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HomeMy WebLinkAboutSubsoil Study~tech SUBSOIL STUDY llcp,vorth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenn·ood Springs, Colorado 81601 Phone: 970-945· 7988 Fax: 970-945-8454 hpgeo@hpgeotech.com FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT A-1, ASPEN GLEN GARFIELD COUNTY, COLORADO JOB NO. 101 293 April 30, 2001 PREPARED FOR: .JIM AND KAREN TUCKER c/o PRI~STON T. PHILLIPS ARCHITECT P.O. BOX 3037 BRIDGEHA.'VIPTON, NEW YORK 11932-3037 () () HEPWORTH-PAWLAK GEOTECHNICAL, INC. April 30, 200 I Jim and Karen Tucker cl o Preston T. Phillips Architect P.O. Box 3037 Bridgehampton, New York 11932-3037 Job No. 101 293 Subject: Report Transmittal, Subsoil Study for Foundation Design, Proposed Residence, Lot A-1, Aspen Glen, Garfield County, Colorado. Dear Mr. and Mrs. Tucker: As requested, we have conducted a subsoil study for the proposed residence at the subject site. Subsurface conditions encountered in the exploratory borings drilled in the proposed building area consist of about 1h to 1 foot of topsoil and 1 ¥2 to 4 feet of stiff sandy clay overlying relatively dense, silty sandy gravel with cobbles and boulders. Groundwater was not encountered in the borings at the tinie of drilling or when checked 14 days later. The proposed residence can be founded on spread footings placed on the natural subsoils and designed for an allowable bearing pressure of 1,500 psf. Footings bearing entirely on the dense gravel soils can be designed for an allowable bearing pressure of 3,000 psf. The report which follows describes our exploration, summarizes our findings, and presents our recommendations. It is important that we provide consultation during design, and field services during construction to review and monitor the implementation of the geotechnical recommendations. If you have any questions regarding this report, please contact us. Sincerely, HEPWORTH -PAWLAK GEOTECHNICAL, INC. Jordy Z. Adamson, Jr., P.E. Rev. by: SLP JZA/ksw ' . () TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 PROPOSED CONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 SITE CONDITIONS ..................... · ...................... 2 SUBSIDENCE POTENTIAL ... .' ................. , ................ 2 FIELD EXPLORATION ........................................ 3 SUBSURFACE CONDITIONS .................................... 3 DESIGN RECOMMENDATIONS ................................... 4 FOUNDATIONS ......................................... 4 FOUNDATION AND RETAINING WALLS ...................... 5 FLOOR SLABS ......................................... 6 UNDERDRAIN SYSTEM ................................... 7 SURFACE DRAINAGE .................................... 7 () LIMITATIONS ............................................... 8 REFERENCES ............................. ; ................. 9 FIGURE 1 -LOCATION OF EXPLORATORY BORINGS FIGURE 2 -LOGS OF EXPLORATORY BORINGS FIGURE 3 -LEGEND AND NOTES FIGURE 4 -SWELL-CONSOLIDATION TEST RESULTS FIGURE 5 -GRADATION TEST RESULTS TABLE I -SUMMARY OF LABORATORY TEST RESULTS () H-P GEOTECH () 0 0 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot A-1, Aspen Glen, Garfield County, Colorado. The project site is shown on Fig. 1. The purpose of the study was to develop recommendations for the foundation design. The study was conducted in accordance with our proposal for geotechnical engineering services to Jim and Karen Tucker dated April 11, 2001. Chen-Northern, Inc., previously conducted a preliminary geotechnical engineering study for the development and another geotechnical engineering study for preliminary plat design under their Job No. 4 112 92, dated December 20, 1991 and May 28, 1993, respectively. A field exploration program consisting of exploratory borings was conducted to obtain information on 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 subsoil conditions encountered. PROPOSED CONSTRUCTION The proposed residence will be a two story, wood frame structure over a partial basement level. Ground floor will be slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 10 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 GEOTECH ·. . . () () (~) -2- SITE CONDITIONS The site was vacant at the time of our field work. The ground surface is relatively flat. There is a gentle slope down to the southeast and the Roaring Fork River borders the southeast side of the lot. There is up to about 2 feet of elevation difference across the proposed building area. Some fill could be located on the lot from overlot grading as part of the subdivision development. Vegetation consists of grass and weeds. Scattered trees are located adjacent the river on the east side of the lot. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen Development. These rocks are a sequence of gypsiferous shale, fme-grained sandstone/siltstone and limestone with some massive beds of gypsum. 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 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 (Chen-Northern, Inc. 1993). These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork Valley. The lot is not located within a broad subsidence area and existing 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. 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 A-1 throughout the service life of the proposed residence, 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. H-P GEOTECH 0 f~ '~J C) - 3 -. FIELD EXPLORATION The field exploration for the project was conducted on April 16, 2001. Three exploratory borings were drilled at the locations shown on Fig. 1 to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight augers powered by a truck-mounted Longyear BK-51HD drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with 1%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 shown on the Logs of Exploratory Borings, Fig. 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 Fig. 2. The subsoils consist of about 1h to l foot of topsoil and 11h to 4 feet of stiff sandy clay overlying relatively dense, silty sandy gravel with cobbles and boulders. Drilling in the dense gravel 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, density and gradation analyses. Results of swell-consolidation testing performed on relatively undisturbed drive samples of the clay soils, presented on Fig. 4, indicate low to moderate compressibility under conditions of loading and wetting. The sample from Boring 2 at 4 feet showed a minor collapse potential (settlement under constant load) when wetted. The sample from Boring 3 at 2 feet showed a low expansion potential when wetted under a constant light surcharge. Results of gradation analyses performed on small diameter drive samples (minus 1 % inch fraction) of the natural H-P GEOTECH ·. -4- Q coarse granular soils are shown on Fig. 5. The laboratory testing is sununarized in Table I. () () No free water was encountered in the borings at the time of drilling or when checked 14 days later. The subsoils were slightly moist to moist. DESIGN RECOMMENDATIONS FOUNDATIONS. Considering the subsoil 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 subsoils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural subsoils should. be designed for an allowable soil bearing pressure of 1,500 psf. Footings which bear entirely on the underlying dense gravels can be designed to impose an allowable soil 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 1 inch and could be differential between footings bearing on the clays and footings bearing on the gravels. 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 bene!lth 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 12 feet. Foundation walls acting as retaining structures should also be H-P GEOTECH ·. 0 5) -5 - designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the undisturbed natural soils. If water seepage is encountered, the footing areas should be dewatered before concrete placement. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. · 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 earth pressure computed on the basis of an equivalent fluid unit weight of 50 pcf for backfill consisting of the on-site soils. Cantilevered retaining structures which are separate from the residence and can be expected to deflect sufficiently to mobilize the full C) active eartli pressure condition should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of 45 pcf for backfill consisting of the on-site soils. The backfill should not contain vegetation, topsoil or oversized rock. () 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 maximum standard Proctor density at a moisture content near optimum. Backfill in pavement and walkway areas should be compacted to at least 95 % 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 H-P GEOTECH - 6 - () material is placed correctly, and could result in distress to facilities constructed on the backfill. 0 () 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 based oi:J. a coefficient of friction of 0.35 for the clays and 0.45 for the gravels. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The CQefficient 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 pas.sive 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. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction. The upper clay soils have variable settlement/heave potential when wetted and slabs placed on the upper clay soils could experience some post construction slab movement. 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% retained on the No. 4 sieve and less than 2% passing the No. 200 sieve. · All fill materials for support of floor slabs should be compacted to at least 95 % of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on-site soils devoid of vegetation, topsoil and oversized rock. H·P GEOTECH •. -7- () . UNDERDRAIN SYSTEM . o Although free water was not encountered during our exploration, it has been our experience in the area and where clay soils are present that local perched groundwater may develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched condition. We recommend below-grade construction, sucli as retaining walls and basement areas, be protected from wetting and hydrostatic pressure buildup by an under drain 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 1 % to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum size of2 inches. The drain grave~ backfill should be at least 1 1/2 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 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 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 3 inches in the first 10 feet ·in paved areas. Free-draining wall backfill should be 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. H-P GEOTECH 0 - 8 - 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 expressed 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 Fig. 1, the proposed type of construction and our experience in the area. 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 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 should provide continued consultation and field 0 services durfug construction to review and monitor the implementation of our recommendations, and to verify that the recommendations have been appropriately () 1 • 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. Sincerely, HEPWORTH -PAWLAK GEOTECHNICAL, INC. Jordy Z. Adamson, Jr., P.E. Reviewed by: Steven L. Pawlak, P.E. JZA/ksw cc: Charles Cunniffe Architects -Attn: Jeff Johnson H-P GEOTECH 0 0 ·~) - 9 - REFERENCES 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. 4 112 92. Chen-Northern, Inc., 1993, Geotechnical Engineering Study for Preliminary Plat Design, Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated May 28, 1993, Job No. 4 112 92. H-P GEOTECH \ J C) () 0 BENCHMARK: GROUND AT ELECTRICAL BOX, ELEV. = 100.0', ASSUMED. LOT A-2 APPROXIMATE SCALE: 1"=401 LOT A-23 r-------1 -1 I BO:NG 1 I I I I I I I I I BUILDING FOOTPRINT. • BORING 2 I I I I • ~ BORING 3 I I BUILDING ENVELOPE I L_~_\ _____ J .. LOT A-1 PROPERTY BOUNDARY ----···----··· ----- ROARING FrJRK RIVER 101 293 HEPWORTH-PAWLAK GEOTECHNICAL, INC. LOCATION OF EXPLORATORY BORINGS GOLF COURSE Fig. 1 t 0 .... 5 GI ~ ..c ... a. ., c 10 15 ( 101 293 BORING 1 ELEV.= 100.4' 25/U 38/6,10/D wo-11.s +4-57 -200-9 15/2.10/0 BORING 2 ELEV.= 100.1' 16/12 21/12 34/12 WC-1.5 +""'55 -200=13 W0=4.2 00=116 -200=49 BORING J ELEV.~ 99.4' 17/12 WC..10.5 DD=a. T 12/2.10/0 Note: Explanation of symbols ls shown on Fig. 3. HEPWORTH-PAWLAK GEOTECHNICAL, INC. LOGS OF EXPLORATORY BORINGS 0 5 ~ : ..c ..... . a. ., c 10 15 Fig. 2 \ -. . LEGEND: TOPSOIL; sandy clay, scattered gravel and cobbles, organic, slightly moist, brown. CLAY (CL}; sandy to very sandy, stiff to very stiff, slightly moist, red, slightly calcareous, porous. GRAVEL (GM}; silty, sandy, with cobbles and boulders, dense, slightly moist, reddish brown. Relatively undisturbed drive sample; 2-lnch l.D. California liner sample. Drive sample; standard penetration test (SPT}, 1 3/B Inch 1.D. split spoon sample, ASTM D-1586. 16112 Drive sample. blow count; Indicates that 16 blows of a 140 pound liar'nmer falling 30 Inches were required to drive the California or SPT sampler 12 inches. T NOTES: Practical drilling refusal. Where shown above bottom of log, indicates that multiple attempts were made to advance the boring. 1. Exploratory borings were drnled on Aprll 16, 2001 with a 4-lnch diameter continuous flight power auger. , ~\. Locations of exploratory borings were measured approximately by pacing from features shown on the site \ _} plan provided. . . ( \_ 3. Elevations of exploratory borings were measured by instrument level and refer. to the Bench Mark shown on Fig. 1. Loge are drawn to depth. 4. The exploratory boring locotlons and elevations should be considered accurate only to the degree implied by the method used. 5. The fines 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 time of drilling or when checked 14 days later. Fluctuatlon In water level may occur with time. 7. Laboratory Testing Results: WC = Water Content ( ,.; } DD = Dry Density ( pcf ) +4 = Percent retained on No. 4 sieve. -200 = Percent passing No. 200 sieve. 101 293 HEPWORTH-PAWLAK GEOTECHNICAL, INC. LEGEND AND NOTES Fig. 3 ( \._ ,. ( \... ' N c 0 ;; "' e a. E 8 N !5 ·o; c Cl 11-w I c ~ ., e a. E 0 CJ 101 ' 0 1 2 3 4 0.1 0 1 2 3 4 0.1 293 Moisture Content = 4.2 percent Dry Density = 116 pct Sample af: Sandy Clay with Gravel Fram: Boring 2 at 4 Feet -~ ... ~ ~ ~ -compression upon !'-.. wetting '\ '~ 1.0 10 100 APPLIED PRESSURE -ksf Moisture Content "" 10.5 percent . Dry Density = 84 pct Sample of: Sandy Cloy Fram: Boring 3 at 2 Feet ~ ., )'\. I-~ -~ (I-\ ' ' Expansion ) upon \ wetting \ 1.0 10 100 APPLIED PRESSURE -ksf HEPWORTH-PAWLAK SWELL CONSOLIDA llON TEST RESULTS Fig. 4 GEOTECHNICAL, INC. ~ :J 1) .. \ '- fi] z ~ LtJ a::: I-z LtJ u a::: LtJ D- fi] z ~ a::: I- ill u ffi a. 101 , I H'IDllllllmR NIAL.SS I RVEANALWS I . 'mfE REAmlCS U.S. STANDARD SERIB I Q£AR SWARE cP£llNGS MHR. 7HR 1100 11/r 3" .... .. 45 Wt 15 MIN. IClllH. 11MJN. 4 MW. 1 llN. ..... ... .., ff• ,. ,. 3/11' 3/4. • 100 10 •• .. .. C> 30 70 z Bi 40 .. < a. .. I-.. z LtJ .. 0 40 fli a. 70 30 ... 20 •• 10 ••• • .... .aa:t ........ .... . rm .at• .... .... .... 1.ta 2.31 4.75 Uu.a 11.0 ;ru 11.2 ,,Ju 203 DIAMETER OF PARTICLES IN MIU.IMETERS CLAY 10 11.T I FINE I IL 'DDAR4 I fjE '"'t""-C@E I Coeaus GRAVEL 57 " SAND 34 " SILT AND CLAY 9 " LIQUID LIMIT " PLASTICITY INDEX " . SAMPLE OF: SUghtly SUty Sandy Gravel FROM: Boring .1 at 4 Feet I H'IDllOlll!1Ell NIAL.SS I llE\IE ANAL'l!IS I --.. U.S. STAtl>ARD liERllS I Cl£IR SQllNIE OPDINGS MHIL 7"" ...... 1100 " ,. 3/r 3/4" , 1/r' 3" II' .. .. 41 YN. 15 MIN. Ir.MN. tliMlf. 4* 1UN. f!IJ ,,. ,. . • 100 10 .. .. .. C> 30 70 z Iii I/) .. .. < a. ..... .. .. Cl 0 .. .. ffi D- 70 !O .. :i: 20 .. 10 100 • .... .... ,DDS .oat .... ...., ,074 .1511 .... .... 1.11 2.31 ..,. ... ,2.5 11.0 37,a 71.2 ,Ju ... DIAMETER OF PARTICLES IN MIWMETERS a.AY TO Sl.i I fjft I Bi , ....... 1 f!HE ~ CDARit I COllllL£5 GRAVEL 55 " SAND 32 " SILT AND CLAY 13 " LIQUID LIMIT " PLASTICITY INDEX " SAMPLE OF: Siity Sandy Gravel FROM: Boring 2 at 6 Feet 293 HEPWORTH-PAWLAK GRADATION TEST RESULTS Fig. 5 GEOTECHNICAL, INC. () 0 0 .., HEPWORTH-PAWLAK GEOTECHNICAL, INC. _.----~· TABLE I JOB NO. 101 293 SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATlJRAL GRADATION PERCENT ATTERBERG LIMITS UNCONFINEO BORING DEPTH MOISTURE DRY GRAVEL SAND · PASSlNG LIQUID Pl.ASTIC COMPRESSIVE SOIL OR ..... CONTENT DENSITY '"' '"' N0.200 LIMIT INDEX STRENGTH BEDROCK TYPE -l'lE.) lpcQ SIEVE I"! I~) IPSFJ 1 4 11.9 57 34 9 Slightly Silty Sandy Gravel 2 4 4.2 116 49 Sandy Clay with Gravel 6 1.5 55 32 13 Silty Sandy Gravel 3 2 10.5 84 Sandy Clay .