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Home My WebLink AboutSubsoil Study for Foundation Design 09.22.17Geotechnlcal Engineering I Engineering Geology Materials Testlng I Environmenlal H.PryKUMAR 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 9457988 Fax (970) 945-8454 Email: hpkglenyood@kumarusa.com Office Locations: Denver iHQ¡, Parker, Colorado Springs, Fort Collins, Glenwood Springs, Surìnmit County, Colorado ST}BSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE, ADU, AND SHOP LOT 13, LOOKOUT MOUNTAIN RANCHES 2575 COANTY ROAD 1.L5 GARFTELÐ COUNTY, COLORADO [0\l i 3 ?0ll PROJECT NO.17-7-683 SEPTEMBER2¿,20t7 PREPARED FOR: CLAYTON SNdITH 648 ALDER RIDGE NEW CASTLE, CO 81647 (rvhitekn ifþra¡rch @ qmail.cr¡nr) TABLE OF CONTENTS 1 PURPOSE ANN SCOPE OF STUDY PROPOSED CONSTRUCTION SITE CONDITIONS FMLD EXPLORATION SUBSURFACE CONDITIONS DESIGN RECOMMENDATIONS ................ RESIDENCE AND SHOP FOUNDATIONS ADU FO{.INDATIONS FOTINDATION AND RETAINING WALLS FLOOR SLABS UNDERDRAIN SYSTEM SURFACE DRAINAGE ... LIMITATIONS FIGURE I - LOCATION OF EXPLORATORY PTTS FIGURE 2 - LOGS OF EXPLORATORY PITS FIGURE 3 - LEGEND AND NOTES FIGURES 4 THROUGH 8 - SWELL-CONSOLIDATION TEST RESULTS TABLE I- SUMMARY OF LABORATORY TEST RESULTS 1 _l _ 1 -,, _ -2- 3 ,........- 8 - H.PÈKUMAR Project No. 17-7-683 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence, ADU, and shop to be Iocated on Lot 13, Lookout Mountain Ranches, 2527 Cowty Road 115, Garfield County, Colorado. The project site is shown on Figure 1. The purpCIse 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 Clayton Smith dated September 6,2017. A field exploration program consisting of exploratory pits 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 rest¡lts of the field exploration and laboratory testing were analyzed to develop recommendations for foundation types, depths and allowable pressures for the proposed building foundations. 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 'We understand that the proposed residence and ADU will be single-story structures over crawlspace and the proposed shop will be a single-story structure. Ground floors will be slab-on- grade. Grading for the structures is assumed to be relatively minor with cut depths between about 2 to 6 feet. We assume relatively iight foundation loadings, typical of the proposed type of construction, The shop is proposed to be built this year and the other 2 structures later. If building locations, grading or loading information are significantly different, we should be notified to re-evaluate the recommendations presented in this report. SITE CONDITIONS Currently the proposed building sites are vacant. A rough graded road connects the building areas. The topography of the site is gently to steeply sloping hills with slopes ranging between H-P*KUMAR Projecl No. 17-7-683 -2- 5Vo to 20Vo. Ycgctation on the site consist r¡f uative glass and brush with scrub oäk, pinon trcos, and juniper trees. FIELD EXPLORATION The field exploration for the project was conducted on September 11,2A17. Five exploratory pits were excavated at the locations shown on Figure 1 to evaluate the subsurface conditions. Two pits were dug at the proposed residence site, one at the proposed ADU site, and two at the proposed shop site. The pits were dug with a steel-tracked backhoe. The pits were logged by a representative of H-P/Kumar. Samples of the subsoils were taken with lelatively undisturbed and disturbed sampling methods. Depths at which the samples were taken are shown on the Logs of Exploratory Pits, 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 encountered at the proposed shop site (Pits 1 and 2) consist of a thin topsoil layer overlying silty sandy clay to depths of 5 and 7 Vz feet (the bottom of the pits). The subsoils encountered at the proposed residence site (Pits 3 and 4) consist of about 2 feet of topsoil overlying silty sandy clay to depths of 3 feet in Pit 3 and 6Yz feet (the botrom of the pir) in Pit 4. In Pit 3, highly calcareous sandy silt and clay was encountered to a depth af 6 Yz feet (he bottom of the pit). The subsoils encountered at the proposed ADU site consist of about I foot of topsoil overlying clayey sandy gravel and silt with cobble to boulder size rock fragments to a depth of 6Vz feet (the bottom of the pit). Laboratory testing performed on samples obtained from the pits included natural moisture content and density and finer than sand size gradation analyses. Results of swell-consolidation testing performed on relatively undisturbed hand driven liner samples, presented on Figures 4 through 8, indicate low to moderate compressibility under toading and typically a minor hydrocompression potential when wetted. A low expansion potential under wetting was H-P*KUMAR Projecl No. 17-7-683 -J- indicated in the sample from Pit 5 at the ADU site. The laboratory testing is summarized in Table 1. No free water was encountered in the pits at the time of excavation and the subsoils were'slightly moist. DESIGN RECOMMENDATIONS RESIDENCE AND SHOP FOUNDATIONS Considering the subsurface conditions encountered in the exploratory pits at the proposed residence and shop sites, and the nature of the proposed construction, we recorrmend the buildings be founded with spread footings bearing on the natural fine-grained soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural clay and silt 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 I inch or less. Additional settlement of about 1 inch could occur if the bearing soils are wetted and precautions should be taken to keep the bearing soils dry. 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 thi's atea. 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 H.PÈKUMAR Project No. 17-7-683 -4- latcral earth prossut'os as disüussutl iu thu "Fuuntlation and Retaining'Walls" section of this report. The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the firm natural soils. The exposed soils in footing area should then be moistened and compacted. A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. ADU FOUNDATIONS Considering the subsurface conditions encountered in the exploratory pit at the proposed ADU site, and the nature of the proposed construction, we recolnmend the building be founded with spread footings bearing on the natural mixed fine-grained and rock fragment soils. The design and construction criteria presented below should be observed fcrr a spread footing foundation system. 1) Footings placed on the undisturbed natural soils should be designed for an allowable bearing pressure of 2,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. Expansive bearing soils may need to be subexcavated below footing arcas and should be further evaluated at the time of construction. 2) The footings should have a minimum width of 16 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 offoundations 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 !2 feet. 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. s) 6) H-PVKUMAR Project No. 17-7-683 5 s)The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the firm natural soils, The exposed soils in footing area should then be moistened and compacted. A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions, 6) 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 least 55 pcf for backfill consisting of the on-site fine-grained soils and at least 45 pcf for backfill consisting of imported granular soils. Cantilevered retaining structures which are separate from the structures 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 50 pcffor backfill consisting ofthe on-site fine-grained soils and at least 40 pcffor backfill consisting of imported granular soils. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge presslìres such as adjacent footings, traffic, construction materials and equipment. The pressures recommended above assume drained conditions behind the walls and a horizontal baekfill 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 walkway areas should be compacted to at least957o 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 H.PlKUMAR Project No. 17'7-683 -6- backfill shr¡uld be expeuted, evsn if the nuteriul is placetl corectly, and could result in distress to facilities constructed on the backfill. The lateral resistance of foundation or retaining wall footings will be a combination of the sliding resistance of the footing on the foundatiòn 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 0.30 for the on-site fine-grained soils and 0.40 for the onsite mixed fine-grained and rock fragment soils. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 300 pcf for the on- site fine-grained soils and 350 pcf for the on-site mixed fine-grained and rock fragment soils. 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 95Va of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils, exclusive of topsoil, arc suitable to support lightly loaded slab-on-grade construction. There could be settlement/heave potential of the slabs if the bearing soils are wetted. 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 ¡educe damage due to shrinkage cracking. The requirements for joint spacing and slab reinforcement should be estabiished 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 Z-inch aggregate with at least SOV> retained on the No. 4 sieve and less tban ZVo passing the No. 200 sieve. The slab subgrade conditions should be evaluated for compressibility/expansion potential at the time of excavation. All fiIl materials for support of floor slabs should be compacted to at least H-PVKUMAR Proiecl No. 17-7-683 95Vo of maximum stand&rd Proctor dcnsity at a moisture content near optimum. Required fill can consist ofthe on-site soils devoid ofvegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our expeúence in the area and where there are clay soils that local pcrchcd 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, crawlspace and basement areas, be protected fi'om wetting and hydrostatic pressure buildup by an underdrain system. The drains should consist of drainpipe placed in the bottom of the wall backfill surounded above the invert level with fi'ee-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 tVo to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2Vo 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 impervions membrane such as 20 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 The following drainage precautions should be observed during construction and maintained at all times after each structure has been completed: 1) 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 95Vo of the maximum standard Proctor density in pavement and slab areas and to at least 9AVo of. the maximum standard Proctor density in landscape areas. ¡{-PVKUMAR Project No. 17-7-683 -8- 3)Tho ground surfaco surur:uuding the ur(turiur uf thu builtling shoultl bu slopcd 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 paved areas. Free-draining wall backfill should be covered with filter fabric and capped with about 2 feet of the on-site soils to reduce surface water infiltration. Roof downspouts and drains should discharge well beyond the limits of all backfill. Landscaping which requircs regular heavy inigation 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 irrigation. 4) 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 waffanty 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 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 pits and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appear diffe¡ent 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 ¡eview and monitor the implementation of our recommendations, and to verify that the recommendations 5) H-PÈKUMAR Project No. 17-7-683 -9- 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, H.FIKUÍVIAR V-n*+L b--.^-(@Robert L. Duran, E.I. Reviewed by: Steven L. Pawlak, RLD/kac $t ''¡:1 H.P*KU]VIAR Project No. 17-7-683 1 æ50 0 150 300 ÂPPROXIMATE SCALE-FEET 17 -7-683 H.PryKUMAR LOCATION OF EXPLORATORY PITS Fig. 1 ! PIT .1 Ptl 2 PIT 5 0 0 WC=10.1 DÐ=98 l I 1 l-rljIJl¡ I- o- L¡lô 5 WC=8.8 DD=89 Ê t-Ir¡IJl! ITF(L l¡lô WC=9.2 DD=87 WC=7.8 DÐ=81 -2QA=71 l0 10 SHOP SITE HOU5E SITE PIT 4 PIT 5 0 0 Þ-U LJl! I :Ë l-..-o- r¡.,Õ WC=3.7 ÐÐ=98 WC=7.0 DD=1 07 !-U LJ LL IIf-fL l¡lÕ q IU 10 HOUSE SITE ADU SITE 17 -7 -683 H-PryKUMAR LOGS OT EXPLORATORY PITS Fis. 2 s LEGEND N IÔPSO|L; ORGANIC SANDY SILT, SL|GHTLY MO|ST, BROWN. CLAY (cL);IL'TY, SANDY, STIFF, SLIGHTLY MOIST, MIXED RED-BROWN, SLIGHÏLY POROUS, LOW PI-ASTICIW. sulxtv CALCAREOUS, þ ii l--r :l-Ll $q..9!.{Y (ML-CL); SANDY, vERY srtFF, SLIGHTLv Motsr, HrcHLy cALcAREous, LTGHTRED TO WHITE, GRAVEL AND slLT (cM-ML); CLAYEY, SANDY, coBBLE To B0ULDER RocK FRAGMENTS, MEDTUM DENSE/VERY STtFF, SLtcHTLy MO|ST, RED. HAND DRIVEN 2_INCH DIAMETER LINER SAMPLE. DISTURBED BULK SAMPLE. NOTES 1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON SEPTEMBER I1,2017, Z. THE EXPLORATORY PITS WERE LOCATED BY THE CLIENT. 3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE NOT MEASURED AND THE LOGS OF THE EXPLORATORY PITS ARE PLOTTEO TO DEPTH. 4, THÊ EXPLORATORY PIT LOCATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO THE DEGREEIMPLIED BY THE METHOÐ USED. 5, THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT THEAPPROXIMATE BoUNDÀRtEs BETwÊEN MATERIAL TypES AND THE TRANstloNs MAy aE GnÀoual. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF DIGGING. PITS WERE BACKFILLED SUBSSOUENT TO SÄMPLING. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (%) (ASTM Ð 2216); DD = DRY DENSTTY (pci) (ASrU O ZZta);_2OO = PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D r r 4o). o- 17 -7 -683 H-PryKUMAR LTGEND AND NOTES Fis. 3, z I s SAMPLE OF:l Sllshlly I Silty Cl Colcoreous Sondy oy FROM:Pit1E^7' WC = 9.2 %, ÐD = E7 pcf AÐDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING F.* \I :li' lñ.r. t.r d0ru oPÞy dry e mEñphr td. û. t..tht ñroñ .ft.l1 nd b rædæd, qc.rt lnldl, ri¡out th. rlfr.ñ .'@l d Xcño. û¡d Ad-. |rc. $d&ndldþ^ brllq Frlqmd l¡óæ.ú.M ith M O-45ß- ^0 JJ ¡! =tn t-z z.oË o_a Joøzoc.: _4 PRESSURE - KSF 10 17-7-683 H-PryKUMAR SWELL-CONSOLIDATION TEST RESULTS Fig. 4 n SAMPLE OF: Slighlly Porous Sondy Sttty' Cloy FROM:Pít2 o-2' WC = 10.1 %, ÐD = 98 pcl ì\ l--t=4 ADDIÏIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING 1 0 N JJ l¿J =an I zo l- ff JoØzoo -1 -¿ -5 -4 _R -6 -7 17 -7 -683 H.PryKUMAR SWELL_CONSOLIDATION TEST RESULTS Fig. 5 SAMPLE OF: Highly Colcoreous Sondy Cloyey Silt FROM:Pit3@4' WC = 8.6 ?6, DA = 69 pcf ¡n ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE OUE TO WETTING : i 1 I I 1 : t ' 1 ^0 j-t l¡l =an t^-zzoÈ 6-1 Ja6zoQ_4 -5 -6 17 -7 -683 H-PTKUIVIAR SWTLL-CONSOLIDATION TTST RESULTS Fig. 6 tI : I(t .r I 5 Ë hd ilü SAMPLE OF: Slightly Colcoreous Sondy Sllty Cloy FROM:Pit4O3' WC = 5.7 %, ÐÐ = 98 pcf ) I : t I ADOITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING 1 JJ l¡J =vt I z0 F ô Jotnzo() 0 -1 -2 -5 -4 -5 -6 -7 t.0 17-7-683 H-PryKUMAR SWELL-CONSOLIDATION TEST RESULTS Fig. 7 3 SAMFLE OF: Sondy Sill ond Cloy with Grovel FROM:Pit5@3' WC = 7.0 "tá, 0O = 107 pcf 1 I l 'ttt EXPANSION UNDER CONSTANT PRESSURE UPON WETTING :! 'f I --.1,- _i I hd ! l r I i i 1 I I I I jo t¡J =vt l.-lz() F ô oØzoo_5 *4 17 -7 -683 H-PryKUMAR SWELL-CONSOLIDATION TEST RESULTS Fig. I H-PtKUMARTABLE 1SUMMARY OF LABORATORY TEST RESULTSProject No. 1 7-7-683SOILTYPESlightly Calcareous SandySlightly Porous Sandy SiltyClayHighly Calcareous SandyClayey SiltHighly Calcareous SandyClayey SiltSlightly Calcareous SandySilty ClaySandy Silt and Clay vrithGravelUNCONFINEDCOMPRESSIVESTRENGTH(PSF)ATTERBERG LIMITSPLASTICINDEX(%lLISUIDLIMIT(%lPERCENTPASSINGNO.200SIEVE7IGRADATIONSAND%tGRAVEL(%',NATURALMOIEÏURECONTENTNATURALDRYDENSIrYPITDEPTH8798898I981079.210.18.87.83.77.07246JJI2J45