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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 SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE AND GARAGE LOT 6, MINEOTA ESTATES MINEOTA DRIVE GARFIELD COUNTY, COLORADO PROJECT NO. 22-7-127 FEBRUARY 24, 2022 PREPARED FOR: CODY AND KARLA FERGUSON P.O. BOX 792 SILT, COLORADO 81652 cf.masonry@yahoo.com Kumar & Associates, Inc. ® Project No. 22-7-127 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY ........................................................................... - 1 - PROPOSED CONSTRUCTION .................................................................................... - 1 - SITE CONDITIONS ....................................................................................................... - 1 - FIELD EXPLORATION ................................................................................................ - 2 - SUBSURFACE CONDITIONS ..................................................................................... - 2 - FOUNDATION BEARING CONDITIONS .................................................................. - 2 - DESIGN RECOMMENDATIONS ............................................................................ - 3 - FOUNDATIONS ........................................................................................................ - 3 - FOUNDATION AND RETAINING WALLS ........................................................... - 4 - FLOOR SLABS .......................................................................................................... - 5 - UNDERDRAIN SYSTEM ......................................................................................... - 6 - SITE GRADING ......................................................................................................... - 6 - SURFACE DRAINAGE ............................................................................................. - 7 - LIMITATIONS ............................................................................................................... - 7 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Kumar & Associates, Inc. ® Project No. 22-7-127 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence and garage to be located on Lot 6, Mineota Estates, Mineota Drive, Garfield County, Colorado. The project site is shown on Figure 1. The purpose of the study was to develop recommendations for foundation design. The study was conducted in accordance with our agreement for geotechnical engineering services to Cody and Karla Ferguson, dated January 18, 2022. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils obtained during the field exploration were tested in the laboratory to determine their classification, compressibility or swell and other engineering characteristics. The results of the field exploration and laboratory testing were analyzed to develop recommendations for foundation types, depths and allowable pressures for the proposed building foundation. This report summarizes the data obtained during this study and presents our conclusions, recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION Design plans for the lot were preliminary at the time of our study. The proposed buildings will be located as shown on Figure 1 and be cut into the north sloping terrain. Excavation for the buildings is expected to be between about 3 to 12 feet below the existing ground surface. For the purpose of our analysis, foundation loadings for the structures were assumed to be relatively light and typical of the proposed type of construction. If building loadings, location or grading plans are significantly different from those described above, we should be notified to re-evaluate the recommendations contained in this report. SITE CONDITIONS The building area was vacant and appeared mostly natural except for a small disturbed area in the general proposed garage site which had probably been flattened by fill grading. The proposed residence site had been staked. There were scattered items including trailers on the property. The ground surface slopes gently to moderately down to the north at grades between about 10 to 15% through the building area and steepens to about 20% uphill of the building area. About 6 to 12 inches of snow covered the lot and vegetation appeared to consist of native grass and weeds. Kumar & Associates, Inc. ® Project No. 22-7-127 - 2 - FIELD EXPLORATION The field exploration for the project was conducted on January 26, 2022. Three exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4-inch diameter continuous flight auger powered by a truck- mounted CME-45B drill rig. The borings were logged by a representative of Kumar & Associates, Inc. Samples of the subsoils were taken with a 2-inch I.D. spoon sampler. The sampler was driven into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D-1586. The penetration resistance values are an indication of the relative density or consistency of the subsoils. Depths at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of the subsurface profiles encountered at the site are shown on Figure 2. Below about 1 foot topsoil or 3 feet of loose organic clay fill in Boring 3, the subsoils consist of stiff to very stiff, silty sandy clay transitioning to medium dense/hard, clayey sand and sandy clay with gravel at depth. Hard to very hard sandstone bedrock was encountered below the sand and clay soil at depths of about 26½ and 17 feet in Borings 1 and 2, respectively. Clay soils like those encountered in the borings can possess an expansion potential when wetted. Laboratory testing performed on samples obtained during the field exploration included natural moisture content and density and finer than sand size gradation analyses. Swell-consolidation testing performed on relatively undisturbed drive samples of the clay soils, presented on Figures 4 and 5, indicate low compressibility under relatively light surcharge loading and minor to low expansion potential when wetted under a constant light surcharge. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at time of drilling and the subsoils and bedrock were slightly moist. FOUNDATION BEARING CONDITIONS The clay and clayey sand soils encountered at the site possess an expansion potential when wetted under light loading. The expansion potential can probably be partly mitigated by load Kumar & Associates, Inc. ® Project No. 22-7-127 - 3 - concentration on spread footings to reduce or prevent swelling in the event of wetting below the foundation bearing level. Surface runoff, landscape irrigation, and utility leakage are possible sources of water which could cause wetting. A lower movement risk foundation would be to place the footings on 3 feet of compacted road base or support the foundation with piers or piles that extend down into the sandstone bedrock. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the buildings can be founded with spread footings placed on undisturbed natural soils with a risk of movement mainly if the bearing soils are wetted. If a deep foundation of piles or piers is desired, we should be contacted for additional analysis and recommendations. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural soils can be designed for an allowable bearing pressure of 2,500 psf. The footings should also be designed for a minimum dead load pressure of 800 psf. In order to satisfy the minimum dead load pressure under lightly loaded areas, it may be necessary to concentrate loads by using a grade beam and pad system. Wall-on-grade construction is not recommended at this site to achieve the minimum dead load. The minimum dead load pressure requirement is not needed if at least 3 feet of structural fill is placed below footing grade. The fill should extend laterally out from the edge of the footing at least 1½ feet on both sides. Structural fill used below the foundation should be a relatively well graded granular material such as CDOT Class 6 road base compacted to at least 98% of standard Proctor density. 2) Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be up to about 1 inch. There could be additional movement of around 1 inch if the bearing soils were to become wet. 3) The footings should have a minimum width of 16 inches for continuous footings and 24 inches for isolated pads. 4) Continuous foundation walls should be reinforced top and bottom to span local anomalies and limit the risk of differential movement. One method of analysis is to design the foundation wall to span an unsupported length of at least 12 feet. Kumar & Associates, Inc. ® Project No. 22-7-127 - 4 - Foundation walls acting as retaining structures should also be designed to resist a lateral earth pressure as discussed in the "Foundation and Retaining Walls" section of this report. 5) Exterior footings and footings beneath unheated areas should be provided with adequate soil cover above their bearing elevation for frost protection. Placement of foundations at least 36 inches below the exterior grade is typically used in this area. 6) Prior to the footing construction, any existing fill, topsoil and loose or disturbed soils should be removed and the footing bearing level extended down to competent bearing soils. 7) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FOUNDATION AND RETAINING WALLS Foundation walls and retaining structures which are laterally supported and can be expected to undergo only a slight amount of deflection should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting of the on-site fine-grained soils and at least 45 pcf for backfill consisting of imported granular materials. Cantilevered retaining structures which are separate from the buildings 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 50 pcf for backfill consisting of the on-site fine-grained soils and at least 40 pcf for backfill consisting of imported granular materials. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The pressures recommended above assume drained conditions behind the walls and a horizontal backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at near optimum moisture content. Backfill placed in pavement areas should be compacted to at least 95% of the maximum standard Proctor density. Care should be taken not to overcompact the backfill or use large equipment near the wall since this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall backfill should Kumar & Associates, Inc. ® Project No. 22-7-127 - 5 - be expected even if the material is placed correctly and could result in distress to facilities constructed on the backfill. The lateral resistance of foundation or retaining wall footings will be a combination of the sliding resistance of the footing on the foundation materials and passive earth pressure against the side of the footing. 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. The coefficient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factors of safety should be included in the design to limit the strain which will occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against the sides of the footings to resist lateral loads should be compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The on-site soils possess an expansion potential and slab heave could occur if the subgrade soils were to become wet. Slab-on-grade construction may be used provided precautions are taken to limit potential movement and the risk of distress to the building is accepted by the owner. A positive way to reduce the risk of slab movement, which is commonly used in the area, is to construct structurally supported floors over crawlspace. Where slab-on-grade is used, we recommend at least 2 feet of relatively well graded granular material such as CDOT Class 6 road base be placed below floor slabs to help mitigate the expansion potential. To reduce the effects of some differential movement, nonstructural floor slabs should be separated from all bearing walls and columns with expansion joints which allow unrestrained vertical movement. Interior non-bearing partitions resting on floor slabs should be provided with a slip joint at the bottom of the wall so that, if the slab moves, the movement cannot be transmitted to the upper structure. This detail is also important for wallboards, stairways and door frames. Slip joints which will allow at least 1½-inches of vertical movement are recommended. Floor slab control joints should be used to reduce damage due to shrinkage cracking. Slab reinforcement and control joints 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 immediately beneath basement level 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. The free-draining gravel will aid in drainage below the slabs and should be connected to the perimeter underdrain system. Kumar & Associates, Inc. ® Project No. 22-7-127 - 6 - Required fill beneath slabs can consist of the on-site gravelly soils or a suitable imported granular material, excluding topsoil and oversized rocks. The fill should be spread in thin horizontal lifts, adjusted to at or above optimum moisture content, and compacted to at least 95% of the maximum standard Proctor density. All vegetation, topsoil and loose or disturbed soil should be removed prior to fill placement. The above recommendations will not prevent slab heave if the expansive soils underlying slabs- on-grade become wet. However, the recommendations will reduce the effects if slab heave occurs. All plumbing lines should be pressure tested before backfilling to help reduce the potential for wetting. UNDERDRAIN SYSTEM Although groundwater 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. Therefore, we recommend below-grade construction, such as crawlspace and basement areas, be protected from wetting by an underdrain system. The drain should also act to prevent buildup of hydrostatic pressures behind foundation walls. The underdrain system should consist of a drainpipe surrounded by free-draining granular material placed at the bottom of the wall backfill. The drain lines should be placed at each level of excavation and at least 1 foot below lowest adjacent finish grade, and sloped at a minimum 1% grade to a suitable gravity outlet. Free-draining granular material used in the drain system 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. The drain gravel should be at least 1½ feet deep. Void form below the foundation can act as a conduit for water flow. An impervious liner such as 20 mil PVC should be placed below the drain gravel in a trough shape and attached to the foundation wall above the void form with mastic to keep drain water from flowing beneath the wall and to other areas of the building. SITE GRADING The risk of construction-induced slope instability at the site appears low provided cut and fill depths are limited to about 8 to 10 feet. 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 existing fill, vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should Kumar & Associates, Inc. ® Project No. 22-7-127 - 7 - be benched into slopes that exceed 20% grade. Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or other means. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after each building has been completed: 1) Excessive wetting or drying of the foundation excavations and underslab areas should be avoided during construction. Drying could increase the expansion potential of the clay soils. 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95% of the maximum standard Proctor density in pavement 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 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 paved areas. 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 foundation walls. Consideration should be given to use of xeriscape to reduce the potential for wetting of soils below the building caused by irrigation. 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 borings drilled at the locations indicated on Figure 1, 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 borings and variations in the subsurface Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 22-7-127 SAMPLE LOCATION NATURAL MOISTURE CONTENT NATURAL DRY DENSITY GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH SOIL TYPE BORING DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%) (ft) (%) (pcf) (%) (%) (psf) 1 2½ 4.3 114 Silty Sandy Clay 15 7.3 120 67 Sandy Silty Clay 2 2½ 4.5 110 Sandy Clay 5 4.6 111 50 Sand and Clay with Gravel 15 5.4 123 54 Sand and Clay with Gravel 3 2½ 5.1 108 50 Sand and Clay 10 8.5 112 Sandy Clay