The URL can be used to link to this page

Home My WebLink AboutSubsoils ReportHel.rvolrh- Pa,r,lirk Gccitechrr ical, Inc 50i0 Courrr-r' R.rrrtl I54 Glcns'oo.J Sprirrgs. Clrlori:dt"r rS 1 6ll Phone' 9?0-945-798.3 HEPWORTH - PAWLAK GEOTECHNICAL Far: 9?t-!-94i-E454 enia i1 : hpgeo@hpgeotech.c ortr SUBSOIL STUDY X'OR FOIINDATION DESIGN PROPOSED RESIDENCE LOT 13, CERTSE RANCH LARKSPUR DRIYE GARFIELD COI.INTY, COLORADO JOB NO. 106 0982. DECEMBER 1I,2006 PREPARED FOR: CRAWT'ORD DESTGN Bt rLD, LLC ATTN: BRAD CRAWT'ORI) P.O. BOX 1236 CARBONDALE, COLORADO 81623 H * s N parker 303-841-7119 . Colorado Str',rings ?i9-633-556? t Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY..-." PROPOSED CONSTRUCTION SI]-R CONDITIONS SUBSIDENCE POTENTIAL ....... FIELD EXPLORATION. SUBSURFACE CONDITIONS...........' FOLINDATJON BEARING CONDITIONS ..-. "'" DESIGN RECOMMENDATIONS.......""... FOUNDATIONS.......... FOTJNDATION AND RETAINING WALLS' FLOOR SLABS LINDERDRAIN SYSTEM SURFACE DRAINAGE .............-....-.... LIMITATIONS,.....-........ FIGURE 1 - LOCATION OF EXPLORATORY tsORINUS FIGURE 2 . LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURES4and5-SWELL-C0NSOLIDATIONTESTRESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS -1- -1- _)_ Sr. t..' -3- J -4- 5 -z- 8- *' *if .&.' &,. .*Cr afr i."tf i: '-..r {ti.r{t.. rf #r- ,o _'?i t] PT]RPOSE AI{D SCOPE OF STTIDY This reportpresents the results ofa subsoil study for a proposed residence to be located on Lot 13, Cerise Ranch, Larkspur Drive, Garfield County, Colorado. The project site is shown on Figure 1. 'fhe pulpose of the study was to develop recommendations for the foundation design. The study was conducted in accordance with our agleement fol geotechnical engineering services to Crawford Design Build, LLC dated November l5' 2006. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurl-ace conclitions- Samples of the subsoils obtained during the field exploration were tested in the laboralory to determine their classiflcation, compressibility or swell arld other engineering characteristics' The results of the field exploration and laboratory testing were analyzed to develop lecommendations for foundation types, clepths and allowablc pressures for the proposed building forurdation- This report summarizes the data obtained during this study and presents our conclusions, design recommendations and other geotechnical engineering considerations based on thc proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION The proposed residence will be a two story, wood frame structure over a walkout basement level located in the northern parr of the building envelope as shown on Figure 1' The attache d garageand basement floors will be slab-on-grade' Grading for the structure will to be relatively minor with cut depths between about 3 to l0 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' , Job No- 106 0982 c&Fte.rl 1 SITE CONDITIONS Lot 13 was vacant at the time of our field exploration. Larkspur Drive is located uphill to thc north of the lot. 'Ihe iot slopes moderately to strongly down to the south at grades between abour gyo and 10%. There is abour 8 f-eet of elevation difference in the building footprint and abour 20 feet across the lot. An abandoned irrigation ditch is located in the upper portion of the building envelope. A currently inactive irrigation ditch is located below the building envelope. Vegetation on the site consists of native grass and weeds. Eagle Valley Evaporite Formation beclrock is visible on the valley hillsjde to the norlh' SUBSIDENCE POTENTIAL Bedrock of the pennsylvanian age Eagle Vailey Evaporite underlies the Cerise Ranch Subdivision. Thesc rocks are a sequence ofgypsiferous shale, fine-grained sandstone and siltstone with sorne massive beds of gypsum and limestonc' 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 condjtinns can cause sinkholes to develop and can produce areas of loc alized subsidence. During previous work in the area, several sinkholes were observed scattered rhroughout the Cerise Ranch Subdivision' These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. Sinkholes were not observecl in the immediate area of the subject lot. No evidence of cavities was encountercd in the subsurface materials; however, the exploratory borings were relatively shallow, for foundation design only. Based on our present knowledge of rhe subsurface conditions at the site, it cannot be said fbr certain that sinkholes will not develop. The risk of future ground subsidence on Lot I 3 throughout the service life ol the proposed residence, in our opinion, is low; however, the owner should be made awate of the potential fbr sinkhole development- If further investigation of possible cavities in the betlr.uck below the sitc is dcsircd, r,ve should be contacted, Job No. 106 0982 c&Btecn -3- FIELD EXPLORATION The field exploration for the project was conducted on November 20, 2006' Two exploratory borings were drilled at thc locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4-inch diameter continuous flight augers poweted by a truck-mounted CME-458 drill rig' The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Slotted PVC pipe, lYz-inch diameter, was installed in the borings to the depths shown on Figure 2. Samples oJ'rhe subsoils were taken with l% inch and 2 inch I'D' spoort sampiers' The samplers were clriven into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This test is similar to lhe 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, Figure 2. Thesamples were returned to our laboralory for review by the project engineer and testing. SUBSTIRFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2' The subsoils consist of about t/z footof topsoil overlying sandy silty clay with scattered gravel lenses/layers. Relativeiy dense, silty sandy gravel containing cobbles was encountered beneath the clay in Boring 1 at a depth of about 2llAfeet down to the maximum explored depth of 23Yrfeet' Laboratory testing performed on samples obtainecl fiom the boiings included natural moisture content, density and finer thm sand size gradation analyses' Results of swell- consolidation testing perlormed on relatively undisturbed drive samples of the clay soils, presented on Figures 4 and 5, indicate moderate to high compressibility under conditions of loading and wetting. The laboratory testing is summarized in Table 1' Job No. 106 0982 c&5tecr, -4- Groundwater was encouptered in the boriirgs at depths of about 16'/z feet at the time of drilling. The groundwater was measured in the borings at depths of I2 and I8% f-eet when checkecl on December I l. 2006. The upper soils were typieally moist to wet near or below the groundrvater Jevel' FOUNDATION BEARING CONDITIONS Based on the subsoil conditions encountered in the borings, a spread footing fbundation bearing on the upper sandy silty ciay soils appears feasible with some risk of differentia-l settlement and blilding clistress, especially if the bearing soils at'e wetted' A deep fbunclation (such as driven piles) which extends down to the relatively dense gravel subsoils could be used to provide a rnoderate load capacity and a low settlernent risk' we should be contacted if a deep fbundation is proposed' DESIGN RECOMMEIIDATIONS FOLTNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the naturc of the proposed construction, we rccofiIlnend ihe building be founded with spread footings bearing on the upper natural soils' The design and construction criteria presented beiow should be observed for a spread footing foundation system. i) Footings placed on the undisturbed natrral soils should be designed for an allowable bearing pressule of 800 psf. To reduce potential groundwater impacts, we recornmend that footing grade be no lower than the water level in the irrigation ditch below the building envclope which appears to be at about elevation 5,975 + feer. Based on experience' we expect inifial scttlement of footings designed ancl constructed as discussed in this section .Iob No. 106 0982 c&Ft=.rt 4) -5- will be about I inch or lcss. There could be about i to 1% inches of additional settlement if the bearing soils become wetted. The footings should have a minimum width of 20 inches for continuous walls and 2 feet for isolated pads. Exterior footings and footings beneath unheated areas should be provided with adequate soil cover above their bearing elevation fbr frost protection- Placement of foundations at least 36 inches below exterior grade is typically used in this at'ea. Continuous foundation walls should be heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at lcast l4 feet. Foundation walls acting as retaining structurcs should also be designed to resist lateral earth pressures as djscussed in the "Foundation and Retaining Walis" section of this rcpofi. The topsoil arrd any loose or disturbed soils should be removed and the footing bearing level extended down to the natural soiis. The exposed soils in footing area should then be moistened and compacted. A representative ofthe geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. s) FOTINDATION AND RETAINING WALLS Foundation walls and retaining structures which are laterally supported and can be expected to undergo only a slight amount cf deflection should be designed for a laterai earth pressure computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting of the on-site soils. Cantilevered retaining structures which are separate frorn the residence and can be expected to deflect sufficiently to mobilize the fulI active earth pressure condition slrould be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site soils. 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, trallic, construction materials and 2) 3) 6) Job No 106 0982 c&Btecn -6- equipment. The pressures Iecommended above asstllne drained conditions behind the walls and a horizontal backfill surface. The buildup of water behind a wall or an upward sloping backfrll surface will increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain should be provided to prevent hydrostatic plessule buildup behind walls. Backfill should be placed jn unifonn lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content near optimurn' Backfill in pavement and walkway areas shoulrJ he compacted to at least 9590 of the maximunt standard Ptuclur density- Care should be taken not to overcompact the backfill or use large equipment near the wall, since this could callse excessive lateral pressure on the wali' Some settlement of deep foundation wall backfill should be expected, even if the material is placed correctly, and could result in clistress to facilities constructed on the backfill. 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 tbotings can be calculated based on a coelficient of friction of 0.35. Passive pre'ssure of cornpacted backfili against the sides of the lootings can be calculated using an equivalent fluid unit weight of 325 pcf. Thc coefficient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factors of saf-ety should be included in the design to limit the strain which will occur at the ultimate strength, particularly in the case of passive resistance. Fill placed againsl the sirles of the footings to resist lateral loads should be compacted to at leastglo/oof the tnaximurn standard Proctor density at a moisturc content near oPttmum. ILOOR SLABS The natural on-site soils, exclnsive of ropsoil, aIe suitable to support lightly loadcd slab- on-grade construction. 'Ihe soils tend to be hydrocompressive, which could result in some slab sefilement and rlistress, especially if the bearing soils become wetted- To recluce the eff-ects of some differential rnovement, floor slabs should be separated from all bearing walls and columns with expansion joints which allow unresl'rained vertical lobNo. 106 0982 e&Ftect't -7 movement. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The requirements for joint spacing and slab reinforcemenr 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 basemenl level slabs to facilitate drainage. This material sl:ould consist of minus 2 inch aggregate with at least 50o/o relained 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 moislure content near optimum. Required fill can consist of imporred granular soils such as'/o-inchroad base devoid of vegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM Although free water was encounlered below the proposed bascment level during our exploration, it has becn our experience in the area that groundwater levels can rise and local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also create a perched condition. We recommend below-grade construction, such as retaining walls, crawlspace and basement areas, be protected from wefting and hydrostatic pressure buildup by an underdrain system" The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above the inverr level r,vith free-clraining granular material. The drain should be placed at each level ofexcavation and at least I foot below lowest adjacent finish grade and sloped al a minimum 1o/o to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less tJ:an Za/o 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 shouid be at least 7Yz feet deep. An impervious membrane, such as 30 mil PVC liner, should be placed beneath the drain gravel in a trough shape and afiached to the foundation wall with mastic to prevent wetting ofthe bearing soils. lob No, I 06 0982 e&Ftecn -8- SUR}-ACE DRAINAGE The followirrg drainage precautions should be observed during construction and maintaincd at all tirnes afler the residence has been completed: l) Inundation ofthe foundation excavations and underslab areas should be avoided during conslruction' Z) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95% of the maximurn standard Proctor density in pavement and slab areas and to at least 90% of the rnaximum standard proctor density in landscape areas, Fiil placed around the building should extend no more than 3 feet about existing grade. Deeper fills rnay be possible but would need to be placed at least a month prior to building construction to reduce polential settlement itnpacts' 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 12 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 wrth about 2 feet of the on- site finer graded soils to reduce surface water infiltration. 4) Roof downspours and drains should discharge wcll beyond the limits olall backfrll. 5) Irrigation sprinkler heads and landscaping which requires regular healry irrigation, such as sod, should be located at least 5 feet from foundation walls. LI}trTATIONS This study has been conductcd in accordance with generally accepted geotechnicai engineering principles and practices in this area at this time' We make no waranfy either express or implied. The conclusions and recommendations submitted in this report are basecl uport Lhc tlata obtained from thc cxploratory borings drilled at the locations Job No. I 06 0982 c&Btecn -9- indicated on Figure 1, fhe proposed type ofconstruction and our experience in thc area. Our services do not include detcrmining the presence, prevention or possibiiity of mold or other biological contaminants (MOBC) developing in the f-uture- If the client is conscrnecl about MOBC, then a professional in this special field of practice should be consulted. Our findings jnciude 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 lrom those described in this report, we should be notifiecl 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 iue not responsible for technical interpretations by others of our information. As the project evolves, we should provide continued consultation and field seruices during construction to review and monitor the implementation of our recommendations, and to verify rhat the recommendatjons 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 lill by a representative of the geotechnical engmeer Respectf ully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Jordy Z. Adamson, Jr., P.E Reviewed by: Daniel E. Hardin, P'E JZAlksw Job No- I 06 0982 c&Btecr' APPROXIMATE SCALE 1" = 30' 5990 - -- \ABKSPUR DRNE 5990 -'+- BORING 2 LOT 13 a BORING 1I IRRIGATION DITCH ABANDONED IRRIGATION DITCH LOT 14 - 5980 5985 - 5985 i LOT 12 5980 -I Figure 1LOCATION OF EXPLORATORY BORINGS HEPIyORT}}PAWIAI( 1 06 0982 PROPOSED RESIDENCE BORING 1 ELEV.:5980' BORING 2 ELEV.: 5986' o) 0)tL I Lo.E (u 0)il q) g) tL co $ a) tu ++ 5990 5985 5980 5975 5970 5965 5960 5955 21112 61',t2 4/12 WC:19.0 DD:108 -20a:87 8/12 wc:20.0 DD:1OB 1112 6112 WC:18.0 DD:1 1 4 Bl12 sl12 Note: Explanation of symbols is shown on Figure 3- 5990 5985 5980 5975 5970 5965 5960 5955 21 - 0 4112 Figure 2LOGS OF EXPLORATORY BORINGSfu1 'Awr-AI< GEOIECFtr|ICAL 106 0982 LEGEND: TOPSOIL; sandy silty clay, scattered gravel, organics, roots, soft, moist, brown. CLAY (CL); silty, sandy, with scattered gravel in Boring 1, more gravelly in Boring 2, medium stiff, soft near groundwater in Boring 2, brown to light brown. GRAVEL (GM); silty, sandy, with cobbles, dense, wet, brown. L ti Relatively undisturbed drive sample; 2-inch l.D. california liner sample. I I Drive sample; standard penetration test (SPT), 1 3/8 inch l.D. split spoon sample, ASTM D-1586r Drive sample blow count; indicates that 4 blows of a 140 pound hammer falling 30 inches were4112 required td orive the California or SPT sampler l2 inches. g' Free water level in boring and number of days following drilling measurement was taken. l- Practicaldrillingrefusal. I FII lndicates slotted PVC pipe installed in boring to depth shown. &J NOTES: '1, Exploratory borings were drilled on November 20, 2006 with 4-inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory borings were obtained by interpolation between contours shown on the site plan provided 4. The exploratory boring iocations and elevations should be considered accurate only to the degree implied by the method used. 5. The lines beiween materials shown on the exploratory boring logs represent ihe approximate boundaries between material types and transitions may be gradual. 6. Water level readings shown on the logs were made at the time and under ihe conditions indicated. Fluctuations in waler level may occur with time. 7. Laboratory Testing Results: WC: Water Content (%) DD : Dry Density (pcf) -200 : Percent passirrg No. 200 sieve a n rffil ffi Figure 3LEGEND AND NOTES106 0982 GgFtoc HEP!YO} T}}.PA T.AK GE TEI 0 2 .l 4 q \oo\ co'a cl)oa EoO 6 7 B 1.0 10 1000.1 APPLIED PRESSURE - ksf 0 1 2 4 o\ c '6 (f) o EoO 5 1.0 10 APPLIED PRESSURE - ksf Moisture Content: 19.0 Dry DensitY : 108 Sample of; 9416y Silty Clay From: Boring 1 at 5 Feet percent pcf Compression _upon wettino ( I ) Moisture Content: 18.0 Dry DensitY : 114 Sample of: $sniy Silty Clay From: Boring 1 at 10 Feet percent pcf 7 No movement upon "wetting ,"-t \ ) Figure 4SWELL-CONSOLIDATION TEST RESU LTSe&crr HEPWoR.N+PAWTAT< GE TECHNICAL i06 0982 0.1 100 0 b92 .1 4 trJ o co'6 a o) o_ Eorl 7 8 Moisture Content : 2A.0 percent Dry Density : 108 PCf Sample of; Sandy Silty Clay with Gravel From: Boring 2 at 10 Feet No movement upon'wetting k_""t \ 1.0 10 APPLIED PRESSURE - ksf Figure 51 06 0982 cHftocrr HEPYYoRrII.PATYLAX GEC'TEGHNjCAL SWELL-CONSOLI DATION TEST RESU LTS 0.1 100 HEPWORTH.PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 106 0982 NATURAL MOISTURE CONTENT NATURAL DRY DENSITY GRADATION RG LIMITS GRAVEL ('h) SAND e/o) PERCENT PASSING NO, 200 SiEVE LIQUiD LI[4IT Pl.ASTIC INDEX UNCONFINED COMPRESSIVE STRENGTH SOIL OR BEDROCK TYPE 'to 19,0 108 87 Sandy Silty Clay 18.0 114 Sandy Silty Clay 20.0 108 ty v Gravel SAMPLE LOCATION DEPTH (ft) 5 I 0 t0 BORING 1 2