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HomeMy WebLinkAboutSubsoil Study for Foundation Design 11.23.2020t(t tmimm*#ü'*" Ân Ëmployoc Otmad Compony 5020 County Road 154 Glenwood Springs, Cû 81601 phone: (970) 945-7988 fax: (9?0) 945-8154 emai I : kagleirwood@;kumarusa.corn www.kuma¡usa.co¡l ûflice Loc¿lions: Denver (HQ). Parl,rer, Color.ìdo Springs. Forl Collins. Clenwood Springs, and Summ:it County, Colorado SUBSOIL STUDY FOR FOTJNDATION DESIGN PROPOSED RESIDENCE LOT WP-9, ASPEN GLEN MIDLAND LOOP GARFIELD COUNTY, COLORADO PROJECT NO.20-7-674 NOVEMBER 23, 2020 PREPARED FOR: MICIIAEL AND BARBARA GERBER 138 CLUB LODGE, ASPEN GLEN CÀRBONDÄLE, COLOR.ADO 81623 Gcrucrr@&eçatn) TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY 1 PROPOSED CONSTRUCTION ....- 1 - SITE CONDITIONS......,....I SUBSIDENCE POTENTIAL DESIGN RECOMMENDATIONS .........,.......- 3 . FOUNDATIONS FIELD EXPLORATION SUBSURFACE CONDITIONS I,'NDERDRATN SYSTEM SURFACE DRAINAGE... LtMrrA'ïlÖNs FIGURE 1 . LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATÕRY BORINGS FIGURE 3 - GRADATiON TEST RESULTS TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS -, - ....- 2 - -?- FOTINDATION AND RETAINING V/ALLS ...............- 4 - FLOOR SLABS .......5 - -{- ..............- 6 - -6- Kumar & Associates, lnc. {i'Froiect No. 20-7-674 PURPOSE AND SCOPE OF STUDY This report prcsents the results of a subsoil study for a proposed residence to be located on Lot WP-9, Aspen G1en, Midland Loop, Garfield County, Colorado. The project site is shown on Figure l. The purpose of the study was to develop recornmendations for the foundation design. The study was conducted in accordance with our proposal for geotechnical engineering services to Miohael and Barbara Gerber dated November 4,?020. A fìeld 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 Iheir classification, gradation and other engineering characteristics. The results of the field exploration and labcratory testing were analyzed to develop recommendations for lbundation types, depths and allowable pressures for the proposed building foundation. This report summarizes the data obtained during this stucly and prcsents our conclusions, design recommendations and other geotechnical engineering considerations based on the prclposed construction and the subsurface couditions encountered. PROPOSED CONSTRUCTION The proposed residence will be a 1 to 2 story structure with an attached garage located as shown on Figure l. Cround floors will be structwal over crawlspace in the living area and slab-on- grade in the garage. Grading fcr the strucfure is assurned to be relatively minor with cut depths between about 3 to 6 tbet. We assume relatively light foundation loadings, typical of the proposed type of construction. lf building loadings, location or grading plans change significantly frorn those described above, we sirould be notified to re-ev¿luate the recorrmendations contained in this repoÍ. SITE CONDITIONS The lot was vacant at the time of the field exploration. The ground surface is gently sloping down to the northeast with about 5 feet of elevation change across the building fooþrint then relatively steep down to the northeast as indicated by the contour lines shown on Figure l, Vegetation consisted of grass and weeds. Kumar & Associates, lnc- 6'Project No. 2A-7'674 -2- SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen development. These rocks are a sequenre ofgypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsurn and limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie porlions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in the area, several sinlcholes were observed scattered throughout the development, mostly east of the Roaring Fork River. The closest mapped sinkhole is located about 600 feet southwest of Lot WP-9. These sinkholes appeff sirnilar to others associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork River valley. Sinklroles were not observed in the immediate area of the 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 WP-9 throughout the service life 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. FIELD EXPLORATION The field exploration for the project was conducted on November 10,2020. Two 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 Kumar & Associates. Sarnples of the subsoils were taken with a 1%inchl.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 penetration resistance values are an indication of the relative density or consistency of the subsoils. Depths at which the samples were take,n and the penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The samples were reh¡med to our laboratory fbr review by the project engineer and testing. Kurnar & Associates, lnc. ¡,Project No, 2A-7-674 -3- SUBST]RFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils encountered, below about Yzfoot of topsoil, consist of relatively dense, slightly silty sandy gravel and cobbles with probable boulders to the maximum drilled depth of 7Yz feet. 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 contett and gradation analyses. Results of gradation analyses performed on small diarneter drive samples (minus l%-inch fraction) of the coarse granular subsoils are shown on Figure 3. The laboratory testing is summarized in Table L No free water was encountered in the borings at the tirne of drilling and the subsoils were slightly moist. DESIGN RßCOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, rve recomrne¡rd the building be founded with spread footings bearing on the natural granular soils. The design and construction criteria presented below should be observed fcrr a spread footing foundation system. 1) Footings placed on the undisfurbed granular soils should be designed for an allowable bearing 3,000 ootings placed within 10 feet of the steep downslope should be an allowable bearing of 2,000 psi Based on experience, we expect settlement of footings constructed as discussed in this section willbe about 1 inch or less. 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 adequatesoilco2ffi|ngelevationforfrostprotection.Placenrent of foundaticns dt least 36 inches bellw exterior grade is typically used in this area, \-----l Kumar & Associates, lnc, er Project No. 20-7-614 -4- 4)Continuous foundation 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 discussed in the "Foundation and Retaining Walls" section of this report. The topsoil and any loose or disturbed soils should be removed ancl the footing bearing level extended down to the relatively dense natural granular soils. Thc exposed soils in footing area should then be moistened and compacted. A representative of the 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 arelaterally 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 50 pcf for backfdl consisting of the on-site gtanular 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 laterai earth pressure computed on the basis of an equivalent fluid unit weight of at leûst 4û pcf for backfill consisting of the on-site granulnr soils. Backfiil should not contain any organics, clay soils or rock larger than about 5 inches. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffrc, construction rnaterials and equiprnent. The pressures ¡ecommended 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 skucture. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Backfill should bc placed in uniform lifts and compacted to at least 90% of the rnaximum standard Proctor density at a moisture content near optimum. Baskfill placed in pavement and walkway areas should be compacted to at least 95% of the maximum standa¡d Proctor density. Care should be taken not to overcompact the backfill or use large equipment near the wall, since this oould causs excessive lateral pressure on lhe wall. Sorne settlement of deep foundation wall backfill should be expectedr even if the material is placed correctly, aud could result in distress to facilitics constructcd on tlrc hackfill. 5) 6) Kumar & Associater, lnc. ø Project No. 20-7-614 -5- The l¿teral 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 0.50. Passive pressure of compacted baokfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 400 pcf. The coefficient of friction and passive pressure vaiues rçcornmended above assume ultimate soil shength. Suitable factors of safety should be included in the design to limit the strain which will oçcur 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 95Vo af tlrre rnaximum stândard Prootor 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-gracle conskuction. To reduce the effects of some differential ûrovement, floor slabs shoulcl 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 requirernents forjoint spâcing 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 materiai should gonsist of minus 2-inoh aggregate with at least 50% retained on the No. 4 sieve and less than?o/o passing the No. 200 sievc. All fili mateúals for support of floor slabs should be compacted to at least95Vo 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. TINDERDRAIN SYSTEM Although free water was not encountered during out 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 creâte a perched condition. lVe 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 backfiil surrounded above the invert level with lìse-draining granular material. The drain should be placed at each level of Kumar & Associates, lnc. o Project No, 20-7-674 -6- excavation and at least I foot below lowest adjacent ñnish gade and sloped at a minimum lo/a Io a suitable gravity outlet, sump and pump or perforated sump/drywell. 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 1% feet deep. SURFACE DRAINAGE The following drainage precautions should be observed dr.ring construction and maintained at all tirues after the residence has been completed: 1) Inundation ofthe foundation excavations and underslab areas should be avoided during construction. 2) Exterior backñll should be adjusted to near optimum moisture and conpacted to at least 95% of the rnaximum 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 *om the foundation in all directions" We recommend a minimum slope of 6 inches in the first 10 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 filtsr fabric and capped with about 2 feet of the on-site üner grained 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 accorda¡rce with generally accepted geotechnical e,lrgineering principles and practices in this arca at this time. TVe make no waranty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the exploratory borings drilled at the locations indicated on Figure 1, 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 variations in the subsurlace Kumar & A$soc¡ates, lnc, +,r Ptoject No. 2g-7-674 -7 - conditions may not become evident until excavation is performed. If conditions encountered during construction appear difierent from those described in this report, we should be notified so that re-evaluation of the recommsndations may be made. This leport has been prepared for the exclusive use by our client for design purposes, lVe are not responsible for technical interpretations by others of our information. As the project evolves, vre should provide continued consultation and field services during construction to review and monitor the implernentation of ourrecommendationso and to veri$ that the recommendations have been appropriately interpreted. Significant design changes rnay require additional analysis ormodifications to the recommendations presented herein. We recommencl on-site observation of excavations and foundation bearing strata and testing of strucrural fiIl by a representative of the geotechnical engineer. Respectfu lly Submitted, Kumar & ÂssocÍate$. Inc. Steven L. Pawlak, P.E. Reviewed by: Daniel E. Hardin, P.E. SLPlkac Cc: Warren Palmer foahnerarchfa.qmail.c ) George Shaver lgeorges haver 1 -8 3 6@ gmail. com ) Kumar & Associates, lnc. e Project No. 20.7.474 ô¡vQ N25'09'56" ilr /32.73'I _--995. . l '- -'-.'l \.) Ï r.9¡" ì"- È ci,i l*ì JOr+ I " i{ "ii= "-\*' (o \) s-\a \ a (s) È i w.- t't:t .- iJ' (g \t L) ,ät Â-\s a, ^-\$ 1 000 I N2g"05'55"r'F.'\ ^r'Æ[ .4q 4. 96' L:37.05' 4/'/ 4.50 BAçK OF rv.ALr{ IITÐLAN' LOOP 3 15.f)50 APPROXIMATE SCALE-FEET 2A-7-67 4 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1 BORING 1 EL. 999.5' BORING 2 EL. 996' 0 U f- l¡J LJtL I:t F* o_ L¡JÕ 77/6,3516 50/5 WC=0.6 +4=67 -200=8 ,- l¿J L!]L I-l- L LUo 5 50/3 tr 4a/6, 50/s WC=1.0 +4=51 -2O0=9 10 10 NOTES TOPSOIL; ORGANIC SANDY SILT AND CLÀY, 0ÀRK BRCIWN. Flo*ourr AND coBBLES (GM-op); sLTGHTLy srLTy, sANÞy, pRoBABLE BouLDERS, DENsE, F#lslrourtv Motsr, BRowN, RoUNDED RocK. I ÐRtvE SAMPLE, 1 3/8-INCH 1.0. SPLÍT SPOON STANDARD PENETRÄTION TEST. .¡"7 7¿ oBtyt SAMPLE BLOW COUNT. ¡¡IDICATES T||AT 27 BLÐWS 0F A 140-PCIUND HAMMER.,, " FALLI¡¡G 50 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 6 INCHES. I PRACTICAL ÀUGER REFUSAL. LEGEND 1. THE EXPLORATORY BORINGS WERE ORILLEÐ ON NOVEMBER 10,2O2O WITH A 4_INCH-DIÀMEIER CONTINUOUS-FLIGHT POWER AUGER. 2. THT LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATËLY BY PACING rROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. J. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INÏERPOLATION BETIVEEN CONTOURS ON THE SITE PLAN PROVIDED. 4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONS'ÐERÊD ACCURATE ONLY TO lHE ÐEGREE IMPLIED BY THE METHOD USED. 5. THE LINES BTTWEEN MATERIALS SHOWN QN THE EXPLORÀTORY BORING LOGS REPRESENÍ THE ÀPPROXIMATE BOUNDÂRIES B'TWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL. 6. CROUNDWATER WAS NOT ENCOUNTERTD IN THE BORINCS AT THE ÏIME OF DRILLING. 7. LASORATORY TEST RESUITS: WC : WATER CONTENT (%) (ASTM T2216}; +4 = PERCENTÂGE RETAINED ON NO. 4 SIEVÊ (ASTM D6913); _ZQQ= PERCENTAGE PASSING NO. 2OO s¡EVE (ASTM Dl 140), 20-7 -67 4 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2 HYDROMETER ANALYSIS SIEVE ANALYSIS rIMå REÂOI{6 HRs 7 HRSvtN ¡ 5 NtN 60MlN r¡ttñ 4[tN lMtfl U.¡. sl,tNDÂFð 55R¡Ss SAND SQIJARf, OPENIÍGS IrlOO I5O '4A t3O lrs i¡lo tO |1 5/8' '/." t 3" 5"6'o lo 2ç 30 40 50 60 ,a ao o to 20 5û æ 50 60 --,: ZO IE p E Id2 100 90 80 70 60 50 ß !o 20 100 90 ôo 70 60 30 10 l0 r - - -J ir Hf,S 7 HNS MÑ ,rrr i. .- .-,,,..-.r.,,J-,r l ¡,¡.1.r---. -, -il---.t-,..rt--r,!8 2,36 1.75 9.5 t9 3E.l 2.O CLAY TO SILT FINE GRAVEL 51 X . SAND 40 LIQUID LIMIÏ SAMPTE OF: Sllghtly Sllty Sondy Grovol TIYE RÉÆIN6S I{EDIUM COARSE FINE I coARsE PLASTICITY INDEX U.S. SNDARD SERIES lt6ó i,5o 1.0 l,¡o flô fro ra GRAVEL SIIT AND CLAY 9 % FROM:Boringl0S' ctEAN SQUARE OPEI{INCS t 3r'a" 5/1" t COEBLES !i s"6 ú{, a ñ Þ Þr o J- l -f ,1-1 t.1-[ -,. ,1,,---.t-:J! J--i-.1-l-1j-: ,,---.o57 ,O75 .15o ,too i .600 1,18 CI.AY TO SILT CRAVEL 67 X SAND 25 LIQUID LIMIÏ SAMPLE OF: Slightly Silfy Sondy Crov.l PLASTICITY INBEX FROM:BorlngZA2.5' S¡LT AND CI-AY 8 % COBBLSS Thes. llsl Esull! opply only lo lh¡ romples whlch *q¡e lc¡led. tho lodlng Þporl ehcll not bq reproduo.d, .xoêpl ln rull, vllhoul lh. wrltl6n oÞpÞvol ol l(umor & Aósûclol.a, lnc. Slry. lnolysl¡ lællng lr Þrlormod ¡n oooordonoo wllh ÀS'fll D6el5, AETM D7928, ÄSl[¡ Cl56 ond/or ASfll 01140. HYOROMETER ANALYSIS SIEVE ANALYSIS SAND GRAVEL COARSEFINEMEDIUM iCOARSE FINE GRADATION TEST RTSULTS Fig. 32A-7 -67 4 Kumar & Associates l(+rtffirffü**TABLE 1SUMMARY OF LABORATORY TEST RESULT$No.20.7.674SOILÏYPESlightly Silty Sandy GravelSlightiy Silty Sandy Gravel(¡stluilcot¡FlilEDcot pREssfi,ESTREIIGTHfYolPLASTICIIIDEXATTERBERG LIIITÍSLrQuþ UfüffFAIPERCENÏPASSTNG r{0.ã,0stEvEIISAI¡Dli/,|4025(%)GRAVEI5167IIATURALDRYDENSTTY(octìloloìI'IATURAtilOISTUREcolrTE}{T0.62Y,ffrlDEPI}I1.05ISAIPLE LOCATIOiIBORITIG2