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Home My WebLink AboutSubsoil Study for Foundation Design 02.16.15HEPWORTH-PAWLAK G EOTECH NICAL Hepworth-Pawlak Geotechnical, Inc 5020 County Road 154 Glenwood Springs, Colorado 81601 Phule. 970-945-7988 Fax: 970-945-8454 Email: hpgeo@hpgeotech.com H R,ECHTVffiä} JUL 2 I 2017 GARFIELD COI"'NTY CIMt\4UNIW DEVELCIPMINTSUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 25, FOUR MILE RANCH RED CLIFF'CIRCLE GARFTELD COUNTY, COLORADO JOB NO. 115 0364 FEBRUARY 16,2015 PREPARED FOR: JANET WOLF' P.O. BOX 746 GLENWOOD SPRTNGS, COLORADO 81602 iwolfcpa@qmail.com TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY PROPOSED CONSTRUCTION SITE CONDTTIONS FIELD EXPLORATION... -l- I SUBSURFACE CONDITIONS FOUNDATION BEARING CONDITIONS..... DESIGN RECOMMENDATIONS ...................... FOUNDATIONS......... FOUNDATION AND RETAINING WALLS FLOOR SLABS UNDERDRAIN SYSTEM SITE GRADING SURFACE DRAINAGE ......... LIMITATIONS 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 _) - -2- -J- _a- _L- 5- 6- -8- ................- I - JobNo. l15 0364 cåBteclT PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 25, Four Mile Ranch, Red Cliff Circle, 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 proposal for geotechnical engineering services to Janet Wolf dated January 26,2015. 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 werc 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 Development plans for the lot were preliminary at the time of our study. In general, the proposed residence will be a single-story structure above a walkout basement level and located within the building envelope shown on Figure 1. The total living area will be between about 2,500 to 3,000 square feet. Ground floor will likely 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. Job No. 115 0364 cåBtecrr -2- SITE CONDITIONS The lot was vacant and covered with patches of snow at the time of our field exploration. The ground surface slopes strongly down to the west at a grade of about 6 to 8% with about 7 feetofelevation difference across the general proposed building area. The surface was wet and soft from recent snow and frost melt. Vegetation consists of grass and moderate sage brush growth. F'IELD EXPLORATION The field exploration for the project was conducted on February 5,9 and 12,2015. Three exploratory borings were drilled at the locations shown on Figure I to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight augers powered by a truck-mounted CME-458 drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with l% 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-l586. 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 Yz foot of topsoil, consist of sandy silty clay to silty clayey sand with gravel down to depths of 9 to 12% feet overlying dense, silty sandy gravel and Job No. 115 0364 cåBtectr -3- cobbles with probably boulders to the boring depths of l}Yzto 16 feet. The upper soils generally have very stiffto hard consistency. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit. 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 of the upper sand, silt and clay soils, presented on Figures 4 and 5, indicate low compressibility under existing low moisture and light loading conditions and minor to low collapse potential (settlement under constant load) when wetted. The samples showed moderate compressibility under additional loading after wetting. The laboratory test results are summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist. F'OUNDATION BE,ARING CONDITIONS The upper sand, silt and clay soils generally have low bearing capacity and moderate settlement potential, mainly when wetted. The underlying dense, coarse granular soils have relatively high bearing capacity and low settlement potential. Spread footings placed on the upper soils can be used for the building support with a risk of settlement and building distress if the bearing soils are wetted and the risk is accepted by the owner. Precautions should be taken to keep the bearing soils dry as described in later sections of this report. Extending the bearing level down to the dense coarse granular soils such as with piers or piles can be used as foundation support and should achieve a low settlement risk foundation. Presented below are recommendations for a shallow spread footing. If a deep foundation is proposed, we should be contacted for additional recommendations. Job No. l15 0364 cå5tecn -4- DESIGN RECOMMENDATIONS FOTINDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the building can be founded with spread footings bearing on the natural soils with a risk of settlement and distress as described below. The design and construction criteria presented below should be observed for a spread footing foundation system. l) 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 section will be about 1 inch or less. Additional settlements of Yzto l% inches could occur if the bearing soils are wetted. Footings that extend down to the dense coarse granular soils can be deigned for an allowable soil bearing pressure of 3,000 psf with minor settlement potential. 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 are preferable over individual pad footings to limit the effects of differential settlement and should be heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 14 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. Job No. l15 0364 cåEtecrr -5- The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the firm natural soils or relatively dense coarse granular soils. The exposed soils in footing area should then be moistened and compacted. A representative ofthe 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 50 pcf f-or backtìll 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 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 backfili 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 and walkway areas should be compacted to at least 95Yo 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. s) 6) JobNo. l15 03óA cåStecrr -6- 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 coeffìcient 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 least95%o 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. There could be some potential for differential movement of the slab if the bearing soils are weffed. 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 forjoint 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 2Yopassingthe No. 200 sieve. JobNo. l15 0364 cåBtec¡ -7 - 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 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 create a perched condition. We recommend below-grade construction, such as retaining walls, crawlspace and basement areaso 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-draìning granular material. The drain should be placed at each level of excavation and at least I foot below lowest adjacent finish grade and sloped at a minimum lYo to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than Z%ópassingthe No. 200 sieve, less than 50Yo passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least IYz feet deep. SITE GRADING We assume the cut and fill depths for the basement level and site grading will not exceed about 8 to l0 feet. Embankment fills should be compacted to at leastg1%o 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%o of the maximum standard Proctor density. The fill should be benched into slopes that exceed 20Yo grade. Permanent unretained cut and fill slopes should be graded at2horizontal to I vertical or flatter and protected against JobNo. l15 0364 cåStecrt -8- erosion by revegetation or other means. This office should review site grading plans for the project prior to construction. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: l) Inundation ofthe 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 95Yo of the maximum standard Proctor density in pavement and slab areas and to at least 90%o 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 l0 feet in unpaved areas and a minimum slope of 3 inches in the first l0 feet in paved areas. Free-draining wall backfill should be capped with at least 2 feet of the on- site finer grained 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 Job No. ll5 0364 cåStecn -9- based upon the data obtained from the exploratory borings drilled at the locations indicated on Figure l, 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 variationò 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 veriff that the recommendations have been appropriately interpreted. Signif,rcant 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. Respectfu lly Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Steven L. Pawlak, P.E. Reviewed by: Daniel E. Hardin, P.E. SLP/ksw JobNo. l15 0364 cåStecn APPROXIMATE SCALE 1":50' 6130 | -')<. -'- J 'gonlruG 2 =1 6130 r¿ I LOf 24 BORING 3 LOT 26 / o BORING 1 --- LOT25 - 6120 &sb ¡ II I ct\çç G\qOLÉ ßÉo 1 15 0364 c$tecrr HEÞìñ,oRIH-PA\,VLÀK GEoTECHNICAL LOCATION OF EXPLORATORY BORINGS Figure 1 BORING 1 ELEV.:6123' BORING 2 ELEV.: 6130' BORING 3 ELEV.:6125' 6'130 6130 20112 WC:9.0 DD:101 6125 47/12 wc:9.6 DD:90 6125 39112 WC:5.3 DD:105 -2OO=47 13112 6120 50/1 35112 wc:10.3 DD:1 1 1 6120 d)o LL Ic .o (ú 0) UJ 14112 WC=9.9 DD:96 o)(l) LL co (d oú61 15 30112 61 15 1416,3014 WC:3.8 DD:123 -20O=32 61 10 50/5 61 10 61 05 6105 Note: Explanation of symbols is shown on Figure 3. 1 15 0364 cåFtecrr HEPVToRTH,PAWLAK GEorEcHNrcaL LOGS OF EXPLORATORY BORINGS Figure 2 E n f-t4 ffi r"-zll t# LEGEND: TOPSOIL; organic sandy silty clay, root zone, brown CLAY (CL); sandy, very stiff to hard, scattered gravel with depth, slightly moist, red-brown, slightly to moderately calcareous, low to medium plastcity. SAND AND CLAY (SC-CL); silty, gravelly, medium dense/very stiff, slightly moist, light brown, slightly to moderately calcareous, low plasticity, stratified. GRAVEL AND COBBLES (GM); silty, sandy, boulders, dense, slightly moist, grey-brown, rounded rock. Relatively undisturbed drive sample; 2-inch LD. California liner sample. 39112 Drive sample; standard penetration test (SPT), 1 3/8 inch l.D. split spoon sample, ASTM D-1586. Drive sample blow count; indicates that 39 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. Practical drilling ref usal. NOTES: 1. Exploratory borings were drilled on February 5, 9 and 12,2015 with 4-inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory borings were obtained by interpolation between contours shown on the site plan provided and checked by instrument level. 4. The exploratory boring locations and elevations 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 free 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 (pcf) -200: Percent passing No. 200 sieve I T 1150364 c$tecrr HTPVIIC'RTH-PÄ'WLAK GEOTECHNICAL LEGEND AND NOTES Figure 3 0 1 2 3 òs C .9Ø <t)o o- Eo(J 4 5 6 7 I 0.1 1.0 10 100 APPLIED PRESSURE - ksf 0 1 \oo\ c .9 U' <t)q) o_ Eo() 2 3 4 5 0.1 1.0 10 100 APPLIED PRESSURE - ksf ) Moisture Content : 9.9 percent Dry Density : 96 pcf Sample of: Very Silty Clayey Sand with Gravel From: Boring 1 at 4 Feet -\ \ \ Compression upon 'wetting \\ \ ( \ \ () Moisture Content = 9.0 Dry Density : 101 Sample of: g¿¡6y Silty Clay From: Boring 2af 2/rFee| percent pcf ) Compression upon wetting \ \ \) 1 15 0364 cåFtecrr HEPWoRIH-PÄWLÄ'K GEoTECHNICÀ.L SWELL-CONSOLI DATION TEST RESULTS Figure 4 0 1 2 òs c .o u)a'q) o- EoO 3 4 5 6 7 0.1 1.0 '10 100 APPLIED PRESSURE - ksf 0 1 rOo\co'6 U)o o_ Eo() 2 3 4 5 0.1 1.0 10 100 APPLIED PRESSURE - ksf Moisture Content = 9.6 Dry Density : 90 Sample of: Sandy Silty Clay From: Boring 2 at 5 Feet percent pcf Compression -uponwetting \\ \ \ $) Moisture Content : 10.3 Dry DensitY : '11'l Sample of: Sandy Silty Clay From: Boring 3 at 5 Feet percent pcf ) Compression upon wetting \ \ $ 1150364 c$tecrr H EPWoRTH-PAWLÄ'K GEorEcHNlcAL SWELL-CONSOLI DATION TEST RESULTS Figure 5 HEPWORTH-PAWLAK GEOTECHNICAL, I NC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 115 0364 SAMPLE LOCAIION NATURAL MOISTURE CONTENT ("Al NATURAL DRY DENSITY locfl GRADAÎION PERCENT PASSING NO. 200 stEvE ATÍERBERG LIMITS UNCONFINED COMPRÊSSIVE STRENGTH {PSF) sotL oR BEDROCK TYPE BORING DEPTH tftt GRAVEL (t SAND l%J LIqUID LIMII PTASTIC INDEX MI 1 2V2 5.3 105 47 Very Silty Clayey Sand with Gravel 4 9.9 96 Sandy Silt and Clay 9 3.8 123 32 Silty Sa¡d with Fine Gravel 2 2V2 9.0 101 Sandy Silty Clay 5 9.6 90 Sandy Silty Clay 3 5 10.3 111 Sandy Silty Clay