Loading...
HomeMy WebLinkAboutSubsoil Study 04.28.2021I(+rt i;ffii[#nftin¡$*"" RECEIVED APR I 4 2ß22 GARFIELD COUNTY COMMUNITY DEVELOPMENT An Employcc Owncd Compony 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email : kaglenwood@kuman¡sa.com www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado April28,202l Gora Group Attn: Mona Gora 32 Court Sfieet, Penthouse Brooklyn, New York II20I mona@soldenlioness'com project No.2r-7-24r Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot GV-13, Aspen Glen Subdivision, Bald Eagle Way and Mariposa, Garfield County, Colorado Dear Mona: As requested, Kumar & Associates, Inc. performed a subsoil study for design of foundations at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to Gora Group dated March 3,202I. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: Plans for the proposed construction were not available at the time of our study. The proposed residence is assumed to be a two-story wood-framed structure over crawlspace or basement with attached garage located on the site as shown on Figure 1. Ground floor are assumed to be a combination of structural over crawlspace and slab-on-grade. Cut depths are expected to range between about 2 to 8 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. If building conditions or foundation loadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report. Site Conditions: The subject site was vacant at the time of our field exploration. Previous over- lot grading from the original subdivision development graded the ground surface relatively flat. Vegetation consists of grass and weeds. Subsidence Potential: Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen Subdivision. 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 a lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in fhe area, several sinkholes were observed scattered throughout the Aspen Glen Subdivision. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork River Valley. The nearest mapped sinkhole is about 1200 feet west of this site. 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 pits were relatively shallow, for foundation design only. Based on our present knowledge of the site, it cannot be said for certain that sinkholes will not develop. [n our opinion, the risk of ground subsidence at Lot GV-l3 is low and similar to other lots in the area but the owner should be aware of the potential for sinkhole development. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about lz feet of topsoil, consist of stiff to very stiff, sandy to very sandy clay down to the maximum explored depth of 8 feet. Results of swell-consolidation testing performed on a relatively undisturbed sample of the clay, presented on Figure 3, indicate low compressibility under existing moisture conditions and light loading and a low collapse potential (settlement under constant load) when wetted and a moderate settlement potential under increased loading. Our experience in the area indicates that the tested sample may have been disturbed. Further evaluation of the clay soils should be conducted at the time of construction. No free water was observed in the pits at the time of excavation and the soils were slightly moist to moist. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, the proposed residence can be supported on spread footings placed on the undisturbed natural soil designed for an allowable soil bearing pressure of 1,500 psf with a risk of foundation movement. The soils tend to compress after wetting and there could be some post-construction foundation settlement. Footings should be a minimum width of 16 inches for continuous walls and2 feet for columns. Loose and disturbed soils and existing fill encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extcndcd down to thc undisturbed natural soils, Exterior footings shoulcl be provìded with aclequate cover above their bearing elevations for fi'ost protcction. Placement of footings at lcast 36 inchcs bclow the exterior grade Kulnar & Associates, lnc. @ Project No, 21-7-241 -3 - is typically used in this area. 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 be designed to resist alateral earth pressure based on an equivalent fluid unit weight of at least 55 pcf for the on-site soil as backfill. Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction with a risk of movement. A positive way to reduce the risk of floor movement commonly used in the arca is to construct structurally supported floors over a crawlspace. 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 relatively well graded gravel, such as road base, should be placed beneath floor slabs for support. This material should consist of minus 2-inch aggregafe with less than 50%o passing the No. 4 sieve and less than I2o/o passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at least 95o/o of maximum standard Proctor density at a moisture content near optimum. Required fiIl can consist of the on- site soils devoid of vegetation, topsoil and oversized rock. Underdrain System: Although free water was not encountered during our exploration, it has been our experience in the areathat 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, basement and crawlspace 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%o to a suitable gravity outlet or sump and pump. 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 IYz feet deep. An impervious membrane such as 20 mil PVC should be placed beneath the Kumar & Associates, lnc. @ Project No. 21-7-241 -4- drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: l) Inundation 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 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. Free-draining wall backfill should be capped with about 2 feet of the on-site, finer graded soils to reduce surface water infiltration. 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 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in pavement and walkway 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 10 feet from the building. Consideration should be given to the use of xeriscape to limit potential wetting of soils below the foundation caused by irrigation. 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 pits excavated at the locations indicated on Figure I and to the depths shown on Figure 2,frT 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 pits 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 at once so re-evaluation of the recommendations may be made. Kumar & Associates, lnc. @ Project No. 2'l-7-241 5 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 veriff that the recommendations have been appropriately interpreted. Significant design changes may require additional analysis or modifications to the recoÍrmendations presented herein. We recommend on-site observation of excavations and foundation bearing strata and testing of structural frll by a representative of the geotechnical engineer. If you have any questions or if we may be of further assistance, please let us know. Respectfully Submitted, Kumar & Associates, James H. Parsons, P Reviewed by: Daniel E. Hardin, P.E. JHP/kac attachments Figure I - Location of Exploratory Pits Figure 2 - Logs of Exploratory Pits Figure 3 - Swell-Consolidation Test Results Table 1 - Summary of Laboratory Test Results cc:Sydney Slossberg ( ) 'I c/r/ot È Kumar & Associates, lnc. ft'Project No. 21-7-241 e PIT 1 @ mÌrãzlßr WffimreffimgmGR[4ñD@m[$7È&Ê .throffi .Èhm H]g W t Ii I Mûæffi ffi4ffiffiffi { 60 APPROXIMATE SCALE_FEET 21 -7 -241 Kumar & Associates LOCATION OF EXPLORATORY PITS Fig. 1 d PIT 1 Ptl 2 0 F L¡l L¡lfL I-¡- o_ t¡Jô WC=6.5 DD=92 F t¡J t¡JtL I-t-o- t¡Jô 5 q ¡NC=4.4 I LL=19 Pl=2 -200=62 10 10 LEGEND TOPSOIL: CLAY, SANDY, SILTY, ROOTS AND ORGANICS, MEDIUM STIFF TO STIFF' SLIGHTLY MOIST, BROWN. CLAY (CL) SANDY TO VERY SANDY, SILTY, SCATTERED GRAVEL, STIFF TO VERY STIFF, SLIGHTLY MOIST, BROWN. F t HAND DRIVE SAMPLE DISTURBED BULK SAMPLE NOTES 1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON MARCH 29, 2021 2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE NOT MEASURED AND THE LOGS OF THE EXPLORATORY PITS ARE PLOTTED TO DEPTH. 4. THE EXPLORATORY PIT LOCATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY THE METHOD USED. 5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATION. PITS WERE BACKFILLED SUBSEQUENT TO SAMPLING. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (%) (ASTM D 2216); DD = DRY DENSITY (pcf) (ASTM D 2216); LL = LIQUID LIMIT (ASTM D 4518); PI = PLASTICITY INDEX (ISTV O 4318); -2Qo= PERCENTAGE PASSING No. 200 SIEVE (ASTM 01140); 21 -7 -241 Kumar & Associates LOGS OF EXPLORATORY PITS Fis. 2 SAMPLE OF: Sondy Sill ond Cloy FROM:Boring2@5' WC = 6.5 %, DD = 92 pcf ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING somplê3 sho¡l not b€iñr€produced, lhofull, Kuñor ond ^lrociotes, lnc. Sw€ll consolidotíon tæting pôrfom€d in occordonce s¡th Æil D-4546. 3 2 1 0 à( J4J -l UJ =vlt-z zotr ô -Jo U1z.o(J_4 -5 -6 -7 -8 1.0 APPLIED PRESSURE - KSF 10 100 21 -7 -241 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 3 I rcnl(urs & Associates, lnc.'Geotechnical and [/aterials Engineersand Environmental ScientistsTABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-241Sandy Silt and ClaySandy Silt and ClaySOIL TYPE(psf)UNCONFINEDCOMPRESSIVESTRENGTHt9262("/"1PLASTICINDEXATTERBERG LIMITS(%)LIQUID LIMITPERCENTPASSING NO.200 stEvESAND(%)GRADATIONGRAVEL("/"192(ocfìNATURALDRYDENSITYlololNATURALMOISTURECONTENT4.46.s(ft)DEPTH6v,J2SAMPLE LOCATIONPIT1