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Home My WebLink AboutSubsoils Study for FoundationH-PryKUMAR 5020 County Road 154 Glenwood Springs, C0 81601 Phone: (970) 945-7988 Fax (970) 945-8454 Email: hpkglenwood@kumarusa.com Geotechnical Engineering I Engineering Geology Materials Testing I Environmental Office Locations: Parker, Glenwood Springs, and Summit County, Colorado STJBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE 982 COUNTY ROAD 245 GARFIELD COUNTY, COLORADO PROJECT NO. L7-7-207 APRIL L4,20L7 PREPARED FOR: DAN SPINK 420 WAGON WHEEL CIRCLE NEW CASTLE, COLORADO 81647 spink308@email.com PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at"982 County Road 245, on Hidden Valley 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 for geotechnical engineering services to Dan Spink dated February 21,2017. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils and bedrock obtained during the field exploration were tested in the laboratory to determine their classification, compressibility or swell 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 residence will be a single story, wood frame structure located on the site as shown on Figure 1. Ground floor is planned to be slab on grade at an elevation slightly above the existing ground surface. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 5 feet. We assume relatively light foundatlon 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. I H-P*TUVIRP Project No.17-7-2A7 -2- SITE CONDITIONS The site is located off County Road 245 at the extreme northern end of Hidden Valley Drive, see Figure 1. The area of the proposed residence is vacant and has undergone some previous grading. A previous residence to the south burned down and has been removed. The ground surface at the proposed residence is relatively flat and slightly sloping down to the southeast' There is a cut into the hillside to the west of the proposed residence up to about 8 feet high and has exposed shale bedrock. Vegetation on the site consists of grass and weeds with juniper trees nearby. FIELD EXPLORATION The field exploration for the project was conducted on March 20,2017. 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 and bedrock were taken with a 2 inch I.D. California Liner sampler. The sampler was driven into the subsoils and bedrock 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 and hardness of the bedrock. 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 profiles encountered at the site are shown on Figure 2. The subsoils encountered consisted of very stiff, sandy silty clay underlain at depths from about 4 to H.PTKUIVIAR Project No.17-7-2O7 -3- 17 feet by hard, claystone shale bedrock. The clay contained scattered shale fragments, and the bedrock was weathered in the upper few feet and shallower to the west near the hillside. The bedrock is the Mancos Shale Formation and extended down to the boring depths of 21 and 16 feet. The claystone shale and soil derived from the shale can, based on our experience in the immediate area, possess an expansion potential when wetted. Laboratory testing performed on samples obtained from the borings included natural moisture content and density, and percent finer than sand size gradation analyses. Results of swell- consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 through 6, generally indicate low compressibility under relatively light surcharge loading and a low to moderate expansion potential when wetted under a constant light surcharge. One sample of the clay soils (Boring 1 at 10') showed a relatively high swell potential. The laboratory testing is summarizedin Table 1. No free water was encountered in the borings at the time of drilling and the subsoils and bedrock were slightly moist. FOUNDATION BEARING CONDITIONS The subsoils and bedrock encountered at the site are expansive. Shallow foundations placed on the expansive materials similar to those encountered at this site can experience movement causing structural distress if the clay/claystone is subjected to changes in moisture content. A drilled pier foundation can be used to penetrate the expansive materials to place the bottom of the piers in azone of relatively stable moisture conditions and make it possible to load the piers sufficiently to resist uplift movements. In addition to their ability to reduce differential movements caused by expansive materials, straight-shaft piers have the advantage of providing relatively high supporting capacity. The piers can be constructed relatively quickly and should experience a relatively small amount of movement. Spread footings can be used for support of the residence with the understanding of a risk of foundation movement and building distress. To reduce the risk of foundation movement and H-P+KUMAR Project No. 17-7-207 -¿ -4- building distress, we recommend spread footings bear on a minimum 3 feet of compacted road base. The road base can consist of CDOT Class 2,5 or 6 material. It is imperative that foundation backfill be adequately compacted, positive exterior surface slope grade be maintained away from the foundation walls and irrigation near foundation walls be limited to reduce the risk of wetting to the bearing materials and distress to the residence. DESIGN RECOMMENDATIONS FOUNDATIONS Provided below are recoÍlmendations fot both.p*ud þoting ?n¿drilled foundation systems. Spread Footings: The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on a minimum 3 feet of properly placed and compacted road base can be designed for an allowable of 2,500 psf 2) Based on experience, we expect settlement or heave of footings designed and constructed as discussed in this section will be up to about 1 inch. There could be some additional movement if the bearing materials below the structural fill were to become wetted. The magnitude of the additional movement would depend on the depth and extend of the wetting but may be on the order of Vz to 1 inch. 3) The footings should have a minimum width of 16 inches for continuous footings and24 inches for isolated pads. 4) Continuous foundation walls should be heavily reinforced top and bottom to span local anomalies and better withstand the effects of some differential movement such as assuming an unsupported length of at least 15 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressures corresponding to an equivalent fluid unit weight of at least 60 pcf. 5) 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 the exterior grade is typically used in this atea. H-P\KUMAR Project No. 17-7-207 5 7) Prior to the footing construction, any existing fill, topsoil and the required depth of soiUshale removed to provide for at least 3 feet of structural fill, and the subgrade moistened to slightly above optimum and compacted. The road base placed as structural fill below the footings should be compacted to atleastgS/o standard Proctor density at a moisture content within about ZVo of optimum. The structural fill should extend at last 2 feet beyond the edges of the footings. A representative of the geotechnical engineer should observe all footing excavations and test structural fill compaction on a regular basis prior to concrete placement to evaluate bearing conditions. Drilled Piers: The design and construction criteria presented below should be observed for a straight-shaft drilled pier foundation system: 1) The piers should be designed for an allowable end bearing pressure of 25,000 psf and an allowable skin friction value of 2,500 psf for that poftion of the pier in bedrock. 2) Piers should also be designed for a minimum dead load pressure of 10,000 psf 3) based on pier end area only. If the minimum dead load requirement cannot be achieved, the pier length should be extended beyond the minimum penetration to make up the dead load deficit. This can be accomplished by assuming one-half the allowable skin friction value given above acts in the direction to resist uplift. Uplift on the piers from structural loading can be resisted by utilizing 75Vo of the allowable skin friction value plus an allowance for the weight of the pier. The piers should be at least 12 inches in diameter and should penetrate at least three pier diameters into the bedrock. A minimum penetration of 10 feet into the bedrock and a minimum pier length of 20 feet are also tecoÍtmended. The Z0 feet minimum depth is measured from the ground surface near the top of pier or adjacent excavation depth, whichever is greater. Piers should be designed to resist lateral loads assuming a modulus of horizontal subgrade reaction of 50 tcf in the clay soils and a modulus of horizontal subgrade reaction of 200 tcf in the bedrock. The modulus values given are for a long, l- foot-wide pier and must be corrected for pier size. 4) 6) 5) H-P\KUMAR Project No.17-7-207 -6- 6)Piers should be reinforced their full length with one #5 reinforcing rod for each 14 inches of pier perimeter to resist tension created by the swelling materials. A 4-inch void form should be provided beneath grade beams to prevent the swelling soil and rock from exerting uplift forces on the grade beams and to concentrate pier loadings. A void form should also be provided beneath pier caps. 8) Concrete utilized in the piers should be a fluid mix with sufficient slump so that concrete will filt the void between the reinforcing steel and the pier hole. IVe recoûrmend a slump in the range of 6 to 8 inches. 9) Pier holes should be properly cleaned prior to the placement of concrete. The drilling contractor should mobilize equipment of sufficient size to effectively drill through possible cemented bedrock zones. 10) Although free water was not encountered in the borings drilted at the site, some seepage in the pier holes may be encountered during drilling. If water cannot be removed prior to placement of concrete, the tremie method should be used after the hole has been cleaned of spoil. In no case should concrete be placed in more than 3 inches of water. I l) Care should be taken to prevent the forming of mushroom-shaped tops of the piers which can increase uplift force on the piers from swelling soils. L2) A representative ofthe geotechnical engineer should observe pier drilling operations on a full-time basis. FLOOR SLABS Floor slabs present a problem where expansive materials are present near floor slab elevation because sufficient dead load cannot be imposed on them to resist the uplift pressure generated when the materials are wetted and expand. 'We recommend that structural floors with crawlspace below be used for all floors in the building that will be sensitive to upward movement. Slab-on-grade construction may be used (such as in the garage area) provided the risk of distress is understood by the owner. 'We recommend placing at least 3 feet of road base as structural fill below floor slabs in order to mitigate slab movement due to expansive soils. 7) H-PÈKUMAR Project No. 17-7-207 -7- To reduce the effects of some differential movement, nonstructural floor slabs should be separated from all bearing walls, columns and partition walls with expansion joints which allow unrestrained vertical movement. Interior non-bearing partitions resting on floor slabs should be provided with a slip joint at the bottom of the wall so that, if the slab moves, the movement cannot be transmitted to the upper structure. This detail is also important for wallboards, stairways and door frames. Slip joints which allow at least 172 inches of vertical movement are recommended. Floor slab control joints should be used to reduce damage due to shrinkage cracking. Joint spacing and slab reinforcement should be established by the designer based on experience and the intended slab use. Required fill beneath slabs can consist of a suitable imported granular material such as CDOT Class 2,5 or 6 road base. The fill should be spread in thin horizontal lifts, adjusted to at or above optimum moisture content, and compacted to 95Vo of the maximum standard Proctor density. Prior to the structural fill placement, all topsoil and loose disturbed soil should be removed and the subgrade moistened to slightly above optimum and compacted. The above recoÍlmendations will not prevent slab heave if the expansive soils underlying the structural fill becomes wet, however, the recommendations will reduce the effects if slab heave occurs. All plumbing lines should be pressure tested before backfilling to help reduce the potential for wetting. UNDERDRAIN SYSTEM An underdrain should not be needed around slab-at-grade or shallow crawlspace areas (less than 4 feet deep) with proper compaction of foundation wall backfill and positive surface slope away from foundation walls. If drilled piers are used as the foundation system along with crawlspace construction, void form below the foundation grade beams can act as a conduit for water flow and a perimeter foundation drain should be provided for this situation. The perimeter foundation drain system, if needed, should consist of a drainpipe surrounded by free-draining granular material placed at the bottom of the wall backfill. The drain lines should be placed at each level ofexcavation and at least I foot below lowest adjacent finish grade, and sloped at a minimum l%o grade to a suitable gravity outlet. Free-draining granular material used H-PTKUMAR Project No.17-7-207 -8- in the drain system should consist of minus 2-inch aggregate with less than 50Vo passing the No. 4 sieve and less than 2Vo passing the No. 200 sieve. The drain gravel should be at least lVz feet deep and covered by filter fabric such as Mirafi 140N. An impervious liner such as 2O or 30 mil PVC should be placed below the drain gravel in a trough shape and attached to the foundation wall above the void form with mastic to keep drain water from flowing beneath the wall and into the crawlspace. SURFACE DRAINAGE Positive surface drainage is a very important aspect of the project to prevent wetting of the bearing materials below the residence. The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Excessive wetting or drying of the foundation excavations and underslab areas should be avoided during construction. Drying could increase the expansion potential of the soils and shale bedrock. 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95Vo of the maximum standard Proctor density in pavement areas and to at Ieast9OVo 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 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. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regula¡ heavy irrigation, such as lawn, and sprinkler heads should be located at least l0 feet from foundation walls. LIIVIITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recornmendations submitted in this report are based upon the data obtained H-P*KUMAR Project No. 17-7-207 -9- from the exploratory borings drilled at the locations indicated on Figure l, 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 f,rndings 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 to be different from those described in this report, we should be notified at once so re-evaluation of the recommendations may be made. This report has been prepared for the exclusive use by our client for design purposes. 'We are not responsible for technical interpretations by others of our information. As the project evolves, we should provide continued consultation and 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 modif,rcations of 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.Pt KUMAR Robert L. Duran, E. I. Reviewed David A. Young, P RLDlksw '/'J'fttr1 H-PèKUIVIAR Project No. 17-7-207 - ¡'<- l l T'Ì I '...'I NEW-CÁSÎLE COLORÂDO {¡i.'. {i .t! æs0 0 50 100 APPROXIMATI SCAI. T--FEFT ,i ,''[.-.,1'"-.f*' ' I . - ,"lll!ùcl VICINITY MAP NOT TO SCALE 17 -7 -207 H-PVKUMAR LOCATION OF TXPLORATORY BORINGS Fig. 1 ii I : BORING 1 BORING 2 0 0 21/12 WC=8.6 DD=l13 -2AO=84 30/12 WC=8.2 DD=116 tJ 28/ 12 WC=9.6 DD= 1 09 50/6 WC=7.6 DD= 1 30 5 10 31 /12 WC=9.1 DD=1 15 to F l¡J LÚ LL I-t-fL l¡Jcl es/12 FlrJ LJl! I :EF o_ LdÕ 15 1544/12 50/1 20 2050/3 aÊ 25 17 -7 -207 H.PryKUMAR LOGS OF EXPLORATORY BORINGS Fig. 2 LEGEND CLAY (CL); SILTY, SANDY, WITH SCATTERED CLAYSTONE SHALE FRAGMENTS, VERY STIFF, SL|GHTLY MOTST, GRAYTSH BROWN, CALCAREOUS AND/OR cypStFEROUS. CLAYSïONI BEDROCK; HARD, SLIGHTLY MOIST, GRAY, CALCAREOUS AND/OR GYPSTFEROUS MANCOS SHALE FORMATION. RELATIVELY UNDISTURBED DR¡VE SAMPLT; 2-INCH LD. CALIFORNIA LINER SAMpLE t1 /12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 21 BLOWS 0F A 14O-POUND HAMMER-,f '- FALLING 30 INCHES WERE REQUIRED TO DRIVE THE CALIFORNIA OR SPT SAMPLER 12 INCHES NOïE_S. 1. THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 20, 2017 WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER. 2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATËLY BY PACING FROM FEATURES SHOWN ON THE FIGURE 1 SITE PLAN. 3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE NOT MEASURED AND THE LOGS OF THE EXPLORATORY BORINGS ARE PLOTTED TO DEPTH. 4. THE EXPLORATORY BORING LOCATIONS 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 = WAIER CONTENT (%) (ASTM D 2216): DD = DRY DENSITY (PCt) (NSTU D 2216); -?.AÕ= PTRCENTAGE PASSING NO. 200 SIEVE (ASTM D 1 140). 17 -7 -207 H.PryKUMAR LEGTND AND NOTES Fig. 3 SAMPLE OF: Sondy Silfy Cloy FROM;Boringl@5' WC = 9.6 %, DD = 109 pcf th. Sr.{ ¡n¡th ol EXPANSION UNDER CONSTANT PRESSURE UPON WETTING 2 JJLJ =tJ7 I zotr Õ Jovtz.oO 0 -l 2 3 -4 PRESSURE -f00 17-7-207 H-PryKU]VIAR SWELL-CONSOLIDATION TEST RESULT Fis. 4 t I 7 6 JJt¡l =tn I z.o t- o =o ar1zoo 5 4 3 2 1 0 -1 -2 -3 t0 r00 SAMPLE OF: Sondy Silty Cloy FROM:Boringf@10' WC = 9.1 ?6, DD = 115 pcf ot ; r r l. EXPANSION UNOER CONSTANT PRESSURE UPON WETTING r îi i i 'i 17 -7 -207 H-PryKU]VIAR SWELL_CONSOLIDATION TEST RESULT Fig. 5 I SAMPLE OF: Sondy Silty Cloy FROM: Boring2@2-5' WC = 8.2 %, DD = 116 pcf ì-I ti EXPANSION UNDER CONSTANT PRESSURE UPON WETTING 2 ñ JJIJ Èvt I zo f- ô =oIAzoO 1 0 -1 -2 -3 f.0 t0 òe JJ l¿! =an I zotr o Jo U'z.o(J 3 2 0 -1 -2 I t.0 - KSF t0 t00 SAMPLE OF: Cloystone FROM:Boring2@5' WC = 7-6 %, DD = 150 pcf EXPANSION UNDER CONSTANT PRESSURE UPON WETTING 17-7-207 H.PryKUMAR SWELL-CONSOLIDATION TEST RESULT Fis. 6 I l-l- N l\Lil v lt-\l\TABLE 1SUMMARY OF LABORATORY TEST RESULTSProject No.17-7-207SOIL TYPESandy Silty ClaySandy Silty ClaySandy Silty ClaySandy Silty ClayClaystone11384UNCONFINEDCOMPRESSIVESTRENGTH(PSF)ATTERBERG LIMITSPLASTICINDEX(o/olLIQUIDLIMIT(o/olPERCENTPASSINGNO.200SIEVEGRADATIONSAND(o/'lGRAVEL(/ùNATURALDRYDENSITY(pcf)109115116130NATURALMOISTURECONTENT(o/"18.69.69,78.27.6SAMPLE LOCATIONDEPTHfft)2.55102.55BORINGI2