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HomeMy WebLinkAboutSub Soil 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 August 6, 2021 Glenn Beaton 2001 Lincoln Street, Unit 2513 Denver, Colorado 80202 gkbdenver@gmail.com Project No. 21-7-605 Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot RR1, Filing 8, Elk Springs, 124 Juniper Drive, Garfield County, Colorado Dear Glenn: 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 July 14, 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 residence will be constructed in three primary areas. The first will be a two-story structure over a slab-on-grade floor. The second will be single-story and structurally supported floor over crawlspace. The third area will be single-story over a full basement. The residence will include an attached garage located on the site as shown on Figure 1. Cut depths are expected to range between about 2 to 8 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. The proposed driveway will be approximately 1,500 feet long connecting to Juniper Drive and have a chip seal surface. 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 subject site was vacant at the time of our field exploration. The ground surface is varied across the lot generally sloping down to the south. An ephemeral drainage crosses the proposed driveway from northwest to southeast through a previously installed CMP culvert crossing. Vegetation consists of grass, weeds and sagebrush with juniper and pinyon trees in the area of the proposed residence. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating 6 exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about ½ to 1½ feet of topsoil, consist of - 2 - Kumar & Associates, Inc. ® Project No. 21-7-605 varied fine-grained soils overlying dense, basalt gravel, cobbles and boulders in a highly calcareous sandy silt matrix to the maximum explored depth of 8 feet. Results of swell- consolidation testing performed on a relatively undisturbed sample of the calcareous sandy silt, presented on Figure 4, indicate low compressibility under existing moisture conditions and light loading and a low collapse potential when wetted. Results of a gradation analysis performed on a sample of the basalt gravel in a sandy silt matrix (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. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, we recommend spread footings placed on the undisturbed natural soil designed for an allowable bearing pressure of 1,500 psf for support of the proposed residence. The soils tend to compress after wetting and there could be around 1 to 1½ inches of post-construction foundation settlement. A lower-risk option would be to remove the upper, fine-grained soils and place spread footings entirely on the underlying basalt rock soils or compacted structural fill. Footings placed entirely on the granular soils or structural fill can be designed for an allowable soil bearing pressure of 2,500 psf. Footings should be a minimum width of 18 inches for continuous walls and 2 feet for columns. Topsoil and loose disturbed soils encountered at the foundation bearing level within the excavation should be removed and the excavation extended down to the undisturbed natural soils. Structural fill should consist of imported granular material such as road base and be compacted to at least 98% of standard Proctor density at near optimum moisture content. 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 12 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 55 pcf for the on-site soil as backfill. Retaining structures which are separate from the residence and can be expected to deflect sufficiently to mobilize the full active earth pressure condition should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site soil as backfill. Resistance to sliding at the bottoms of the footings can be calculated based on a coefficient of friction of 0.35. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction. 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 - 3 - Kumar & Associates, Inc. ® Project No. 21-7-605 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 relatively well graded gravel should be placed beneath slabs-on-grade for support. This material should consist of minus 2-inch aggregate with less than 50% passing the No. 4 sieve and less than 12% passing the No. 200 sieve. For basement level slabs the gravel should be a free draining material with 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 devoid of vegetation, topsoil and oversized rock. Underdrain System: Although free water was not encountered during our exploration, it has been our experience in the area and where clay soils are present that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched condition. We recommend below-grade construction, such as retaining walls, crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. 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. Surface Drainage: Proper surface grading and drainage will be critical to keeping the bearing sols dry and limiting foundation settlement. 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 may be needed uphill to direct surface runoff around the residence. - 4 - Kumar & Associates, Inc. ® Project No. 21-7-605 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation should be located at least 10 feet from the building. Consideration should be given to the use of xeriscape to limit potential wetting of soils below the foundation caused by irrigation. Drive Pavement: These recommendations are for typical residence traffic, including fire trucks. Additional or heavy construction traffic could result in increased wear and possible damage to the driveway surface and repairs may be required post construction. Assuming the discussed chip-seal driveway we believe the gravel pavement section can consist of 10 inches of CDOT Class 6 road base placed on the native subgrade below the topsoil. Additional fill to establish subgrade can consist of the onsite sols devoid of organics or rocks larger than 4 inches moisture conditioned to near optimum and compacted to 95 percent of maximum standard Proctor density. Prior to placing the pavement section, the entire subgrade area should be scarified, moisture conditioned and compacted. The pavement subgrade should be proofrolled. Areas which deform excessively under heavy wheel loads are not stable and should be removed and replaced to achieve a stable subgrade prior to placing base course material. The collection and diversion of surface drainage away from paved areas is extremely important to the satisfactory performance of gravel pavement. Drainage design should provide for the removal of water from the driveway area and prevent wetting of the subgrade soils. Uphill roadside ditches should have an invert level at least 1 foot below the road base. 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. Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 21-7-605 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 2 7.6 93 77 34 15 Sandy Clay 2 2½ to 3½ 8.5 32 48 20 Calcareous Silty Sand and Gravel 5 2½ 10.0 72 Calcareous Sandy Silt