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HomeMy WebLinkAboutSubsoil Study for Foundation Design 04.01.2021 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 LOT 3, PINYON MESA PINYON MESA DRIVE GARFIELD COUNTY, COLORADO PROJECT NO. 21-7-169 APRIL 1, 2021 PREPARED FOR: URIEL MELLIN 144 CLIFFROSE WAY GLENWOOD SPRINGS, COLORADO 81601 (uriel.mellin@hotmail.com) Kumar & Associates, Inc. ® Project No. 21-7-169 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 - PROPOSED CONSTRUCTION ................................................................................................ - 1 - SITE CONDITIONS ................................................................................................................... - 1 - SUBSIDENCE POTENTIAL ..................................................................................................... - 2 - FIELD EXPLORATION ............................................................................................................ - 2 - SUBSURFACE CONDITIONS ................................................................................................. - 3 - FOUNDATION BEARING CONDITIONS .............................................................................. - 3 - 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. 21-7-169 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 3, Pinyon Mesa, Pinyon Mesa Drive, Garfield County, Colorado. The project site is shown on Figure 1. The purpose of the study was to develop recommendations for the foundation design. The study was conducted in accordance with our agreement for geotechnical engineering services to Uriel Mellin dated February 1, 2021. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils and bedrock 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, design recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION The proposed residence will be a two-story wood frame structure with a lower level cut into the hillside and attached garage at the main level. Ground floors could be a combination of structural over crawlspace and slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 9 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. If building loadings, location or grading plans change significantly from those described above, we should be notified to re-evaluate the recommendations contained in this report. SITE CONDITIONS The subject site was vacant at the time of our field exploration. A natural ridge top is near the uphill, western building envelope line. The ground surface is moderately to strongly sloping down to the east away from the ridge top. Vegetation consists of grass and weeds. - 2 - Kumar & Associates, Inc. ® Project No. 21-7-169 SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot as encountered in Boring 1. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in the area, several sinkholes were observed scattered throughout the lower Roaring Fork River Valley. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in this area. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory borings were relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot 3 throughout the service life of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. FIELD EXPLORATION The field exploration for the project was conducted on March 15 and 18, 2021. Two 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 augers 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 and hardness of the bedrock. 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. - 3 - Kumar & Associates, Inc. ® Project No. 21-7-169 SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsurface conditions were variable and below about ½ foot of topsoil consist of stiff to very stiff sandy clay to between 6½ and 24½ feet where bedrock was encountered to the maximum explored depth of 31 feet. Boring 1 encountered hard, gypsum bedrock from 24½ to 31 feet. Boring 2 encountered weathered siltstone/claystone from 6½ to 13 feet where it transitions to very hard siltstone bedrock to 21 feet deep. Laboratory testing performed on samples obtained from the borings included natural moisture content and density and finer than sand size gradation analyses. Results of swell-consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 and 5, indicate low compressibility under existing low moisture condition and light loading. The clay samples showed low expansion potential when wetted. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils and bedrock materials were slightly moist. FOUNDATION BEARING CONDITIONS Our experience indicates the sandy clay soils expected at typical shallow foundation depth tend to settle when they become wetted under load but should be further evaluated for compressibility and/or expansion potential at the time of excavation. The proposed residence can be supported on spread footings bearing on the clay soils and siltstone/claystone bedrock with a risk of differential movement mainly if the clay soils are wetted. Spread footings that transition between soils and bedrock may have a risk of differential movement possibly resulting in distress to the residence. Spread footings that transition between soils and bedrock should be heavily reinforced to reduce the risk of movement. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the building be founded with spread footings bearing on the natural clay soils or bedrock with a risk of settlement, mainly if the bearing soils become wetted. - 4 - Kumar & Associates, Inc. ® Project No. 21-7-169 The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on natural soils or bedrock should be designed for an allowable bearing pressure of 1,500 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. Additional settlement of about ½ to 1½ inches could occur if the clay soils below the bearing level become wetted. A ⅓ increase in the allowable bearing pressure can be taken for toe pressure of eccentrically loaded footings. 2) The footings should have a minimum width of 18 inches for continuous walls and 2 feet for isolated pads. 3) 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 exterior grade is typically used in this area. 4) 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. The foundation should be configured in a “box like” shape to help resist differential movements. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) Topsoil and any loose disturbed soils should be and the footing bearing level extended down to the firm natural soils or bedrock. The exposed soils in the footing area should then be moistened and compacted. 6) 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. Cantilevered retaining structures which are separate from the residence and can be expected to deflect sufficiently to mobilize the full active earth pressure - 5 - Kumar & Associates, Inc. ® Project No. 21-7-169 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 fine-grained soils. 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 a moisture content near optimum. Backfill placed in pavement and walkway 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 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 325 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 a compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction with a risk of movement if the bearing material are wetted. To reduce the effects of some differential movement, floor slabs should be separated from all bearing walls and columns - 6 - Kumar & Associates, Inc. ® Project No. 21-7-169 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 basement level slabs to facilitate drainage. This material should consist of minus 2-inch aggregate with at least 50% retained on 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 or imported granular 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 where there are clay soils and shallow bedrock 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. An impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. SITE GRADING The risk of construction-induced slope instability at the site appears low provided the building is located as planned and cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about 10 feet. Embankment fills should be compacted - 7 - Kumar & Associates, Inc. ® Project No. 21-7-169 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 should be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or other means. This office should review site grading plans for the project prior to construction. SURFACE DRAINAGE It will be critical to the long-term building performance to keep the bearing soils dry. 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. 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. 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. 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 or 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. Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 21-7-169 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 4 12.2 100 Sandy Clay 7 11.4 109 86 Sandy Clay 10 11.0 104 Sandy Clay 2 4 10.7 104 Sandy Clay 7 4.6 118 Siltstone/Claystone