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Home My WebLink AboutSubsoil Study for Foundation Design 12.29.17H.PVKUMAR Geotechnical Engineering I Englneerlng Geology Materials Testlng I Environmental 5020 County Road 1S4 Glenwood Springs, CO 81601 Phone: (920) 945-7988 Fax (970)94ffi454 Email: hpkglenwood@kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado December 29,2017 David Rosenfeld 279Blae Heron Vista Glenwood Springs, Colorado 81601 drosenfe@hotmail.com Subject: Projecr No.t7-T-832 subsoil study for Foundation Design, Proposed Residence, Lot 65, Filing 5, Elk Springs, Crescent Place, Garfield County, Colorado Gentlemen: As requested, H-P/Kumar performed a subsoil study for design of foundations at the subject site. The study was conducted in accordance with our proposal for geotechnical engineering services to you dated June 21,2AÛ. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: Design plans for the residence were preliminary at the time of our study. The proposed residence will be approximately located as shown on Figure I and have a footprint of around 1,000 square feet above a walkout, garden level basement. Ground floors will be structutal over crawlspace or slab-on-grade. Cut depths are assumed to range between about 2 to 6 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 lot slopes gently down to the southeast and was vacant at the time of our field exploration. The site is vegetated with pinon and juniper trees in the building area with sagebrush and grass to the north and west. Scattered boulders were obsèrved at the ground surface of the lot. Subsidence Potential: The lot is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. Dissolution of the gypsum under certain -2- conditions can cause sinkholes to develop and can produce areas oflocalized subsidence. During previous work in the general area, sinkholes have been observed outside the Elk Springs Subdivision. Sinkholes were not observed in the immediate area of the subject lot. The pits were relatively shallow, for foundation design only, and no evidence of voids was observed in the pits. Based on our present knowledge of the site, it cannot be said for certain that sinkholes will not develop. In our opinion, the risk of ground subsidence at Lot 65 throughout the service life of the residence 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 l. The logs of the pits are presented on Figure 2. The subsoils encountered, below about one foot of topsoil, consisted of I to lr/z feet of stiff, sandy silty clay with gravel and cobbles overlying relatively dense, basalt rocks from gravel to boulder size in a white, sandy silt matrix down to the bottom of the pits at 2Vzto 3 feet where refusal to digging with the backhoe was encountered. Results of swell- consolidation testing performed on a relatively undisturbecl sample of the sandy silty clay, presented on Figure 3, generally indicate low to moderate compressibility under conditions of loading and wetting. The laboratory test results are summarizedinTable 1. No free water was observed in the pits at the time of excavation and the soils were slightly moist. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, tve recommend spread footings placed on the undisturbed natural soil below the topsoil designed for an allowable soil bearing pressure of 2,000 psf for support of the proposed residence. The upper clay and silt matrix soils tend to compress after wetting and there could be post-construction foundation settlement of around r/zto I inch depending on depth of wetting. Footings should be a minimum width of l6 inches for continuous walls and,2 feet for columns. The topsoil and loose disturbed soils encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural soils. Voids created by boulder removal can be backfilled with concrete or structural fill, such as road base compacted to at least 98Vo of. standard Proctor density at near optimum moisture contenl Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least 36 inches below the exterior grade 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 L2 feet. Foundation walls acting as retaining H-P+¡<¡l¡v¡¡P Project No. .17-7-832 -3- structures should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site soil as backfill excluding topsoil and rocks larger than 6 inches. 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 movemenq 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 less than SOVr passing the No. 4 sieve and less than 27o passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at least91%o of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on- site soils devoid ofvegetation, topsoil and oversized rock. 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 also create a perched condition. W'e 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 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 lfto to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 27o passingthe No. 200 sieve, less than 507o passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel bacKill should be at least l/z feet deep. 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. H.PVKUIVIAR Project No. 17-7-832 -4- 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95Vo of the maximum standard Proctor density in pavement and slab areas and to at least 90Vo 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 l0 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 l0 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, 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 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. 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 fîeld services during construction to review and monitor the implementation of our recommendations, and to verify 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 H.PèKUMAR Project No. 17-7-832 5 of excavations and foundation bearing strata and testing of structural fÏll by a representative of the geotechnical engineer. If you have any questions or if we may be of further assistance, please let us know. Respectfu lly Submitted, FI-P* KUM.AR Steven L. SLP/kac Attachments: Figure I Pits Figure 2 -Logs Exploratory Pits Figure 3 - Swell-Consolidation Test Results Table 1 - Summary of Laboratory Test Results t.üã2¡r H-PèKUIVIAR Project No. 17-7-832 Ngg"lïzîW-- :Çrr ff \æ-- îw Ê F*' * .&\ -.F h...' \\ â ñ Itl 87t 7, ¡ 449.68 ,I,lß tl ler z Ør ã5 HãTHT 2gÛ.87'' f 924,,og Pfrf9rEÛ¿¡tt renÆt{T h\I ÆrâfsgÉ NüEår frwfi fþfr ft€gTHÊ|TON + T,OT 6#gr rof s0.FT, t\ql APPROXIMATE SCALE-FEET 17-7-832 H.PryKU]VIAR LOCATION OF EXPLORATORY PITS Fig. 1 Ê å Ë PIT 1 Ptl 2 1 1 40). o t- lÁJ l¿¡t¡- I IF-fL lr.lò F- r¡J r¡J LL IT|-IL t¡Jo WC=9.9 DD=99 WC=12.1 DD=72 -20O=41 0 55 LEEEND ñ TOPSOTL; ORGAN|C SANDY StLTy CLAY, SLtcHTLy MO|ST, SROWN. 9.SY..f.cÐ.;.!4.{-Dl: S_çATTERED cRAvEL AND coBBLEs, vERy ST|FF, SL|GHTLY MotsT, BRowN,MEDIUM PLASTICIW, BLOCKY. [ZqF{.t Bou,lqERs ¿N9.slLr MArRlx (GM-ML); HlcHLy cALcAREous, cLAyEy, DENSE/HARD, K,,,lSLIGHTLY MOIST, GRAYr/WHITE. I F HAND DRtvEN 2-tNcH DTAMETER LTNER sAMpLE. l- pnrcrrcAL DrGGrNc REFUSÄL oN BouLDERs. NOTES- 1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON DECEMBER 8, 2017. 2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEÀSUREO APPROXIMATELY BY PACING FROMFEATURES SHOWN ON THE SITE PIAN 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 THg DEGREEIMPLIED BY THE METHOD USED. 5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANStTtoNs MÃy AE cnAoUaL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF DIGGING. PITS WERE BACKFILLED SUBSSQUENT TO SAMPLING. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (,l) (ASIM D 2216); DD = DRY DENSITY (PCf) (ASTM O 2216); -200 = PERCENTAGE PASSING NO. 200 STEVE (ASTM D 17-7-832 H-PryKU]VIAR LOGS OF EXPLORATORY PITS Fig. 2 SAMPLE OF: Sondy Sllty Cloy FROM:Pltlgl.5' lilC = 9.9 21, DD = 99 pcf <ì I I J : I i 't I I I I I i I ¡ _.1 I I i I I I I EXPANSION UNDER CONSTANT PRESSURE UPON WETTING o }Q j-1 l¡l =tt7)-z 2o l- !-so anzoc) -4 17-7-832 H-PryKUMAR SWELL-CONSOLIDATION TIST RESULTS Fig. 3 H-P*KUMARTABLE 1SUMMARY OF LABORATORY TEST RESULTSProject No. 1 7-7-832SOILTYPESandy Silty ClayCalcareous Sandy SiltMatrixUNCONFINEDCOMPRESSIVESTRENGTH(PSRPLASTtCINDEX(o/"1LIQUIDLIMIT(%lPERCENTFASSINGNO.200S!EVE44SAND('/"1GRAVEL(vùNATURALDRYDENSITYlocfl9972NATURALIiIOISTURECOÍTITENT(o/ol9.912.1.OCATIONDEPTHfñrIVz2V2SAIIPLEPIT1