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Home My WebLink AboutSubsoil Study for Foundation Design 06.14.2021lGrt$;ffiåffiß:nlix'å*-' Ån Employco Owncd Sompany 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email : kaglenwood@kumarusa,com wwrv. kumarusa.com Ofüce Locations: Denver (HQ), Parker; Colorado Springs, Þ'ort Collins, Glenwood Springs, and Summit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED ACCESSORY DWELLING UNIT 1655 COUNTY ROAD 109 GARFIELD COUNTY, COLORADO PROJECT NO.21-7-348 JUNE 14,2021 PREPARED FOR: RIDGE RUNNER CONSTRUCTION ATTN: BRENT LOUGH 1655 COUNTY ROAD 109 GLENWOOD SPRTNGS, COLORADO 81601 b I rid seru n n er(As m a il. co m TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY PROPOSED CONSTRUCTION SITE CONDITIONS SUBSIDENCE POTENTIAL FIELD EXPLORATION SUBSURFACE CONDITIONS FOLTNDATION BEARING CONDITIONS ....... DESIGN RECOMMENDATIONS ..... FOUNDATIONS 1 I a aJ REINFORCED MAT SLAB F OLINDATION ALTERNATIVE FOUNDATION AND RETAINING WALLS.. 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 ....- I - _?- _7 - Kumar & Associates, lnc. @ Project No. 21'7-348 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed Accessory Dwelling Unit (ADU) to be located at 1655 County Road 109, Garfield County, Colorado. The project site is shown on Figure 1. 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 Ridge Runner Construction dated April 13, 2021. H-P Geotech (now Kumar & Associates) performed a subsoil study for the existing house on this site in a report dated January 27,2015, Job No. 114-1874. A field 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 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 The proposed building will be a single story wood frame structure, with a footprint of approximately 28 feet by 36 feet plus a detached front porch and a detached patio in the rear. Ground floor is proposed to consist of a structural slab-on-grade with no basement or crawlspace. Grading for the structures is assumed to be relatively minor with cut depths between about 2 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 subject site was developed with a one and two story residence and a barnboth located uphill to the west of ADU site. The overall slope in the proposed building area is strongly sloping down to the east. The lot becomes steeper uphill to the west of the building area. Eagle Valley Kumar & Associates, lnc. o Project No. 21-7-348 -2- Evaporite bedrock is exposed on the hillside to the west. Ephemeral drainage channels cut into the steeper terrain above the lot appear to drain onto the lot. Vegetation consists of grass, weeds, sagebrush, and juniper bushes. 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 oflocalized subsidence. During previous work in the area, several sinkholes have been identified in the Roaring Fork River Valley. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Eagle Valley. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials. 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 this site throughout the service life of the proposed building, 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 fîeld exploration for the project was conducted on April21,2021. Two exploratory borings were drilled at the locations shown on Figure I 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, Inc. Samples of the subsoils were taken with 1% 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. 21-7-348 -3- shown on the Logs of Exploratory Borings, Figure 2. The samples were retumed 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 consist of about I foot of sandy, gravelly organic topsoil overlying alluvial fan deposits composed of interlayered loose to dense, slightly moist, silty clayey sand and gravelly silty clayey sand. Claystone/Siltstone bedrock was apparently encountered at about 106 feet in Boring l, although intact samples of the formation rock were not recovered. Drilling in the hard claystone/siltstone with auger equipment was difficult at depth and drilling refusal was encountered at 108 feet in Boring 1. In Borin92,the same soil profile was encountered and drilling was terminated at 2l feet. Laboratory testing performed on samples obtained from the borings included natural moisture content, density and finer than sand size gradation analyses. Results of swell-consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 and 5, indicate a low to moderate collapse potential (settlement under constant load) when wetted. The laboratory testing is summarized in Table l. Based on the moisture content, density, and percent finer than sand size gradation analyses, the soils underlying the site have a low to moderate collapse potential (settlement under constant load) when wetted. No free water was encountered in the borings at the time of drilling and the subsoils were generally slightly moist. FOUNDATION BEARING CONDITIONS The soils at the site consist of low to moderate collapse potential interlayered silty clayey sands, and silty clayey sand with gravel composed of Eagle Valley Evaporite fragments. Based on previous experience in the area, we estimate that these soils have an overall settlement potential of about 2Yo or more if wetted. Depending on the depth of future wetting, the potential settlement could be 6 to l8 inches. In order to mitigate the effects of this potential settlement, the building can be supported on a heavily reinforced mat slab or post-tensioned slab foundation designed to withstand relatively large differential settlement. Alternatively, relatively deep piles, such as micro-piles end-bearing in the underlying bedrock could be used. Recommendations for these two foundation alternatives are provided below. The floor for a pile foundation should be Kumar & Associates, lnc. o Project No. 21-7-348 -4- structurally supported over a crawlspace. Satisfactory performance of the foundation will require good surface drainage away from the house, and the elimination of all irrigation within l5 feet of the building will be critical to prevent wetting of the bearing soils. The precautions contained inthe Surface Drainage Recommendations section below should be followed. DESIGN RECOMMENDATIONS FOLTNDATIONS Considering the subsurface conditions encountered in the exploratory borings, our experiences with the previous building on the property and the nature of the proposed construction, we recommend the building be founded with a heavily reinforced mat slab. REINFORCED MAT SLAB The design and construction criteria presented below should be observed for a mat slab foundation system. 1) A mat slab a minimum 5 feet depth of on-site compacted soils shouldon be designed for an allowable bearing pressure of 1,090-8! Based on experience, we recommend the mat be reinforced to allow an unsupported corner of at least 10 feet. 2) We expect the mat slab will be 2 feet thick if conventionall y reinforced. A post-tensioned slab would be thinner. The slab should extend out to support any structural supports such as attached deck/porch columns. 3) The mat slab should be provided with adequate soil cover above its bearing elevation for frost protection. Placement of foundations at lgg!.fS$Slow exterior grade is typically used in this area. As an alternative, shallow mat slab edges can be protected against frost by providing insulation in accordance with the 2009 International Residential Code. 4) Continuous foundation walls (grade beams) should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 14 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. 5) Any existing fill, topsoil, any loose disturbed soils, and the upper soils should be removed to a minimum depth of 5 feet below the mat slab bearing level and to at Kumar & Associates, lnc. @ Project No. 21-7-348 5 6) least 5 feet beyond the mat slab edges. The exposed soils in excavation area should then be moistened and compacted. The on-site soils should be replaced, compacted to at least 98% of standard Proctor density within 2% of optimum moisture content. A representative of the geotechnical engineer should observe the mat slab subgrade excavation and evaluate structural frll compaction prior to concrete placement. FOUNDATION ALTERNATIVE As an alternative, the proposed ADU could be supported on micro-piles drilled down into the underlying bedrock. The micro-pile capacity can be calculated based on an end-bearing capacity of 10,000 psf and a skin friction of 2,000 psf for that portion of the pile in bedrock. Downdrag due to potential settlement of the upper 50 feet of soil can be taken as 1,000 psf for the outside surface area of the pile. A pipe sleeve in the upper part of the pile could be needed to reduce the downdrag on the pile. We should review the micro-pile design prior to construction. 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 eafth pressure computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting of the on-site soils. Cantilevered retaining structures which are separate from the additions 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 backflrll consisting of the on-site soils. Retaining structures separate from the proposed structures can be supported on spread footings designed for an allowable soil bearing pressure of 1,200 psf provided that they can tolerate the relatively large potential settlements at this site. All foundation and retaining 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. Kumar & Associates, Inc. o Project No. 21-7-348 6- Backfîll should be placed in uniform lifts and compacted to at least 90o/o of the maximum standard Proctor density at a moisture content near optimum. Backfill in pavement and walkway areas should be compacted to at leastg5o/o 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 lateralpressure 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 backfill. 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 pressute 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. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 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 95% of the maximum standard Proctor density at a moisture content near optimum. TJNDERDRAIN SYSTEM The proposed mat slab should not need an underdrain system. We recommend below-grade construction, such as retaining walls and crawlspace areas be protected from wetting and hydrostatic pressure buildup by an underdrain system. If installed, the drains should consist of drainpipe placed in the bottom of the wallbackfill 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 lYo to a suitable gravity outlet or sump and pump. Free-draining granular material used in the underdrain system should contain less than 2o/o passing the 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 drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. Kumar & Associates, lnc. @ Project No, 21-7-348 -7 - SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the building has been completed: l) Inundation ofthe foundation excavations and underslab areas should be avoided during construction and at all times thereafter. 2) Exterior backfìll 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 90Yo of the maximum standard Proctor density in landscape areas. 3) The ground surface surounding 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 paved areas. Unlined drainage swales should have a minimum grade of 4Yu Free-draining wall backfill (if any) should be covered with filter fabric and capped with at least 2 to 3 feet of the on-site soils to reduce surface water inflr ltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation (sod) should preferably not be installed at the site. If used, it should be located at least 15 feet from the building perimeters. Preferably, xeriscape that requires minimal irrigation should be used to reduce the potential for wetting of soils below the building caused by irrigation. LIMITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this arca atthis 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 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 frndings include interpolation and extrapolation of the subsurface conditions identified at the exploratory borings and variations in Kumar & Associates, lnc. @ Project No. 21-7-348 -8- the subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appear different from those dessribed 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 review and monitor the implementation of our recommendations, and to veri$' 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 of excavations and foundation bearing strata and testing of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, Kqåxela! & "&ss*¡eËgf*n, Ëlrc, David A. Noteboom, Staff Engineer Reviewed by: Daniel E. Hardin, P DN/kac *1tsltç Hu**ar & Åse*ø9at*s, *ns" ii'Prcje*t S*" 2'Ë"T"3dS 50 APPROXIMATE SCALE_FEET 21 -7 -348 Kumar & Associates LOCATION OF TXPLORATORY BORINGS Fig. 1 N I\¡ IG æ BORING 1 1 oo' BORING 2 1 00' 0 0 12/12 WC=2.0 6/12 10/t2 13/t2 22/12 wC=3.1 -200=10 DD=108 -200=24 s/12 18/12 WC=2.4 DD=f09 SC=-4.8 10 32/12 wC:3.2 DD=l 15 -200=44 ts IItsd o 15 37 /12 WC=J.8 15 21 /12 DD=l 1 I -200=60 20 63/12 wC=1.2 2D DD=l I 9 -200=25 3t /12 WC=3.8 ùD=122 SC=-1.4 30 27 /12 35 70- 40 51 /12 wC=6.6 20/12 DD=l 26 -200:J1 45 80- 50 85- t 4/12 55 90- ts U I I L ô 60 95- 49/tz 65 100 - 105 -70 110 - xc 3 À) Pq oØo a. oø -o C)Ø o r !5Ð +on @oÐzôØ T G l.\) ñ$ LEGEND N TOPSO|L; GRAVELLY, CLAYEY, SILTY SAND 0RGANICS, FIRM, SLIGHTLY M0|ST, BROWN. SAND AND (SC-SM); CLAYEY, SILTY, SCATTERED GRAVEL T0 GRAVELLY L00SE T0 DENSE, SLIGHTLY MOIST, BROWN TAN. cLAysToNE/STLTSToNE (SC/MS) BEDRoCK. DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE. I DRTVE SAMpLE, 1 5/8-INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST. ".2.^ DRIVE SAMPLE BLOW COUNT. INDICATES THAT 12 BLOWS OF A 14o-POUND HAMMER ''/ '' FATLTNG 30 TNCHES WERE REQUIRED To DRtvE THE sAMPLER t2 tNcHEs. f rnacrtclL AUcER REFUSAL. NOTES 1. THE EXPLORATORY BORINGS WERE DRILLED ON APRIL 21,2021 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 MEASURED BY HAND LEVEL AND BOTH BORINGS WERE AT THE SAME LEVEL. 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 TRANSITIONS 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 DENSTTY (pct) (aSrV D2216); -2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1140); Sc = PERCENT SWELL (+) oR cONSOLlDATloN (-) UPON WETTING UNDER CONSTANT LoAD (ASTM D 4s46, METHOD B). LEGEND AND NOTTS Flg.321 -7 -348 Kumar & Associates ¡ g 1 0 -1 -2 às -5 j-t l¡J =I,t1 r_5 zo t- ô:-oo U1zo(Jj -8 -9 -10 -11 I,O APPLIED PRESSURE - KSF t0 SAMPLE OF: Silty Cloyey Sond FROM:Boringl@7.5' tNC = 2,4 %, DD = 109 pcf ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING I \ \ \ rh.rr r.st r.!u¡s oPPy onry ro ¡n. .omÞb¡ irrt6d, ft. t.stlñE r.9orl rholl not ba r.produc.d, .xoôpt ln lulì, elthoút th€ wrlll€ñ opprovol of Kum¡ ond Asroclol!6, lnc. Swrll Cùsol¡dollon t.ll¡ng p.dormod ln dccırdô¡c. wllh ASIM D-4516. I ) 21 -7 -348 Kumar & Associates SWELL-CONSOLIDATION TTST RESULTS Fig.4 SAMPLE OF: Silty Cloyey Sond FROM:Boring2@2A' WC = 3.8 %, DD = 122 pcf ; .. ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING i Th.8€ to6t rcsulþ opply onrY to tho lomplos t€d6d. ft! t.dlng ropod Êholl not b3 rrproduc€d, .xcspt in lull, without th€ wdtton opprovol of Kumor ond tu6oclot.., lnc. Sw6ll Consolldotloñ toding ptdorñôd ln 6..ôrddñcã rìth ñM D-4546. 1 ñ J.J t¡J =UI I zotr ôfo U1zo(J 0 -1 .L -5 -4 -5 -6 APPLIED PRESSURE - KSF 10 100 21 -7 -348 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig, 5 lGrtåiffiå'ffi{üiiiffü**'TABLE 1SUMMARY OF LABORATORY TEST RESULTSSilty Clayey SandSilty Clayey SandSilty Clayey SandVery Sandy Silt and ClaySilty Clayey SandSilty Clayey SandSilty Clayey SandSilty Clayey SandSOIL TYPE- 4.8- 1.4P/.1CONSOLIDATION(ololPLASTICINDEXATTERBERG LIMITS(ololLIQUID LIMIT60253440PERCENTPASSING NO.200 srEVE2444SAND("/"1GRADATION("/"1GRAVEL126t22focflNATURALDRYDENSITY10810951I1181192.02.43.23.8J.¿6.63.13.8t%tNATURALMOISTURECONTENT7v,l05120750120{ft)DEPTH2%BORINGI2SAMPLE LOCATIONect No. 21-7-348