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HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.24.17H.PTKUMAR 5020 County Fload 154 Glenwood Springs, CO 81601 Phone: (970) 945-7998 Fax (970) 945-8454 Email: hpkglenwood@kumarusa.com Geotechnical Engineering I Engineering Geology Materials Testing I Environmental Office Locations: Parker, Glenwood Springs, and Silverthorne, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 31,ITERON CROSSING AT IRONBRIDGE RIVER BEND WAY GaRFIELD COUNTY, COLORADO PROJECT NO. t7-7-t29 JANUARY 24,2017 PREPARED FOR: RM CONSTRUCTION ATTN: TOM DOOGAN " 5O3O COUNTY ROAD'154 GLENWOOD SPRINGS, CO 81601M TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY PROPOSED CONSTRUCTION SITE CONDITIONS SUBSIDENCE POTENTIAL....., FMLD EXPLORATION SUBSURFACE CONDITIONS FOIJNDATION BEARING CONDMIONS DESIGN RECOMMENDATIONS ......... FOUNDATIONS FLOOR SLABS I.JNDERDRAIN SYSTEM.. SURFACE DRAINAGE LIMITATIONS FIGURE 1 - LOCATION OF EXPLORATORY BORING FIGURE 2 -LOG OF EXPLORATORY BORING FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS 1 ...........- 2 - .......- 3 - .....- 3 - I -2- -4- 4 4 5 5 6 ....- 6 - H-P \ KUMAR Project No. 17-7-129 PURPOSE AND SCOPE OF STUDY This report presents the results ofa subsoil study for a proposed residence to be located on Lot 31, Heron Crossing at lronbridge, River Bend V/ay, Garf,reld County, Colorado. The project site is shown on Figure 1. The purpose of the stlrdy was to develop recommendations for the foundation design. The study was conducted in accordance with our agreement for geotechnical engineering services to RM Construction dated January 16,2017. Hepworth-Pawlak Geotechnical (now H-P/Kumar) previously conducted a preliminary subsoil study in the Heron Crossing at konbridge development area and presented the findings in a report dated Febnrary 28,2014, Job No. 113 471A. An exploratory boring was drilled 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 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 Building plans were conceptual at the time of our study. In general, the proposed residence will be a one-story structure with an attached slab-on-grade garage located as shown on Figtrre L Ground floor of the residence will be structural over crawlspace. Grading for the structure is assumed to be relatively minor with cut depths between about 3 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. H-P+ KUMAR Project No. 17-7-129 -2- SITE CONDITIONS The lot was vacant and covercd with about 2 inches of snow and scattered weeds and grass at the time of the field exploration. The ground surface had been stripped of topsoil. The ground surface gently slopes down to the north. River Bend Way is asphalt paved and follows the northeast boundary of the lot. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge Development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. There is a possibiiity 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 of localized subsidence. Several sinkholes were observed during geological assessments conducted for the konbridge development. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork Valley. A sinkhole opened in the cart storage pa*ing lot in January 2005 and iregular bedrock conditions have been identified in the affordable housing site located roughly 500 to 1,000 feet south of the cunent development area. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory boring was relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot 31 throughout the service life ofthe proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If ftrrther investigation of possible cavities in the bedrock below the site is desired, we should be contacted. H-P* KUMAR Project No. 17-7-129 -3- FIELD EXPLORATION The field exploration for the project was conducted on January 17,2017. One exploratory boring was drilled at the location shown on Figure I to evaluate the subsurface conditions. The boring was advanced with 4 inch diameter continuous flight augers powered by a truck-mounted cME-458 drill rig. The boring was logged by a representative of H-p/Kumar. Samples of the subsoils were taken with l% inch and 2 inch I.D. spoon samplers. The samplers were driven into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D-15g6. The penetration resistance values ale 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 Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACB CONDITIONS A graphic log of the subsurface conditions encountered at the site is shown on Figgre 2. The subsoils consist ofabout I 1 feet ofvery stiff, sandy silt and clay overlying dense, silty sandy gravel with probable cobbles and boulders. Laboratory testing performed on samples obtained from the boring included natural moisture content and density, and finer than sand size (minus No. 200 sieve) gradation analyses. Resglts of swell-consolidation testing performed on relatively undisturbed drive samples of the upper silt and clay soil, presented on Figure 3, indicate low to moderate compressibility under conditions of loading and wetting with a nil to low swell potential when wetted. The laboratory testing is summarized in Table 1, No free water was encountered in the boring at the time of drilling and the subsoils were slightly morst. H-P + KUMAR Project No. 17-7-129 -4- F'OUNDATION BEARING CONDITIONS The upper sandy silt and clay soils have low bearing capacity and low to moderate compressibility mainly when loaded after wetting. Shallow spread footings placed on the natural silt and clay soils can be used with a risk of movement as described below. The footing bearing level should be at least 4 feet below existing ground surface so there is no more than 7 feet of compressible soils below the bearing level. Use of a deep foundation placed on the underlying dense gravel soils could be used to achieve a low settlement risk. The expansion measured on mainly the sample from 5 feet depth appears to be an anomaly but the expansion potential should be ftuther evaluated at the time of construction. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encor¡ntered in the exploratory boring and the nature of the proposed construction, we tecommend the building be founded with spread footings bearing on the natural silt and clay soils. If a deep foundation is desired, we should be contacted for supplemental recommendations. 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 1,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be abor-rt % to I inch or less. Additional differential movement up to I inch could occur if the bearing soils are wetted. 2) The footings should have a minimum width of 20 inches for continuo¡rs 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 H-P\ KUMAR Project No.17-7-129 -5- of foundations 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 14 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral earth pressure coffesponding to an equivalent fluid unit weight of at least 55 pcf for on-site silt and clay soil as backfill. Any existing fill, topsoil and any loose or disturbed soils should be removed. The exposed soils in footing area should then be moistened and compacted. A representative ofthe geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. The natural on-site soils, exclusive of topsoil, are suitable to support lightty loaded slab-on-grade construction with a movement risk 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 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. All fill materials for support of floor slabs should be compacted to at least 957a of maximum standard Proctor density at a moistrlre content near optimum. Reqr-rired fill can consist of the on- site silt and clay soils devoid of vegetation and topsoil. UNDERDRAIN SYSTEM It is our understanding the finished floor elevation at the lowest level will be at or above the surrounding grade. Therefore, a foundation drain system is not required. 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 mnoff can create a perched condition. We recommend below-grade construction, such as retaining walls and basement 5) 6) FLOOR SLABS H-P* KUMAR Project No. 17.7-129 -6- areas, be protected from wetting and hydrostatic pressure buildup by an underdrain and wall drain system. An underdrain should not be plovided around the crawlspace to help prevent surface water infiltration down to the bearing soils. If the finished floor elevation of the proposed structure has a basement level, we should be contacted to provide recommendations for an underdrain system. All earth retaining stmctures should be properly drained. SURFACE DRAINAGE Proper surface grading and drainage will be critical to the satisfactory performance of the building. The following drainage precatttions 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 moisturc and compacted to at least 95Vo of the maximum standard Proctor density in pavement and slab areas and to at least 907o 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 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in paved areas. Drainage swales should have a minimum slope of 3Vo. 4) Roof gutters should be provided with downspouts and drains that discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation should be located at least 10 feet from foundation walls. Consideration should be given to Lrse of xeriscape 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 area at this time. 'We make no warranty either express or implied H-P * KUMAR Project No. 17-7-129 -7 - The conclusions and recommenclations submittecl in this report are based upon the data obtained from the exploratory boring drilled at the location 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 findings include interpolation and extrapolation of the subsurface conditions identified at the exploratory boring and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encountered during construction appear different from those described in this report, we should be notified so that re-evaluation of the recommendations may be made. This report has been prepared for the exclusive use by our client for design purposes. We are not responsible for technical interpretations by others of our information. As the project evolves, we shottld 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 anaiysis or modifications to the recommendations presented herein. 'We recommend on-site observation ofexcavations and foundation bearing strata and testing ofstructural fill by a representative of the geotechnical engineer. Respectftrlly Submitted, I.I-P* KUM,AR Ålrrrt* r/r(Nt\@ Shane M. Mello, Staff Engineer Reviewed by: Steven L. Pawlak, P SMM/ksw H.P I KUMAR Project No.17-7-129 RIVER BEND WAY LOT 30 LOT 52 - I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I PROPOSED BUILDING AREA a BORING 1 LOT 51 EUILDING SETBACK LINE -1 IL-- I I -J SCALE-FEET 17-7 -129 H-PryKUMAR LOCATION OF EXPLORATORY BORING Fig. 1 BORING 1 LEGEND 0 SILT AND CLÄY (ML-CL); SANDY, VERY STIFF, SUGHTLY MO|ST, SROWN, SUGHTLY POROUS. t6/12 WC=6.3 lF n*outl (cM); srLTy, sANDy, coBBLEs AND BouLDrRs [:..H rnoamLE, DENSE, SLIGHTLY M0|ST, BRoWN, RoUNDED RocK.0D=1 01 5 2t/12 WC=7.0 DD=l 09 F i DRIVE SAMPLT, z-INCH I.D. CALIFORNIA LINER SAMPLE t-l¡! LJlr It¡-o-l¡lo 0RtvE sAMpLE, I 5/S-|NCH t.D. SpLtT SP0ON STANDARD PENEIRATTON TEST. 10 2a/n WC=4.4 0D=l 08 -200=57 16712DR|VE SAMPLT BLOW COUNT. INDICATES ÎHAT 16 BL0WS OF¡-,'-A 110-POUND HAMMER FALLING 50 INCHES WERE RTAUIRED TO DRIVE THE SAMPLER 12 INCHES. 15 NOTES so/4 THE EXPLORATORY BORING WAS DRILLED ON JANUARY 17, 2017 WIÏH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUCER. 2, THE LOCATION OF THE EXPLORATORY BORING WAS MIASURTD APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITT PLAN PROVIDED. 20 3. THE ELEVATION OF THE EXPLORATORY BORING WAS NOT MIASURED AND THE LOG OF THE IXPLORATORY BORING IS PLOTTED TO DEPTH. 4, THE IXPLORÂTORY BORING LOCATION SHOULD BE CONSIDEREO ACCURATE ONLY TO THE DEGRTE ÍMPLIED BY THE METHOD USEÓ. 5. ÏHE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOG REPRESilT THE APPROXIMATE BOUNDARIES BETWIEN MATERIAL TYPTS ANO THE TRÄNSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORING AT THE TIMT OF DRILLING. 7, LABORATORY TEST RESULTS: WC = WATER coNTtNT (%) (ASTM 0 22t6); DD = DRy DENstry (pcf) (tSru o zzto);' -2OO = PERCENTAGE PASSING NO. 2OO SIEVT (ASTM D If4O). 17 -7 -129 H-PryKUMAR LOG OF EXPLORATORY BORING Fig. 2 SAMPLE OF: Sondy Sill ond Ctoy FROM:Boring 1@ 2.5' WC = 6.5 %, DD = 101 pcf EXPANSION UNDER CONSTANT PRESSURE UPON WETTING JJ l¡J =ltt I zo F- ô Jo t/1zoo JJ t¡J =UI I zo l- o Jo altz()() 1 0 -1 -2 -? 2 0 -1 -2 t0 SAMPLE 0F: Sondy Silt ond Cloy FR0M:Boring 1 @ 5' WC = 7.0 %, DD = 1Og pcf lnd h 17 -7 -129 H.PryKUMAR SWELL-CONSOLIDATION TEST RESULTS Fis. 3 H.P*KUMARTABLE 1SUMMARY OF LABORATORY TEST RESULTSProject No. 17-7-129SOIL TYPESandy Silt ancl ClaySandy Silt and ClaySandy Silt and ClayUNCONFINEDCOMPRESSIVESTRENGTHIPSF}ATTERBERG LIMITSPLASTICINDEX(%lLIQUIDLIMIT(%lPERCENTPASSINGNO.200SIEVE57GRADATIONSAND(V"lGRAVEL%t6.3101NATURALDRYDENS]TY{pcf}109108"NATURALMOISTURECONTENT(o/"17.44.4SAMPLE LOCATIONEORINGDEPTH2%5I01