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HomeMy WebLinkAboutSubsoils Report 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 SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT F-19, ASPEN GLEN DIAMOND A RANCH ROAD GARFIELD COUNTY, COLORADO PROJECT NO. 21-7-232 APRIL 19, 2021 PREPARED FOR: WHITNEY WARD 918 BROOKIE CARBONDALE, COLORADO 81623 woward@me.com Kumar & Associates, Inc. ® Project No. 21-7-232 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 ............................................................................................ - 4 - FOUNDATIONS .................................................................................................................... - 4 - FOUNDATION AND RETAINING WALLS ....................................................................... - 5 - FLOOR SLABS ...................................................................................................................... - 6 - UNDERDRAIN SYSTEM ..................................................................................................... - 6 - SURFACE DRAINAGE ......................................................................................................... - 7 - LIMITATIONS ........................................................................................................................... - 8 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS FIGURES 5 and 6 - GRADATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Kumar & Associates, Inc. ® Project No. 21-7-232 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot F-19, Aspen Glen, Diamond A Ranch Road, 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 Whitney Ward dated March 2, 2021. 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, design recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION Design plans for the proposed residence were not available at the time of our study. The proposed residence is assumed to be a one or two story wood frame structure with attached garage possibly over a basement level. Ground floors could be slab-on-grade or structural over crawlspace. Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 10 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. The ground surface is relatively flat with about 1 to 2 feet of elevation difference across the building envelope. Vegetation - 2 - Kumar & Associates, Inc. ® Project No. 21-7-232 consists of grass. An artificial pond is located across the golf course to the northwest. A subsidence area was previously mapped in the northeast part of the lot just outside of the building envelope as shown on Figure 1. Site grading as part of the original subdivision development placed fill material in the subsidence area to level the lot. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen Subdivision. 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. 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 Aspen Glen Development. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork Valley. A subsidence area was previously observed northeast 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 or ground will not subside in the unstable soil area. The risk of future ground subsidence within the designated building envelope area of Lot F-19 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 and future ground subsidence especially in the northern area of the lot, outside of the building envelope. 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 23 and 24, 2021. 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 augers powered by a truck-mounted CME-45B drill rig. The borings were logged by a representative of Kumar & Associates, Inc. - 3 - Kumar & Associates, Inc. ® Project No. 21-7-232 Samples of the subsoils were taken with 1⅜ inch and 2 inch I.D. spoon samplers. The samplers were 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 conditions encountered at the site are shown on Figure 2. The subsoils were variable and below about ½ foot of topsoil consist of up to 6½ feet of granular fill overlying very stiff, sandy clay with scattered gravel down to a depth of 4½ to 10 feet. Below the clay, dense, silty sandy gravel with cobbles and probable boulders was encountered down to the maximum explored depth of 14 feet. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in Borings 2 and 3. Laboratory testing performed on samples obtained from the borings included natural moisture content and density, gradation analyses and Atterberg limits. Results of swell-consolidation testing performed on a relatively undisturbed drive sample of the clay, presented on Figure 4, indicate low to moderate compressibility under conditions of loading and wetting. Results of gradation analyses performed on small diameter drive samples (minus 1½-inch fraction) of the coarse granular subsoils are shown on Figures 5 and 6. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist. FOUNDATION BEARING CONDITIONS The upper fill and clay soils encountered in the borings down to depths of about 5 to 10 feet possess a low bearing capacity and typically low to moderate settlement potential when under loading. The dense gravel soils encountered in the borings below 5 to 10 feet possess moderate bearing capacity and typically low settlement potential. Spread footings placed on the upper - 4 - Kumar & Associates, Inc. ® Project No. 21-7-232 soils will have a risk of foundation movement possibly resulting in distress to the proposed residence. We recommend removal of the upper fill and natural clay soils and placing spread footings or compacted structural fill on the underlying gravel soils for support of the proposed residence. The borings drilled on the site do not indicate influence from the sinkhole and subsidence area mapped northeast of the building envelope. If deeper than anticipated clay soils or soft areas are encountered during construction, we should be contacted to further evaluate the bearing conditions. Future ground subsidence in the unstable soil area shown on Figure 1 could indicate sinkhole reactivation and the need for mitigation such as deep compaction grouting. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, we recommend the building be founded with spread footings placed on the natural dense gravel soils or compacted structural fill bearing on the natural granular soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed entirely on the undisturbed natural granular soils should be designed for an allowable bearing pressure of 3,000 psf. Footings placed on compacted structural fill should be designed for an allowable bearing pressure of 2,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. 2) The footings should have a minimum width of 16 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 12 feet. - 5 - Kumar & Associates, Inc. ® Project No. 21-7-232 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) All existing fill, clay soils and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural granular soils. The exposed soils in footing area should then be moistened and compacted. Structural fill placed to reestablish design bearing level should be compacted to at least 98% of standard Proctor density. 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 50 pcf for backfill consisting of the on-site soils. Cantilevered 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 40 pcf for backfill consisting of the on-site soils. Backfill should be a predominantly granular soil devoid of organics, clay soils and rock large than 6 inches. 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 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 - 6 - Kumar & Associates, Inc. ® Project No. 21-7-232 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 0.45. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 400 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 granular material 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. 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 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 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 that local perched groundwater can develop during times of heavy precipitation or - 7 - Kumar & Associates, Inc. ® Project No. 21-7-232 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 or drywell. 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 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 6 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 finer graded 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 5 feet from foundation walls. Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates Kumar & Associates TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Project No. 21-7-232 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 7 & 10 combined 1.9 42 43 15 Silty Sand and Gravel 2 4 2.1 51 32 17 Silty Sand and Gravel (Fill) 7 17.7 90 38 21 Slightly Sandy Clay 3 6 15.7 110 Slightly Sandy Clay 7½ 17.4 113 90 Slightly Sandy Clay 10 3.9 43 39 18 Silty Sand and Gravel