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HomeMy WebLinkAboutSubsoil Study for Foundation Design~tech April 20, 2001 Danyel Fraker P.O. Box 131 Carbondale, Colorado 81623 llt-pirnrlh-l'awluk Gtntcchnkal. Inc. Sll:?O Count) l<uatl 15-4 G lu11111ud ~ prin}t.\, C olur:1tlo I! 1611 I Phone: lliO·t.1-'5· 798S Fux: 9it1·9-'S·ll-'5.$ hpi:l'o tit h11g~u1cd1 .com Job No 101 262 -Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 48, Filing 5, Los Amigos, Garfield County, Colorado Dear Mr. Fraker: As reques1ed, Hepworth-Pawlak Geo1ecbnical. Inc . performed a subsoil study for design of foundations at the subjecc site . The study was conducted in accordance with our agreement for geotechnical engineering services to you dated April 2, 2001. The data obcained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The proposed residence will be two story wood frame construction with an attached garage and located in the northeast part of the building envelope as shown on Fig. 1. Ground floor for the residence will be structural over crawlspace and the garage floor will be slab-on-grade. Cut depths arc expected co range between about 3 to 4 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. If building conditions or foundation loadings are significantly different from those described above, we should be notified co re -evaluate the recommendations presented in this report. Site Conditions: Lot 48 is located on the south side of Elfin Place. The ground surface is moderately sloping down co the south with about 4 to 6 feec of elevation difterence across the building area . Vegetation consists of a sparse pinon and juniper forest with a ground cover of grass, weeds and sage brush . Numerous basalt cobbles and boulders are exposed on the ground surface. Subsurface Conditions : The subsurface conditions at che site were evaluated by excavating three exploratory pits at the approximate locations shown on Fig . l. The logs of the pits are presented on Fig. 2. The subsoils encountered, below about 111:? feet of topsoil, consist of basalt cobbles and boulders in a silty sand matrix. Results of swell -consolidation testing perfonned on relatively undisturbed samples of che silty sand matrix soils, presented on Fig. 3, indicate low compressibility under existing moisture conditions and light loading, low collapse potential (senlcment under constant load) Danyel Fraker April 20, 2001 Page 2 when wened, and moderate compressibility under additional loading. Results of a gradation analysis performed on a sample of the basalt cobbles (minus 5 inch fraction) obtained from the site are presemcd on Fig . 4 . No free water was observed in the pits at the time of excavation and the soils were slightly moist to moist. Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, we recommend spread footings placed on the undisturbed natural soil designed for an allowable soil bearing pressure of 2,500 psf for support of the proposed residence. The matrL~ soils cend to compress after wetting and there could be some post-construction foundation senlement . Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed soils at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural soils . Voids created by boulder removal should be backfilled with compacted gravelly soil or with concrete. Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least 36 inches below the exterior grade is typically used in this area. 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 be designed to resist a later.tl earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site soil devoid of vegetation, topsoil and oversized rock as backfill . Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction. To reduce the effocts of some differential movement, floor slabs should be separated from all bearing walls and columns with expansion joints which allow unrestrained venical 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 des igner based on experience and the intended slab use. A minimum 4 inch layer of free-draining gravel should be placed beneath basement kvel slabs to facilitate drainage. This material should consist of minus 2 inch aggregate with less than 50% passing the No. 4 sieve and less than 2 3 passing the No. 200 sieve . All fi.11 materials for support of floor slabs should be compacted to at least 95 % of maximum standard Proctor density at a moisrure content near optimum. Required fill can consist of the on-site soils devoid of vegetation, topsoil and oversized rock. Underdr:iin 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 runoff can create a perched condition. We recommend below-grade construction, such as retaining walls and basement areas, be protected from wetting and hydrostatic H·P GEOTECH Danyel Fraker April 20, 2001 Page 3 pressure buildup by an underdrain system. Shallow crawlspace areas should not need a perimeter underdrain provided the foundation wall backfill is properly compacted and has a positive slope to prevenc ponding. The drains should consist of drainpipe placed in the bottom of lhe wall backfill surrounded above the invert level with free-draining granular material. Tbe drain should be placed at each le vel of excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum I% co a suitable gra\•ity outlet. Free-draining granular material used in the undcrdrain system should contain less than 2 % passing the No. 200 sieve, less than 50~ passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least 1 'h feet deep. -:- Surface Drainage: The following drainage precautions should be observed during construction and maimained at all times after the residence has been completed: 1) Inundation of the foundmion 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 dens icy in pavement and slab areas and to at least 90% of the maximum standard Proctor densicy in landscape areas. Free-draining wall backfilJ should be capped with about 2 feet of the on -site, finer graded soils to reduce surface water infiltration. 3) The ground surface surrounding the exterior of the building should be sloped to drain away from the foundation in an directions. We recommend a minimum slope of 12 inches in the first IO feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in pavement and walkway areas. A swale will be needed uphill to direct surface runoff around the residence. 4) Roof downspouts and drains should discharge well beyond lhe limits of all backfill. Limitations: This study bas been conducted in accordance with generally accepted geotechnical engineering principles and practices in lhis area at this time. We make no warrancy either expressed or implied. The conclusions and recommendations submitted in this repon are based upon the data ob£ained from the exploratory pits excavated at lhe locations indicated on Fig. 1 and to the depths shown on Fig. 2, 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 pits 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 at once so re-evaluation of lhe reconunendations may be made. H·P G EOTECH Danyel Fraker April 20, 2001 Page .t 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 e vol ves, we should provide continued consultation and field services during construction to rev iew 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 modifications to the recommendations presented herein. We recommend on-site observation of excavations and foundation bearing strata and tec;ting of structural fill by a representative of the geotechnical engineer. If you have any questions or if we may be of further assistance, please let us knifo. Sincerely, HEPWORTH -PAWLAK GEOTECHNICAL. IN C. ~~y__v// ~ '( ,.:::.....,._.-/ Louis E. Eller ' LEE/ksw anacluncats H-P GEOTECH LOT 47 101 262 LOT BOUNDARY APPROXIMATE SCALE ,. = 50' 100 -1-----____:----------PIT 2 • ~ PIT 3 go--......_ --- - - - - - - - - - -AREA - - - --. - - -BUILDING \-- LOT 49 \ \ 90 80 -t LOT 48 • PIT 1 \ , --,,, ----\-80 10 .... I ---------------:u:o1Nc ~ ' ENVELOPE -.... -.... L .............. _ ...... _ -----70 HEPWORTH-PAWLAK GEOTECHNICAL, INC. --------- LOCA 110N OF EXPLORATORY PITS Fig. 1 :5 a. Ill c a 5 PIT 1 ELEV.-. 84' - I +'4-80 _ ~ -200-a PIT 2 ELEV.= 94' WC-9.7 00•90 -200-20 WC-10 .8 DD•!ID PIT 3 ELEV.• 96' 10 LEGEND . T NOTES : TOPSOIL; sandy silt, organic, firm, moist, dork brown. BASALT COBBLES AND BOULDERS (GM); in o s !l ty send matrix, dense, slightly moist to moist, llght brown , colcoreous, 2" Diameter hand driven llner sompl e. Disturbed bul k sample. Proctlcol digging refusal with backhoe in bosolt bou lders. 1. Exploratory pits were excavated on AprU 10, 2001 with o Cot 416c backhoe. a 5 10 2. Locations of exploratory pits were measured approximately by peeing from features on the site plan provided. 3 . Elevations of the exploratory pits were obtained by Interpolation between contours on the site pion provided. 4. The exploratory pit locations and elevations should be considered accurate only to t he degree implied by the method used. 5. The lines between malerlols shown on the exploratory pit logs represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered In the pits ct the time of excavating. Fluctuations In water level may occur wlth time. 7. Laboratory Testing Results: WC = Water Content ( % ) DO = Dry Density ( pcf ) +4 a Percent retained on No. 4 sieve -200 = Percent passing No. 200 sieve 101 262 HEPWORTH-PAWLAK GEOTECHNICAL, INC. LOGS OF EXP LORA TORY PITS Fig. 2 ..... GI v i..... .s= -c.. Ill 0 Moisture Content • 9.7 percent Dry Density = 90 pct Sample of: Silty Sand Matrix From: Pit 2 at 5 Feet 0 r-,_ ~ ,_""' H~ c , lh-,__ 0 ~ -... ""' .... Compression . ii "' upon II) !'..... ... wetting Q. 2 ~ "~ (.) 3 I\. I) 4 0.1 1.0 10 100 APPLIED PRESSURE -ksf Moisture Content -10.8 percent Ory Density .. 60 pcf Sample of: Silty Sand Matrix Fram: Pit 2 at 7 Feet 0 --r--r-I'-""'~ 111) l Compression upon 2 wetting ~ -~ c ~ 0 3 ti ~ "' ~ Cl ... Q. E 4 ~ 0 0 0 5 0.1 1.0 10 100 APPLIED PRESSURE -ksf 101 262 HEPWORTH-PAWLAK SWELL CONSOUDA TION TEST RESULTS Fig. 3 GEO TECHNICAL, INC. 24 Ill. 1 Ill O 4' 11111. !!I MUI. -· 1111JN. 4 Wk T Ull. pt10 11111 11!0 Pl Ill 18 14 l/r l/•" 1 1tr T s•e• r 1111 10 20 lD 0 ~ 40 ~ w ~ I-:II) z LU (.J c:: w 0. IO 10 ea IO ICIO 101 262 .001 .cm .m!I .111111 .011 .Ml .m• .150 .lOll ..IOO DIAMETER OF PARTICLES IN MILLIMETERS GRAVEL BO % SAND 11 SILT AND CLAY 9 LIQUID LIMIT PLASTICITY INDEX SAMPLE OF: Silty Sandy Grovel and Cobbles FROM: Pit 1 at 1.5 thru 3 Feet HEPWORTH-PAWLAK GEOTECHNICAL, INC. GRADATION TEST RESULTS :m 10 0 1L2 ISZ 2Al 127 Fig. 4 ~ z Ui V> < a. 1-z w u Q: ~ a. HEPWORTH-PAWLAK GEOTECHNICAL, INC . TABLE I JOO NO 101 262 SUMMARY OF LABORATORY TEST RESULTS SAMrlt IOCAllO" Nf\lllHAL NAlUHAI flRAO.t.llOll f'fflCrl'H II n~RDfAO ll',llTS ur1cormru u Pll OEPIH MOISTURE DAY GRl\VEL SArlO P/1 ~5ttJI; l lO UIO 1'1.ASllC COMPRU ~IVI SOii 11R U•ett CONl{NI 0CllSll1 I'll.I ,,.., PI O ZOO UMll ~'lDEX SIRlNO rtl OCOltOCK IYl'C t'l.I tricU SILVf .: .... f1l,I IPSfl 1 1 % • 3 80 11 9 Silty Gravel and Cobbles 2 5 9 .7 90 20 Silty Sand Matrix 7 10.8 80 Silty Sand Matrix .