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Home My WebLink AboutSubsoil Study for Foundation Design 07.25.2014H !ì.¡ .,,,j, i'.,,-¡,1 : i',.',.,1, i',1. l',. 'l ,::r il. Ilii ,, j-;'.,¡.-.r ì,.,i .la.i ', j il'" ì1 !,t t r,,4t{t iT:!1. i'.qw :, Å F ç [:{}l å:{" l € }l I t Ã l- July 25,2014 Jim Gornick P.O. Box 1296 Glenwood Springs, Colorado 81602 jgr¡:dp}-sls-çJ¡,us.¡:-gl Job No.l14286ê^ Subject:subsoil study for Foundatioh Design, Proposed Residencc, Lot 46, Four Mile Ranch, 110 Maroon Drive, Garfìeld County, Colorado Dear Mr. Gornick: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design of foundations at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated July 21,2014. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. proposed Construction: The proposed residence will be one and two story wood frame construction over a crawlspace or possible basement with an attached gàîage. The site is shown on Figure l. Ground floors will be slab-on-grade. Cut depths are expôcted to range between about 3 to 9 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. If building conditions or foundation loadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report. Site Conditions: The vacant lot is a west facing hillside and slopes moderately down to the west. The tot is vegetated with sage brush, grass and weeds in the building a¡ea and grass with weeds in the uphill front portion. The house on Lot 4?, to the south, is under construction. t*'Þþ-u,r,. ,#,",,'i;iîlrlrt:r' ",, r ii¡,i,i.{;,!.!1v i v.,ttfi,6¡,,t. [;r¡"krr. ]¡1-¡i{l.7tlç r {lr¡lt¡l'r¡rl¿r Sp¡t¡1¡. ;lL}-¡iì-i:(-il ¡ Srtr'r:lit{ì1¡ìt q?Lr-4ó¡i lçSiJ 2- subsidence Potential: Four Mile Ranch is underlain by pennsylvania Age Eagle Valley Evaporite bedrock' The evaporite contains gypsum deposits. Dissolution of the gypsum under ceftain conditions can cause sinkholes to develop and can produce areas of localized subsidence. sinkholes were not observed in the immediate area of the subjectlot' The pits were relatively shallow, for foundation design only. Based on our present knowledge of the site, it cannot be said for certain that sinkholes will not develop. In our opinion, the risk of ground subsidence at Lot 46 throughout the service lif.e of the building is low and similar to other lots in the area, but the owner should be aware of the potential for sinkhole clevelopment. subsurface conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits at the approximate locations shown on Figure l. The logs of the pits are presented on Figure 2. The subsoils encountered, below about I to I Yt feet of topsoil, consist of sandy silt and clay with scattered cobbles. A silt and sancl layer was observed at the bottom of Pit I at 7 feet. Results of swell-consolidation testing performed on relatively undisturbed sampres of the sandy silt and clay, presented on Figures 3 and 4, indicate low compressibility under existing moisture conditions and light loading with a moderate collapse potential (settlernent under constant load) when wetted. The samples showed moderate to relatively high compressibility upon increased loacling af'ter wetting. No free water was observed in the pits at the time of excavation and the soils were slightly moist. Results of a gradation analysis performed on a sampre of slightly gravelly silty sand (minus l% inchfraction) obtained from the site are presented on Figure 5. Foundation Recommendations: considering the subsoil conditions encountered in the exploratory pits and the nature of the proposecl construction, we recommerrd spread footings placed on 3 feet of structural fill overlying tlie unclisturbecl naturat soil 6esigned f-or an allowable soil bearing pressure of 1,200 psf for support of the proposecl residence. The soils tend to compress after wetting and there could be some post-construction foundation settlement. To reduce the risk of distress due to bearing soi ls settlement, we recommend the soils be sub_excavated at least 3 feet below design footi ng grade and be rep laced with structural {ill, moistened and compacted to at least 98% of the maximum Job No. I 14 28(tA cåFtech -J Standard proctor density at a moisture content near optimum. The on-site soils devoid oftopsoil and oversized rock can be used as structural frll. In the event ofsubsurface wetting of the bearing soils, settlement of 1 inch or more should be expectcd. it will be critical to the pcrfornrance of the f'oundation to keep the bearing soils fi.om getting wet. If this risk of settlement is not acceptable to the owner, a deep foundation suclr as pile .r piers could be constructed' In order to provide recommendations for a cleeper foundation, drilling borings on the site would be required. If the risk of settlement is acceptable, footings should be a minimum width of 20 inches for continuous walls and 2 feet ftrr columns' Topsoil and ltlose or disturbecl soils encountered at the foundation bearing level within the excavation should be removecl and the footing bearing level extendecl clown to the undisturbed sandy silt ancl clay soils. Exterior ftrotings should be provicled with adequate cover above their bearing elevations fbr frost protection. placement of fbotings at least 36 inches below the exterior grade is typically used in this area. continuous foundation walls should be reinfbrced top ancl bottom to span local anomalies such as by assuming an unsupported length of at least l4 feet. Foundation walls acting as retaining structures should be designed to resist a lateral earth pressure basecl on an equivalent fluid unit weight of at least 55 pcf for the on-site soil as backfill. A representative of the geotechnical enginccr should observe all footing excavations prior to concrete placement to evaluate bearing conditions. Floor slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loacled slab-on-grade construction. If the soils underlying the floor slabs are allowed to get wet, settlement of the slabs shoulcl be expected. As an alternative, structural floors could be consttucted over a crawlspace. To reduce the effects of sorne differential tnovernent' floor slabs should be separated from all bearing wails ancl columns with expansion joints which allow unrestrained vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The requirernents 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 nrinus 2 inch aggregate with ress than 50% passing the No. 4 sieve and ress than2v, passing the No. 200 sieve. Job No. I l4 2864 cåFtecr" -4- All fill materials for support of floor slabs should be compacted to at least g5% of maxi¡num standard Proctor density at a moisture content near optinium. Required fill can consist of the on-site soils devoiclof vegetation, topsoil and oversized rock. Underdrain System: Although free water was not encountered during our exploration, it has been our experience in mountainous areas that local perchecl groundwater can develop cluring times of heavy precipitation or seasonal runofT. Frozen ground during spring runoff can create a perched condition. We recommend below-grade construction, such as retairring walls, crawlspace and basement areas, be protected fiom wetting and hydrostatic pressure buildup by an urclerclrain system. The clrains should consist of drainpipe placed in the bottom of the wall backfill surrounded above the invsrt level with free-draining granular material. The drain should be placed at each level ofexcavation ancl at least I foot below lowest adjacent finish gtade and slopecl at a tninimunt lVo to a suitable gravity outlet. Free-draining granular materiat used in the underdrain system should contain less than 2% passing the No. 200 sieve, less than 50olo passing the No. 4 sieve anrl have a maximum size of 2 i'ches. The drain gravel backfill sliould be at least lt/z feefdeep. 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 wetti'g.f the bearing soils, Surf'ace Drainage: The f-ollowing drainage precautions should be observed during construction and maintained at all times after the resiclence has been cornpleted: I) Inundation ofthe foundation excavations and underslab areas should be avoided during construction. 2) Exterior backfill should be adjustecl to near optimum moisture a¡tl compacted to at least 95% of the maximum standard proctor density in pavement and slab areas and to at least 90% of the maximum stanclard Proctor density in landscape areas. Free-draining wall backfill shoulcl be capped with about 2 feet of the on-site, finer gracled soils to recluce surf'ace water infrltration. 3) The grouncl surtbce surrounding the exterior of the builcling should be slo¡red to clrain away f}.m the fìrunclation in all directions. we Job No.I l4 2864 cåFteol -5- 4) s) reconlmend a minimum slope of l2 inches in the first l0 feet in unpaved areas and a minimum slope of 3 inches in the fìrst l0 feet in pavement and walkway areas. A swale may be needecr uphill to direct surf'ace runoff around the residcnce. Roof downspouts ancl drains should discharge wellbeyond the limits of all backtill. Landscaping which requires regular heavy irrigation should be located at least l0 feet fì'om the building. consicleration shoulcl be given to the use of xeriscape to limit potential wetting of soils below the foundation caused by inigation. Limitations: This study has been condusted in accordance with generally accepted geotechnical engineering principles and practices in this area at this tirne. We make no waffanty either express or irnplied. The conclusions an<l recommenrlations submitted in this report are based upon the clata obtained fiorn the exploratory pits excavated at the locations indicated on Figure I antl to the depths shown on Figure 2, 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 contarninants (MOBC) developing in the f'uture. If the client is concerned about MOBC, then a profèssional in this special held of practice should be consulted. Our fìnclings include interpolarion ancl oxtrapolation of the subsurfàce conditions identifìed at the exploratory pits anclvariations in the subsurtbce conclitions tnay not become eviclent until sxcavation is performed. lf conditions encounterecl during construction appear <liffcrent fiom those descritrecl in this report, we should be notified at oncc so re-evaluation of the recomlnendations rnay be rnade. 'l-his report has been prepared fbr the exclusive use by our ctie¡rt for design purposes. We are not responsible for technical interpretations by others of our information. As the project evolves, we should provide continuecl co¡rsultation ancl fìelcl services clur-ing construction to review and monitor the irnplernentation of our recornmenclations, and to verify that the reconrlnendations have been appropriately interpreted. Significant design changes rnay require aclditieinal analysis clr mocli{Ìcations to the recornmendations ¡rresenteclherein' We recommcncl on-site otrservation of excavations ancl fbunclation Jol¡ No. I11286A cåFtectr 6- bearing strata and testing ofstructural fill by a representative ofthe geotechnical engtneer. If you have any questions or if we rnay be of further assistance, please let us know Respectfully Submifted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Louis Eller Reviewed by: f Daniel E. Har.din, p.E. LEE/ksw attachments Figure I - Location of Exploratory pits Figure 2 _ Logs of Exploratory pits Figures 3 and 4 _ Swell_Consolidation Test Results Figure 5 * Gradation Test Results Table I _ Summary of Laboratory Testing Job No. ll4 2g6A c&Ftec¡ MABOON DRryE APPROXIMATË SCALEl": 60' f PIT 2 BUILDING ENVELOPE PIT 1 LOT 46 LOT 45 LOT 47 HP GEOTECH JOB NO. 106 0498 I I OPEN SPACE 114 286A LOCATION OF EXPLORATORY PITS Figure 1 PIT 1 PIT 2 0 0 oq) tL ¡c Q 0)Õ 5 WC=6.6 DD=94 o)oLL I -c o. 0)rl 5 - ì +¿=r¿ - r -200=21 WC 5 .2 DD=93 10 LEGEND:10 TOPSOIL; organic sandy siit and clay, firm, stighily moist, dark brown. sllr AND CLAY (ML-CL); sandy, scattered cobbles, medium stiff to stiff, slighrly moisr, tight brown SAND AND slLT (SM-ML); slightly gravelly, medium dense/stiff, stighfly mo¡st, tighr brown þ I 2" Diameter hand driven liner sample. I I J Disturbed bulk sample. NOTES: l. Exploratory pits were excavated on Jury zi, z0r4with a cat s04D mini_excavalor. ' båiHl: of exploralory pits were measured approximately by pacing from features shown on the site pran 3' Elevations of exploratory pits were not measured and the logs of exploratory pits are drawn to depth.4 The exploratory pit locations should be considered accurate only to the degree implied by the method used.t niJlLTì,åiyï:ifl:îfl:JH#rïJ[îexprorarory *ü;;sen*he approximare boundaries berween 6' No free water was encountered in the pits at the time of excavating. Fluctuation in water level may occur with time7, Laboratory Ïesting Results: WC : Waler Content (%) DD : Dry Densfty (pcf) ' +4 : Percent retained on the No. 4 sieve-200 : Percent passing No. 200 sieve- - 114 286A LOGS OF EXPLORATORY PITS Figure z cC)oo_()a-(úÕ(o*.3.ò ã_=c)il ı8àro-uRı EccE t ör/l à .. -U!ç'^,øO--Y -c otO-r jg ã ..6LJ=;sååFI///c'ı()h a',- C.=5 Rão5ltr\\\\Jooc:euJE:)v)cÉ0*OUJJo_o_Oc(cY)c{c/)sO"¿ uorssetduuo3$(of--@o)(oac{srU)FJ)ØLUGþ'-U)tllt--zotrr]foU)zo()IJJUJ=U)cr)ol_pLL Moisture Content : 5,2 Dry Density : 93 Sample of: g¿¡6y Silt and Clay From: Pit 2 at6yzFeet percent pcf Compress wetting 0 n qo'ı U)c)a Ë at 2 ? 4 5 o 7 I I 1,0 t0 APPLIED PBESSURE - ksf 0.1 100 114 2B6A SWELL-CONSOLI DATION TEST RESULTS Figure 4 24 liR. 7 HR 0 45 MlN. 15 lvllN TIME READINGS U,S. STANDARD SFRIËS #100 #sa #30 t/16 #8 CLEAR SOUARE OPENINGS 60MtNrgMtN.4 MrN. 1 MtN. #200 #4 3/8" 3/4" 1 112 3" 5"6' 8,100 10 90 2A 80 30 7A ôl¿J 40zal-- LJE F50z LJOvl¡JÕ- 60 602 6vl & 50 F-z. L¡lOu t¡J 40 fL 70 80 30 2A 90 10 100 0001 .002 005 .009 .019 037 .074 150 300 .600 1.18 2 36 DIAMETER OF PARTICLES IN MILT.IMETERS 4.75 9.5 19.0 37.5lo È 76.2 152 2A3 127 ÛLAY TÕ S'LI coBsa Is GRAVEL 12 %SAND 67 YÖ SILT AND CLAY 21 7O LIQUID LIMIT O/O PLASTICITY INDËX O/O FROMTPit 1 at 7 to B FeetSAMPLE OF: Slightty Graveily Sitty Sand 114 286A GRADATION TEST RESULTS Figure 5 HEPWORTH-PAWLAK GEOTECHNICAL, INC,TABLE 1SUMMARY OF LABORATORY TEST RESULTSJob No. 1L42B6ASOIL ORB€DROCK TYPESandy Silt and ClaySandSandy Silt and ClayUNCONFINEDCOMPRËSSIVESTRÊNGTH(PSF)NATURAIDRYDENSTTYPERCENTPASSINGNO. 200SIEVELIMffSGRAVELSANDUQUIDLIMITPt-ASTiCINDE((vo)(o/o)2I67I)9493NATURALMOISTURECONTENT(o/ø\6.6\)PITDEPTH31t27to86y2I2