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HomeMy WebLinkAboutSubsoil Study for Foundation Design 04.24.2019t(+rt:*;, lftÍmr & åsÉ$slãb6, lñG, 5020 County Road 154Geotechn¡câlândMaterlalsEnqíneers Glenwood Springs, CO AtOOt ånd Env¡ronmðntal$cientiub phonå: (gi') S¿S-ZSSS fax: (970) 945-8454 email : kaglenwood@kumarusâ.com Ån Hnployco (hvngd Ëompony www.kumarusa.çotr Ofüce Locations: Denver {HQ}, Farker, Çolorado Springs, Fort Coliins, Glenwood Springs, and $ummit County, tolorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 430 SPRTNG RIDGE RESERVE HIDDEN VALLEY DRIVE GARFIELD COUNTY, COLORADO PROJECT NO. 19-7-236 APRIL 24,2019 PREPARED FOR: CARRIE DION 46107 US HIGH\ryAY ó GLENIryOOD SPRINGS, COLORADO 81601 c a rrie f¡ii dzremo deli n g.c o nn TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY PROPOSED CONSTRUCTION SITE CONDrTIONS......,....... .- 1 - 1 I GEOLOCY ,,.2 FIELD EXPLORATION -2- SUBSURFACE CONDITIONS ...2- FOT'NDATION BEARING CONDITIONS ....3 . DES IGN RECOMMENDATIONS ....................... FOUNDATIONS FOUNDATION AND RETAINING WALLS FLOOR SLABS T.INDERDRATN SYSTEM ......... SITE GRADING........... SURFACE DRAINAGE............. LIMITATTONS 7- FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 . LOGS OF EXPLORATORY BORINGS FIGURE 3 . LEGEND AND NOTES FIGURE 4. SWELL-CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Kumar & Aasociates, lnc.Project No. 1$.7.236 PURPOSE AND SCOPE OF STUDY This rcport presents the results of a subsoil study for a proposed residence to be located on Lot 43, Spring Ridge Reserve, Hidden Valley Drive, 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 Carrie Dion dated April 16, 2019. A field exploration prcgram consisting of exploratory borings was conducted to obtain information on the subsurface conditions, Samples of the subsoils and bedrock 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 recomrnendations and other geotechnical engineering consiclerations based on the proposed construction and the subsurface conrÌitions encountered. PROPOSED CONSTRUCTION The proposed residence will be a one-story structure with an attached garage and walkout basement level. Grading for the structure is assumed to be relatively minor with cut depths between about 8 to l0 feet. We assume relatively light foundation loaclings, typical of the proposed type of construction. If building loadings, Iocation or grading plans change significantly from those described above, we shoul<i be notified to rc-evaluate the recommendations contained in this report. SITE CCINDITIONS The lot was vacant at the time of our field exploration. The ground surface appears mostly natural with possible fill near llidden Valiey Drive. The ground surface slopes down to the west at about 12lo 15 percent in the prnposed building area and is nloderately steep near the roadway Kumar & Associales, lnc.Project f{o, 19.7-?36 n Vegetation consists of grass and weeds. The surrounding lots have one to two'stoly single- family residences. Sandstone bedrock outcrops were observed adjaoent to the castern side of Hidden Valley Drive. GEOLOGY Accorcling to the Geologic Map of the Cattle Creek Quaclrangle, Garfield County, Coloraclo, by Kirkham. R.M., Streufert, R.K., Hemborg. Thomas, and Stellìng, dated 2014, the site soils consist of alluvium and colluvium ofthe Holocene age underlain by Maroon Formation sandstone, FIELD EXPLORATION The fìeld exploration for the project was conducted on April 19, 2019. Two exploratory borings were drjlied at the locations shown on Figure 1 to evaluate the subsurface conditions. The building outline had been marked by others, The borings were advanced with 4-inch diarneter continuous flight augers powered by a truck-mounted CME-458 drill rig. The borings were logged by a representative of Kumar & Associates. 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-l586. The peuetration resistance values are ar1 indication of the relative density or consistency of the subsoils and hardness of the bedrock. 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 retumed 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 encountered, below aboutt/z fbot of topsoil, consist of loose to mediurn dense/medium stiff t<l stiff sand and silt, underlain by hard to very hard sandstone bedrock. A thin layer of weathered bedrock was encourltcred within Boring 2. Kumar & Aseoeialos, lnc,Project No. 19-7-236 -3- Laboratory testing perfonned on samples obtained from the borings included natural moisture content and clensity and finer than sand-size gradation analyses. Results of swell-consolidation testing performed on a relatively undisturbecl drive sample of very silty sand, presented on Figure 4, indicate low to moderate compressibility under conditions eif loading and wetting. 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 moist. FOUNDATION BEARING CONDITIONS At assumed excavation depths, we expect the subglade will transition natural sand and silt soils and sanclstone bedrock. The natural soils at the site possess low bearing capacity, relatively low settlement potential and the bedrock possesses a high bearing capacity, Both the natural soil and bedrock are considered competent bearing materials fcrr the support of shallow fcrundations and slabs-on-grade provided the subgrade remain undisturbed during construction. The topsoil is not oonsidered suitable for the support of shallow foundations and slabs-on-grade due to its potential cornpressibility and should be fi.rlly removed from the building footprint, Excavation within the very hard sandstone bedrock will be diftìcult and may require specialized heavy duty equipment. DESIGN RECOMMENDATIONS FOLINDATIONS Consiclering 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 granular soils and bedrock. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural granular soils should be designed for an allowable bearing pressure g!jsq!,p$!. Footings placed on sandstone bedrock should be designed for an allowable bearing pressure of 4,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about I inch or less. Kumar & Associatcs, Inc.Project No, 19-7"236 -4- 2)Thefootingsshou1dhaveanrinimumwidthofrybrcontinuouswallsanc1 2 feg!þr isolated pads. \r-- 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+ 4) area, Continuous founclation 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 discussetl in the "Foundation and Retaining Vy'alls" section of this report, The topsoil and any loose or disturbed soils should be removed and thc footing bearing level extended down to the natural granular soils or bedrock. The exposed soils in footing area should thcn bc moistencd and compacted, A representative of the geotechnical engineer sliould observe all footing excavations prior to concrete placement to evaluate bearing conditions. FOI-INDATION AND RETAINÏNG WALLS lìoundation walls and retaining structures which are laterally supported and can be expected to undergo only a slight amount of deflection should be clesigned 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 rnobilize the full active earth pressure condition should be designed t'or 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. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction materials arrd 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 buiidup behind walls. exterior grade is t¡pically used in this s) 6) Kumar & As*ocialos, lne.Project No. 19"7"236 -5- Backfill shoulcl be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content near optimum. Backfill placed in pavement and walkway areas should be compacted to at least g5% of the maximum standarcl 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 comectly, and could result in distress to facilities construoted on the backfill. Backfill should not contain organics, debris or rock larger than about 6 inches. 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 f iction of 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent f'luid unit weight of 375 pcf. The coeffìcient of friction and passive prÊssure values recommended above assumc ultimate soil strenglh. 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 rnoisture 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 movennent. Floor slab control joints should be used to reduce clamage due to shrinkage cracking. 'Ihe requirements for joint spacing and slab reinforcement should be established by the designer based on experience and the intended slab use. A minirnum 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 sievc and less than2o/o passing the No. 200 sieve. Kumar & As*oeiales, lnc.Project No. 19"7-?36 -6- All fill materials for support of floor slabs should be compacted to at leastg1Yo 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. LINDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in mountainous areas and where bedrock is shallow that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoffcan 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 I foot below lowest adjacent finish grade and sloped at a minimum l%oto a suitable gravity outlet. 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% feet deep. SITE GRADING The risk of construction-induced slope instability at the site appears low provided cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about l0 to 12 feet" Fills should be limited to about I to 10 feet deep. Embankment fills should be compacted to at least 95% of ths 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% of the maximum standard Proctor density The fill should be benched into slopes that exceed 2A% graðe. Permanent unretained cut and fill slopes should be graded at 2 horizontal to I vertical or flatter and protected against erosion by revegetation or other means. The risk of slope instability will be increased if seepage is encountered in cuts and flatter slopes may be neÇessary, If seepage is encountered in pennanent cuts, an investigation should be conducted to determine if the seepage Kumar & Aseociates, lnc,Project Ho. '19-7-236 -7 - will adversely affect the cut stability. 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 comploted: 1) Inundation ofthe foundation excavations and underslab areas should be avoidecl 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 90a/o of tlne maximum standard Proctor density in landscape areas, 3) The ground surface surrounding the exteríor 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 I 0 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 frlter fabric and capped witli 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 inigation shoulcl be located at least l0 feet from foundation walls. LIMITATIONS This study has been conducted in accordance with gerrerally accepted geotechnical engineeling principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recommendations subrnitted in this report are based upon the data obtained frorn the exploratory borings drilled excavated 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 rnold or other biological contaminants (MOBC) developing irr the future. If the client is concemed about MOBC, then a professional in this special field ofpractice should be consulted. Our findings include interpolation and Kumar & Associates, lnc.Project No, 1S.7"236 -8- extrapolation of the subsurface conditíons identified at the exploratory borings and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encounterecl 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 ofexcavations and foundation bearing strata and testing ofstructural fìll by a representative of the geotechnical engineer. Respectfully Submitted, Kumar & Ässociates, Ine. Shane J. Robat, P.E. Project Manager Reviewed by: Steven L. Pawlak, P SJR/kac Kumar & Associâtcg, lnc.Project lr¡o, 19.7.23û ¡-l¡JuL!¡uJolltr¡lFxoolÈILJIlôlo(,=e.oÍDlËieéã&I3éíEIII.,t'-,r,aì,,-ì.'..;f- -ìrf:.,,I¡ıáô¿zs!ãts-effi-:- -l(,zÉ.oElìIii_-.iiLIÞ'3ffiL.rc¡rv)c\¡I¡'\ICt)oo.gooau,oðfúE5vtn()zÊomu.ot-É.oJo-Xf¿ll+oz.otr()oJ<t)f!aó¿t,s - s¡o¿ 'c! ¡ 3 ! ¡ L .4 BORING 1 EL. 1 00' BORING EL, 106 2 0 0 5/ 12 Y,,lC=12.7 DD='l I 5 30/12 tNC=4,7 DD=1 1 4 -200= 1 0 5 10/1 WC= Â 2 4.8 50/ 4 DD= l 04 -200=44 t-IJ l4l l! I fF-ô- l¡Jô 10 50/4 10 l-t¡Jl¡lu I F.L L¡Jâ 50/ 1 15 1 a0/3 1550/ 1 20 2A 19-7 -236 Kumar & Associates LOGS OF TXPLORATORY BORINGS Fig. 2 I _L-ESENÞ"." ñN TOPSOIL; ORGANIC, SANDY SILT, FIRM, DARK BROWN, MO|ST. SAND AND SILT (SM-ML); LOosE TO MËDIUM DTNSE/MEDIUM STIFF To sTlFF, SLIGHTLY MOIST TO MOIST, RED. T--Å lÁäi:l tiL WEATHERED SANDSTONË; MEÐIUM HARD, MOIST, RED. Sff.:â lliili:li:1 [Lr;.iäJ SANDSTONE BEDROCK; VERY HARÞ, SLIGHTLY MOIST, RED. MAROON FORMATI0N. DRIVE SAMPLE, ?*INCH I.D. CALIFORNIA LINER SAMPLE. q71' DRIVE SAMPLE ELOW COUNI. INDICATES THAT 5 BLOWS OF A'14O-POUND HAMMER",'. FALLING 30 INCHIS WERE REQUIRED TO DRIVE THE SAMPLER 12 INcHEs. NOTES 1. THE EXPLORATORY BORINGS WERE DRILLED ON APRIL 19,2019 WITH A 4-INCH_DIAMETER CONTINUOUS*FLIGHT POWER AUGER, 2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MËASURED APPROXIMATELY BY PACING FROM TEATURES SHOWN ON THE SITE PLAN PROVIDED. THE BUILDING CORNERS HAD BEEN STAKED. 3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFER TO BORING 1 AS 100,, ASSUMËD, 4. THE EXPLORATORY BORING LOCATIONS AND EIEVATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO IHE DEGREE IMPTIED BY THE METHOD USED. 5. T]JE LINES BETWEEN MATERIALS SHOWN CIN THE EXPLORATORY BORING LOGS REPRËSENT THE APPROXIMATE BOUNDARIES BEIWEEN MATER¡AL TYPES AND THE TRANSITIONS MAY BE GRADUAL 6. GROUNDWATER WAS NOT ENCOUNTERED IN THË BORINGS AT THË TIME OF DRILLING 7, LABORATORY TE$T RESULTS: wc = wATER CONÍENT (%) (ASTM 0221 6); DD = DRY DENSITY (pcf) (ASTM D2216); -ZOQ= PERCENTAGE PASSING NO. 200 SIEVE (ASTM Dl 140) 19-7-236 Kumar & Associates LEGTND AND NOTTS Fig. 3 ?- p É I I Ë å I FI {i SAMPLE OF: Very Sllty Sond FROMIBorlngl@2.5' tNC = 12.7 %, DD = I 15 pcf ol NO MOVEMENT UPON WETTINC 1 0 >t J.J -ll¿¡3 tJ1 t-2 za $-s0utzo{J-4 I 19-7-236 Kumar & Associates SWTLL-CONSOLIDATION TEST RESULTS Fi1, 4 l(+rtffiffitli'**"-:iTABLE 1SUMMARY OF LABORATORY TEST RESULTS19.7.236SOIL TYPËVery Silty SandVery Siþ Sand'Weathered Sandstone(psf)UilCOilF[IEDcoilPRESSf\rESÏRËHGTHPljSilCn¡DÐ(t%lATTERBERG LIil]IrSL¡QUIT}UMT{0/JPERCEI{TPASStilG ilO.ã!0$a,E4401sAltt)(%)GRADÂTIôTI{%}GRAVETlocfìI¡ATURAtDRYoElr$û1151CI4tt4t2.74.84.7I¡ATURALHOISTURECOI{TENTBORINGLOCATtOi¡ÐEPTH2t/252y,12