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HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.19.2021I Grt äåiþïfi:iiffffiif r#ü *"' An Employee Owned Compony 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email : kaglenwood@kumarusa.com www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT F-17, FILING I, ASPEN GLBN 109 WEST DIAMOND A RANCH ROAD GARFIELD COUNTY, COLORADO PROJECT NO.20-7-771 JANUARY 19,2021 PREPARED FOR: BELLA VILLA BUILDERS ATTN: RUSSELL BURTON P.O. BOX 875 CONIFER, COLORADO 80433 russell. bu rton@bellavilla builders. net TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY. PROPOSED CONSTRUCTION ... SITE CONDITIONS.. SUBSIDENCE POTENTIAL. FIELD EXPLORATION.. SUBSURFACE CONDITIONS DESIGN RECOMMENDATIONS ............ FOTINDATIONS .... FOUNDATION AND RETAINING WALLS.. FLOOR SLABS TINDERDRAIN SYSTEM ........... SURFACE DRAINAGE............... LIMITATIONS REFERENCES FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - GRADATION TEST RESULTS APPENDIX 1 I 1 I .-2- 1 ..-2- -6- -7 J tJ 4 5 5 6 Kumar & Associates, lnc. @ Project No.20-7-771 PURPOSE AND SCOPE OF STUDY This report presents the results ofa subsoil study for a proposed residence to be located on LotF-lT,Filing 1, Aspen Glen, 109 V/est Diamond A Ranch Road, 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 f'or geotechnical engineering services to Bella Villa Builders dated December 21, 2020. 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 analyzedto 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 residence will be a two-story wood-frame structure over basement level with attached garage. The basement and gafage floors will be slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 4 to l0 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 vacant and covered with 4 inches of snow at the time of our field exploration. The ground surface is gently sloping down to the south at a grade of about 2 percent. Vegetation consists of grass and weeds. Kumar & Associates, lnc. o Project No.20-7-771 1 SUBSIDENCE POTBNTIAL The Aspen Glen Subdivision is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. 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, several broad subsidence areas and smaller size sinkhole areas were observed scattered throughout the Aspen Glen development, predominantly on the east side of the Roaring Fork River (Chen-Northem, Inc. 1993). Lot F-17 is mapped as being just outside the west side of a broad depression. Sinkholes were mapped about 250 feet east and 250 feet north of the subject lot. 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 F-17 is low and similar to other lots in the area but the owner should be aware of the potential for sinkhole development. We have in the attached appendix, the Chen-Northem recommendations for building in a broad surface depression area. We believe these recommendations are conservative but will reduce structural distress in the event of future ground movement and should be considered in the design. FIELD EXPLORATION The field exploration for the project was conducted on January 8,2021. 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 Kumar & Associates, Inc. Samples of the subsoils were taken with a 7%inch I.D. spoon sampler. The sampler was 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. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils consist of about Yzfootof topsoil overlying dense, silty sand and gravel to the maximum Kumar & Associates, lnc. @ Project No, 20-7-771 -3- drilled depth of ll% feet. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in both borings at about 1 I feet. Laboratory testing performed on samples obtained from the borings included natural moisture content and gradation analyses. Results of gradation analyses performed on small diameter drive samples (minus l%-inchfraction) of the coarse granular subsoils are shown on Figure 3. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist. DESIGN RECOMMENDATIONS FOIINDATIONS Considering the subsurface conditions encountered in the.exploratory borings and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the natural granular soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural granular soils should be designed for an allowable bearing pressure of 3,000 psf._Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about I inch or less. 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 of foundations at least 36 inches below exterior grade is typically used in this area. 4) Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 10 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. Kumar & Associates, lnc. @ Project No.20-7-77'l -4- Topsoil and any loose disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural granular 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. FOLINDATION 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 45 pcf for backfill consisting of the on-site granular 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 35 pcf for backfill consisting of the on-site granular soils. 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. Backfill should be placed in uniform lifts and compacted to at least 90% of the maxlmum standard Proctor density at a moisture content near optimum. Backfill placed in pavement and walkway areas should be compacted to at least 95o/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 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 backfill. Backfill should not contain organics, debris or rock larger than about 6 inches. 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 s) 6) Kumar & Associates, lnc. @ Project No. 20-7-771 -5- based on a coefficient of friction of 0.50. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 425 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 a granular material compacted to at least 95o/o of the maximum standard Proctor density at a moisture content near optimum. 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 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 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 50%o retained on the No. 4 sieve and less than2Yo passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on- site granular soils devoid of vegetation, topsoil and oversized rock. UNDE,RDRAIN 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 runoffcan create a perched condition. V/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 1 foot below lowest adjacent finish grade and sloped at a minimum 10% to a suitable gravity outlet or drywell. Free-draining granular material used in the underdrain Kumar & Associates, lnc. @ Project No. 20-7-771 -6- system should contain less than 2o/o passingthe No. 200 sieve, less than 50olo passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least I% feet deep. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times 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 near optimum moisture and compacted to at least 95o/o 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. 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 6 inches in the first 10 feet in unpaved areas and a minimum slope of 2Y, 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, finer graded, soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. LIMITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area 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 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 Kumar & Associates, lnc. @ Project No, 20-7-771 4 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. \Me 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 implernentation of our recoffunendations, and to veriff 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 frll by a representative of the geotechnical engineer. Respectfully Submitted, K¡¡mar & Associafes, [nc. James H. Parsons, E.I. Reviewed by: ¡ 'r, j: 't ,,t- -: Daniel E. Hardin, JHP/kac RE,FERENCES: Chen-Northern, Inc. 1991, Preliminary Geotechnical Engineering Study, Proposed Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated December 20,1991, Job No. 4112 92. Chen-Northem, Inc. 1993, Geotechnical Engineering Studyfor Preliminary Plat Design, Aspen Glen Development, Garfield county, colorado, prepared for Aspen Glen company, dated May 28, 1993, Job No. 4 ll2 92. Xumar & Asscciat*s, lnc.Projeet Ho. 20-7"771 Jonuory r9,2021 - llrlToñ7-77 tRôrldànè.-D¡oño¡d A Ro¡chb.'a\\\ÀtIc!UI¡T@oo4,z6)ff-ÞÐPtoIôttr,'r.l\\tt\@ovz.6)-Or$r.ËFEEt€å'il,T\\übtff+2ı&I¡{Ëhriã*rå)¡r¡."rh%@!7åNJON-E-oñoxÞ--lTNll)c)rrrlI-flrrtrnI3a*e-N)OI!I!Ixc3g)-eoU'Øoo.0)oØt-OC)--lOz.O-lrnX-ut-OÐ--.{OnæOnz.6)U)Tl(o ¿ ì à E ı I i BORING 1 EL. 6065' BORING 2 EL. 6066' 00 5 50/ 12 58/ 12 WC=2.0 +4=54 -2OA=10 50 / 4.5 50/5 q f-t! LJtL I-F o_ L¡lo 61/12 50/ 4 50 / 4.5 10 10 so/3 15 15 N TOPSOIL; CLAY, SANDY, ORGANICS, FIRM, MOIST' DARK BROWN GRAVEL (GM); SANDY, SLIGHTLY SILTY, WITH C0BBLES, DENSE, SLIGHTLY MOIST, BROWN i DRTVE SAMPLE, 1 3/8-INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST. F^IA^ DRIVE SAMPLE BLOW COUNT' INDICATES THAT 50 BLOWS OF A 14o_POUND HAMMER3v/ t¿ FALLTNG 30 TNCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES. NOTES 1 . THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 8, 2O2O WITH A 4_INCH_DIAMETER CONTINUOUS_FLIGHT POWER AUGER. 2. THE LOCATIONS OF THE EXPTORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN PROVIDED. 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); +4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913); _2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM 01140). t- l,¡J L¡J LL I-t-(L t¡Jô Fig. 2LOGS OF TXPLORATORY BORINGSKumar & Associates20-7 -77 1 P t ı ¡ e E I ã SIÊVE ANALYSISHYDROMETER ANALYSIS SOUARE OPENINGSU.S, STANDÁRO SÉRIES 24 HRS 7 HRS1( V¡É i5 VIN IIME REAOINGS 6oYrN f gyrN 4MrN lUlX lj ' '',J 1 '/;-t. '¡il t + L ti:lLii r I l I :l ¡: TTLl 100 90 ao 70 60 50 40 30 20 t0 o o 10 20 30 10 50 60 70 ao 90 2 = f' loo .o75 r .600 125 l9 200 .o0t .009 152 DIAMETER OF PARTICLES IN MI RS CLAY TO SILT COBBLES GRAVEL 51 % SAND LIQUID LIMIÏ SAMPLE OF: Slightly Silty Sondy Grovel 36% PLASTICITY INDEX SILT AND CLAY 10 % FROM:Boringl@4' th€sê l€sl r€sulls oÞply only to lh€ somol€s which w€r€ l€sl€d. Ih€ lcslinq rcporl sholl not b. raProduccd, cxcepi in full, without lhc wr¡tl6ñ ooorovol of Kumor & Associotos, lnc. sieve onolvsis lsst¡nq i3 performód ¡n occordoncá wlth AST-M D6913, ASTM 07928' ASIM Cl56 ond/or ASTM D'll'{0. GRAVELSAND FI NE COARSEMEDTUM lCOAnSeFINE Fig.3GRADATION TEST RESULTSKumar & Associates20-7 -77 1 APPEF{ÐIX 1 8 practical due to the depth of the sinkioles. The grouting procedure should help reduce the setllement risk but not tota.liy eliminate it. Therefore, we beiieve that avoiding the sinkholes by building setback is the lower risk and the more appropriate approach thar should be nken. : Based on our hndings, development within the ground surface depression areas (shown on Fig. 1) should be feasible provided appropriate mitigalive designs are implemented fo¡ the residential buitdings, utilities a¡d roadways as described beiow. The appropriate level of the mitigatíve designs depend on the potential ground defr:rmalion, the building t¡,'pe, iocalion a¡d configuralion and ievel of tolerable maintenance (maidy fo¡ roadways and utilities). Building design considerations include use of a relativeiy rigid foundation, (such as a stiffened siab or râft) and a simply shaped buildirg footprint to reduce potential damage in the event of differenrial movement. These design concepts would be included in the engineered foundations for residences located i¡l the depression areâs. Utilities should be designed and constructed to be relatively flexible and ailow fo¡ differential movement without rupturing. Where possible, settlement sensitive main urility iines should be routed outside of the ground surface depression areas. Roadways ca¡¡ be convenlionally designed and constructed with provisions for maintenance if subsidence related dist¡ess is experienced. There a¡e several geofechnical design concepts which can be used to mitigate potential subsidence damage to residendal buildings and undergrounci urilities. Special mitigative designs for a specific lot should be developed by the owner's a¡chitect a¡d structurai engineer and should be based on the type of building proposed and the site specific foundation conditions. The following design concepts are presented to assist jn evaiuating design oplions prior to site 16r<.rr,nq | ìç,nee, \ anO li¡rcntrstsChenèNorthern lnc. i t t I I I I I I 9 specific investigations for a¡ individuaì building site. The concept for underground utili¡ies should be incorporated into the urility design by the developer. Building Configurations: The extent of damage to a building subjected to the surface effects of subsidence may be reduced by implementing several architectural measures in the building design. These measures would include the following: * Relatively flexible strucfur¿l systems such as wood frame construction, flexible exterior siding, and dry wall interior partitions are preferable to less flerible masonry structural system and exterior sidings. * Interior non-be¿ring partitions resting on the floor slab should be provided with slip joints so that slab movements are not lransmitted to the upper strucfure. * The buitding should be a low structure preferably ümited to one or fwo stories. * The building should have ¡elarively smaij plan dimensions of 60 feet or less. If this is not practical then the building should be divided into independent modules. x The building configuration should be a simple rectangular configuration with straight foundation walls and a minimum of side projections from the main building. x The ground floor should be on a single level rather than using a split level design. * Basements are parricularly susceptible to subsidence damage and are not recommended unless the entire foundation is at basement level and designed for lateral earth loading. a ChenèNorthern,Inc.Coñsrjl¡'ôq Énq,oee¡s Anı Scrent¡sts T I T I t t I I I " I I I l0 Building Foundations: A raft foundation with a bearing level nea¡ the exterior grade appeårs to be an appropriate foundation system for reducing the vulnerabiliry of buildings to differential subsidence damage. Typicai shallow spread footings would be a relatively flexible system and a rigid system is preferable for the larger magnirude deformatioris. Foundation syster.n considerations are outlined below: * A raft foundation system is the preferable system and should be designed according to the site specific soil bearing conditions. x The bottorn surface of the raft should be smooth and free of vertical projections. * The raft should be separated from the bearing soils by placing the raft on a minimum 4-inch thick compacted, clean sand. A polyethylene sheet should be placed between the raft and the sand layer. * The use of deep foundation walls should be minimized to the extent practical. The soil pressure equal to at least twice the "a[ rest" eâ¡th pressure (on the order of 80 to i00 pcf equivalent fluid unit weight) should be assumed to act on all vertical surfaces in contact with the foundation soils. * The bearing elevation of the raft should be placed below frost depth o¡ sufficient soil cover should be provided for frost protection. Underground Urilities: Underground utiiities are susceptible to the affects of a¡ea subsidence. As outlined below there are several mitigative design concepts which can be used to reduce the potential for damage. In our opinion the mitigation measures should be used where underground utiiities are iocated in the ground surface depression areas shown on Fig i. Consu¡t'nq Enq,¡eers a.,c Sa e,r,rst(ChenèNorthern,Inc. * * t t I I I I * * * ll Flexible joinrs should be used between adjacent pipe segments for both gravity and pressure lines. positive restråints should be provided in pressure lines to prevent pipe separation' A flexiblejoint should be provided as close as practica.l to any building, manhole, or other rigid structural connection. A soii cushion in the immediate vicinify of the pipe should be provided by not over-compacting the backfill soils close to the pipe' Check valves shculd be placed at appropriate locatjons on all gas and water mains to permìt intemrption of flow in case of subsidence distress. DEBRIS FLOW RISK AND MITIGATION Hazard Evaluation: This study shows that the alluviat and debris fans along the western side of the development are potential sites of water flooding and debris flows' The area evaluated is shown on the attached Fig lA. A summary of the basins and fans evaluatel is presented on the attached Table II. The calculated flow depths and volumes are based on hydrological data provided by Schmueser Gordon Meyer, Inc' potential water floods, with high sediment concentrations, should be considered for all of the basins upslope of the fans. Appropriate surface water hydrorogic methods should be used to evaluate the flood hazards on ail fans. Fa¡s I and 2 in the southern part of the a¡ea are not subject to debris flows, but debris flows should be considered on Fans 3 through 25 ød the a¡ea to the north (see Fig. 1A)- Based on numerical debris flow modeling, we have designated three potential hazard Consuil,nq f ^_e'neers 3ôd Screnl,S:sChenèNorthern,lnc.