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HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.23.2018H.PVKUMAR Geotechnlcal Engineerlng I Engineering Geology Matarials Tesllng I Envlronmentral 5020 County Road 154 Glenwood Springs, C0 91601 Phone: (970) 945-7988 Fax {970) g4s-9454 Email: hpkglenwood@kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado STJBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 15, FILING I, PINYON MESA 145 SAGE MEADO\ry ROAD GARFIELD COUNTY, COLORADO PROJECT NO. 18-7-s86 ocToBER 23,2018 PREPARED FOR: FERNANDO HERNANDEZ P.O. BOX 2333 GLENWOOD SPRTNGS, COLORADO 8MA2 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY..,.. PROPOSED CONSTRUCTION SITE CONDMIONS SUBSIDENCE POTENTIAL FIELD EXPLORATION. SUBSURFACE CONDITIONS FOUNDATION BEARING CONDITIONS DESIGN RECOMMENDATIONS ................ FOUNDATIONS FOI.INDATION AND RETAINING WALLS ...... FLOOR SLABS UNDERDRAIN SYSTEM ............ SURFACE DRAINAGE LIMITATIONS FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURES 4 AND 5 _ SWELL-CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS a .....- 2 - I I a-J- -3- -8- H-P!KUMAR Project No. 18-7-586 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 15, Filing 1, Pinyon Mesa, 145 Sage Meadow Road, Garfield County, Colorado. The project site is shown on Figure l. 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 Fernando Hernandez dated September 19, 2018. Subsurface evaluation 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, expansion-compression potential and other engineering characteristics. The results of the field exploration and laboratory testing were analyzed to 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 lot were conceptual at the time of our study. The proposed residence is assumed to be a two-story structure with a basement, attached garage and slab-on-grade ground floors. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 10 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 property is vacant and vegetated with grass and weeds with sage brush on the north part, The front part of the lot was graded for the road construction and vegetation was sparse. The ground surface is moderately sloping in the northern part to gently sloping in the southern building envelope area down to the southwest. H.PVKUMAR Project No. 18-7-586 -2- SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian Age Eagle Valley Evaporite underlies the Pinyon Mesa Development. These rocks are a sequence of gypsiferious shale, fine-grained sandstone/siltstone and limestone with some massive beds of gypsum. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite unde¡lie portions of the property. 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 Rive¡ valley. No evidence of subsidence or sinkholes was observed on the property or 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 at the site throughout the service life of the proposed structure, in our opinion is low, however the owner should be 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 September 20,2A18. 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 H-P/Kumar. 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 shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. H.PVKUMAR Project No. 18-7-586 -3 SURSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils, below a thin root zone, consist of mixed sand, silt and clay with gravel to about 11 feet in Boring I underlain by sandy silt and clay to about 23 feet where relatively dense, silty sand and gravel with cobbles was encountered to the boring depths of 26 and 31 feet. Laboratory testing performed on samples obtained from the borings included natural moisture content and density and finer than sand size gradation analyses. Results of swell-consolidation tests performed on relatively undisturbed samples taken from the borings, presented on Figures 4 and 5 typically show low compressibility under light loading and natural low moisture and moderate compressibility under additional loading after wetting. The sample from I5 feet in Boring 1 and 5 feet in Boring 2 showed a low expansion potential when wetted. The laboratory testing is summarized in Table L No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The upper mixed sand, silt and clay soils encountered in the borings at typical shallow foundation depth are relatively low density and mainly tend to settle when they become wetted. Two of the tested samples indicated a low expansion potential but ou¡ experience on nearby lots is that the subsoils are settlement prone. A shallow foundation placed on these soils will have a risk of settlement if the they become wetted and care should be taken in the surface and subsurface drainage around the house to prevent the soils from becoming wet. It will be critical to the long-term performance of the structure that the recommendations for surface grading and drainage contained in this report be followed. The amount of settlement, if the bearing soils become wet, will mainly be related to the depth and extent of subsurface wetting but may result in settlements of around I to 2 inches which could cause building distress. Mitigation methods such as a deep foundation (piles or piers extending down around 25 feetbelow existing ground surface) or removing and replacing the bearing soils as compacted structural fîll could be used to H-PVKUMAR Project No. '18-7-586 -4- support the proposed house with a lower risk of settlement. If a deep foundation is desired, we should be contacted to provide additional design recommendations. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the building can be founded with spread footings bearing on the natural soils at basement level with a risk of movement. Compacted structural fill should be used for shallow depth footings such as for the garage. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the natural soils at basement level or on compacted structural fill in shallow cut areas should be designed for an allowable bearing pressure of 1,500psf. The garage footing areas should be sub-excavated down about 7 feet 2) below existing ground surface and the excavated soil replaced as compacted structural fill back to design grade. The sub-excavation should extend down at least 4 feet below the footing bearing level. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about I inch or less. Additional differential settlements of about 7z to 1 inch could occur if the bearing soils are wetted. The footings should have a minimum width of 20 inches for continuous walls and 4) 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 belory exterior grade is typically used in this area. Continuous foundation walls should be heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 14 feet. The foundation should be configured in a box like shape to help resist differential 3) H-PVKUMAR Project No. 18-7-586 -5- s) movements. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. The topsoil, sub-excavation depth and any loose or disturbed soils should be removed below the foundation area. The exposed soils in footing areas after sub- excavation should then be moistened and compacted. Structural fill should consist of low permeable soil (such as the on-site sandy silt and clay soils) compacted to at least 987o of standard Proctor density within 27o of optimum moisture content. The structural ñll should extend laterally beyond the footing edges equal to at least Vzthe fùl depth below the footing. A representative of the geotechnical engineer should evaluate the fill placement for compaction and observe all footing excavations prior to concrete placement. FOUNDATION AND RETAINING WALLS 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 leas¡ 55 pcf for backfill consisting of the on-site fine-grained soils. Cantilevered retaining structures which are separate from 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 computed on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site fine-grained soils. All foundation and relaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The pressures recommended above assume drained conditions behind the walls and a horizontal backfill surface. The buildup of water behind a wall or an upward sloping backfill surface 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 90Vo of the maximum standard Proctor density at a moisture content near optimum. Backfill placed in pavement and 6) H-P*KUMAR Project No. 18-7-586 -6- walkway areas should be compacted to at least95Vo of the maximum standard Proctor density. Care should be taken not to overcompact the backfill or use large equipment near the wall, since this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall backfill should be expected, even if the material is placed correctly, and could result in distress to facilities constructed on the backñll. 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 0.35. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight af 325 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 compacted to at least 95Vo of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-on-grade construcfion with a settlement risk similar to the foundation if the underlying soils are wetted. To reduce the effects of some differential movement, floor slabs should be separated from all beáring 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 507o retained on the No. 4 sieve and less thanTVo passing the No. 200 sieve. All ñ11 materials for support of floor slabs should be compacted to at least95Vo of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on- site soils devoid of vegetation and topsoil. H-PVKUMAR Project No. 18-7-586 -7 - UNDERDRAIN SYSTEM Although free water was not encounlered during our èxploration, it has been our experience in the area and where clay soils are present, 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 and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. An underdrain should not be provided around slab-at-grade garage and shallow crawlspace areas to help limit potential wetting of bearing soils from shallow water sources. The drains should consist of drainpipe placed in the bottom of the wall backfill surounded 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 rninimum LVo to a suitable gravity outlet or sump and pump. Free-draining granular material used in the underdrain system should contain less than 27o passing the No. 200 sieve, less than 507o passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least lVz feet deep. An impervious msmb¡ane such as 30 mil PVC should be placed beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. SURFACE DRAINAGE Proper surface grading and drainage will be critical to keeping the bearing soils dry and limiting building settlement. The following drainage precautions should be observed during construction and maintained continuously after the residence has been completed: 1) Inundation ofthe foundation excavations and underslab areas should be avoided during construction. 2) Exterior backfill should be adjusted to neff optimum moisture and compacted to at least 957o of the maximum standard Proctor density in pavement and slab areas and to at least 9O7o af. 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 12 inches in the first 10 feet in unpaved areas and a minimum slope of H.P!KUMAR Project No. 18-7-586 -8- 4) 3 inches in the first 10 feet in paved areas. Free-draining wall backfill should be covered with filter fabric and capped with at least 2 feet of the on-site soils to reduce surface water infil[ation. Roof downspou¡s and drains shouid discharge well beyond the limits of all backñll. Graded surface swales should have a minimum slope of 37o. Landscaping which requires regular hôavy irrigation should be located at least 10 feet from foundation walls. Consideration should be given to use of xeriscape to reduce the potential for wetting of soils below the building caused by inigation. LIlVtrTATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at the time of this study. We make no wamanty either express or implied. The conclusions and recomnendations 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 inclt¡de 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 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 info¡mation. 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 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 5) H-P*KUMAR Project No. 18.7-586 -9- of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, H-P* KUMAR Steven L. Pawlak, P Reviewed by: Daniel E. Hardin, P.E. SLP/kac Cc: Brian Kurtz (kurtzengineer'@yahoo.cgm) t I I ô222 H-PVKUMAR Project No. 18-7-586 f ö -o4Ö LOT 15 LOT 14 LOT 17 ( PROPERTY CORNTR PIN SACE MEADOW ROAD r---*---1 Uz. J I)î æ. Ld ô_z3 r -'l I I I I LOT 15 LOT 21 LOT 20 25 0 LOT 16 8oRtNe 2 O LOT 15 o eon¡xo I APPROXIMATE SCALT-FTET 1 8-7-586 H-PryKUMAR LOCATION OF EXPLORATORY BORINGS Fig. 1 BORING 1 EL. 1 01 .5'BORING 2ËL. 105.5' 0 0 3A/12 WCx4.7 DD=97 -ZQA=57 2A/ 12 WC:5.9 DD= I 0't -200=69 5 517/12 18/ 12 WC=6.0 D0= 1 05 10 l026/12 WC=2.5 DD=l 10 -200= 1 9 17 /12 IJ 15|-L' t!L I =F UÕ 33/ 12 tNC=7.7 DD= 1 07 17/12 WC:6.6 DD= 1 01 tsIJ LJL I-Fû-U 20 20/ 12 24/12 25 53/12 40 /6,20 / 2 30 ?n44/12 35 1 8-7-586 H-PVKUMAR LOGS OF TXPLORATORY BORINGS Fig. 2 e a I 3 :! LEGEND N ntlt TOPSOIL; ORGANIC SANDY SILT AND CLAY, ROOT ZONE. :1I?.1N!_ S]LT (SM-ML); SII9TTLY CLAYÊY, WITH GRAVEL, SCATTERED SASALT COBBLES,MEDIUM DENSE, SLIGHTLY MOIST, LIGHT BROWN. sANÐ (SM); StLTy, flNE GRAVEL, MEDTUM DEN5E, SLtcHTLy MOIST, BROWN. CLAY AND SILT BROWN, SLIGHTL SAND AND CRAVEL MOIST, MIXTD BROW i.c!.lvt!)',,sfNDY, srlFF To VERY srFF wrrH DEPTH, sLrcHTLy MorsT, L|GHTY CALCARËOUS, (sM-GM); slLTY, BASALT COBBLES, MEÐtUM DENSE ro DENSE, SLTGHTLY N. RELATIVELY UN0ISTURBED DR¡VE SAMPLE; 2-|NCH t.D. CALTFORNTA LTNER SAMpLt I I DRrvE sAMpLE; sTANDARD pENtîRAïroN TEsT (spT), 1 3/B INCH r.D. splrr spooNI sAMpLE, ASTM' D-1s86. so" r z p{ffi.=1$\T,.Iä'^å?i-l;åi,?JEå'?¡ 'JäJ.'f*3'3x'.ì,3å-å f1'¡??'ii"3l#ii ,N.HES _lgolEg_ 1' THE TXPLORATORY BORINCS WERE DRILLED ON SEPTEMBER 20,2018 WITH A 4-INCH D¡AMETERCONTINUOUS FL'GHT POWTR AUGER. 2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MTASURED APPROXIMATELY BY PACINCFROM FEATURES SHOWN ON THE SITË PLAN PROVIDEO. 3, THE ¡LEVATIONS OI THE EXPLORATORY BORINGS WERT MEASUREÐ BY INSTRUMENT LEVEL ANDREFIR TO THT BINCHMARK ON F'G, 1. 4. THE EXPLORATORY BORING LOCATIONS AND ELTVATIONS SHOULD BE CONSIÐERED ACCURATEONLY TO IHE DEGREE IMPLIED BY THE METHOD USTD. 5. THT LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRTSËNT THEAPPRoxIMATÊ BoUNDARtES sETwEÊN MATERTAL TypEs AND THt rRANStrtoNs uay si óä¡ou¡t-. 6' GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING. 7, LAEORATORY TEST RESULTS: WC = }VATER CONTENT (%) (ASTM A 2?16);DD = ORY DENSÍTY (PCf) (ASTM D 2216); -2OO= PERCENTAGE PASS'NO NO. 2OO SITVE (ASTM D 1 1 4o). 1 8-7-586 H-PVKUMAR LTGTND AND NOTES Fig. 3 i- SAMPLE OF: Sondy Sitty Ctoy FROM:Boringl@,l5' tNC = 7.7 %, DÐ = 107 pcf ! ! i I I ;l ,lrl i I- EXPÄNSION UNDER CONSTANT PRESSURE UPON WETTINC i I I : t i liJJ ùJ =llt I z.ot- o =oln oo ñ JJ l¿l =UI I zotr ô =() v1zo(J 2 0 -l -2 -3 2 0 -1 * KSr -2 SÀMPLE OF: Sondy Sili cnd Ctoy FROM:Boring2@5' WC = 6.0 %, 9Ð = 105 pcf I I:ll EXPANSION UNDER COÑSTANT PRESSURE UPON WETTING I:l'1 :l:, i -3 t.0 1 8-7 -586 H.PryKUMAR SWELL-CONSOLIDATION TEST RTSULTS Fig. 4 E SAMPLE OF: Sondy Síll ond Cloy FR0M:8oring2@.15' WC = 6,6 %, DÐ = lOt pcf : NO MOVEMENT UPON WETTING I l I l o àq J) l¡J ÈIN t-z zo t- ô_? o UIzoo_4 -5 -6 t00 1 8-7*586 H-PVKUMAR SWELL-CONSOL¡DATION TTST RESULTS Fig. 5 H-P*KUMARTABLE 1SUMMARY OF LABORATORY TEST RESULTSProject No. l8-7-586SOILWPEVery Sandy Silt with GravelSilty Sand with GravelSandy Silty ClaySandy Silt and ClaySandy Silt and ClaySandy Silt and ClayUNCONFINEDCOMPRESSIVESTRENGTHlosflPERCENTPASSINGNO.200SIËVELIQUIDLIMITPLAST]CINDËX571969GRADATIONSAND(o/'lGRAVEL(%tNATURALMOISTURECONTENTNATURALDRYDENSITY9711010710I10510I4.72.57.75.96.06.6BORINGDEPTH12Y,10I52Y,5152