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HomeMy WebLinkAboutSubsoil for Foundation DesignSUBSOIL STUDY Hqmmrh·Pil11:Llk Oc11rcchnic·il, Inc 5020 Count' RuaJ 1;~ Glcrurnll<l Sprm;?s. Columdu 8160! Ph1mc 9i(l 9-l5·i98R Fa:.. 970.94;.s.JH ~mu\ hpl!t.'tt'!ilhpi;cot1.'1.h wm FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 32, HERON CROSSING AT IRONBRIDGE RIVER BEND \VAY GARFIELD COUNTY, COLORADO JO:P N0.116 210A JUNE9,2016 PREPARED FOR: RM CONSTRUCTION ATTN : ERIC LINTJER 5030 COUNTY ROAD 154 GLENWOOD SPRINGS, COLORADO 81601 eric@bulldwithrm.com Parker 303-841-7119 ° Colorndo Springs 719-633-5562 • Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY ............................................................................ -l - PROPOSED CONSTRUCTION •..............•.....•...........................•..........................•....... - l - SITE CONDITIONS ......................................................................................................... - 2 - SUBSIDENCE POTEITTIAL ...••.•..........•..•.•..............•................•..........•.••............•.•..... -2- 'FIELD EXPLORATION ................................................................................................... 3 .. SUBSURFACE CONDITIONS ........................................................................................ -3 - FOUNDATION BEARING CONDITIONS .................................................................. • 4- DESIGN RE.COMMENDATIONS ........•.........•.•.............••••..............•.•.•.........•.•..•..•...... -4 - FOUNDATIONS ......................................................................................................... - 4 - FLOOR SLABS .••...........••...••.•.......•..•••...•.•..•.•..•.••••..••.•••••.......•.•••••••.....•.•.•....••..•.•••... -5 - UNDERDRAIN SYSTEM .......................................................................................... -6 - SURFACE DRA.Il'fAGE .............................................................................................. -6 - L™IT ATIONS ···········-···································································································-7 - FIGURE 1 -LOCATION OF EXPLORATORY BORINGS FIGURE 2 -LOOS OF EXPLORATORY BORINGS FIGURE 3 -LEGEND AND NOTES FIGURES 4 AND 5 -SWEL~CONSOLIDATION TEST RESULTS TABLE 1-SUMMARYOFLABORATORYTESTRESULTS .. PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for n proposed residence to be located on Lot 32, Heron Crossing at lronbridge, River Bend Way, 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 RM Construction dated May 24, 2016. We previously conducted a preliminary subsoil study in the Heron Crossing at lronbridge development area and presented our findings in n report dated February 28, 2014, Job No. 113 47 lA. A field exploration program consisting of exploratory borings was conducted lo 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 presenls our conclusions, design recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION Development plnns for the lot were conceptual at the time of our study. In general, the proposed residence will be a single story, wood frame structure located in the building envelope shown on Figure 1. Ground floor will be structural over crawlspace in the living area and slab-on-grade in the garage . Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 5 feet. We assume relatively light foundation loadings, typical of lhe proposed type of construction. Job No 116 210A -2- If building loadings. location or grading plans change significantly from those described nbove, we should be notified to re-eva1uate the recommendations contained in this report. SITE CONDITIONS The Jot was vacant and had scattered grass and weeds with old hydroseed in the northern (front) part at the time of our field exploration. The ground surface had been stripped of topsoil and the subdivision had been graded. The ground surface slopes gently down to the north with about 2 feet of elevation difference across the lot. River Bend Way is asphalt paved and folJows the northern boundary of the lot. SUBSIDENCE POTENTIAL Bedro_ck of the Pennsylvanian age Eagle Vnlley Evaporite underlies the Ironbridge development. These rocks are n 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 cnn produce areas of localized subsidence. Several sinkholes were observed during geologic assessments conducted for the Ironbridge development. These sinkholes appe~d similar to others associated with the Eagle Valley Evapc>ritc in areas of the Roaring Fork River valley. A sink.bole opened in the cart storage parking lot in January 2005 and irregular bedrock conditions have been identified in the affordnble housing site located roughly 500 to 1,000 feet south of the current 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 borings were relatively shallow, for foundation design only. Bns~d 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 32 throughout the service life of the proposed residence, in our opinion, is low; however, the owner sho1:1ld be made aware Joh No. 116 2IOA -3 - 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 field exploration for the project was conducted on May 25, 2016. Two exploratory borings were drilled at the locations shown on Figure 1 to evalu:ite the subsurface conditions. The borings were advanced with 4 -inch diameter continuous flight augers powered by a truck-mounted CME-45B drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with P/a 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 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 8 to 1 OV7 feet of stiff, slightly moist, sandy. silty clay overlying dense, slightly silty, sandy gravel and cobbles with boulders. Drilling in the coarse granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit. Laboratory testing performed on samples obtained from the borings included natural moisture content and density and finer than sand size gradation analyses. Results of sweU-consolidation testing performed on relatively undisturbed drive samples of the clay Job No . 116 '.?IOA -4- and silt soils, presented on Figure 4 and 5, indicate low to moderate compressibility under conditions of loading and wetting in the sample from Boring 1 al 2¥1 feet. The sample from Boring 2 at S feet showed a low collapse potential (settlement under constant loading) when wetted and high compressibility under additional loading. The laboratory testing is summarized in Table I. No free water was encountered in the borings nt the time of drilling and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The upper snndy, silty clay soils have low bearing capacity and low to moderate compressibility mainly when wetted. Shallow spread footing5 placed on the natural clay soils can be used with a risk of settlement as described below. The footing bearing level should be at least 3 feet below existing ground surface so there is no more than 7 feet of compressible soils below the bearing level. Use of n deep foundation placed on the underlying dense gravel nnd cobbles soils could be used to achieve a low settlement risk. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the building can be founded with spread footings bearing on the natural clay and silt soils with a settlement risk. If n 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. l) Footings placed on the undisturbed natural soils should be designed for an allowable bearing pressure of 1,000 psf. Based on experience. we expect Job No . l 16 llOA -5- inilial settlement of footings designed and constructed as discussed in this section will be about Vl to I inch or less. Additional differential settlement up to about I inch could occur if the bearing soils are wetted. 2) The footings should have a minimum width of 20 inches for continuous walJs and 2 feel 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 heavily reinforced top and bottom to span local anomalies such as by assuming an unsupporled length of at least 14 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral eanh pressure corresponding to an equivalent fluid unit weight of at least 55 pcf for the onsite clay and silt soil as backfill. 5) Any existing fill, topsoil and loose or disturbed soils should be removed in the footing nreas. The exposed soils in footing area should then be moistened and compacted. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FLOOR SLABS The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab- on-grade construction with a settlement 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. Aoor 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. Job No. l 16 210A -6- All fill materials for support of floor slabs should be compacted to nt least 95% of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on-site clay nnd silt soils devoid of vegetation and topsoil. UNDERDRAIN SYSTEM It is our understand ing that the finished floor elevation at the lowest level will be al or above the surrounding grade. Therefore, a foundation drain system is not required. It has been our experience in the area that local perched groundwa ter 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 and basement areas, be protected from wetting nnd hydrostatic pressure buildup by an underdrain and wall drain system. An undcrdrain should not be provided 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 structures should be properly drained . SURFACE DRAINAGE Providing proper surface grading nnd drainage is very important to the satisfactory performance of the building. The following drainage precautions should be observed during construction and maintained at all times after the ~sidence has been completed: 1) Inundation of the 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 95% of the maximum standard Proctor density in pavement and slab areas and to at least 90% of the maximum standard Proctor density in landscape areas. 3) The ground sudnce surrounding the exterior of the building should be sloped to drain away from the foundation in all directions. We Job No. ll6 210A -7- 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 IO feet in paved areas . Drainage swales should have a minimum slope of 3%. 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 such as sod should be located at least IO feet from foundations. Consideration should be given to use of xerisc<ipe lo reduce the potential for wetting of soils below the building caused by irrigation . LIMITATIONS This study has been conducted in accordance with generJlly accepted geotechnical engineering principles and practices in this area at this 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 I, 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 chis 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 perfonned. 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 exclus ive use by our client for design purposes. We are not responsible for technical interpretations by others of our infonnation. 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 Jcb No 116 210A. -8· 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 n representative of the geotechnicnl engineer. Respectfully Submitted, SLP/ksw cc: Job No . 116 2lOA 116 210A RIVER BEND WAY APPROXIMATE SCALE 1· = 30' ------~ LOT33 LOCATION OF EXPLORATORY BORINGS Figure 1 0 5 ~ I ..c:. a. ~ 10 15 116 210A BORING 1 ELEV.= 5931 .5' 9/12 WC=7.3 DO::i:98 11/12 WC=72 00=94 ·200•76 1516.31/6 1916.50/3 BORING2 ELEV.= 5930 5' 111'2 WCcs66 00•100 ·200=83 10/12 WC •7.0 00.,.90 47112 501'2 Note: Explanation of symbols is shown on Figure 3. LOGS OF EXPLORATORY BORINGS 0 5 ~ • £ a. Cl) c 10 15 Figure 2 LEGEND: Q CLAY (CL); silty, sandy, stilf, slightly moist, brown, slightly porous ~ GRAVEL AND COBBLES (GM·GP); silty, sandy, boulders . dense, slightly moist, brown, rounded rock p Relatively undisturbed drive sample; 2-inch 1.0. California liner sample. ~ Drive sample; standard penetration test (SPl), 1 3/8inch1 .0. split spoon sample, ASTM D-1586. 10112 Drive sample blow count; indicates that 10 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. T Practical drilling rerusel. NOTES: 1. Exploratory borings were drilled on May 25, 2016 with 4-inch diameter continuous flight power auger. 2. Locations or exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3 . Elevations or exploratory borings were approximated from contours shown on Figure 1. 4. The exploratory boring locations and elevations should be considered accurate only lo lhe degree implied by the method used . 5. The tines between materials shown on the exploratory boring logs represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the borings at the ti me of drilling . Fluctuation in water level may occur with lime. 7. Laboratory Testing Results : WC = Water Content (%) DD = Dry Density (pcl) -200 = Percent passing No . 200 sieve 116 210A ~ Heoworth-Pawlak Geotec:hnlcal LEGEND AND NOTES Figure 3 Moisture Content = 7.3 percent Dty Density ... 98 pcf Sample of: Sandy Silty Clay From: Boring 1 at 2 ~ Feel 0 ~ -i,...-.... -r--1--~ i"'o-( 1 n" " ,--i"'o ... ,. ,.. _No movement !'\. upon ~ 2 welling c ~ .Q \ Cl) en 3 Q) 0. E \ 8 4 1 5 1[> . . 01 t.O 10 100 APPLIED PRESSURE-ksf 116 210A catech SWELL-CONSOLIDATION TEST RESULTS Figure 4 HSJllWO!nl+PAWLAK G£0nECHNIC:.U. Moisture Content -7.0 percent Dry Density = 90 pcf Sample of: Sandy Silty Clay From: Boring 2 al 5 Feet 0 -----... :·o 1 .----~ Compression v ..... i--~'"'upon .c:::. i--i.-wetting '" 2 "' I c:; \ 0 'iii tJI 3 CJ ...... a. \ E 0 C) 4 5 \ 6 \ 7 \ \ a I 9 \ \ 10 11 \ I~ 12 OT 10 10 100 APPLIED PRESSURE • ksf 116210A ~ch SWELL-CONSOLIDATION TEST RESULTS Figure 5 HEJ>WORTH-PA~K G£0Tl'.C:HNIC:AL HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE1 · Job No. 116 210A SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG UMTTS UNCONFINED PERCENT MOISTURE ORV GRAVEL SAND PASStNG LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY (%) N0.200 LIMIT INDEX STRENGTH BEDROCK TYPE (%) uu "'' I Den SIEVE (%) '"'' lPSs:\ 1 2Vi 7.3 98 Sandy, Silty Clay 5 7.2 94 76 Sandy, Silty Clay 2 21h 6.6 100 83 Sandy, Silty Clay 5 7.0 90 Sandy, Silty Clay