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HomeMy WebLinkAboutOriginal Subsoil Study for Foundation Design 03.21.2000H Hcpra'orth-l'an'lak (Jeottchnical, I nc 5020 Count.y Ro¡d 154 (ìlcnruoocl Springs, (lolorrdo 8ló(lI I'lrone: 970-945-7988 l.'axr 9?0-945-{145¿tr Job No. 100 227 March 21,20t0 Lonnie and Anita Bones 1041 Heritage Drive Carbondale, Colorado 81623 Subject:Subsoil Study for Foundation Design and Percolation Test, Proposed Residence, Lot 6, Teller Springs, County Road 109, Garfield County, Colorado. Dear Mr. and Mrs. Bones: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study and percolâtion test for foundation and septic disposal designs at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated February 28, 2000. The data obtained ând our recofftmendations based on the proposecl çonstruction and subsurface conditions encountered are presentecl in this report. Proposed Construction: The proposed residence will be a two story wood frame structure located in the eastern (downhill) portion of the lot as shown on Fig, 1. Ground floors will be sfructural over crawlspace or a slab-on-grade basement level. Cut depths are expected to range between about 4 to 8 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. The septiÇ disposal system is proposed to be located about 40 to 50 feet to the northwest of the proposed building area' If huilding conditions or foundation loadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report. Site Conditions: The site wäs vacant at the time of our field work, The property is a previously irrigated field. An irrigation ditch transecfs the middle portion of the lot. The ground surface is relatively flat with a slight slope down to the east. There is about 2 ta 3 feet of elevation difference in the proposed building area. A steep slope down to a lower terrace and the Roaring Fork River is located to the east of the lot. The lot is vegetated with sagebrush, grass and weeds. Cri4twl gile g¿rorl (e**i* *rc4ø/ruaùaa.) Lonnie and Anita Bones March 2I,2tt0 Page 2 Subsidence Potential: Teller Springs is underlain by Pennsylvania-age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits, Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce localized subsidence. During previous work in the atea, a few sinkholes were observed in the terraces close to the Roaring Fork River at Teller Springs. Sinkholes were not observed in the immediate area of fhe subject lot. The exploratory pits were relatively shallow, for foundation design only. Based on our present knowledge of the site, it cannot be said for cerfain that sinkholes will not develop. In our opinion, the risk of ground subsidence at Lot 6 during the service life of the residence low, but the owner should be aware of the potential for sinkhole development. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits in the building area and one profile pit in the septic disposal area at the approximate locations shown on Fig. l. The logs of the pits are presented on Fig. 2. The subsoils encountered, below about I foot of topsoil, generally consist of relatively dense slightly silty sandy gravel with cobbles and scattered boulders. About I th ts 2 feet of stiff sandy silty clay was ençountered between the topsoil and gravels in Pit I and the Profile Pit. Results of swell-consolidation testing performed on a relatively undisturbed sample of the clay, presented on Fig. 3, indicate low compressibility under existing moisture conditions and light loading and a low expansion potential when wetted. Results of a gradation analysis performed on a sampte of the gravel (minus 5 inch fraction) obtained from the site are presented on Fig, 4. The laboratory testing is summarized on Table I. No free water was observed in the pits at the time of excavation and the soils were slightly moist to rnoist. Founflation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, \rye recommend spread footings placed on the undisturbed natural gravels designed for an allowable soil bearing pressure of 3,000 psf for support of the proposed residence. The upper clay soils tend to be expansive after wetting and they should be removed from beneath footing areas. Footings should be a minimum width of 1ó inches for continuous walls and 2 feet forÆ Loose disturbed soils and clay encountered at bearing level within the ld be e to the undisturbed provided with cover above 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 12 feet. Foundation walls acting as retaining H-P GËOTECH t Lonnie and Anita Bones March 21, 2000 Page 3 structures should be designed to resist a lateral earfh pressure based oÍ ân equivalent fluid unit weight of at least 50 pcf for the on-site soils, excluding oversized rock, as backfill. Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction. The upper clay soils appear expansive when wetted which could result in some floor slab heave. The clays could be removed to prevent potential heave. To reduce the effects of some differential movemenl, 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 esfablished 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 nraterial should consist of minus 2 inch aggregate with less than 507o passing the No. 4 sieve and less than2% passing the No. 200 sieve, All fill materials for support of floor slabs should be compacted to at least 95To of maximum standard Proctor density at a moisture content near optimum, Required fill can consist of the on-site gravels devoid of vegetation, topsoil and oversized rock. Urrderdrain System: Although free water was not encountered during our exploration, it has been our experience in area that local perched groundwater can develop during times of heavy preeipitation or seâsonal runoff. Frozen ground during spring runoff cancreateaperchedcondition.Wdecorrstruction,suchas çta:vls pa gr llcl-b-11e¡re $¡rrea s,-þ-i bu an underdrain . An underdrain should not be needed for crawlspace areas shallower than about feet prov a surface slope is maintained arou nd .¡he bu and the exterior wall properly 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 excavatiolt and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 7Vo 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 of 2 inches. The drain gravel backfill should be at least lVz feet deep. H-P Georecr Lonnie and Anita Bones March 2l,2OOO Page 4 Surface Drainage: The following drainage precautions 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 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. Free-draining wall backfill 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 all directions. We recommend a minimum slope of 6 inches in the first 1û feet in unpaved areas and a minimum slope of 3 inches in the first I0 feet in pavement and walkway åreas. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. Percolation Testing: Percolation tests were conducted on March 16, 2000 to evaluate rhe feasibility of an infiltration septic disposal system at the site. One profile pit and three percolation holes were dug at the locations shown on Fig. 1. The test holes (nonrinal 12 inch diameter by 12 inch deep) were hand dug at the bottom of shallow backhoe pits and were soaked with water one day prior to testing. The soils exposed in the percolation holes are similar to those exposed in the Profile Pit shown on Iìig. 2 and consist of about 1 foot of topsoil and 2 feet of sandy silty clay overlying slightly silty sandy gravel with cobbles and scattered boulders. The percolation test results are presented in Table II. The percolation fest results indicate an infiltration rate lretween 7 and 13 minutes per inch with an average of9 per inch . Based on the subsurface conditions ençountered and the percolation test results, the tested area should be suitable for a convenfional infiltration septic disposal system. LirnitatÍons: This study has been conducted in accordance with generally accepted geotechnical engineering principles and practiÇes in this area at this time. We make no warranty either expressed or implied. The conclusions and recortmendations submitted in this report are based upon the data obtained from the exploratory pits excavated at the locations indicated on Fig. 1, the proposed type of çonstruction 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 H-P GrorrcH Lonnie and Anita Bones March 21,24CI0 Page 5 may not become evident until excavation is performed. If conditions encountered during consfruction appear 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 desigtt 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 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 geotçÇhnical engineer. If you have any questions or if we may be of further assistance, please lef us know. Sincerely, HEPWORTH - PAWLAK INC Iardy Z. Reviewed by J ïÞ^^/ Steven L. Pawlak, P.E. JZAlksm attachments R NAt I 29 a a-.,p H-P GEoTECH APPROXIMATE SCALL 1" = 150' couvTY fto¿o tog , , I I I , I I ,I I I I , I I LOT 6 I I f ,I t ,I LOT 7 I , LOT 5 ,I I t I I I I I t , I I I I l t I I t I l I I IP ^ 1 ,I T LE I , P 1Ià PR? d>r P1 T , PIT T I t t t I PI T 2 I ),\\ 10a 227 HEPWORTH _ PAWLAK GEOTECHNICAL, INC. LOCATON OF EXPLORATORY PITS AND PERCOLAT]ON TEST HOLES Fis. 1 PIT 1 PIT 2 PROFILE PIT 0 oq,u- I ..Ê CLtô tl0-8.4 DD-l12 -20û*84 t - J +4-68 -2OOÉf 0 oou- I .c. o-oo 5 5 10 10 LEGEND: TOPSOII sondy sílty cloy, orgonic, firm, moist. brown. CLAy (CL): eilty, sond¡ etiff, slightly moist, brown. slightly colcoreous, slÎghtly Porous. ffi GRA\ËL (Gp-GM)¡ Ëondy, slîghtly eîlty, with cobbles ond scottered boulderê, dense, slfghtly moist, brown. þ f: 2' Diometer hond drlven liner somple. I J Disturbed bulk ecmPle NOÏES: 1. Explorotory plts wsrs excsvoted on Morch 16, 2000 with o bockhoe' 2. Locoilons of explorotory plts were messured opproxlmotely by pocing frorn feotures on the sÎte Plon Provlded. S. Elevotions of explorotory plts were not meosured ond logs of explorotory plts ore drown to depth' 4. The explorotory pit locotions should be considered occurote only to the degree lmplied by the method used. 5. The llnes between motsrlols shown on the explorotory pît logs rep.resent the opproxìmote boundories between moterlol types ond tronsitions moy be groduol- 6. No free woter wos encountared ln the pits of the tlme of excovotlng. Fluctuotíons in froter level moy occur with time' 7. LoborotorY Testing Results: WC = Woter Content ( 74 ) DD = Dry DensÍty ( Pcf )f4 = Pircent retolned on No. 4 sieve -200 = Percont posslng No. 200 sieve ffi ffi Fig. 2LOGS OF EXPLORATORY PITSHEPWORTH PAWLAK GEOTECHNICAL, INC.1AO 227 Moisture Content - 8,4 Dry Density E 112 percent pcf Somplo of: Sondy Silty Cloy From¡ Pit 1 ot 2 Feet \ )\\ \ Exponslon -upon wettîng \I N cô a^co CLx l¡J I ç .g¡ta,oLo. Eo C) 1 0 1 2 o.'t i.0 10 APPLIED FRESSURE - ksf 'to0 100 227 HEPWORTH _ PAT,VLAK GEOTECHNICAL, INC.SWELL_CONSOLIDATION TEST RESULTS Fis. 3 A¡¡ALIËIg 1llÆ RÉA0NCS u,8. Sf^t{¡^fto 24 HR.¡s t¡I.7HR 15 r¡lN.t0 r'o ltxt 00 f0 20to 70 t0 ()4ôoIAvl IL Ejo l¡J()ut¡,& to ô l¿J¡l{l ZaF-l¡JÉ. 508 bJ()&LJo- 60 70 ¡û t0 20 ¡o t0 1m 0 ,OT+ .räo .Jll0 .!00 t-lË ¿J6 1.15 e.5rr,3 lg.0 t?,s 76.2 þ157 2At.m! .oo2 .0Û5 .oø .ole 'o:17 DIAMETER OF PARTICLES IN MILLTMETERS cuY ro $lf co!8r¡s GRAVËL 68 %SAND 22 %SILT AND CLAY 10 % LIQUID LIMIT 7"PLASTICITY INOEX z FROM: Pït 2 ot 3 to 4 Feet fllaE SAMPLE OF:Slightly SiltY SondY Grovel with Cobbles Fig. 4GRADATION TEST RESULTSHEPWORTH PAWLAK GEOTECHNICAL, INC.100 227 HEPWORTH-PAWLAK GEOTECHNICAL, rNc.TABLË ISUMMARY OF LABORATORY TEST RËSULTSJOB NO. 1AO 227.sol[ oßICDROO( TYÞESandy Silty ClaySlightly Silty SandyGravel w¡th Cobblesul{coilFlr,tEocouPnEssnÆSTRÊÍIGTHtpsF!ATr€Rl€nG UifirsPt¡lsÌrcrroEx(96'uourDuufrtiûlPERCÊflTPASSlrGNO.20ostËvc8410GRAOATIOiIs f{Þt%t22GñAVEL{%l68NAYURÂLDñYDËNSITVlpcfl112ffÂÌrnALMOrSïU8EÛOT{TETÚTt*t8.4SAMPLÉ LOCAI¡OÍ{DtPtHlf¡crl23to4ñf12 HEPWORTH.PAWLAK GEOTECHNICAL, INC. TABLE II PERCOLATION TEST RESULTS JOB NO. 1AO 227 percotation test holes were dug in the bottom of backhoe pits and soaked by the cllent prior to our field work. Percolation tests were conducted on March 16, 2000. The average percolation rates were based on the last two readings of each test' I Note: HOLE NO.HOLE DEPTH ilNCHESI IENGTH OF INTERVAL lMtN) WATER DEPTH AT START OF INTERVAL fiNCHESI WATER DEPTH AT END OF INTERVAT I¡NCHES) DROP IN WATER TEVEI {rNcHES) AVERAGE PERCOLATION RATÊ (MrN./lNCHt P-1 48 15 water added water added I 5 3t4 2114 I 6 3t4 2 314 3 6 3114 2 314 s 314 7 1 3.¡4 7 5 2 5 31t4 't 314 P.?46 15 water added water added I 7 2 7 7 4 3 7 1t2 5 2112 I3t4 7 ll2 21t4 7 112 5 21t2 5 3 2 P-3 ã0 I5 water added water added I 61t2 1 112 13 6112 3114 3 1/4 7 5 112 1 1t2 e 314 I3t4 1 I3/4 7 112 1 114 7112 61t2 I