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HomeMy WebLinkAboutSubsoil Studyl(+rtirJþilfi'åìÌ'fË:i'[iË;å'*" An Employcc Owncd Compony 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 ernail : kaglenwood@kumarusa.com wwwkumarusa.com Office Locations: Denver (HQ), Parke¿ Colorado Springs, Fort Collins, Glenwood Springs, and Surnmit County, Cololado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT H-8 47 SPIRE RIDGE WAY ASPEN GLEN SUBDIVISION GARFIELD COUNTY, COLORADO PROJECT NO. 21-7-852 JANUARY 17,2022 PREPARED FOR: SMITH MOUNTAIN BUILDERS ATTN: ZACHSMITH 1011 HERITAGE DRIVE CARBONDALE' COLORADO 81623 smith mountainbuilders@.sm ail.com TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY PROPOSED CONSTRUCTION SITE CONDITIONS SUBSIDENCE POTENTIAL... FIELD EXPLORATION... SUBSURFACE CONDITIONS FOUNDATION BEARING CONDITIONS DESIGN RECOMMENDATIONS . FOUNDATIONS FLOOR SLABS IINDERDRAIN SYSTEM .... SURFACE DRAINAGE............... LIMITATIONS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS 1 a 3- 6- -3 - -J- -3- 5 5 -6- FIGURE I - LOCATION OF EXPLORATORY BORINGS 1 1 1 Kumar & Associates, lnc. @ Project No. 21-7-852 PURPOSE AND SCOPE OF STUDY This report presents the results ofa subsoil study for a proposed residence to be located on Lot H-8 within Aspen Glen Subdivision, 47 Spire Ridge Way, Garfield County, Colorado. The project site is shown on Figure 1. The purpose of the study was to develop recommendations for foundation design. The study was conducted in general accordance with our agreement for geotechnical engineering services to Smith Mountain Builders dated October 28,2021 A freld 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 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 Development plans for the proposed residence were not available at the time of our study. V/e assume similar construction to that in the area which typically consists of a I or 2-story structure with an attached garage. The ground floors will be structural over crawlspace or slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 5 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 and vegetated with grass and weeds at the time of our field exploration. The terrain ranges from relatively flat to gently sloping down to the east, north, and west with around 3 feet of elevation difference across the general building envelope. Spire Ridge Way is to the Kumar & Associates, lnc, @ Project No. 21-7-852 1 east, a vacarÍ lot is to the south, Aspen Glen golf course is to the west, and a single-family residence is to the north. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian-age Eagle Valley Evaporite underlies the subject site. 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, sinkholes have been observed scattered throughout the lower Roaring Fork Valley. The nearest mapped sinkhole is about 1,000 feet southeast of this lot. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in this area. 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 at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot H-8 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 I0,202I. 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 & Associates. Samples of the subsoils were taken with l%-inch and 2-inch I.D. 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 Kumar & Associates, lnc. @ Project No.21-7-852 -3 - 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, below about I foot of topsoil, consist of about 4/zto 6/zfeet of stiff, sandy silty clay, underlain by dense, silty cobbly sandy gravel with probable boulders. Drilling in the dense, coarse granular soils with auger equipment was difficult due to the cobbles and probable small boulders and drilling refusal was encountered in the deposit at depths of about 9 and 1 1 feet. Laboratory testing performed on samples obtained from the borings included natural moisture content and swell-consolidation. Results of a swell-consolidation testing performed on a relatively undisturbed drive sample of the clay soil, presented on Figure 4, indicate low compressibility under existing low moisture conditions and light loading and moderate to high compressibility after wetting under increased loading. The natural moisture content and relatively low dry density indicates the clay soils may be prone to settlement (collapse potential) when wetted. Our experience in this area indicates the soils typically have a low collapse potential when wetted under load. The laboratory test results are summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The natural sandy silty clay soils within about the upper 5%to7/zfeet are typically low density and compressible especially when wetted. The underlying coarse granular soils possess a moderate bearing capacity and a relatively low settlement potential. At assumed excavation depths, we expect the subgrade will expose sandy silty clay soils and, possibly, sandy, cobbly gravel soils. Spread footings should be feasible for foundation support of the residence with a risk of settlement if the clay soils become wet. DESIGN RECOMMENDATIONS FOUNDATIONS 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 Kumar & Associates, lnc. o Project No, 21-7-852 -4- on the natural soils or properly compacted structural fill. The bearing level could be extended through the clay soils down to the underlying dense, coarse granular soils to reduce the settlement potential and should be fuither evaluated at the time of construction. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural clay soils should be designed for an allowable bearing pressure of 1,500 psf. Footings placed entirely on the natural granular soils or compacted structural fill should be designed for an allowable bearing pressure of 3,000 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. There could be additional settlement of around lzto I inch if the clay bearing soils are wetted. 2) The footings should have a minimum width of 18 inches for continuous walls and 2 feel 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 feef. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressure corresponding to an equivalent fluid unit weight of at least 55 pcf for the on-site fine-grained soil as backfill. 5) The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the natural soils. The exposed soils in footing area should then be moistened and compacted to a minimum of 95Yo of the standard Proctor density. If needed, structural hll used to reestablish design footing bearing level in sub-excavated clay soil areas should consist of imported 3/o-inch road base and extend at least I% feet beyond footing edges and be compacted to at least 98% of standard Proctor density at near optimum moisture content. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. Kumar & Associates, lnc. @ Project No. 2'l-7-852 5 FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction with the accepted risk of movement with clay soil subgrade. The risk of movement can be reduced by placing slabs-on-grade on a minimum of 2 feet of compacted structural fill or by using structural floors over crawlspace, which is commonly done in the area. The structural fill should consist of CDOT Class 5 or 6 base course material. 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 sand and gravel base course should be placed beneath floor slabs-at-grade for support. This 4-inch thickness can be included in the recommended2 feet of base course below the slabs. A minimum 4-inch layer of free-draining gravel should be placed beneath basement level slabs (if any) 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 than 2Yo passingthe No. 200 sieve. 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, below the recommended depth of base course, can consist of the on-site soils devoid of debris, topsoil and oversized rocks þlus 4-inch). I.INDERDRAIN 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 also create a perched condition. We recommend below-grade construction, such as retaining walls and deep crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. An underdrain should not be placed around shallow crawlspace areas to help limit the potential for wetting the bearing soils. 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 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to Kumar & Associates, lnc, @ Project No. 21-7-852 -6- a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2o/o passingthe 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 llz feet deep. SURFACE DRAINAGE Providing proper surface grading and drainage will be very important to limiting potential wetting of the bearing soils and potential building movement and distress. 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 95o/o of the maximum standard Proctor density in pavement and slab areas and to at least 90To 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. V/e 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. 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 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 submitted in this report are based 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 presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is concemed 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 Kumar & Associates, lnc, o Project No, 21-7-852 -7 - 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 recoÍlmendations 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 recommendations, and to verifu that the recoÍrmendations 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. fnc. Mark Gayeski, Reviewed By: Daniel E. DEHlkac T e:r(>øf> Kumar & Associates, lnc.'l Project No.21-7-852 \ \ '.E BORING 1o Êi Ë ,ìt Ét È 6ocÞ ""é åi! ê ) ,606Àt 'ts Þo, \l ¡ )-: tì rÀ r,i s qô.,4t % ':,:" BORING 2o ""d, t.'tr 1.,''l i'.'. [. 1 ''I 4 uñ' %o, -! 6 I*:: ! t ï-- .it ¡t '-È :È ':. : i¡. 15 0 15 30 APPROXIMATE SCALE_FEET I s \\ ¡\ s 21 -7 -852 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1 ı .; É BORING 1 EL. 6063.5' BORING 2 EL. 6062' 0 0 5 à !- LJ L'JtL I-t"- o_t¡lo 13/ 12 WC= 1 0.0 DD=92 11 /6, 35/6 F- t¡J t¡JtL ITF o_ LJÕ 50/ 4.s 10 1050/5.s 15 15 21 -7 -852 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2 9 I I Ê ¡ 8,,' JIrÌ ,i 5'í 5s LEGEND TOPSOIL; CLAY, SANDY, SILTY, FIRM, MOIST, BROWN TO DARK BROWN CLAY (CL); SILTY, SANDY, STIFF, SLIGHTLY MolST, BROWN, TRACE POROSITY AND TRACE CALCAREOUSNESS. GRAVEL (GM); SANDY, SILTY WITH COBBLES AND PROBABLE SMATL BOULDERS, DENSE, SLIGHTLY MOIST, TAN AND LIGHT GRAYISH-TAN. DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE. i DR|VE SAMPLE, 1 5/8-rNCH l.D. SPLIr-SPOoN STANDARD PENETRATION TEST A2I ^ DRIVE SAMPLE BLOW COUNT. INDICATES THAT 15 BLOWS OF A 14o-POUND HAMMERtr/ tz FALLTNc Jo TNcHES wERE REQUIRED To DRtvE THE sAMpLER 12 tNcHES. I PRACTICAL AUGER REFUSAL. NOTES THE EXPLORATORY BORINGS WERE DRILLED ON NOVEMBER 10, 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. 5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN PROVIDED. 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 TRANSIÏIONS 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); DD = DRY DENSITY (PCt) (ISTV D2216), 21 -7 -852 Kumar & Associates LEGEND AND NOTES Fig.3 E ò I SAMPLE OF: Sondy Silty Cloy FROM:Boringl@5' WC = 1O.O %, DD = 92 pcf NO MOVEMENT UPON WETTING in tæt rdultg tdt d. ftg w¡th 1 J) t¡J =tt1 I z.otr ô =o t/1zo() 0 -l -2 -5 -4 -5 -6 -7 t PRESSURE - KSF 10 100 21 -7 -852 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 4 I (+rt irffi [åifËfni':,Í å *' "TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.2l-7-852Sandy Silty ClaySOIL TYPElpsf)UNCONFINEDCOMPRESSIVESTRENGTH("/"1PLASTICINDEXATTERBERG LIMITS(ololLIQUID LIMITPERCENTPASSING NO.200 stEvESAND(%)GRADATION$tGRAVELlpcf)NATURALDRYDENSI'fY10.092IololNATURALMOISTURECONTENT(fr)DEPTH5SAMPLE LOCATIONBORING1