The URL can be used to link to this page

Home My WebLink AboutPreliminary Geotechnical Study 12.29.00-H Hepworth-Parvlak Geotechnical, fnc. 5020 County Road 154 Glenvood Springs, Colorado 81601 Phone: 970.945-7988 Fax: 970.945-8d54 hpgeo@hpgeotech.com {J ) RECEIVED sEP 2 7 20,t8 GARFIELD COUNTY COMMUNITY DEVELOPMENT PRELIMINARY GEOTECHMCAL STUDY PROFOSDD GILEAD GARDENS STJBDTYTSION 1s77 COITNTY ROAD 335 GARF',IELD COUI{TY, COLORADO JOB NO. L00 67? DECEMBER 29,200b PREPARED FORI BOTINDARIES UNLIMITED, INC. ÀTTN: BRIICE LE\ryIS 0401 corjNrv R0AD r49B GLENTyOOD SPRINGS, COLORADO 816r)1 "å ',,-/ IrEpwoRTH - pAWtAK GEOTECHMCAL, INC. Ðecember 29,2000 Boundaries Unlimited, Inc. Attn: Bruce Lewis 0401 County Road 1498 Glenwood Springs, Colorado 81601 Job No.lO0 672 ) Subjeet:Report Transmittal, Preliminary Geotechnical Strrdy, Proposed Gilead Gardens Subdivision, L577 County Road 335, Garfield County, Colorado Dear Bruce: As requæted., we have conducted a geotecbnical study for the proposed subdivision at the Subject site. Development of the properry shorrldbe suiable based on the geologic and geotechnical conditions. Development of the individual lots should consider potential storm water and associated dehris flows from the hillside drairages to the southeast. Subsrufaee conditions encountered in the exploratory boriags drilled tbroughout the proposed dgvelopment area consist of stiff tð very siif chys and medium ñ stiffsaudy clay and silt with gravel overlying dense river gravel alluvium. Groundwater was not encountered in the borings and the soils were slightly moist to very moist with depth.. Samples çf th. e clays encountered on Lots 6, ? and I showed an expansion potentialwhenwettedunderlightloading -r,rq,^.rn*ty L0+ -1 Spread footings placcd sn the natural subsoìls and desïgned fp{anallowable bearing piessure in rhe range of 1,500 psf to 3,000 psf appear suita¡tÞ,tr building support. The clay soils encountered on Lots 7 and 8, and_Egþab$-I¡t 6, should be completely removed from beneath footing and floor slab areæ due to their expansion potential. Foundations for the buildings on Lots 7 and I can be located as close as 10 feet from the top of the steep escarpment provided the footings are placed on the natural dense gravel aliuvium. The ri:port which follows describes our exploration, summadzes our findings, and presents our recomm.endatïons suitable for planning and preliminary desþ. It is important that rve 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 thì'q ¡sps¡¡, please contâct us. Sincerely, HEP\MORTH - PAWLAK GEOTECHNICAL, INC. "Po-n/A StevenL. Pawlak, P.E, Rev. by: DEH SLP/djg/ksw ¿ () TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY PROPOSED DEVEI.,oPMENT 1 I 1SITE CONDITIONS GEOLOGTC SET?ING . .. FORMATIONROCK COLORADO RTVER ALLWIUM ALLWIALFANS. I,OESS FIELD EXPLORATION, . . . . SUBSURF^å.CB CONDITIONS GEOLO GIC SITE ASSF,SSME¡TT TERRACE ESCARPMENT MOISTURE SENSTTIVE SOILS STORM 1VATER FLOOÐING EARTHQUAKES RADIATION POTENTI.{L PRELTMINÀRY DESIGN RECOMMENÐATTONS FOT]NDATIONS .. . . FLOORSLABS UNDERDR.A,IN SYSTEM SITEGRADING ... SURFACE ÐR.A.INAGE SEPÏIC DISPOSAL SUBSIJRFÀCE PROFILE LIMITATIONS REFERENCE .... FIG{JREI-GEOTOGYMAP FIGURËS Z&,3 - LOGS OF E}GLORATORYBORINGS FIGURE4 - I,EGEND AND NOTËS FIGURES 5.7- SIVELLCONSOLIDATTON TF,ST RESUUTS TABLE I - SUMMARY OF LABORATORY TEST RÉSIJLTS 2 2 2 3 J 3 4 5 5 5 6 6 6 7 7 7 8 I 9 9 I ( 'ü lJ-Þ lîcnrcnu J () PTJRPOSE, AND SCOPE OF'STIIDY This report presents the results of a preliminary geoteehnical study for the proposed Gilead Gardens Subdivision located at 1577 County Road 335, Garfield Counff, Colorado. The project site is'shown on Fig, 1. The pu{pose of the srudy was to evaluate the geologic and subsurfase couditions and their potential impacts oû the project. The study was conducted as requested by Bruce Lewis iu addition to our previous radiation potential lettgr, dated August 18, 2000. A field exploration pro$am consisting s¡ s leconnaissance and exploratory borings wæ conducted to obtain information on the site and subsurface conditions. Samples of the subsoils obtained during th¡ field exptoration were tested in the laboratory to determine their classificatio¡1, compressibility or swçll and othe¡ engineering characteristics. The results of the field exploration and laboratory testing were anaþzed to deveþ recommentlations for project planaing and preliminary design. This report summarizes the data otitained during this study and presents our conclusions and recornmendations based on the pro¡osed deveþment and subsurface conditions encountered. PROPOSED DEI¡ELOPMENT The proposed development consists of an 8 lot subdivision as shown on Fig. L. The lots will range in size from roughly 3 to 7 acres and be developed with single family homes. Äccess to the lots willbe partty offof County Road 335 and from a nelv opsite road, see Fig. 1. We assume the residences will be typical of the area and be one to two stqries with a partial or full basement, The developmént will be serviced by on-site wells and i¡dividual septic disposal systems. The buildings on Lots 7 and 8 are proposed to be built close to the top of the steep dowa slope to the Colorado River. If development plans change significantþ from those described, we should be notified to re-evaluate the recommendations presented in this report. SITE CO¡{DITIONS The proposed development corsists of about 35 acres located in the western half of Section 7 ,T 6 S, R 91 'W. Most of the property consists of irrigated pâ$flre that is. crossed by fleld ditches. Lots 1 and 5 have existing residences which a¡e located above \r' H-P G¡orçcu (J J Thc high terrace deposits are giacial oufwash that were deposited during the Bull Lake glacial period (Scott and Sh¡oba,1997). The intermediate teffûce deposits are glacial outwash that was deposited during the Pinedale glacial period. Deposits below the low teuace are Pinedale and Holocene gravels. Borings on tbe interrnediate terrace Qû show that the soil profrle consists of an upper sandy clay that is from |to 5 feet thick overlying relatively clean to silty sand and gravel with rounded cobbles antl boulders. The laboratory tests show that the upper sandy clay on the intermediate terrace has a moderate to high expansion potential when wetted. The intenuediate terrace is covered by alluvial fan deposíts in the southeastern part of the property. ALLWIAL FANS Geologicaìly young a{uvial fa¡s (QaÐ that are largely the produet of occasional debris flows and floods assocíated with intense thunderstorms cover the intermediate tenace in the southeastern part of the property. To our knowledge historic debris flows have not occurrod ou.the fa¡ls but historic debris flows have occurred on similar fans ia the general area. The drainage basins abcive the fans are relatively sm,all and cover about 7 to ?3 acres. At the borings, the fan deposits were between 1.7 and 26r/z feet deep and underlain by Colorado River alluvium. The laboratory tests show that the alluvial fan deposits äre not expansive but have low to moderate compressibility under conditions of wetting and loading LOESS Regional mapping shows that wind deposited, non-stratified sandy silt loess deposits (Qlo) covcr the intermediate terace to the west of the project area and the high terrâce to the southeast of the property (Scott and Shroba, L997). Loess is not present in the proþosed building areas. FMLD ET(PTORATION The field exploration for the projeet wæ conducted on October 31 and November l, 2000. Eight exploratory borings we¡e drilled at the locations shown on Fig. 1 to eyaluate the subsurface conditions.U H.P GEoTFÍ:H t;-5- GEOLOGIC SITE ASSESSMDNIT Geologic.conditions are not prçsent in the area which would make the proposed development infeasible. Geologic conditio¡s that should be considered in projecr planning and design ard their expec.ted influence on the proposed development are discussed below. TERRACE ESCARPMENT The steep terrace escarpmetrt between temaces Qtr and QÞ forms the noilh sid.e of the building areas ot Lots 7 aad 8. The escarpment is about 25 feet high at an aveiage slope oTabout1}To (!2/ahornontalto 1 yertical). We understand that current plans are to build the residences on Lots 7 andS close to the ûop of the escârpmeût and coßtruÊt a pedestrian trail down the escarpmetrt to the river. The gravel terrace that makes up most of the steep escaq)ment appears relatively stable and should support lightly loaded footings with'appropriate setback from. the slope. setback recommendations and other gra.ding precautions for l¡ts 7 and I are presented in the Preliminary Design Recommeda¿iozs section of this report. The 100 year flood plain limit of the Colorado River roughly follows the roe of the steep terrace escarpment and there is a potential for long term erosion. atrd undercutting of the slope. A civil engineer experienced, with zurface water hydrology shoulcl evaluate potential flood flow aud erosion potential at the base of the terrace escarpmentandprovideerosionprotectiondesþasneeded. : MOISTURE SËNSTTIVE SOILS The alluvial fan soils that cover most of the development are typicalþ moisture sensitive and could have a collapse potential, especially Ín the a¡eas that have not been inigated. The laboratory testing shows that the clay soils overlyiig the QÞ terrace have a swell potential when wetted. Foundation design considerations will be needed to mitigate possible adverse effects of the moistwe se,lrsitive soils. Preliminary foundation desÍgn consideratíors are discussed in the Prelimírnry Desígn RecommendafÍo¿s section of this report. \,/ tU H-P Gpnrrcpr 'J -7- PRELIMINARY DESIGN RECOMMENDATTONS The conclusions aud recommendatious presented below a¡e bæed on the proposed deveþmeut, subsurface conditions encountered in the exploratory þp¡ings, and our experienci ín the a¡ea. The recommend.ations are generally suitable for planning and preliminary design aud site specific studies should be conducted for individual lot development. Site specific foundation designrecommendations and setback criteria are given for Lots 7 and 8, FOUNDATIONS Bearing.conditions will vary depending on the specific location of the building on the property, Based on the nature of the proposed construction, spread footings bearing on the natural subsoils should be suitable for buitding suppo$s. We expect that footings placed on fhe alluvial fan soils (Lots 1-5) can be sized for an allowable bearing pressure on the.order of 1500 psf and footings placed on the dense gravel alluvium (Lots 7 and 8 and possibly IÆt 6) can be sized for an allowable bearing pressure of 3,000 þsf. The expansive clays at Lots 7 and I and possibly Lot 6 should be removed aud the footings extended down to bear on the dense gravel alluvium. Foundations for the buitdings on Lots 7 and I can be locateð as close as 10 feet from the top of the steep e$carpment ptooidtd the footings are placed on the natural dênse gravel alluvium. The actual setback should be verified at the time of construction. Footings placed withìn 20 feet of the escarpment should be sized for an allowable bearing pressure of 2,000 psf and footings at a greater distance for 3,000 psf. Foundation walls should be designed to span local anomalies and to resist lateral earth loadings when acting as retaining sfuçtures. Founilation walls bacldilled with the on+íte ctaþ should be designed to resist a lateral earth preserve of 60 pcf equivatent fluid uuit weight. Below grade areas and retaining walls should be protected frorr wettùrg and hydrostatie loading by use of an underdrain system. The footings should have a minímum depth of 36 inches for frost protection. FLOOR SLABS Slab-on-grade construction should be feasible for bearÍug ou the dense gravel alluvium or the sandy clay and silt soils. (v. H-P Georec¡r I'J -9- SURFACE DRAINAGE The grading plan for the subdivision ohould consider potential storm water flows from the hiìlside drainages to the soutþeast of the property. 'Water should ûot be allowed to pond or flow up against buildings which could impacr slope stability and foundations. Fouádation wall backfill should be well compacted and have a positive slope away from the buildiug for a distance of L0 feet ro limït infilhation into the bearing soils. Roof downspouts and drains should discharge well beyond the limits of ali backfiIl and landscape irrigation should be restricted. Surface water should not be concentrated and directed on to the steep escarpment sþe of Lots 7 and I without adequate erosion protection. SEPTIC DISPOSAL SUBSURFACE PROFILE We understand that percolation testing will be performed tbroughout the deveþment area to evaluate the feasibilrty of infiltration septic disposal systems at each of the proposed lots. We alio understand that the residences on Lots I and 5 have operating infiltration septic disposal systens approved by the Couuty. Groundwater or bedrock were not encountered to the drilled depths of generally I feet oi greater at each of the lofb. The disposal systems at l.ots 6, 7 and 8 may,need to be based in the underlying gravel alluvium due to possible slow percolation rates of the upper clay soils. The systems on Lots ? and 8 will need to be set.back from the steep escarpment in acçordance' with county regulations LIMITATIONS This study has been conducted according to generally accepted geotechnicat engineeriug principles and practices in this area at this time. lVe make tro wa1¡anfy either expressed or implied. The conclusions and.recommeudations submitted in this report are based upon the data obt¿ined from the field resonnaissance, review of published geologic reports, the exploratory borings located as shown on Fig. L, ttre proposed type of construction aud our experience in the area. Our fifilings include interpolation and extrapolation of þ subsurface conditións identified at the exploræory borings and va¡iations in the subsurface conditions may not become evident until excavation is performed. tv ¡.1-Þ lìF¿'\lË.lJ U (J Ð$LA¡¡An0N Qaf -Allt¡vlal Fan Qc - Gollwium Qo -Loess Qtl -lsrTenace Qfl -lnÞrmediaþTenasÊ Q6 -HighTe¡race Tw -lryasafifiFm. ----¡'- Gonþct (approx) Br. Boring(appmx,) Fobrrlial Debfis Flory BasinÀn¡hmo fFì\\l7 --(:.. Qtl Qtl ê Qaf Qcftw 1 I i!I o ðî \III % l¡r7 atz QlolQE Qe/tw Qlo/Qt3 AIo/QtS Qcflw I r0 4{lltfi. l--.-¡-¡ Sc¡ls I ln.s4{l(}ñ. Qaf Qaf Qlo/Qt3 Gilead &dens Subdiv[Son- Geology [iap100672 HEPìTIIORT}I+AWI.AI\ GEO'IËCHNICAL Inc. t,V. Fb. r ( VI 00 5 t BORÏÀIG 6 EI,EV.= 5510' LOT 2 8lL2 6lLz 7lL2 wG=1?.6 DÐ=109 BORING 7 ELEV.= 55L4' LOT 2 BORING 8 ELE\¡. = 5514 ' I:OT 3 BORTNG 9 EÏrEV. = 55L4' IJOT 4 lL/12 aLlx2 I{C=13 .6 DD=112 9ltz Wc=L3.5 DD=112 l.tl12 LI/L2 lfc=15.9 DD=113 -2Q0='19 I¡L=25 PI=7 ?515,LA lÐ ¡J 0,(¡) tlt I ÉIJIÊ ]Joo ftr I ,qr, O¿(l)'a 10 15 25 30 ÞROBLBIJE BOUTJDER 6lLz a4l12 9ll-z 9lLz t0l.Lz 7 /3,L0/0 Nobe¡ ErçlarraEíorr of elrnboLs íe shown on Fig. 4. 10 L5 20 25 30 20 100 612 T{EÞT¡ÍORTH - PAT,ÍLAT( GE$TECTTNICATJ, ÍNC.IJOGS OF EXPT'ORATORY BORTT{GS Fí9.3 l4oisture Contenú = 1S.2 percerrt Dry lJensity = LLg percent Sample of: slightly Sandy CIay From: Boring 2 at 2 Feet \ \ \ \ (¡ \ E>riansion upotl wetÈíng \ \ 5 4 3 2 1 0 1 2 d\o Éo..l t¡s rú fr Êt I Éo.r{ o UIo ¡{ O¡ É,oU t" 0 1 2 dlo Éorrl v) É fit B t{ I Êo,r{ au 0)T 9,Ëo U o 1 1.0 10 .âPPìLrIEÐ PRESSURE - ksf l_0 APPTJTED PRESST]RE . Ksf L00 Moisture ConEenE = 13.4 percent Ðry Density = 11.7 percetrþ Samp1e of : Sli.ghtly Sandy Clay From! Boring 4 aÈ 2 Feet ì L Erçransion upon l¡retting 100 672 HEPWORTH-PAVITTAK GEOTECHNTCAL, INe.gWEtIr CONSOüIDATION TEST RES(IIrTS Fís.5 0.1 1.O rbo ( ( { \, 0 1 2 dp Éo..J üt v,o T{g o cJ ? Moi sture ContenE = L3.6 percerrt Drlr Density = tLZ percents Sample of : Sandy SílL and Ci"ay Frorn: Boring 9 at 10 Feet /ar'{*No movement uporrwetting \\ \I 10 .ÀPPIJIEÐ PRESSURE - ksf L00 672 HEPWORTH-PAT¡ilJAK GEOTECTTNICAI,, INC.SI{ELL CONSOL,IDAEIoN TEST RESUITrS Fis. ? 0.1 L.0 100