HomeMy WebLinkAboutSubsoil StudylcrtKurnr & Associates, lnc.'Geotechnical and Materials Engineenand Environmental ScientistsAn Em$oyoe 0rnsd CompcnyOffrce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, ColoradoSUBSOIL STT]DYFOR FOTJNDATION DESIGNPROPOSED RESIDENCETIPPET LANELOT 14, BAGLEY SUBDIVISION EXEMPTIONGARFIELD COT]NTY, COLORADOPROJECT NO.20-7-467SEPTEMBER 30, 2020PREPARED FOR:DOUG LAHO475 PANORAMIC DRIVESILT, COLORADO 81652(douelaho@smail.com)5020 County Road 154Glenwood Springs, CO 81601phone: (970)945-7988fax: (970) 945-8454email : kaglenwood@kumarusa.comwww.kumarusa.com
TABLE OF CONTENTSPURPOSE AND SCOPE OF STUDY ....PROPOSED CONSTRUCTION ....SITE CONDITIONS.FIELD EXPLORATION.SUBSURFACE CONDITIONS ...FOUNDATION BEARING CONDITIONSDESIGN RECOMMENDATIONS ............FOI.]NDATIONSFOUNDATION AND RETAINING WALLSFLOOR SLABSTINDERDRAIN SYSTEM .............SURFACE DRAINAGELIMITATIONS......FIGURE 1 - LOCATION OF EXPLORATORY BORINGSFIGURE 2 - LOGS OF EXPLORATORY BORINGSFIGURE 3 - LEGEND AND NOTESFIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTSTABLE 1- SUMMARY OF LABORATORY TEST RESULTS.- 1 --1--1-...-2 --2-aJJ4556-6-1Kumar & Associates, lnc. 6Project No.20-7467
PURPOSE AND SCOPE OF STT]DYThis report presents the results ofa subsoil study for a proposed residence to be located onLot 14, Bagley Subdivision Exemption, Tippet Lane, Garfield County, Colorado. The projectsite is shown on Figure 1. The purpose of the study was to develop recommendations for thefoundation design. The study was conducted in accordance with our agreement for geotechnicalengineering services to Doug Laho dated August 18,2020.A field exploration program consisting of exploratory borings was conducted to obtaininformation on the subsurface conditions. Samples of the subsoils and bedrock obtained duringthe field exploration were tested in the laboratory to determine their classif,rcation,compressibility or swell and other engineering characteristics. The results of the fieldexploration and laboratory testing were analyzedto develop recommendations for foundationtypes, depths and allowable pressures for the proposed building foundation. This reportsummarizes the data obtained during this study and presents our conclusions, designrecommendations and other geotechnical engineering considerations based on the proposedconstruction and the subsurface conditions encountered.PROPOSED CONSTRUCTIONThe proposed residence will be a one and two story structure with attached garage. Groundfloors are assumed to be structural over crawlspace for the living areas and slab-on-grade for thegatage. Grading for the structure is assumed to be relatively minor with cut depths betweenabout 2 to 6 feet. V/e assume relatively light foundation loadings, typical of the proposed type ofconstruction.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 CONDITIONSThe subject site was an irrigated field at the time of our field exploration. The ground surfacewas sloping down to the southwest at a grade of around 15 percent. Vegetation consists of grassFIELD EXPLORATIONThe field exploration for the project was conducted on August 27,2020. Two exploratoryborings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditionsKumar & Associates, lnc. @Project No.20-7467
-2-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 Kumar &Associates, Inc.Samples of the subsoils were taken with I% inch and 2 inch I.D. spoon samplers. The samplerswere driven into the subsoils at various depths with blows from a 140 pound hammer falling 30inches. This test is similar to the standard penetration test described by ASTM Method D-l586The penetration resistance values are an indication of the relative density or consistency of thesubsoils and hardness of the bedrock. Depths at which the samples were taken and thepenetration resistance values are showñ:on the Logs of Exploratory Borings, Figure 2. Thesamples were returned to our laboratory for review by the project engineer and testing.SUBSURFACE CONDITIONSGraphic logs of the subsurface conditions encountered at the site are shown on Figure 2. Thesubsoils consist of about I foot of topsoil (root zone) overlying medium stiff, slightly sandy clayto 2 feet in Boring 1 and down to 13 feet in Boring 2. Medium dense sand and clay wasencountered in Boring 1 from 2 to 14 feet deep. Weathered sandstone bedrock was encounteredin Boring I at 14 feet deep and in Boring 2 at 13 feet deep. The sandstone bedrock becameharder with depth down to the maximum explored depth of 26 feet.Laboratory testing performed on samples obtained from the borings included natural moisturecontent, density and finer than sand grain size gradation analyses. Results of swell-consolidationtesting performed on a relatively undisturbed drive sample of the sand and clay from Boring 1,presented on Figure 4, indicate low compressibility under existing moisture conditions and lightloading and a low expansion potential when wetted under constant light surcharge. Results ofswell-consolidation testing performed on a relatively undisturbed drive sample of the slightlysandy clay from Boring 2, presented on Figure 3, indicate low to moderate compressibility underexisting moisture conditions and light loading and a low collapse potential (settlement underconstant load) when wetted. The laboratory testing is summarizedinTable 1.No free water was encountered in the borings atthe time of drilling and the subsoils wereslightly moist to moist.FOT]NDATION BEARING CONDITIONSThe slightly sandy clay and sand and clay soils encountered at the site possess a low bearingcapacity and variable swell or collapse potential, when wetted. Spread footings placed on theKumar & Associates, lnc. @Project No.20-7467
-J-sand and clay subsoils can be used for support of the proposed residence with a risk ofdifferential movement, especially if the subsoils become wetted. A lower risk option would beto extend the bearing level down to the underlying sandstone bedrock with a deep foundationsystem such as helical piers or drilled piers. Provided below are recommendations for a spreadfooting foundation system. If recommendations for helical piers or drilled piers are desired weshould be contacted to provide them.DESIGN RECOMMENDATIONSFOUNDATIONSConsidering the subsurface conditions encountered in the exploratory borings and the nature ofthe proposed construction, the building can be founded with spread footings bearing on thenatural soils with a risk of some post-construction movement in the event of subsurface wetting.The design and construction criteria presented below should be observed for a spread footingfoundation system.1) Footings placed on the undisturbed natural soils should be designed for anallowable bearing pressure oljj99$ Based on experienco, we expectmovement of footings designed and constructed as discussed in this section willbe about I inch or less. There could be some additional long term foundationsettlement if the bearing soils become wetted. The magnitude of the additionalsettlement would depend on the depth and extent of wetting but could be up toabout I inch.2) The footings should have a minimum width of 18 inches for continuous walls and2 feet for isolated pads.3) Exterior footings and footings beneath unheated areas should be provided withadequate soil cover above their bearing elevation for frost protection. Placementof foundationsatl6 inchesbelow exterior grade is typically used in thisàteà.4)Continuous foundation walls should be heavily reinforced top and bottom to spanlocal anomalies such as by assuming an unsupported length of at least 14 feet.Foundation walls acting as retaining structures should also be designed to resistlateral earth pressures as discussed in the "Foundation and Retaining'Walls"section of this report.All topsoil and any loose or disturbed soils should be removed and the footingbearing level extended down to the natural soils. The exposed soils in footings)Kumar & Associates, Inc. @Project No.20.7467
-4-area should then be moistened and compacted. If water seepage is encountered,the footing areas should be dewatered before concrete placement.A representative of the geotechnical engineer should observe all footingexcavations prior to concrete placement to evaluate bearing conditions.FOUNDATION AND RETAINING WALLSFoundation walls and retaining structures which are laterally supported and can be expected toundergo only a slight amount of deflection should be designed for a lateral earth pressurecomputed on the basis of an equivalent fluid unit weight of at least 50 pcf for backfill consistingof the on-site soils. Cantilevered retaining structures which are separate from the residence andcan be expected to deflect sufficiently to mobilize the full active earth pressure condition shouldbe designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weightof at least 40 pcf for backfill consisting of the on-site soils.All foundation and retaining structures should be designed for appropriate hydrostatic andsurcharge pressures such as adjacent footings, traffic, construction materials and equipment. Thepressures recommended above assume drained conditions behind the walls and a horizontalbackfill surface. The buildup of water behind a wall or an upward sloping backfill surface willincrease the lateral pressure imposed on a foundation wall or retaining structure. An underdrainshould be provided to prevent hydrostatic pressure buildup behind walls.Backfill should be placed in uniform lifts and compacted to at least 90o/o of the maximumstandard Proctor density at a moisture content near optimum. Backfill in pavement and walkwayareas should be compacted to at least 95Yo of the maximum standard Proctor density. Careshould be taken not to overcompact the backfill or use large equipment near the wall, since thiscould cause excessive lateral pressure on the wall. Some settlement of deep foundation wallbackfill should be expected, even if the material is placed correctly, and could result in distress tofacilities constructed on the backfill.The lateral resistance of foundation or retaining wall footings will be a combination of thesliding resistance of the footing on the foundation materials and passive earth pressure againstthe side of the footing. Resistance to sliding at the bottoms of the footings can be calculatedbased on a coefftcient of friction of 0.35. Passive pressure of compacted backfill against thesides of the footings can be calculated using an equivalent fluid unit weight of 375 pcf. Thecoefficient of friction and passive pressure values recommended above assume ultimate soilstrength. Suitable factors of safety should be included in the design to limit the strain which will6)Kumar & Associates, lnc.6Project No.20-7467
-5-occur at the ultimate strength, particularly in the case of passive resistance. Fill placed againstthe sides of the footings to resist lateral loads should be a nonexpansive material compacted to atleast95Yo of the maximum standard Proctor density at a moisture content near optimum.FLOOR SLABSThe natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-gradeconstruction. To reduce the effects of some differential movement, floor slabs should beseparated from all bearing walls and columns with expansion joints which allow unrestrainedvertical movement. Floor slab control joints should be used to reduce damage due to shrinkagecracking. The requirements for joint spacing and slab reinforcement should be established by thedesigner based on experience and the intended slab use. A minimum 4 inch layer of gravelshould be placed beneath slabs to provide support. This material should consist of minus 2-inchaggregate with at least 50o/o retained on the No. 4 sieve and less than l2Yo passing the No. 200sieve.All fill materials for support of floor slabs should be compacted to at least 95o/o of maximumstandard 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 SYSTEMAlthough free water was not encountered during our exploration, it has been our experience inthe area that local perched groundwater can develop during times of heavy precipitation orseasonal runoff. Frozen ground during spring runoff can create a perched condition. Werecommend below-grade construction, such as retaining walls and crawlspace areas, be protectedfrom wetting and hydrostatic pressure buildup by an underdrain system.The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded abovethe invert level with free-draining granular material. The drain should be placed at each level ofexcavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% toa suitable gravity outlet. Free-draining granular material used in the underdrain system shouldcontain less than 2o/o passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have amaximum size of 2 inches. The drain gravel backfill should be at least L%feetdeep. Animpervious membrane such as 20 mll PVC should be placed beneath the drain gravel in a troughshape and attached to the foundation wall with mastic to prevent wetting of the bearing soils.Kumar & Associates, lnc. @Project No.20-7467
-6-SURFACE DRAINAGEIt will be critical to the long term performance of the proposed residence to construct andmaintain effective surface drainage away from the house. The following drainage precautionsshould be observed during construction and maintained at all times after the residence has beencompleted:1) Inundation of the foundation excavations and underslab areas should be avoidedduring construction.2) Exterior backfill should be adjusted to near optimum moisture and compacted toat least 95o/o of the maximum standard Proctor density in pavement and slab areasand to at least 90o/o of the maximum standard Proctor density in landscape areas.3) The ground surface surrounding the exterior of the building should be sloped todrain away from the foundation in all directions. We recommend a minimumslope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of3 inches in the first 10 feet in paved areas. Free-draining wall backfill should becapped with about 2 feet of the on-site finer graded soils to reduce surface waterinfiltration.4) Roof downspouts and drains should discharge well beyond the limits of allbackfill.5) Landscaping which requires regular heavy irrigation should be located at least10 feet from foundation walls. Consideration should be given to use of xeriscapeto reduce the potential for wetting of soils below the building caused by inigation.LIMITATIONSThis study has been conducted in accordance with generally accepted geotechnical engineeringprinciples 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 obtainedfrom the exploratory borings drilled atthe locations indicated on Figure 1, the proposed type ofconstruction and our experience in the area. Our services do not include determining thepresence, prevention or possibility of mold or other biological contaminants (MOBC) developingin the future. If the client is concerned about MOBC, then a professional in this special field ofpractice should be consulted. Our findings include interpolation and extrapolation of thesubsurface conditions identified at the exploratory borings and variations in the subsurfaceconditions may not become evident until excavation is performed. If conditions encounteredduring construction appear different from those described in this report, we should be notified sothat re-evaluation of the recommendations may be made.Kumar & Associates, lnc. @Project No. 20-7-467
-7 -This report has been prepared for the exclusive use by our client for design pu{poses. We are notresponsible for technical interpretations by others of our information. As the project evolves, weshould provide continued consultation and field services during construction to review andmonitor the implementation of our recommerìdations, and to veriry that the recommendationshave been appropriately interpreted. Significant design changes may require additional analysisor modifications to the recommendations presented herein. Vy'e recommend on-site observationof excavations and foundation bearíng strata and testing of skuctural filI by a representative ofthe geotechnical engineer.Respectfully Submiued,Kumar & .A.ssocíates, Inc.frltu/Ja#es H. Parsons, E.I.Reviewed by:"qlDaniel E. Ha¡din, PJHP/kacKumar & Associates, lnc. eProject lio. 20.7.¡lô7
BENCHMARK:GROUND LEVEL AT COTTONWOODTREE, EL=100', ASSUMEDffi.1000100APPROXIMATE SCALE-FEETBORING 12BORIFig. 1LOCATION OF TXPLORATORY BORINGSKumar & Associates20-7 -467
ËIBORING 1EL. 94'BORING 2EL. 83.5'001o/ 127/125517/12WC=8.4DD=113s/12WC=11.3DD= 1 05101028/12WC=7.1DD=l 1 1-200= 1 Is/12WC=15.0DD=1 19-200=89t-t¡Jt¡JtLI:Et-(Lt¡Jô1515Ft¡Jt¡JtLI-FfLL¡Joso/1.5tNC=7.4DD=116so/s.5WC=6.0DD=1 26202050/1.550/22525so/170/123050Fig. 2LOGS OF TXPLORATORY BORINGSKumar & Associates20-7 -467
EtILEGENDTOPSOIL; CLAY, SLIGHTLY SANDY, ORGAN|CS, MEDIUM STIFF, SLIGHTLY MO|ST, DARKBROWN.CLAY (CL); SLIGHTLY SANDY TO SANDY, SCATTERED GRAVEL, STIFF, SLIGHTLY MOIST, DARKBROWN.SAND AND CLAY (SC-CL); SCATTERED GRAVEL, STIFF T0 VERY STIFF, SLIGHTLY MOIST,LIGHT BROWN.SANDSTONE AND CLAYSTONE; HARD, SLIGHTLY MOIST, BROWN AND GRAY. WASATCH ANDOHIO CREEK FORMATION.DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLEiDRTVE SAMPIE, 1 3/9-|NCH t.D. SpLtT SPOON STANDARD PENETRATTON TEST1î/1, DRIVE SAMPLE BLOW COUNT. INDICATES THAT 10 BLOWS 0F A 140-POUND HAMMER'-I'- FALLING 50 INCHES WERE REQUIRED TO DRIVE THE SAMPLER.I2 INCHES.---> DEPTH AT WHICH BORING CAVED FOLLOWING DRILLING.NOTESI. THE EXPLORATORY BORINGS WERE DRILLED ON AUGUST 27,2O2O WITH A 4_INCH-DIAMETERCONTINUOUS-FLIGHT POWER AUGER.2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACINGFROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFERTO THE BENCHMARK ON FIG. 1.4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATEONLY TO THE DEGREE IMPLIED BY THE METHOD USED.5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THEAPPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.7. LABORATORY TEST RESULTS:WC = WATER CONTENT (%) (ASTM D2216);DD = DRY DENSITY (PCI) (ISÏV D2216);_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM 01140).Fig. 3LEGEND AND NOTESKumar & Associates20-7 -467
SAMPLE OF: Sond ond CloyFROM:Boringl@5'WC = 8.4 %, DD = 113 pcf¡ñ' i ...t,...""..).EXPANSION UNDER CONSTANTPRESSURE UPON WETTINGI.i. ....i....r....r. .......... 1........JJL¡J=tJ1Iz.oFo=ov1z.o(J10-1-2-3-41.0APPLIED PRESSURE - KSF100Fig. 4SWELL-CONSOLIDATION TEST RESULTSKumar & Associates20-7 -467Ë¡
tË*ISAMPLE OF: Sondy CloyFROM:Boring2@5'WC = 1 1.3 %, DD = 105 pcfI.. ............1........ .....1ADDITIONAL COMPRESSIONUNDER CONSTANT PRESSUREDUE TO WETTINGI,lrlwlthInnot bê rêproducêd,tè*d. lhe2JJl¡J=UIIz.otro=o1/1zo()0-2-4-6-8-10-12-141.0APPLIED PRESSURE - KSF10100Fig. 5SWELL-CONSOLIDATION TEST RESULTSKumar & Associates20-7 -467
I (lrt i;,.rifiliniÉtrr i'iiå *' "TABLE 1SUMMARY OF LABORATORY TEST RESULTS'Weathered Sandstonet266.015Sandy Clay89t1915.010Sandy Clay105I 1.35218SOIL TYPE1("/"1l:/"1SANDGRAVELW'eathered SandstoneSand and ClaySand and Clay1161131117.48.47.r15510ATTERBERG LIMITSLIQUID LIMITSAMPLE LOCATIONDEPTHBORINGPERCENTPASSING NO.200 stEvEUNCONFINEDCOMPRESSIVESTRENGTHNATURALDRYDENSITYNATURALMOISTURECONTENTPLASTICINDEXPNo.20-7-467