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HomeMy WebLinkAboutSoils Report 11.26.2014HEPWORTH-PAWLAK GEOTECHNICAL SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 3, DUTCH MAJOR EXEMPTION 997 COUNTY ROAD 229 GARFIELD COUNTY, COLORADO JOB NO. 114 505A NOVEMBER 26, 2014 PREPARED FOR: TOM JURMU 1902 COUNTY ROAD 214 SILT, COLORADO 81652 ton jurn1E1 !! um1n11.CUE11 �',ui kor 303-$41,71P-) • C't lvr.adIi ,Trn1gs, ! I9.611-5562 • ti11% crrhorne 9'/0.465- �} �) TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - I - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - FIELD EXPLORATION _- 2 - SUBSURFACE CONDITIONS - 2 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 3 - FOUNDATIONS - 3 - FOUNDATION AND RETAINING WALLS - 4 - FLOOR SLABS - 5 - UNDERDRAIN SYSTEM - 6 - SURFACE DRAINAGE - 6 - 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 I- 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 on Lot 3, Dutch Major Exemption, 997 County Road 229, 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 Tom Jurmu dated November 6, 2014. 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 The proposed residence will be a 1 and 2 story structure with a partial walkout lower level and located in the southern part of the lot as shown on Figure 1. The garage will be located at the main level. Ground floors will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 6 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. If building Ioadings, location or grading plans change significantly from those described above, we should be notified to re-evaluate the recommendations contained in this report. Job No. 114 505A Get.tech -2 - SITE CONDITIONS The building site was vacant pasture at the time of our field exploration. A well had been drilled near the middle of the Iot as shown on Figure 1. The ground surface is gently rolling with a gentle slope down to the south and west. Field irrigation ditches cross the area including one in the middle of the garage area. Vegetation consists of grass and weeds. FIELD EXPLORATION The field exploration for the project was conducted on November 1 1, 2014. 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 Hepworth-Pawlak Geotechnical, 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 conditions encountered at the site are shown on Figure 2. The subsoils, below about 8 inches of topsoil, consist of 5 to 11 feet of medium stiff to stiff, sandy silt and clay overlying medium dense, silty to clayey sand with scattered gravel to the boring depths of 21 feet. Job No, 114 505A GecPtech -3 - Laboratory testing performed on samples obtained from the borings included natural moisture content and density, finer than sand size gradation analysis and unconfined compressive strength. Results of swell -consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 and 5, indicate low to moderate compressibility under loading, and a minor collapse potential (settlement under constant load) when wetted. The unconfined compression test indicated the clay soil has stiff consistency. 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 subsoils have relatively low bearing capacity and generally low settlement potential under light loading. Shallow spread footings placed on the natural soils should be suitable for foundation support with some settlement risk, mainly under wetted bearing conditions. Precautions should be taken to keep the bearing soils dry. 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 bearing on the natural soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural soils 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 Job No. 114 505A ~!tech -4 - section will be about 1 inch or less. There could be additional differential settlement of12 to 1 inch if the bearing soils are wetted. 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 rade is typically used in this area. 4) Continuous foundation walls should be 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 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 and the footing bearing level extended down to the firm natural soils. The exposed soils in 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 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 backfin consisting of the on-site soils. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction materials and Job Nu. 114'505A ~'Ptech -5 - 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 Iifts and compacted to at least 90% of the maximum standard Proctor density at near optimum moisture content. Backfill placed in pavement and walkway areas should be compacted to at least 95°0 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 0.30. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 30 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 Job No, 114 505A Ge Gh -6 - 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 the basement slab 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° o of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on site soils devoid of vegetation and topsoil. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in the area and where there are clay soils 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 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 interior sump. 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 1' 2 feet deep. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: Job No. 114 505A Gec�stech -7- I) 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 I0 feet in paved areas. Free -draining wall backfill should be capped with at least 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 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. Utile 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 Joh No. 1 14 505A — — Gle tech -8 - 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 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, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Steven L. Pawlak, P.E. Reviewed by: Daniel E. Hardin, P.E. SLP/ksw Joh No. 114 56A Gtech APPROXIMATE SCALE 1" = 120' Ce EXISTING WELL LOT 3 DUTCH MAJOR EXEMPTION BORING 1 • BUILDING AREA BORING 2 • NOTE: BORINGS WERE DRILLED IN DESIGNATED BUILDING AREA, 114 505A H Hepworth—Powlak Geotechnical IRRIGATION EASEMENT 0 -------WAVERL.INE I LOT 4 TO COUNTY ROAD 229 -m.-- LOCATION OF EXPLORATORY BORINGS Figure 1 5 a) 1.1 0. O 0 5 10 15 20 BORING 1 BORING 2 /7 10/12 'WC=81 jDD=104 20/12 17/12 .• .▪ _ WC=88 ;;•. DD 113 14/12 •r, is 27/12 .., / i 7/12 6/12 WC 12.9 DD 99 i 13,12 WC= 151 DD= 115 -200=73 UC=4,800 17/12 16/12 0 5 10 15 20 25 25 114 505A Note: Explanation of symbols is shown on Figure 3. Hepworth—Puwlok Geotechnical LOGS OF EXPLORATORY BORINGS Figure 2 m L a O LEGEND: TOPSOIL; organic sandy silt, brown. SILT AND CLAY (ML -CL); sandy, medium stiff to stiff: slightly moist to moist, light brown to brown, low plasticity, calcareous. SAND (SM -SC); silty to clayey, scattered gravel, medium dense, moist, brown. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. 10/12 Drive sample blow count; indicates that 10 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. NOTES: 1. Exploratory borings were drilled on November 10, 2014 with 4 -inch diameter continuous flight power auger. 2. The exploratory borings were located in the building area designated by the client. 3. Elevations of exploratory borings were not measured and the logs of exploratory borings are drawn to depth. 4. The exploratory boring locations should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory boring logs represent the approximate boundaries between material types and transitions may be gradual. 6. No tree water was encountered in the borings at the time of drilling. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pct) -200 = Percent passing No. 200 sieve UC = Unconfined Compressive Strength (psf) 114 505A GgiSteCh HEP W ORTH•PA W LAK GEOTECHNICAL LEGEND AND NOTES Figure 3 Compression % 0 1 2 3 0 3 Moisture Content = 8.1 percent Dry Density - 104 pcf Sample of: Sandy Silt and CLay From: Boring 1 at 2 Y Feet _Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content - 8.8 percent Dry Density = 113 pcf Sample of: Silty Clayey Sand From: Boring 1 at 10 Feet r Compression upon wetting 0.1 114 505A H 1.0 10 APPLIED PRESSURE - ksf Hepworth—Pawlak Geotechnical SWELL -CONSOLIDATION TEST RESULTS 100 Figure 4 Compression 0 1 2 3 4 Moisture Content 12.9 percent Dry Density 99 pcf Sample of: Sandy Silt and Clay From: Boring 2 at 5 Feet Compression upon wetting 0.1 114 505A H 10 Hepworth—Pawlak Geotechnical 10 APPLIED PRESSURE kst SWELL -CONSOLIDATION TEST RESULTS 100 Figure 5 Job No. 114 505A cn Lfl LU 1� CC LO rCr CC 0 Aa1.1 LC } SOIL OR BEDROCK TYPE 1 Sandy Silt and Clay Silty Clayey Sand 11 Sandy Silt and Clay 11 Sandy Silty Clay 11 i 1 1 1 00 ATTERBERG LIMITS z 0 O L7 F LIQUID LIMIT J Q H 0 Ise a a° DRY DENSITY Z CL z Z 0 0 SAMPLE LOCATION CI. x 0. W 0 z 0 m 00