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Home My WebLink AboutSubsoil Study for Foundation Design 08.31.2007l"lcprvorth,l)owh[ Õccttçhnrcll, lnc. 5Lì20 Çouruy Road 154 Clt nrvoorJ Sixingr, Color¡tlo 8ló01 I'ho¡rc 9?0.945,7988 F¡r,920.9.15.84í4 * mail hpgcei'thpgeorcclr. currr HEPWORTH. PAWLAX GEOTECHNICAL August 31,20A7 Randy Jacobsen 157 Creekside Court Glenwood Springs, Colorado 8160l Subject: H { Job No. I 07 0660 !u!¡oi! study for Foundatiol pesigj proposed Residence, Lor gl, Filing 6, Elk springs subdivision, Elk spriirgs Drive, Garfield county, coloradã t Dear Mr. Jacobsen: As requesled, Hepworth-Parvlak Geotechnical, Inc. performed a subsoil srudy for design of foundations at the subject site. The study rvas conducted in accordance wilh our agreement for geotechnical engineering services to you dated Augu s129,2007. The dara obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Heprvorth-Pandak Geotechnical lnc., previously performed a prelirninary geotechnicalstudy for Filings 6 through 9 of the Elk Springs subdivision (formerly Los Arnigos Ranch PUD) and presenred our finclings in a report dated February 14, 1997, Job No. 196 6l?. Proposed Constructlou The proposed residcnce will consist of o one to llvo story wood framc struclure rvith an attaclted gflrâge. Cround floor will be slructural over a crarvlspacc for lhe living nrea and slab-on-gracle in the garûge, Cut deprhs arc assumed to range betrveen ¡bout 3 to 6 feet. Foundation loadings for this type of construction are relatively light and typical of the proposed typc ofconstruction. lf building conditions or foundation loadíngs are significantly dífferent fronr those described aþovc, rve should be notified to re-cvatuate the rccommenclarions presented in this report. Sitc Conditions¡ Lot 8l is located on the rvesl side of Elk Springs Drive and w's *acûnt at thc timc of our field exploration. l'hc ground surface in thc building enyelope is relatively flat rvith a gentle slope dorvn lo the rvest. Vegetation consists of scattcred P,trkcr 101"841'7119 ' Cokrratlo S¡rrings 7lr9"61]"5f6: . Silyr.'rr¡.¡r¡re g?0,46S.lggg -2- coluûms. Loose and disturbed soils encountered at the foundation bearing juniper trees, sagebrush, grass and weeds. Scattered basalt cobbles were cxposed on the ground surfbce. Subsurface Conditions: The subsurface conditions at the site rvere evaluated by excavating four exploratory pits at the approximate tocations shown on Figure l. The logs of the pits are presented on Figure 2. The subsoils encounrered, belorv about I ro 2 feet of topsoil, consist of 2 to 3 %leetof loose to rnedium dense, gravelly silt and sand, Relatively dense, basalt boulders and cobbles in a silty sand nlatrix were encountered beneath the silt and sand and depths of 3 to 4Yz feet. Digging in the dense rock rvas diflicult and practical digging refusal wÂs encountered in rhe pirs. Results of srvell. consolidation lesting performed on a relatively undisturbed sample of the silt soils, presented on Figure 3, indicote lorv contpressibility under existing moisture conditions and light loading and a lorv collapse potential (settlement under constant load) rvhen rvetted' 'l'he sample shorved moderate compressibility upon increosed loacling afler rvctting. Results of a gradation analysis performed on a sample of the gravel soils (minus 3 inch fraction) obtained from the site are presented on Figure 4. No free rvater rvas observed in the pits at lhe ti¡ne of excavation and the soils were stightly moist, Foundation Reeommendations: Considering the subsoil conditions encounterecl in rhe exploratory pits and the nature of the proposed consruction, we recomme nd spread footings placed on the undisturbed natural soil designed for an allowable soil bearing pressure of2,000 psffor support ofthe proposed residcnce. The upper- fìner grained soi ls and matrix soils could be compressible after wetting and there could be some post- conslruction foundation settlement. The settlement could be clifferential betrveen footings bearing on the upper finer graded soils and footings bearing o¡r the basalt rock soils. Footings should be a minimum widtlr of 6 inches for continuous walls and 2 feet for -- level rvithin the excavation should be removed and the footing bearing level extended clown to the undisturbed natural soils. Excavations into the basalt rock soils may require rock excrivating techniques such as blasting and chipping. Our experience in Elk Springs has been that a conventional large lrackhoe can excavate up to two feet deeper than thc refusal depth encountered in our pits. Voids created from boulder removal at footing grade should be backfilled rvith compacted sand and gravel, such as roarl base, or with concrete, Exterior footings should be prnvided with adequate cover above their bearing Job No.107 0660 cåFtecrr ) -3- elevations for fiost protection. Placement of footings at least 36 inches belorv the exterior grade is typically used in this area. Continuous foundation rvalls shouid be reinforced top and botfom to span local anomalies such as by assuming an unsupported length of ar least l0 feet. Foundation rvalls acling as retaining structures should be designed to resisr a lateralearth pressure based on an equivalent f}.rid unit weight of at least 50 pcf fbr the on- dite soilas backlìll. t'loor Slrbs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade construction. To rcduce the effects of some differential movemen!, fìoor slabs should be separated from all bearing rvalls anclcolun'¡ns with expansion joints rvhich allorv unrestrained vertical movement. Floor slal¡ controljoints .should be used to reduce damage due to shrinliage cracking, The requiremcnrs for joinr spacin! and slab reinforcement should be established by tlre designer based on cxperience and the intended slab use. A minimum 4 inch layer of sand and gravel, such as roacl base, should be placed beneath interior slabs-on-grade as a leveling course ancl lor subgradc support. Th¡s material should consist of minus 2 inch aggregate rvirh less rhan i0% passing the No. 4 sieve and less than l2% passing rhe No. 200 sieve, ,41¡ f¡ll materials for support of floor slabs shoulcl be compacted to at least g5% of maximum standard Proctor density at a moisture conte¡:t near optimum. Required fill can consist of the on'sits soils or a suitable imported moteriât suclr as road base devoid of vegetation, topsoil and oversized rock, Underdrain System: Although free water was not encountered during our exploration, it has been our experience in the area that local perched groundrvatcr can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoffcan create a perched condítion. We recomrnend belorv-grade construction, such as retaining rvalls and cralvlspace ateâs, be protected from rvetting and hydrostatic pressure buildup by an underdrain system. The drains should consist of drainpipe placed in the bottom of the wall backfill sunounded above the inve¡t level with free-draining granular material. The drain should be placed at each level ofexcavation and at least I foot belorv lorvest adjacent finish grade and sloped at a minimum lYo to a suitable gravity outlet. Free-draining granular Job No.107 0660 eåFæcrr -4- material used in the underdrain system should contain tess 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 backfïll should be at least I |/l feet deep. Su¡face Drainage The lollorving drainage precautions shoulcl be observed during construction and maintained at all times affer the residence has been completed: l) Inundation of the founclation e.\caval¡ons and underslab areas should be avoided durin g 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 al least 90% of the maximum standard Proctor dcnsity in landscapc areffi. Free-draining rvall backfill should be capped with about 2 feet of the on-site, finer graded soils to reduce surface rvater infiltration. 3) The ground surface sunounding the exterior of the building should be sloped to drain away lrom the foundation in all directions. we recommend a minimum slope of 6 inches in the first l0 feet in unpaved are¡s and a minimunl slope of 3 inches in the fìrst l0 feet in pavement and rvalkway areâs. A swale may be neecled uphilt to direct surface runoff around the residence. 4) Roof downspouts and drains slrould discharge rvell beyond the limirs of all backfill. LimltatÍons¡ This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no rvananty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obta¡ned frorn the exploratory pits excavated at t¡e locations indicated on Figure I and to the depths shown on Figure 2, the proposed type of construction' and our experience i¡r 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 specíal field of practice should be consutted. Our fìndings include inferpolafion and extrapolation of the subsurface conditions iclentified at the exploratory pits and variations in the subsurface conditions may nol become evident unlil excavation is performed. If Job No. I 07 0660 cåFtecrr j í ) -5- conditions encountered during construction appear different fro¡n those described in this report, we should be notilied al once so re-evaluation of the recommendations may be made. 'l'his 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, rve should provide continued consultatiorr and fietd services during construction to review and monitor the implemenlation of our recommendations, and to veri$ that the recommendations have been appropriately interpreted. Significant design changes may require ndditional analysis or modifications lo the recommendations prcsented herein. We recomnlend on-site observation of excavations and foundation bearing strûta and testing of structurol fìll by a representative of the geotechnical engineer. I f you have any questions or i f rve nray be of further assistance, please let us knorv Respeclfrrlly Submitted, HEPWORTH - PAWLAK GEOTECFINICAL, INC. Jordy Z. Adamson, Jr., P.E. Reviewed by: DanielE. Hardin, P,E. lZNcay attachmenls Figure I - Location of Exploratory Pits Figure 2 * Logs of Exploratory Pits Figure 3 - Swell-Consolidation Test Results Figure 4 * Gradation Test Results Job No,107 0660 cåFtecn APPROXIMATE SCALE 1" '. 100' ELK SPRING$ ORIVE LOT 81 BENCH MARKI GROUNDAT BUILDING ENVELOPE CORNER;ELEV. ' 10oO, ASSUMSD. --*çrPIT 1Ir I I I I I I I I LOT 83 t PIT 2 LOT 80I , PIT 3 tt L PlT 4***-J COMMONAREA 107 0660 LOCATION OF EXPLORATOHY PITS Figure 1 Á 0 PIT 1 ELEV.= 92' PIT 2 ELEV,- 92' PIT 3 ELEV.* 97,3' I I +ar$S .200- 10 PIT 4 ËLHV.= 98.8' 0 WC-10.3 0D-78 o)ot¡. I -tr o.¡,ô I I -Joü t o"0,f) 5 5 J 10 10 LEGEND: TOPSOIL; organic sandy slll and clay, firm, slightly moist, dark brown. SILT AND SAND (MþSM): gravelly, loose to medium dense, slightly rno¡sl, calcareous, lighl brown. ffi BASALT COBBLES AND BOULDERS {GM}; in a sllty sand malrix, dense, slightly moist, calcareous, light brown. 2* 0lameter hand driven llner sample. Disturbed bulk sample, Practical digging refusal with backhoe. NOTES: 1. Exploratory pits were excavated on August 30. 2007 with a Caterpillar 305c backhoe. 2. Locations of exploratory píls were measured âpproximately by pacing hom fealures shown on the site plan provfded. 3. Ëlevations of exploratory pits were measured by lnshument level and reler to the Bench Mark shown on Flgure 1. Logs are drawn to depth. 4. The exploratory pit locations and elevations should be considered accurate only to lhe degree implled by the method used. 5. The lines belween malerials shown on the explorålory pit logs represent the approximale boundaries betweenmåter¡allypes and lransilions may be gradual. 6. No heo waler was encounlered in lhe pits al the time of oxcavating. Flucluatíon in water level may occur wílh time. 7, Laboratory Testing Flesults: WC o Water Conlent (%) DD = Dry Densiry (pcf) +4 = Percent relafned on lhe No. 4 sleve -200 = Fercent passlng No. 200 sieve I I J þ l: 't07 CI660 LOGS OF EXPLORATORY PITS Figure 2 pefcent pcf Moisture Content = 10.4 Dry Density = 78 Sample of: Sandy Silt From; Pit 1 at 2 Feet il \ 1 Compression _upon wetting \il \il \\\ I , 10 APPLIED PBESSURE. ksl 0 àe Ê .9atu, 0)a Eoo 2 1 3 4 5 6 1001,00.1 107 0660 SWELL-CONSOLIDATION TEST RESULTS Figure 3 24T1A, 7 HR 45 MlN. t5 MtN. TMERE^ÞNGS U,S, STANDARD SERIES #50 #30 #16 #8 CTEAR SOUANE OPENINGS 0 I MIN ##4 3/8' 314' I 112' 3. 5.6. 8. t0 20 30 't00 90 60 70 f¡y40 al--l¡JGÞ50z t¡J(J 0rl¡lo- 60 602 Øv, 0. 50 l-z l¡J() e, l¿J 40 lL 70 80 90 r00 30 20 10 .001 .@2 .005 .009 .019 ,0t7 074 .150 ,gO0 .600 01.18 2.36 4.75 9,5 t9,0 3?.5 78.2 152 203t2.5 127 DIAME-IER OF PARIITLES IN M'I.IIMEÍEFS q¡YlOgt"t cogE Ê9 GRAVEL 56 %SAND 34 o/"SILTANDCLAY 10 % LIQUID LIMIT V"PI.ASTICITY INDEX O/O FROM:Pit 3 at 3 to 3/a Feet -}....''.'''..# SAMPLE OF: Slightly Si[y Sandy Gravol 107 0660 GRADATION TEST HESULTS Figure 4