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HomeMy WebLinkAboutSoils Report 07.18.2016H-PKUMAR Geotechnical Engineering 1 Engineering Geology Materials Testing 1 Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Parker, Glenwood Springs, and Silverthorne, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 24, PINYON MESA SAGE MEADOW ROAD GARFIELD COUNTY, COLORADO PROJECT NO. 16-7-167 JULY 18, 2016 PREPARED FOR: SCOTT DILLARD 21 COUNTY ROAD 126 GLENWOOD SPRINGS, COLORADO 81601 scottdilla rdreal for @ gmai I.com TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4 - FOUNDATION AND RETAINING WALLS - 6 - FLOOR SLABS -7- UNDERDRAIN SYSTEM - 7 - SURFACE DRAINAGE - 8 - LIMITATIONS - 9 - FIGURE 1 - LOCATION OF EXPLORATORY BORING FIGURE 2 - LOG OF EXPLORATORY BORING FIGURE 3 - LEGEND AND NOTES FIGURES 4 AND 5 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot 24, Pinyon Mesa, Sage Meadow 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 professional services to Scott Dillard dated July 7, 2016. An exploratory boring was drilled 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 The proposed residence is assumed to be a two story structure with an attached garage. Ground floor will be over a basement or crawlspace. The garage and basement floors will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 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. -2 SITE CONDITIONS The lot was vacant at the time of our investigation. The site is generally flat with a gentle slope down to the west with less than 2 feet of elevation change across the site. Vegetation on the site consists of grass and weeds throughout the northern portion of the lot and sagebrush dominating the southern portion. 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. 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, sinkholes have been observed scattered throughout the lower Roaring Fork River valley. 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 boring was 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 24 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 July 11, 2016. An exploratory boring was drilled at the location shown on Figure 1 to evaluate the subsurface H-P1KUMAR -3 - conditions. The boring was advanced with 4 -inch diameter continuous flight augers powered by a truck -mounted CME -45B drill rig. The boring was logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with a 2 inch California 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 Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The subsoils encountered, below a thin root zone, consist of about 33 feet of stiff, clayey sandy silt with scattered gravels. Below 33 feet, weathered siltstone was encountered down to the maximum drilled depth of 38 feet. Laboratory testing performed on samples obtained from the borings included natural moisture content and gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 and 5, indicate low compressibility under light loading with a moderate to high collapse potential (settlement under constant load) when wetted. The samples were highly compressible under increased loading after wetting. The laboratory testing is summarized in Table 1. No free water was encountered in the boring at the time of drilling and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The sandy clayey silt soils encountered at typical shallow foundation depth tend to settle when they become wetted. A shallow foundation placed on the clayey silt soils will have -4- a high risk of settlement if the soils become wetted and care should be taken in the surface and subsurface drainage around the house to prevent the soils from becoming wet. It will be critical to the long term performance of the structure that the recommendations for surface drainage and subsurface drainage contained in this report be followed. The amount of settlement, if the bearing soils become wet, will be related to the depth and extent of subsurface wetting. We expect that initial settlements will be less than 1 inch. If wetting of the shallow soils occurs, additional settlements of 2 to 3 inches could occur. Settlement in the event of subsurface wetting will likely cause building distress and mitigation methods such as deep compaction, a deep foundation (such as piles or piers extending down roughly 40 feet below existing ground surface) or a heavily reinforced mat foundation, on the order of 2 feet thick, and designed by the structural engineer should be used to support the proposed house. If a deep foundation or mat foundation is desired, we should be contacted to provide further design recommendations. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, the building can be founded with spread footings bearing on compacted structural fill with a risk of settlement, mainly if the bearing soils become wetted, and provided the risk is acceptable to the owner. Control of surface and subsurface runoff will be critical to the long-term performance of a shallow spread footing foundation system. The garage and crawlspace footing areas should be sub - excavated down about 8 to 10 feet below existing ground surface and the excavated soil replaced compacted back to design bearing level but to a depth of at least 6 feet below footing bearing level. Basement level footings should be placed on at least 3 feet of compacted fill. The design and construction criteria presented below should be observed for a spread footing foundation system. -5- 1) Footings placed on a minimum 6 feet of compacted structural fill for the garage and crawlspace and at least 3 feet of compacted structural fill for the basement level of the residence should be designed for an allowable bearing pressure of 1,200 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 settlements of about 1 to 11 inches could occur if the clayey silt soils below the bearing level become wetted. A 1/3 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 24 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) The topsoil and any loose or disturbed soils should be removed below the building area. The exposed soils in footing areas after sub -excavation to design grades should then be moistened and compacted. Structural fill should consist of low permeable soil (such as the on-site sandy silt and clay soils) compacted to at least 98% standard Proctor density within 2% of optimum moisture content. The structural fill should extend laterally beyond the footing edges equal to about 1/2 the fill depth below the footing. 6) A representative of the geotechnical engineer should evaluate the structural fill as it is placed for compaction and observe all footing excavations prior to fill placement to evaluate bearing conditions. -6 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 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 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 -7 - 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 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 12% 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 seasonal runoff. Frozen ground during spring runoff can also create a H-Pk -8 - 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 sump and pump. 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 11/2 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. SURFACE DRAINAGE It will be critical to the 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. -9 - Free -draining wall backfill should be 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 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 boring drilled at the location 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 boring 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 so that re-evaluation of the recommendations may be made. This report has been prepared for the exclusive use by our client for design purposes. We are not responsible for technical interpretations by others of our information. As the project evolves, we should provide continued consultation and field services during construction to review and monitor the implementation of our recommendations, and to verify that the recommendations have been appropriately interpreted. Significant design changes may require additional analysis or modifications to the recommendations - 10 - presented herein. We recommend on-site observation of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, H -P* KU'MAR Tom C. Brunner Reviewed by: Daniel E. Hardin, P. E. TCB/ksw ym 24443 z, s`�'ONAL 04 APPROXIMATE SCALE 1"=40' ......,,... 1 T 1 I • BORING 1 1 1 LOT 24 LOT 26 LOT 25 HP GEOTECH JOB NO 115 028A L E J 7. 1 LOT 31 HP GEOTECH JOB NO 114 111A 1 LOT 32 I r 1 1 1 1 1 1 LOT 23 HP GEOTECH JOB NO 108 569A 16-7-167 H - P KU MAR LOCATION OF EXPLORATORY BORINGS Figure 1 a) LL s a a) 0 0 - 5 - 10 15 20 25 30 35 BORING 1 37/12 - 6/12 12/12 WC=5,5 DD=88 /— 35/12 j' 10/12 /WC=6.9 DD=86 /// -200=89 11/12 - 15/12 WC=6.5 DD=91 f — 45/12 WC=6.9 DD=103 -200=89 - 36/12 100/1 f 0 5 10 15 20 25 30 35 40 40 NOTE: Explanation of symbols is shown on Figure 3. Depth - Feet 16-7-167 H -P- KU MAR LOG OF EXPLORATORY BORING Figure 2 LEGEND: SILT (ML); sandy to very sandy, clayey, slightly gravelly, scattered cobbles, medium stiff to stiff, slightly miost, light brown. WEATHERED SILTSTONE/CLAYSTONE; hard to very hard, slightly moist, gray. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. 39/12 Drive sample blow count; indicates that 39 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. NOTES: 1. The exploratory boring was drilled on July 11, 2016 with a 4 -inch diameter continuous flight power auger. 2. The exploratory boring location was measured approximately by pacing from features shown on the site plan provided. 3. The exploratory boring elevation was not measured and the log of exploratory boring is drawn to depth. 4. The exploratory boring location should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory boring log represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the boring at the time of drilling. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) -200 = Percent passing No. 200 sieve 16-7-167 H-P� KUMAR Engines-:n..q 1 E — LEGEND AND NOTES Figure 3 Figure 4 1 Compression 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Moisture Content = 5.5 percent Dry Density = 88 pcf Sample of: Sandy Silt and Clay From: Boring 1 at 7 Feet 2 ',Compression upon wetting 1 =MI \\ 0.1 1.0 APPLIED PRESSURE - ksf 10 100 16-7-167 H -P ` KU MAR SWELL -CONSOLIDATION TEST RESULTS Compression % W— — — — (0 CO v 0 01 A CO N 1 0 ivtoisture Content — 6.5 percent Dry Density = 91 pcf Sample c& Sandy Silt From: Boring 1 at 20 Feet (' Compression upon etti ng . \ 0.1 1.0 10 100 APPLIED PRESSURE - ksf 16-7-167 H-P` ICU MAR SWELL-CONSOLIDATION TEST RESULTS Figure 5 HP KUMAR TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 16-7-167 SAMPLE LOCATION NATURAL NATURAL MOISTURE DRY CONTENT DENSITY i (..b) (pcf) GRADATION PERCENT PASSING 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE ' STRENGTH (PSF) SOIL OR BEDROCK TYPE BORING DEPTH (ft) GRAVEL (%) �o) SAND ( o �o) LIQUID 1 PLASTIC LIMIT INDEX (0 n) {"4,} 1 7 5.5 88 Sandy Silt and Clay 12 6.9 86 89 Sandy Silty and Clay 20 6.5 91 Sandy Silt 25 6.9 103 89 Sandy Silt