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Home My WebLink AboutOriginal Subsoil Study for Foundation Design 11.15.2006HEPWORTH - PAWLAK GEOTECHNICAL Hepu'rrrrh-Prrt,hk Georechn ical, I nc 5020 CtrLmtv Road I54 Glerrrvoorl Sr'rings, Color;rdo 6 I 601 Phone 970-945-7988 Fax: 970-945-8454 ernai l; hpgeo@hpge(rrech-c()rt1 SIIBSOIL STUDY FOR X'OT]NI}ATION DESIGN PROPOSED RESIDENCT, LOT 65, IROI{BRIDGE PIIASE 2 GARFIELD COTINTY, COLORADO JOB NO. 106 0791 NO\|EMBER 15,2006 PREPARED FOR: rRoIrBRrItcE HOMES, I,LC AITN: JODI TIffiVIS}EN 4lO IRONBRIDGE DRIVE GLENWOOD SPRINGS, COLORADO 81601 H I I :i Parker 303-841'7119 r Colorado Springs 719-633-5562 r Sih'errhorne 970-466-19S9 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY PROPOSED CONSTRUCTION.. - SITE CONDITIONS STIBSIDENCE POTENTIAL .. -... FIELD EXPLORATION...-... SUBSURFACE CONDIT]ONS FOLINDATION BEARING CONDITIONS DESIGN RECOMMENDATIONS............. FOLINDATIONS.... FOUNDATION AND RETAINING WALLS. FLOOR SLABS I]NDERDRAIN SYSTEM SURFACE DRAINAGE "........-..... LIMITATIONS .....,........... FIGURE I - LOCATION OF EXPLORATORY BORINGS FIGIIRE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 . LEGEND AND NOTES FIGURE 4 * GRADATION ]EST RESULTS TABLE I - SUMMARY OF LABORATORY TEST RESULTS -l - I ! I : a : ...- 2 - -2- -3 - -L- -4 -a_ -a_ -{_ _'7 _ -"7 _ -R_ -8- PTIRPOSE AND SCOPN OF STUDY This report presents the results of a subsoil study fbr a proposed residence to be located on Lot 65, Ironbridge Phase 2, River Bend Way, Garfield County, Colorado. The project site is shown on Figure l. The purpose of the study was to develop recommendations for the fotrndation design, The study was ccnducted in accordance with our agreement for geotechnical engineering services to lronbridge Homes, LLC dated August 31,2006. Hepworth-Pawlak Geotechnical previously conducted geotechnical engineering srudies for the subdivision development and presented the findings in reports dared October 2g, 1997 and F'ebruary 12,7998, Job No. 197 327. A field exploration program consisting of exploratory bcrings was conducted to obtain information on the subsurface conclitions. Samples of the subsoils obtained during the field exploration were tested in the laboralory to determine their classification and other engineering characteristics. The results of the field exploration and laboratory testing werc analyzed to develop reconunendations for foundation types, depths and ailowable pressures for the proposed building foundation. this report summarizes the data obtained during this study and presents our conclusions, design recommendations and other geotechnical engineering considerations based on the proposed construction and the subsurface conditions encountered. PROPOSED CONSTRUCTION The proposed residence on the lot had not been determined at the time of our study. We assurne the residence will be a 1 to 2 story structure above a walkout basement level and located in the eastem part of the building envelope shown on Figure 1. The garage will probably be at the main level. Ground floors will probably be slab-on-grade. Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 6 feet. We assume relatively light foundation loadings, typical ofthe proposed type of construction. Job No-r 06 079t ceFtecrr -2- If building loadings, Iocation or grading plans change significantly frorn those described above, we should be notified to re-evaluate the recommendations contained in this report. SITE CONI}ITIT}NS The lot is located on ujlingly rloping *lh r l0 ro 15 feet higher than the Roaring Fork River. The ground surface is relatively flat with a slope down to the east and about 6 feet of elevation difference across the assumed building area. The lot in the general building area has been disturbed and partly stripped of vegetation. In general, the natural terrain is somewhat irregular from debris flow deposition on the alluvial fan. The fan surface in places contains minor ridges and gullies and terminates at a scatp about l0 feet high down to the flood fringe of the river just east of the lot. Vegetation consists of grass, weeds and sage brush. The areas near the lot have also been recently disturbed by the subdivision development including rough grading for River Bend Way. SUBSIDNNCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge Development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution ofthe gypsum under certain conditions can cause sinkholes to Cevelop and can produce areas of localized subsidence. $uring previous studies for the subdivision development, several sinkholes were observed scattered throughout the Ironbridge Development. These sinkholes appear similarto others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. The closest mapped sinkhole is located about 300 fbet north of Lot 65 in the downhill corner of Lot 63. Numerous shallow sinkholes were observed in undisturbed areas of the adjacent I'ot 66 and similar depressions rnay have existed on Lot 65 now covered by the gtround disturbanee, fhese depressions appear to be from subsurface erosion (piping) of the fine-grained alluvial fan soils into coarser snils. These small sinkholes were described JobNo.106079t ceFtectr -J- in our February 12,1998 report and it was concluded that they were probably not caused by piping into the underlying formation rock. Sinkholes related to the underlying formation rock were not obseryed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; howevero the exploratory borings were relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it camot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot 65 throughout the service life of the proposed residence, in our opinicn, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigatir:n of possible cavities in the bedrock below the site is desired, we should be contacted. FIELD HPLORATION The field exploration for the project was conducted on September 8 and 19, 2006. Four exploratory borings were drilled at the locations shown on Figure I to evaluate the subsurface conditions. The borings were advanced with 4 inch diarneter continuous flight augers powered by a track-mounted CME 45 drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Sampies of the subsurface materials were taken with 1% inch and 2 inch I.D. spoon sarnplers. The samplers were driven into the subsurface materials at various depths with blo"vs frcrn a 140 po'.md hammer falling 30 inehes. This test is similar to the st.mdard penetration test described by ASTM Method D-1586. The penetration resistance values are an indication of the relative density or consistency of the subsoils and hardness of the bedrock. Depth.s at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The samples were retumed to our laboratory for review by the project engineer and testing. Job No- 106 0791 4 ST]BSI'RFACE CO}IDITIONS Graphic logs ofthe subsurface conditions encountered at the site are shown on Figure 2. The subsoils consist of about Il foot of topsoil overlying medium dense, siity sand with gravel and possible cobbles overlying dense, sandy gravel and cobbles with boulders. Drilling in the coarser soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was eneountered in the deposit. Below the dense granular deposits at depths of about l4Yzfeet at Boring 2, weathered and soft claystone-gypsum to hard claystone-siltstone bedrock was encountered to the drilled depth of 20 feet. Laboratory testing performed on sarnples obtained from thc borings included natural moistwe content and gradation analyses" Results of gradation analyses performed on a srnall diameter drive sample minus {lYzinch fraction) of the upper granular soils are shown on Figure 4. The laboratory testing is summarized in Table L Free water was encountered at a depth of about l4 feet and near the cantact of the soils with the bedrock. The upper soils were slightly moist to rnoist wirh depth. TOUIYDATION BIARING CONDITIONS The soils at expected cut depth mainly consist of medium dense, silty sand and gravel with some potential to collapse (settle under constant load) when wetted. Shallow spread footings placed on the natwal soils should be feasible for the building support with a risk of settlement and distress. Ihe risk of settiement appears- low due to the relatively shallow depth of the compressible soils but the settlement potential and the presence of possible voids in the subsurface should be furt'her evaluated at the time of construction. DESIGN RECOMME,IITDATIONS FOLINDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, we reconunend the building be founded with spread footings bearing on the natural soils. JobNo.1A60791 ceFtecrt 5 The design and construction criteria presented below should be observed for a spread lboting foundation system. 1) Footings placed on the undisturbed natural soils should be designed for an allowable bearing pressure of 1,500 psf. Based on expenence, we expect initial settlernent of footings designed and constructed as discussed in this section will be about I inch or less" Additional settlement of about t/z inch could occur if the bearing soils at shallow depth are wetted. 2) 1'he footings should have a minimum width of l6 inches for continuous walls and 2 feet for isolated pads. 3) Exterior footings and footings beneath unheated areas should be provided with adequate soil cover above their bearing elevation for frost protection. Placement of foundations at least 36 inches below exterior grade is typically used in this area. 4) Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsiupported length of at least 12 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earthpressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the natural gtanular sails. The exposed soils in footing area should then be moistened and compacted. 6) A represen'ntive of the geoteclr.nical engineer should abserye all footing excavations prior to concrete placement to evaluate bearing conditions. FOUNDATION AND RETAINING WALLS Foundation walls and retaining structures which are laterally supported and can be expected to undergo only a slight amount of deflection should be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at ieast 50 pcf for backfill consisting of the on-site granular soils. Cantilevered retaining structures which are separate frorn the residence irnd can be expected to deflect sufficiently to JobNo.1060791 ceFtecrr -6- mobilize the full active earth pressure condition should be designed for a lateral earth presswe computed on the basis of an equivalent fluid unit weight of at least 40 pcf for backfill consisting of the on-site granular soils. All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction rnaterials and equipment. The pressures recornmended above assume drained conditions behind the walls and a horizontal backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will increase the lateral pressure irnposed on a foundation wall or retaining structure. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Backfill should be piaced in uniform lifts and compacted to at least 90% of t1e maximum slandard Proctor density at near optimurn moislure content. Backfill in pavement and walkway areas should be compacted to at least 95% af the maximum standard Proctor density. Care should be taken not to overcompact the backfill or use large equipment near the wall, since this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall backfill should be expected, even if the material is placed correctly, and could result in distress to facilities constructed on the backfill. The lateral resistance of foundation or retaining wall footings will be a combination of t6e sliding resistance of the footing on the fbundation materials and passive earth pressure against the side of the footing. Resistance to sliding at lhe bottoms ofthe frrotings can be calculated based on a coefficient of friction of 0.4. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The coefficient of friction and passive pressure values recomrnended above assume ultimate soil strength. Suitable factors of safety should be included in the design to limit the strain which will occru at the ultimate strength, particularly in the case of passive resistance. Fill placed against the sides of the footings tc resist lateral loads should be compacted to at least 95% ofthe maximum standard Proctcr density at a moisture content near optimum. JobNo.1060791 G&&ecrr FLOOR SLABS f he natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab- on-grade construction. To reduce the effects sf some differential rnovement, floor slabs should be separated from ali bearing walls and columns with expansion joints which allow unresffained vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The requirements for joint spacing and slab reinforcement sheiuld 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 basement level slabs to facilitate drainage. This material should consist of minus 2 inch aggegate with at least 500/o retained on the No. 4 sieve and less than2o/a passing the No. 200 sieve" All fill materials for support of floor slabs should be compacted to at least 957o of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on-site granular soils devoid of vegetation, topsoil and oversized rock. I'NDERDRAIN SYSTEM Although free water was encountered below proposed excavation depths, it has been our experience in the area that the groundwaler level can rise and local perched groundwater can develop dwing times of heavy precipitation or seasonal runoff. Frozen ppound during spring runoffcan create a perched condition. We recornmend below-grade construction, such as retaining r,lalls and basement areas, be protected from',irefiing and h1'drostatic prcssure buildup by an underdrain system. The drains should consist of drainpipe placed in the bottom ofthe wall backfill surrounded above the invert level with free*draining granular material. The drain should be placed at each level of excavation and at least I foot below lowest adjacent finish grade and sloped at a minimum lolo to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 27o passing the No. 200 sieve, less than 50% passing the No.4 sieve and have a maximum si'r,s of 2 inches. The drain gravel backfill should be at least l%feet deep. An impervious membrane, such as a -7 - JobNo.1060791 c&Fecr, -8- 30 mil PVC liner, should be placed beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. SURFACE DRAJNAGE The following drainage precautions should be observed during construction and rnaintained at all tjmes after the residence has been completed: l) Inundation ofthe foundation excavations and underslab areas should be avoided during construction. 2) Exterior backfill should be a{usted to neiu optimum moisture and compacted to at least 95Ta of the maximum standard Proctor density in pavement and slab meas andto at least 90% ofthe maximum standard Proctor density in landscape areas. 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 minimurn slope of l2 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first I0 feet in paved areas- Free-draining wall backfill should be capped with about 2 feet ofthe on- site finer grained soils to reduce surlace water infiltration- 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation and sprinkler heads should be located at least l0 feet from foundation walls. LIMITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We rnake no warranty either express or implied. The conclusions and recommendations submitred in thjs report are based upon the data obtained from the exploratory borings drilled at the locations indicated on Figure l, the proposed type of construction and our experience in the area. Our services do not iaclude determining the presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is JobNo.1060791 G8&ecrt -9 - concerned about MOBC, then a professional in this special field of practice should be consulted. Our findings include 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 construclion appear different from those described in this report, we should be notified so that re-evaluation of the recommendations may be rnade. This report has been prepared for the exclusive use by our client for design purposes. We are not responsible for technical interpretations by ofhers of our information. As the project evolves, we should provide continued consultation and field services during construction to review and monitor the irnplementation of our recommendations, and to verify that the recommendations have been appropriafely interpreted. Significant desigp changes may require addirional 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 geotechnical engineer. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECFINICAL, INC. Steven L. Pawlak, P.E Reviewed by: Daniel E. Hardin, P.E, SLP/vad cc: High Country Engineering- Attn: Scott Gregory 1 :: JobNo. I060791 106 0791 Figure 1 APPROXIMATE SCALE 1" - 40' 5925 - LOT 64 5930 I '"!-.*'* FOAB/NGFOfrKB/Wfr 5925 4al\.- > ...*:*'---*--u- lrI BORING 1 a BORING 2 , I I I l7I I 5930 I la*-f- | L__5935 5940 LOT 66 a BORING 4 7tn \/\?___r BORING 3 LOT 65 I . /t\_,/ I I I I L-. _\\\_ -*/ _L '\\_\__ -\- j\ I I I J 5935 / BEND WAY RIYEB Figure 2 BORING 1 ELEV.= 5926' BORING 2 ELEV.:5926' BORING 3 ELEV.= 5930' BORING 4 ELEV.:5930.5' 5935 5930 5925 5920 5910 5905 50/10 36112 WC=2.7 +4:22 -20o=3s 50/6 s0/5 WC:3.8 -200=26 50/6 't3112 50/6 3A112 WC=2.6 -2OA=27 5935 5930 5925 5920 5915 5910 5905 c)oLL Ico CU (l) tri 5915 0)q) LL Ic .a G' o)iil I I I I I I Note: Explanation of symbols is shown on Figure 3. LEGEND: H ffi HSr ffi ffi F I TOPSOIL; organic sandy silt, brown. SAND (SM);silty, gravelly, possible cobbles, rnedium dense, slightly moist, brown, subangular rocks. GRAVEL AND COBBLES (GM-GP); sandy, slightly silty, small boulders, dense, rounded rocks. CLAYSTONE-SILTSTONE BEDHOCK; with gypsum, soft and weathered to hard with depth, moist, gray-black. Eagle Valley Evaporite. Relatively undisturbed drive sample; z-inch l.D. California liner sample. 36/12 Drive sample; standard penetration test ($PT), 1 318 inch LD. split spoon sample, ASTM D-1586. Drive sample blow count; indicates that 36 blows of a 140 pound harnrner falling 30 inches were required to drive the California or $PT sarnpler 12 inches. Free water level in boring at time of drilling. Depth at which boring had caved when checked on September 25, 2006. I -f- Praclical drilling refusal. Where shown above bottom of log, indicates that rnultiple attempts were I I made to advance the boring. II | ruorrs, I | 1. Exploratory borings were drilled on $eptember 8 and 19, 2006 with 4-inch diarneter continuous flight power auger. I | 2, Locations of exploratory borings were rneasured approximately by pacing from features shown on the sile planI provided. I | 3. Elevations oi exptoratory borings were obtained by interpolation between contours shown on the site plan provided i ancJ checkeci by instrument ievei. I o. ,n* exploratory boring locations and elevalions should be considered accurate only to the degree implied by theI rnethod used.I I s. fne lines betraeen materials shown on the exploratory boring logs represent the approximate boundaries between I material types and transition$ may be gradual. I O. Wut", level readings shown on the logs were made at the time and under the cnnditions indicated. Fluctuations in I water level may occur with time, I z. Unoratory Testing Results: I WC : Water Content (%) I +4: Percent retained on the No. 4 sieve| -2OA: Percent passing No. 200 sieve I LEGEND AND NOTES Figure 3 T]ME HEADINGS U-S- STANDAED SEHIES #r00 #50 .001 .oo2 .005.009 .019 .037 .074 .150 .300 ,600 1.18 z.ffi D'qMETER OF PARNCLES IN MILLIMEIERS GHAVEL 22 %SAND 43 "/" LIQUID LIMIT o/o 4.75 9.5 19.0 12.5 coSSLES SILTANDCLAY 35 % PIASTICITY INDEX % FROM: Boring 2 aL 1/z Feet CLEAR 5'6, 8'100 90 80 70 602 (no o- 50F ztd C)(E td 400 ofrJ 40z t--ld 0a F50zlrjc) E,ld(L60 30 20 70 80 90 10 0 106 0791 g HEPrbiI}] GRADATION TEST RESULTS Figure 4 SAMPLE OF: Silty Sand with Gravel H EPWORTH.PAWLAK EEOTETH NICAL, I NI. TABLE 1 SUMMARY OF LABORATORY TESI RESULTS iob No, 106 0791 sotL 0R EEOROIK TYPE UNCONFINEE IO}.lPRESSIVE STRENGTH PLASTIC INDEX LIOUIO LIMIT .al PERCENT PA5SIN6 NO, 200 stEyE 26 27 NATURAL OHY 0il{slfi NATURAL PlOISTURE CONTENT 0tPTH {feet} L0t4 80R]NG