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HomeMy WebLinkAboutSubsoil StudyH SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT M46, ROARING FORK MESA ASPEN GLEN SUBDIVISION GARFIELD COUNTY, COLORADO JOB NO. 114 1l3A MAY 28,2014 PREPARED FOR: ALISON FOTO P.O. BOX 7911 ASPEN, COLORADO 8TóT2 ( ¿rlia glevfrir g nrai l.c o nr) l {c¡.ru,,1 1 1-,-¡',t, l:rli ( ìc, :tcelrnlrrlI, Irtr i¡irr ( lt {i¡ri\ li, 'rr.l I i,1 ( ì1,--¡r'r,r'.1 Si.r inr:., (j',l,,r,ril,, 5 lorri {ì}1,,i1' ; rli¡ì-tl4 j - f tiS;: 1-:rr ; i.¡1.', .l+t -¡+ ;+ r tri.r!l: ìr¡'¡L, r€ì11.,:fr't (ilt.!!,tìì H EPWORTH.PAWLAK GãOTECHN ICAL l"r'Lcr' ltì'b1l"ill9 ( .,,1,'l'rì,1,, Sl'l'int. i l,)-r, ì l-5i(Ija Silr crtll"r'rrc q7r)-.1(ìS- Iq.lq TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY..... PROPOSED CONSTRUCTI ON. SITE CONDITIONS. SUBSIDENCE POTENTIAL FIELD EXPLORATION. SUBSURFACE CONDITIONS.. FOUNDATION BEARING CONDITIONS DESIGN RECOMMENDATIONS FOIINDATIONS FOI.INDATION AND RETAINING WALLS. FLOOR SLABS...... UNDERDRAIN SYSTEM SITE GRADING SURFACE DRAINAGE. LIMITATIONS REFERENCES FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FTGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - GRADATION TEST RESULTS I 1 -z^ 2- .......... - 3 - .-3- -4- 4 4 5 6 7 7 I -8- ..-10- PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at LotM46, Roaríng Fork Mesa, Aspen Glen Subdivision, Garfield Count¡ 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 you dated April 1A,2074. Chen-Northem, Inc. previously conducted a preliminary geoteclrnical engineering study for the development (Chen-Northem, 1991) and another geotechnical engineering study for preliminary plat design (Chen-Northern, 1993). A field exploration program consisting of exploratory borings was conducted to obtain infomation 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 ofthe field exploration and laboratory testing were analyzed to develop recofiÌrnendations for foundation types, depths and allowable pressures for the proposed building foundation. This report summarizes the data obtained during this study and presents ow conclusions, design recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTTON The proposed residence will be one and two story wood fiame construction above a walkout basement with an attached garage. Basement and garage floors will be slab-on- grade. Grading for the structure is assumed to be relatively minor with cut depths befween about 3 to 9 feet. We assume relatively light foundation loadings, typical ofthe proposed type of construction. If building loadings, Iocation 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 I l3A cåEtecn /) - L- SITE CONDITIONS The lot is vaca¡t and vegetated with sparse grass and weeds. The ground surface is relatively flat with a gentle slope down to the east and appears to have been graded during subdivision development with fill placed across the entire lot. The slope becomes very steep down to the east at the rear of the buildíng envelope. The Roaring Fork River is located east and several topographic benches below the site. The Eagle Valley Evaporite Formation is exposed on a hillside just uphill and west of the site along County Road 109. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies most of the lower Roaring Fork River valley and the Aspen Glen Subdivision. These rocks are a sequ€nce of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous studíes in the alea, several sinkholes were observed scattered throughout the Aspen Glen development (Chen-Northem, Inc., l99t). These sinkholes appe¿u similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork Valley. Sinkholes were not observed in the immediate area ofthe subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory borings were relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot M46 throughout the seruice iife of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. Job No. I 14 I l3A c,&Etech â-.1- FIELD EXPLORATION The field exploration for the project was conducted on April 14, 2014. Two exploratory borings were drilled at the locations shown on Figure I to evaluate the subsurface conditions. The borings were advânced with 4 inch diameter continuous flight augers powered by a truck-mounted CME-458 drill rig. The borings were logged by a repres entative o f Hepworth-P awlak Geotechnical, Inc. Samples ofthe subsoils were taken with a l3Á nchl.D. spoon sampler. The sampler was driven into the subsoils at various deptbs 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 ofthe subsoils. Depths at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Fþre 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs ofthe subsurface conditions encountered at the site are shown on Figure 2. The subsoils consist of about I ro lVz feet of man-placed fiIl overlyíng slightly silty sandy gravel with cobbles and small boulders. 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 gradation analyses. Results of gradation analyses performed on small diameter drive samples (minus 1% inch fraction) of the coarse granular subsoils are shown on Figure 4. The laboratory testing is surnmarized tnTable 1. Job No. I 14 I l3A cåEteclr -4- No free water was encountered in the borings at the time of drilling or when checked 43 days later and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The natural gnnular soils are adequate for support of spread footing foundations and slab-on-grade floors. The man-placed fill encountered at the site was relatively shallow and should be removed from beneath footings and slabs. DESIGN RECOMMENDATIONS FOLINDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, r¡/e recoiltmend the building be founded with spread footings bearing on the natural granular soils. The design and construction criteria presented below should be observed for a spread fo oting foundation system. 1) Footings placed on the undisturbed natural glanular soils should be designed for an allowable bearing pressure of 2,500 psf Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. 2) The footings shouid have a minirnumwidthof 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 tlpically used in this area. 4) Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assumíng an unsupported length of at least 10 Job No. I 14 1134 c,äBtecrr 5 s) 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. All existing fill, topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural granular soils. The exposed soils in footing area should then be moistened and compacted. A representative of the geotechnical engineer should obsele all footing excavations prior to concrete placement to evaluate bearing conditions. FOI-INDATION 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 45 pcf for backfill consisting of the on-site granuiar 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 40 pcf for backfill consisting ofthe on-site granular soils. All foundation and retaining sttuctures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The pressures recontmended 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. Backfïll should be placed in uniform lifts and compacted to at least 90%o of the maximum standard Proctor density at a moisture content near optimurn. Backfill in pavernent and 6) Job No. 114 I l3A c,äEtecn -6- walkway ateas should be compacted to at Teast 95To of the maxirnum 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 footíng. Resistance to sliding at the bottoms of the footings can be calculated based on a coefficient of friction of 0.50. Passive pressure of compacted backfrll against the sides of the footings can be calculated using an equivalent fluid unit weight of 400 pcf The coefñcient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factors of safety should be included in thc design to lirnit the strain which will occur at the ultimate strength, particularly in thc case ofpassive resistance. Fill placed against the sides of the footings to resist lateral loads should be a 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 man-placed fill and topsoil, are suitable to support lightly loaded slab-on-grade construction. To reduce the effects of some difflerential 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 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 minus 2 nch aggregate with at least 50% retained on the No. 4 sieve and less than 2% passing the No. 200 sieve. Job No. I 14 I l3A eåEtecrr -7 - All fill materìals for support of floor slabs should be compacted to at least 95%o of maximum standard Proctor density at a rnoisture content near optimum. Required fill can consist ofthe on-site glanular soils devoid ofvegetation, topsoil and oversized rock. LINDERDRAIN 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 runoffl Frozen ground during spring runoffcan also 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 I foot below lowest adjacent finish grade and sloped at a minimum l%, to a suitable gravity outlet. Free-draining granular material used in the underdrain systern 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 l% feet deep. SITË GRADING The risk of construction-induced slope instability at the site appears low provided the building is located above the steep slope as planned and cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about I to l0 feet. Fills should be limited at the downhill side of the residence where the slope steepens. Embankment fills should be cornpacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefuily prepared by rernoving all vegetation and topsoil and Job No. 114 I l3A cåEtecn -8- compacting to at least 95Yo of the maximum standard Proctor density. The fill should be benched into the portions of the hillside exceeding 20io grade. Permanent unretained cut and fill slopes should be graded at2horizontal to 1 vertical or flatter and protected against ercsion by revegetation or other means. The risk of slope instability will be increased if seepage is encountered in cuts and flatter slopes may be neeessary. If seepage is encountered in permanent cuts, an investigation should be conducted to determine if the seepage will adversely afflect the cut stability. This offrce 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: 1) Inundation ofthe foundation excavations and underslab areas should be avo ided during construction. 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 954/o of the maxirnum 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 l2 inches in the first 10 feet in unpaved areas and a rninimum slope of 3 inches in the first 10 feet in paved. areas. Free-draining wall backfill should be capped with 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. LIMITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area aI this time. We make no wan'anty either Job No. I 14 I 134 c,äFtecrr -9 express or impliecl. The conclusions and recommendations submitted in this repoÍ are based upon the data obtainecl fì'om the exploratory borings drilled at the locations indicated on Figure 1, the proposecl type of construction and our ex¡rerience in the area. Our services do not include detennining tlie plesence, prevention or possibility of rnolti or other biological contaminants (MOBC) cleveloping in the future. If the client is concerned about MOBC, then a professional in this special freld of practice should be consultecl. Our findings inclucle interpolation and extrapolation of the subsurface conditions identified at the exploratory borings and variations in the subsurface conditions may not become evident until excavation is perfirnnecl. If conclitioris encountered during construction appear diff-erent li'om those described in this report, we should be notified so that re-evaluation of the recommenclations mav be made. This report has been prepared fol the exclusive use by our client for design purposes. We are not responsible for technical interpretations by others of our infonnation. As the project evolves, rve should provide continuecl consultation and field services during constructicln to revier,v and lnonitor the irnplernentation olour recommendations, and to veri$, that the recotnmendations have been appropriately interpretecl. Significant clesign changes may require adclitional analysis or modifications to the recornmendations ¡rresented herein. We recommencl on-site observation of excavations ancl founclation bearing strata and testing of structural fìll by a representative of the geotechnical engineer. Respectflilly Submittecl, HEPWORTH . PAWLAK GEOTECHNICAL, INC uis E. Eller Reviewecl by: DanielE. Harclin, P.E. LEE/ljg oô çla/, Job No. ll+ I l3A il,'iL Geótech - 10- REFERENCES chen-Northern, Inc., r99r, Preliminary Geotechnical Engineering sturly, proposed Aspen Glen Development, Garfield county, colorado, prepared for.Aspen Glen Company, dated December 20, 1991, Job No. 4 Llz92. Chen-Northern, Inc., 1993, Geotechnical Engineering Studyfor Preliminary Plat Desígn, Aspen Glen Development, Ga(ìeld counþ, colorado, prepared for Aspen Gren Company, datecl May 28, I 993, Job No. 4 112 92. Job No. tt4 1l3A c$teclr APPFOXIMATE SCALE 1" : 20',LOT M8 1 É¡.tÉt^É> ü1\tñ ffo*,nrÑffig I BORING 2 a I I I t \I \ LOT M46 I I BORING 1 a I LOT M45 I LOT M47I I BENCH MARK: GROUNDAT PROPERTY CORNEF; ELEV. : 100.0', ASSUMED. GOLDEN STONE 3A114 11 LOCATION OF EXPLORATORY BORINGS Figure 1 BORING 1 ELEV.:97,9' BORING 2 ELEV.:93.9' 100 100 9s 55112 WC=2,2 +4:57 -200:8 95 o o) LL Ico ı o LIJ 56!12 o) c) TL I Co (ú (l) ul90 WC:2.0 +4:56 -200:10 90 9A112 85 85 Note: Explanation of symbols ís shown on Figure 3. 3A114 11 LOGS OF EXPLORATORY BORINGS Figure 2 LEGEND: m FILL; sandy silt and clay with cobbles and small boulders, medium dense, moist, rnixed brown and red-brown, GRAVËI, COBBLES AND BOULDERS (GM-GP); sandy, slightly silty, dense, slightly moist, light brown, subrounded rocks. I Drive sample; standard penetration test (SPT), 1 3/8 inch l.D. split spoon sample, ASTM D-l586. 55/12 Drive sample blow count; indicates that 55 blows of a 140 pound hammer falfing 30 inches were required to drive the SPT sampler 12 inches. Practical refusal to auger drilling. î NOTES: 1. Exploratory borings were drilled on Aprit 14,2014 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 províded. 3. Elevations of exploratory borings were measured by instrument level and refer to the Bench Mark shown on Figure 1 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 approxímate boundaries between material types and transitions may be graduaf. ô. No free water was encountered in the borings at the time of drilling or when checked 43 days later. Fluctuation in water level may occur with tÍme. 7. Laboratory Testing Results: WC : Water Content (7") +4 : Percent retained on the No.4 sieve -20A : Percent passing No. 200 sieve LEGEND AND NOTES114113A Figure 3 TIME BEADINGS U.S, STANDARD SERIES? ¡"rR 15 MtN_60MlNr9MrN.4 MlN. 1 MtN. #2A0 *10A #50 #30 #16 #8 #4 CLEAR SQUARE OPENINGS 318' gl4u 1 U2" 3" 5"6' 152 127 8" CIz U) CN ù Fz LUOc I.IJo- Õ LUz F l"-rlE Fz Lu(.)E Lll o_ 10 20 30 40 50 60 70 80 90 100 r00 s0 BO 70 5rÌ 40 30 20 t0 0 .m2.001 .005 .0@ .019 .037 .O7A ,tso .3m .600 ¡,,18 2,36 4.7s 9.5 lz.s tg.o 37.5 76.2 DIAMEÏER OF PARÏCLES IN MILLIMETERS +.-+--_ +-t-+- - ---+ f+ ------.,i-I --------+--...--- 7 HB TIME READINGS 15 MlN. 60MtN1gMtN,4 MtN. 1 MlN. #200 #10û U,S. STANDARO SERIES #50 #30 #16 #8 CÔBBLES CLEAR SQUABE OPENINGS 3/8' A/4' 1 1/2" 3u 5'6" CLAY TO SILT GRAVEL 57 7o LIQUID LIMIT O/O SAMPLË OF: Slightly Sitty Sandy Gravet SAND 35 % S¡LTANDCLAY 8 % PLASTICITY INDEX % FROM: Boring 1 at2åFeet 90 80 CI704ØU)60Í Fsoñ () 40fi o_ 30 24 45 0 10 ô20 LU230 t- H40 F250 UJOE60 LIJrL #4 o 100 70 80 s0 100 20 10 0 .001 .AoZ .00s,009 .019 .097 .074 .1s0 .300 .600 1.18 2.96 4.25 DIAMETER OF PABTICLES IN MILLIMETERS 9.q2.519.0 37.5 76.2 1t52 2o3 FINE t I l. +tJ-|^,tt '# -¡+l._+-;-- 1- --f_-_+- CL,AY 1O SILI GRAVEL 56 % LIQUID LIMIT SAMPLE OF o//o SAND 34 0/" PLASTICITY INDEX coBELrs SILTAND CLAY 10 O/O o/o 2at?and 5 Feet CombinedGravelFROM: 3A114 11 GRADAT¡ON TEST RESULTS Figure 4