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Home My WebLink AboutSubsoil Study for Foundation Design 02.22.2021rcrf iiçlfl'ff:ir:ffini'iiå*'" 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 Oftice Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 38, PHASE 3,IRONBRTDGE BLUE HERON DRIVE GARFIELD COUNTY, COLORADO PROJECT NO. 20-7-785 FEBRUARY 22,2021 PREPARED FOR: SCIB, LLC ATTN: LUKE GOSDA 0115 BOOMERANG ROAD, SUITE 52018 ASPEN, COLORADO 81601 luke.gosda@sunriseco.com TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY . PROPOSED CONSTRUCTION ... SITE CONDITIONS GEOLOGY FIELD EXPLORATION SUBSURFACE CONDITIONS ........ FOUNDATION BEARING CONDITIONS DESIGN RECOMMENDATIONS FOUNDATIONS FLOOR SLABS UNDERDRAIN SYSTEM SIIRFACE DRATNAGE... LIMITATIONS FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS FIGURE 5 - GRADATION TEST RESULTS TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS ..-2- -3- .) 3- 4 4 5 5 6 ..-6- I I I Kumar & Associates, lnc. @ Project No.20-7-785 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 38, Phase 3, Ironbridge, Blue Heron Drive, Garfield County, Colorado. The project site is shown on Figure 1. The pu{pose 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 December 3I,2020. Exploratory borings were drilled to obtain information 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 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 At the time of our study,lans for the residence had not The building is proposed within the building envelope shown on Figure 1. For the purposes of our analysis, we assume the proposed residence will be a wood frame structure over a crawlspace with an attached slab-on-grade garage. Grading for the structure will be relatively minor with cut depths between about 2 to 3 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 site was vacant at the time of our field exploration, and covered with about 6 inches of snow The lot slopes gently down to the northwest across the buildingarea and also drops moderately Kumar & Associates, lnc, @ Project No. 20-7-785 a down to the northeast about 5 t'eet to gently sloping, relatively natural terrain. Vegetation consists of sparse weeds. The downhill side of Blue Heron Drive appears to be a fill bench for residence construction placed during the subdivision development. The Roaring Fork River is located downhill about Yn mile to the north. GEOLOGY The geologic conditions were described in a previous report conducted f'or planning and preliminary design of the overall subdivision development by Hepworth-Pawlak Geotechnical (now Kumar & Associates) dated October 29,1991, Job No. 197 327. The natural soils on the lot mainly consist of sandy silt and clay alluvial fan deposits overlying gravel terrace alluvium of the Roaring Fork River. The river alluvium is mainly a clast-supported deposit of rounded gravel, cobbles, and boulders typically up to about 2 to 3 fcct in sizc in a silty sand matrix and overlies siltstone/claystone bedrock. Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge subdivision. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. A sinkhole occurred in the parking lot adjoining the golf cart storage tent in January, 2005 located about %mile to the south of Lot 38 which was backfilled and compaction grouted. To our knowledge, that sinkhole has not shown signs of reactivation such as ground subsidence since the remediation. Sinkholes possibly related to the Evaporite were not observed in the immediate area of the subject lot. Based on our present knowledgc of thc subsurface conditions at the site, it cannot be said for certain that sinkholes related to the underlying Evaporite will not develop. The risk of future ground subsidence on Lot 38 throughout the service life of the proposed building, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If fuither investigation of possible cavities in the bedrock below the site is desired, we should be contacted. FII,LD DXPLORATION The fielcl exploration for thc projcct rvas conducted on January 12,202L Tu'o exploratory borings were drilled at the locations shown on Figure I to evaluate the subsurface conditions Kumar & Associates, lnc. @ Project No. 20-7.785 -3 - 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. Samples of the subsoils were taken with I% inch and 2-inch LD. spoon samplers. The samplers were 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-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 returned 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 consist of about 3Yz feet of relatively dense, mixed clay, silt, sand and gravel fill overlying about 2 fo 4 feet of stiff, sandy silty clay. Dense, slightly silty sandy gravel with cobbles was encountered below the silty clay soils at depths of 6 to 8 feet down to the bottom of the borings of 16 feet. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and possible boulders. Laboratory testing performed on samples obtained from the borings included natural moisture content and density and gradation analyses. Results of swell-consolidation testing performed on a relatively undisturbed drive sample of the silty clay soils, presented on Figure 4, indicate low compressibility under light loading and a minor expansion potential when wetted under light loading. The laboratory test results are summarized in Table 1. No free water was encountered in the borings at the time of drilling the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS Spread footing foundations placed on the relatively dense fill soil above the natural silty clay soils or on the natural silty clay soils should be adequate for support of the proposed residence Kumar & Associates, lnc. o Project No.20-7'785 -4- with relatively low settlement potential. Although the silty clay soil sample showed a minor expansion potential when wetted, our experience in this area is that thc silty clay soils in this arca are typically not expansive. Footings bearing entirely on the dense gravel soils should have a low settlement risk. The bearing condition of the soils exposed in the excavation should be further evaluated at the time of construction. DESIGN RECOMMENDATIONS FOLINDATIONS 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 relatively dense fill soils or the natural silty clay soils if encountered. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the relatively dense fill soils or the underlying stiff silty clay soils should be designed for an allowable bearing pressure of 1,500 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. Post-construction settlement could be around 1 inch mainly if the bearing soils are wetted. 2) The footings should have a minimum width of 18 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 typically used in this area. 4) Continuous foundation walls should be heavily 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 a lateral earth pressure coffesponding to an equivalent fluid unit weight of at least 55 pcf for the onsite soils as backfill. 5) Any topsoil and loose disturbed soils should be removed and the footing bearing level extended down to the relatively dense filI soils or to the stiff silty clay soil Kumar & Associates, lnc. o Project No.20.7.785 -5- beneath the fill. The exposed soils in footing area should then be moistened and compacted. Additional structural f,rll can consist of the onsite soils compacted to at least 98o/o of standard Proctor density atnear optimum moisture content. New structural flrll should extend laterally beyond the footing edges a distance equal to at least one-half the fill depth below the footing. A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FLOOR SLABS The 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 road base gravel should be placed beneath garage level slabs. This material should consist of minus 2-inch aggregate with at least 50o/o retained on the No. 4 sieve and less than l2Yo passing the No. 200 sleve. 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 onsite soils devoid of vegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM The proposed shallow (less than 4 feet) crawlspace and slab-on-grade garage should not require a perimeter underdrain system provided that the site grading recommendations contained in this report are followed. We recommend that below-grade construction, such as retaining walls, deep crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. If installed, 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 6) Kumar & Associates, lnc, @ Project No. 20-7-785 -6- placed at each level of excavation and at least 1 foot below lowest adjacent finish gracle and sloped at a minimum lYo to a suitable gravity outlet or drywell based in the gravel soils. 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 shoulcl be at least lYz feet deep. An impervious membrane such as 30 mil PVC should be placed beneath the drain gravel in a trough shape and attached to the for¡nclation wall with mastic to prevent wetting of the bearing soils. SURFACE DRAINAGE Proper surface grading and drainage will be critical to prevent wetting of the bearing soils and satisfactory performance of the foundation. The following drainage precautions should be observed during construction and maintainecl at all times after the resiclence has been completecl: 1) lnundation ofthe 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 95o/o of the maximum 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. W.e recommend a minimum slope of 12 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 (if any) should be capped with about 2 feet of the on-site soils to reduce surface water infiltration. Graded swales should have a minimum slope of 3%. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation should be located at least l0 feet from foundation walls. Consideration should be given to use of xerigcape to reduce the potential for wetting of soils below the building caused by inigation, LtMI'l'^'t'toNs This study has been oonduotod in accordance r,vith generally accepted geotechnical enginccring principles and practices in this area at this tirne. We rnake no warranty either express or implierl. Kumar & Associates, lnc. @ Project No. 20-7-785 -7 - 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 subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appeü 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 veri$ 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, [nc.l# David A. Noteboom, Staff Engineer Reviewed by: Steven L. Paw SLPlkac ,l CT'15222 Kumar & Associates, lnc.6 Project No. 20.7.785 ;il .iì LOT 39 i:t¡.:i Ì¡i,a'Ì]iì1 arrrlt BORING 2 LOT /ti:-.4 :,::-! I ï -.1 il¡ BENCHMARK: AT SEWER MANHOLE RIM EL. lOO' ASSUMED "'. .:: I ii ¡,r. I -i_ì." ê¿ È-0g t*, o+rÞt 20 rf.a) .Jt LOT 38 APPROXIMATE SCALE_FEET 0 20-7 -785 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig.1 ò ¿ e € {l I BORING 1 EL. 99.0' BORING 2 EL. 98.5' 0 0 27/12 1e/12 5 e/12 WC=8.4 -200=58 17 /12 WC=9.3 -20Q=64 5 11/12 tNC=14.2 DD= 1 08 s8/ 12 t- LJt!tL I :EFfL t¡Jô F t¡J L¡l LL I :EF- o_ LdÕ 10 10 47 /12 s8/12 15 1538/6, 50/5 52/ 12 20 20 WC=2.2 *4=51 -200= 1 5 20-7 -785 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2 Ê '{t ò LEGEND N TOPSOIL: CLAYEY SILTY, SAND, ROOTS AND ORGANICS, FIRM, MOIST, BROWN. FILL: SILTY CLAYEY SAND AND GRAVEL, MEDIUM DENSE, SLIGHTLY MO|ST, BROWN. 7tL/tlrl CLAY (CL): SANDY, SILTY, WITH SCATTERED GRAVEL, STIFF TO VERY STIFF, SLIGHTLY MOIST, BROWN. GRAVEL (GM-GP): SANDY, StLTy, DENSE, SL|GHTLY MO|ST, BROWN. ROUNDED ROCK DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE i DRIVE SAMPLE, 1 5/8-|NCH t.D. SpLtT SPOON STANDARD PENETRATTON TEST t-7 /1t DRIVE SAMPLE BLOW COUNT. INDICATES THAT 27 BLOWS OF A '140-POUND HAMMER FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES. NOTES 1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 12,2021 WITH A 4-INCH DIAMETER CONTINUOUS-FLIGHT POWER AUGER. 2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY INSTRUMENT 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 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)i DD = DRY DENSITY (PCt) (NSTU D2216);+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ISTV OOSIS); -200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM 011a0); 20-7 -785 Kumar & Associates LEGTND AND NOTES Fig. 3 I SAMPLE 0F; Sondy Silty Cloy FROM:Boringl@-7' WC = 1 4.2 %, DD = 108 pcf .l-, I l l i i of EXPANSION UNDER CONSTANT PRESSURE UPON WETTING l'- ; i'l iliirìi I .: L ) l l 1 i I I .l I l l l l i t I I I I ii -1_ ; :-+- l'1-' i 1 l ì-- r i I ii l l i ì - . i Ìr: :i 1 JJ t¡J =(r, I z.otr o =o U)z.o(J 0 -1 -2 -3 -4 1.0 APPLIED PRESSURE - KSF 10 100 20-7 -785 Kumar & Associates SWELL_CONSOLIDAÏION TEST RESULTS Fig. 4 E 3 I HYDROMETER ÀNALYSIS SIEVE ANALYSIS U.S. STANDARO SERIES CLEAR SQUARE 'IME FEADINGS 7 HRSHRS --- ]_l - f t00 90 80 70 60 60 10 30 20 to o o 'to 20 50 10 50 60 70 80 90 roo ? .ot7 .075 .125 2,O DIAMETER OF CLAY TO SILT COBBLES GRAVEL 51 % SAND LIQUID LIMIT SAMPLE OF: Sllly Sondy Grovel 34% PLASTICITY INDEX SILT AND CLAY 15 % FROM: Boring 2 @ 7' &. 10' (Comblned) Thoso losl rosulls qpply only lo lh€ eqmplee whloh w€ro l.sl€d. Thr losllng roporl sholl nol br rrproduc.d,trurpl lr full, wlllruul llr¡ wrlllcrr opprovql ol Kumor & Assoclolos, lnc. S¡éva ôtlôlysls tðllll1o 13 Þô{òrñ€d ln occordonco wlth ASTM D6915, ASTM D7928, ASTM C156 qnd,/or ASTM Dt1,l0. SAND GRAVEL FINE MEDIUM COARSE FINE COARSE 20-7 -785 Kumar & Associates GRADATION TEST RTSULTS Fis. 5 lcrt Kurmr & Asmciates, lnc.@ Geotechnical and Materials Engineers and Environmental Scientists TABLE 1 SUMMARY OF LABORATORY TEST RESULTS No.20-7-785 AÏERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH PERCENT PASSING NO. 2()() SIEVE LIQUID LIMIT PLASTIC INDEX SOIL TYPE 58 Sandy Silty Clay Sandy Silty Clay 64 Sandy Silty Clay with Gravel 15 Silty Sandy Gravel SAND (:/"1 34I5 GRADATION (:/,1 GRAVEL locfl NATURAL DRY DENS]TY 108 9.3 2.2 t%l NATURAL MOISTURE CONTENT 8.4 t4.2 (ft) DEPTH 4 7 4 7 andI0 combined SAMPLE LOCATION BORING 1 2