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HomeMy WebLinkAboutSubsoil Study for Foundation Design 05.05.2021,lc,l lfumar & Asmclates, lnc.@ Geotechnical and Materials Engineers and Environmental Scientists An Employcc Owncd Compony 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email : kaglenwood@kumarusa.com www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Sumrnit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 610 TRONBRTDGE' PHASE rrl RIVER BEND \üAY GARFIELD COUNTY, COLORADO PROJECT NO.2t-7-277 MAY 5,2021 PREPARED FOR: SCIB, LLC ATTN: LUKE GOSDA 0115 BOOMERANG ROAD, SUITE 52018 ASPEN, COLORADO 81611 luke. gosda@sunriseco.com TART,E OF CONTN,NTS PURPOSE AND SCOPE OF STUDY PROPOSED CONSTRUCTION SITE CONDITIONS SUBSIDENCE POTENTIAL... FItrLD EXPLORATION. SUBSURFACE CONDITIONS DESIGN RECOMMENDATIONS FOUNDATIONS FOUNDATION AND RETAINING WALLS. FLOOR SLABS.... UNDERDRAIN SYSTEM.... SURFACE DRAINAGE........ LIMITATIONS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - GRADATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS 1 1 -1 ,| a-L- -3 - -3 - -3- -4- -5- -5- -6- -6- FIGURB 1 - LOCATION OF EXPLORATORY BORINGS Kumar & Associates, lnc. @ Project No.2l-7-277 PURPOSE AND SCOPE OF STUDY This report presents the results ofa subsoil study for a proposed residence to be located on Lot íl,Ironbridge, River Bend Way, 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 SCIB, LLC dated March 16,202T. A held exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils obtained during the f,reld exploration were tested in the laboratory to determine their classification and other engineering characteristics. The results of the field exploration and laboratory testing were analyzedto 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 recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION Plans for the proposed residence were preliminary at the time of our study. The proposed residence is assumed to be a single-story wood-frame structure with attached garage. Ground floors could be a combination of slab-on-grade and structural over crawlspace. Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 4 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. 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 CONDITIONS The subject site was vacant at the time of our held exploration. The ground surface is sloping down to the northeast at an estimated grade of arouq$ 5 percenl A detention pond for storm watcr flows has bccn built just south of the lot. Vegetation consists of grass and weeds with landscaped trees near the roundabout. Kumar & Aggoclâtes, lnc. n'Projoct No,21-7-277 -2- SUBSIDENCE POTENTIAL Bcdrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge development. These rocks are a sequence 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 work in the area, several sinldroles were observed scattered throughorrt the Tronhridge development, These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork Valley. Sinkholes 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 atthe site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot 61 throughout the service life of the proposed residence, in our opinion, is low and similar to other lots in the area of similar subsurface profiles; however, the owner should be made aware of the potential f'or sinkhole development. If fuither investigation of possiblc cavitics in the bedrock below the site is desired, we should be contacted. FIELD EXPLORATION The field exploration for the project was conducted on March 22,2021. Two exploratory borings were drilled at the locations shown on Figure 1 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 & ^ --^^:^+^^ T-^¡Lùù\rul6lùçù, lllv. Samples of the subsoils were taken with a I%-inch I.D. spoon sampler. The sampler was driven into the subsoils at various depths with blows from a 14O-pound hammer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D-1586. The penetration resistance values are an indication of the relative density or consistency of the subsoils. 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. Kumar & Associates, lnc. @ Project No.21-7-277 -3 - SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils consist of about I to tYz feet of topsoil overlying dense, slightly silty sandy gravel with cobbles and probable boulders down to the maximum explored depth of 7 feet. A layer of clay was encountered in Boring 1 from I to l% feet deep. Drilling in the dense granular soils with auger equipment was diffrcult 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 a gradation analysis performed on small diameter drive samples (minus llt-inch fraction) of the coarse granular subsoils are shown on Figure 3. The laboratory testing is summarizedin Table 1 No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist. DESIGN RECOMMENDATIONS FOLI-NDATIONS Considering 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. The design and construction criteria presented below should be observed for l) Footings placed on the undisturbed natural granular soils should be designed for Based on experience, we expect settlement of footings designed ild;ıÑ¡dcted as discussed in this section will be about 1 inch or less. The footings should have aminimum width of 16 inches for continuous walls and 2 feet for isolated pads. 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 typically used in this area. 2) 3) rlrrl ;,t':., " ;i;ltì iil rrt tl l l [t¡',',r,rl] 1,;l iir'l ì irr\;ìi i ;irí,li: i, i,¡ i ii i: l,i lr i iij l,r,:l ì Kumar & Associates, lnc. @ Project No.2'l-7-277 -4- 4)Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an ultsupported length of at least 10 l'eet. Fuultlation walls ac[ing as retaining struotures should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. The topsoil, upper clay soils and any loose 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 observe all footing excavations prior to concrete placement to evaluate bearing conditions. FOL]NDATION AND RETAINING V/ALLS 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 50 pcf for backfill consisting of the on-site granular soils. Cantilevered retaining structures which are separate trom 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 computecl on the basis of an equivalent fluid unit weìght of at least 40 pcf for backf,rll consisting of the on-site granular soils. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffrc, construction materials and equipment. The pressures recommended above assume drained conditions behind the walls and a horizontal backfill surfacc. The buildup of water behind a wall or an upward slopilg liaukfill surfat c 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. Backfill should be placed in uniform lifts and compacted to at least 90o/o 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 95%o of the maximum standard Proctor density. Care should be taken not to overcompact the backfîll or use large equipment near the wall, since this could cause excessive lateral pressure on the wall. Some settlement of deep f'ounclation wall - backf,rll should be expected, even if the rnaterial is placed comectly, and could result in distress to facilities constructed on the backfill. s) 6) Kumar & Associates, lnc. @ Project No.2'l-7-277 5 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 friction of 0.50. Passive pressure of compacted backfill 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 values recommended above assume ultimate soil strength. 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 95o/o of the maximum standard Proctor 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-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 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-inch aggregate with at least 50o/o retained on the No. 4 sieve and less than2o/o passing the No. 200 sieve. The gravel layer beneath slabs-at-grade such as the garage, should consist of T¿-inch road base. All fill materials for support of floor slabs should be compacted to at least 95Yo 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. Clay soils stripped from the site should be used in non-structural graded areas. UNDERDRAIN 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 runoff. Frozen ground during spring runoff can create a perched condition. We recommend below-grade construction, such as retaining walls, crawlspace and basement areas (if any), be protected from wetting and hydrostatic pressure buildup by an underdrain system. Kumar & Associates, lnc. o Project No.21-7-277 -6- The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded abovc the invert level with fi'ee-draining granular rnaterial. 'Ilhe drain shoulcl be plaoecl at ea<;h level of cxuavatiort and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 102ó to a suitable gravity outlet or drywell. þree-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 IYzfeef deep. SLIRFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95Yo of the maximum standard Proctor density in pavement and slab areas and 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 to drain away from the foundation in all directions. We recommend a minimum slope of 6 inches in the first l0 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 filter fabric and capped with about 2 feet of the on-site finer graded soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5\ 'l qnrlcnqnina rr¡hinh ro^rriroo ¡ac,,lor hoo.r., iminofin- .h^"1'1 L^ l^^^+^'l o+ laao+rvYurrvo rvåursr rrwsvJ ¡¡rróqrrvrr r¡¡vulv vw rvw4lwg qt MùL 5 feet from foundation walls. LIMITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recommendations submittcd in this rcport arc bascd upon the data obtained from the exploratory borings drilled 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 Kumar & Associates, lnc, @ Project No.21-7-277 -7 - 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 subsurface conditions may not become evident until excavation is performed. If conditions encountered 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 recoÍrmendations, and to verifu 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 of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, Kumar & Associates, James H. Parsons, P Reviewed by: Steven L. Pawlak, P.E. JHP/kac lt 9(7 Kumar & Associates, lnc.6 Project No.21-7-277 ,Í / LQT 2'7 ( ì: I la. i.( t""" BENCHMARK: GRAVEL SURFACE AT CORNER STAKE EL. lOO' ASSUMED F¡i tfai iìt i -,4 BORING 1 ,l ,0T 60 LOT 61 d ,ci:tsìiti'o BORING 2 )6. I ?502550 APPROXIMATE SCALE-FEET 21 -7 -277 Kumar & Associates LOCATION OF TXPLORATORY BORINGS Fig. 1 I I I ñ BORING 1 EL. '1 01 .5' BORING 2 EL. 1 01' U U 20/6,50/s 50/ 12 F t¡J t¡.jt! I-Fo- L¡lo à 36/6,50/3 40/6,50/s 5 t- t¡Jt! LL I-f-È t¡Jô 10 10 LEGEND TOPSOIL; CLAY, SANDY, SCATTERED GRAVEL, ORGANICS, MOIST, FIRM, BROWN CLAY (CL); SANDY, HARD, MOIST, BROWN GRAVEL (GM); SANDY, SILTY, COBBLES, PROBABLE BOULDERS, DENSE, SLIGHTLY MOIST, MIXED BROWN AND GRAY. ROUNDED ROCK. I DRTVE SAMPLE, 1 3/8-INCH l.D. SPLIT SPOON STANDARD PENETRATIoN TEST 50/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 50 BLOWS OF A 14o-POUND HAMMER FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES. I PRACTICAL AUGER REFUSAL. NOTES 1. THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 22, 2021 WITH A 4-INCH DIAMETER CONTINUOUS-FLIGHT POWER AUGER. 2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY TAPING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFER TO THE BENCHMARK ON FIG. 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 APPROXIMATE 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); +4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6915); -2oO= PERCENTAGE PASSING No. 200 SIEVE (ASTM D11a0); WC=2.0 +4=62 -200=9 21 -7 -277 Kumar & Associates LOGS EXPLORATORY BORINGS Fig. 2 8 g HYDROMETER ANALYSIS SIEVE ANALYSIS U.S. SÍANOARÞ SERIES CLEAR SQUARE OPENINôS tltô t/A6 t t/tâ TIME READIN6S HR6 MIN 7 HNEts LIN I I / l l ì I i ¡ I I i =2 Íoo 90 80 70 50 40 30 20 fo o o f0 20 30 40 50 60 70 80 90 100 - .oo9 .019 .o57 .125 DIAM OF PARTICLES IN MI CLAY TO SILT COBBLES GRAVEL 62 % SAND LIQUID LIMIÏ SÂMPLE OF: Slightly Silty Sondy Grovol 29% PLASTICITY INDEX SILT AND CLAY 9 % FROM: Boring 2 @ 1 'ond 4' (combincd) Th€se lssl rssulls opply only lo lhe somples which were lesled. The l€sllng rôporl sholl nol bo roproduc6d, excopl ln full, wllhout lh6 wrlllenqpprovol of Kumor & Assgclqleg, lnc, Slov€ onolysls tosllng ls performed ln oooordonoo wlth ASTM D6913, ASTM D7928, ASTM C136 ond/or ASTM 01140, SAND GRAVEL FIN E MEDIUM COARSE FI NE COARSE 21 -7 -277 Kumar & Associates GRADATION TEST RESULTS Fis. 5 l(+rt$,'içli'.ffiî:Ëtr*'"'Ëü'*''TABLE 1SUMMARY OF LABORATORY TEST RESULTSSOIL TYPEATTERBERG LIMITSLIQUID LIMITUNCONFINEDCOMPRESSIVESTRENGTHPLASTICINDEXSlightly Silty Sandy GravelPERCENTPASSING NO.200 stEvE9SAND%tGRADATIONGRAVEL(%)(ocflNATURALDRYDENSITY2.02962P/"1NATURALMOISTURECONTENT| &,4Combinedlft)DEPTHSAMPLE LOCATIONBORING2No.21-7-277