush at the time of our site visit. The ground surface is strongly to gently sloping down generally to the west with around 12 feet of elevation difference across the residence footprint. A few inches of snow covered the ground surface at the time of our site visit. Subsurface Conditions: The subsurface conditions at the site were evaluated by observing one exploratory pit at the approximate location shown on Figure 1. The log of the pit is presented on - 2 - Kumar & Associates, Inc. ® Project No. 21-7-932 Figure 2. The subsoils encountered, below a thin topsoil layer, consist of interlayered sand (medium dense) and silt (firm) with gravel and scattered cobble rock fragments with depth. The soils were underlain by weathered to hard sandstone/siltstone bedrock at a depth of about 5 feet and extended down to about 7½ feet where practical digging refusal was encountered. No free water was observed in the pit at the time of excavation and the soils and bedrock were slightly moist. Laboratory testing performed on a sample obtained from the pit included natural moisture content and density, swell-consolidation, and percent silt and clay-sized particles. Results of a swell-consolidation test performed on a relatively undisturbed liner sample of sandy silt soil, presented on Figure 3, indicate slight compressibility under existing low moisture conditions and light loading and low compressibility under conditions of loading and wetting. Natural moisture and dry density test results indicate the sand and silt soils generally possess a moderate collapse potential when wetted. The laboratory testing is summarized in Table 1. Engineering Analysis: The natural sand and silt soils possess a collapse potential and generally tend to settle especially when wetted under load. The underlying sandstone and siltstone possess a high bearing capacity and a comparatively low risk of settlement. Very hard, potentially cemented bedrock may be encountered during excavation, especially at the deeper cuts planned into the hillside, and the depth to bedrock will likely be shallower upslope from the exploratory pit. We anticipate excavations into the comparatively unweathered bedrock will be difficult and could require additional excavation methods, such as chipping or rock splitting. Foundation Recommendations: Considering the subsurface conditions encountered in the exploratory pit, the presence of compressible soils, and the nature of the proposed construction, we recommend spread footings placed entirely on the underlying, undisturbed natural sandstone/siltstone materials designed for an allowable bearing pressure of 4,000 psf for support of the proposed residence. Foundation settlements should be minor, less than 1 inch. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. The topsoil, sand and silt soils, and loose disturbed rock encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural bedrock. Structural fill such as road base placed to at least 98% standard Proctor density and to at least one-half the fill depth below the footing beyond the footing edges can be used to reestablish design bearing level. Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least - 3 - Kumar & Associates, Inc. ® Project No. 21-7-932 36 inches below the exterior grade is typically used in this area. It may be acceptable to reduce frost depth where very hard cemented rock is encountered but still meet the requirements of the structural engineer. 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 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 and well broken rock or imported granular material such as base course as backfill. Floor Slabs: The natural on-site soils and bedrock, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction with the accepted risk of movement with the sand and silt subgrade. The risk of movement can be reduced by placing slabs-on-grade on a minimum of 2 feet of compacted structural fill or using a structurally supported floor over a crawlspace. The structural fill below floors can consist of the onsite soils or imported CDOT Class 5 or 6 base course material. 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 free-draining gravel should be placed beneath interior slabs to facilitate drainage and can be included in the recommended 2 feet of base course below slabs-on-grade. This material should consist of minus 2-inch aggregate with less than 50% passing the No. 4 sieve and less than 2% passing the No. 200 sieve. 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 and well broken rock devoid of vegetation, topsoil and oversized rock larger than 6 inches or imported granular material such as road base. Underdrain System: Although free water was not encountered during our exploration, it has been our experience in the area and where bedrock is shallow that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also create a perched condition. We recommend below-grade construction, such as retaining walls and deep crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. - 4 - Kumar & Associates, Inc. ® Project No. 21-7-932 The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above the invert level with free-draining granular material. The drain should be placed at each level of excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least 1½ feet deep. Site Grading: The risk of construction-induced slope instability at the site appears low provided cut and fill depths are limited. We assume the cut depths for the main level will not exceed about 15 feet. Fills should be limited to about 8 to 10 feet deep. Embankment fills should be compacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should be benched into the portions of the hillside exceeding 20% grade. Permanent unretained cut and fill slopes in soil should be graded at 2 horizontal to 1 vertical (2H:1V) or flatter and protected against erosion by revegetation or other means. Cut slopes in bedrock can be made at 1½H to 1V as shown on Figure 1. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. 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. Free-draining wall backfill should be covered with filter fabric and capped with about 2 feet of the on-site, finer graded soils to reduce surface water infiltration. 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 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in pavement and walkway areas. A swale will be needed uphill to direct surface runoff around the residence. 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 Kumar & Associates Kumar & Associates Kumar & Associates TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 21-7-932 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) 1 1½ 5.4 97 65 Sandy Silt