Loading...
HomeMy WebLinkAboutGeotechnical Study.pdfHEPWORTH-PAWLAK GEOTECHNICAL, INC. 5020 Road 154 Glenwood Springs, CO 81601 Fax 970 94;.9454 Phone 970 945-7988 PRELIMINARYGEOTECHNICALSTUDY PROPOSED RESIDENTIAL DEVELOPMENT ST. FINNBAR FARM SUBDIVISION NORTH OF ROARING FORK RIVER WEST OF 100 ROAD, GARFIELD COUNTY, COLORADO JOB NO. 198 483 AUGUST 4, 1998 PREPARED FOR: ST. FINNBAR LAND COMPANY CIO LAND DESIGN PARTNERSHIP ATTN: RON LISTON 918 COOPER A VENUE GLENWOOD SPRINGS, COLORADO 81601 HEPWORTH -PAWLAK GEOTECBNICAL, INC. August 4, 1998 St. Finnbar Land Company c/o Land Design Partnership Attn: Ron Liston 918 Cooper Avenue Glenwood Springs, Colorado 81601 Job No.198 483 Subject: Report Transmittal, Preliminary Geotechnical Study, Proposed St. Finnbar Fann Subdivision, North of Roaring Fork River, West of 100 Road, Garfield County, Colorado. Gentlemen: As requested, we have conducted a geotechnical study for the proposed St. Finnbar Farm Subdivision. The property is suitable for the proposed development based on geologic and geotechnical conditions. Subsurface conditions encountered in the exploratory borings drilled in the general proposed development area consist of 1 to l ':12 feet of topsoil overlying dense sandy gravel with cobbles and boulders. Groundwater was encountered between 3 and 7 feet in the borings. Spread footings placed on the natural subsoils and designed for an allowable bearing pressure of 3,000 psf to 4,000 psf appear suitable at the building sites. We recommend that foundations be kept shallow to avoid groundwater impacts. The report which follows describes our investigation, summarizes our fmdings, and presents our recommendations suitable for planning and preliminary design. It is important that we 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 this report, please contact us. Sincerely, HEPWORTH -PAWLAK GEOTECHNICAL, INC. Daniel E. Hardin, P.E. Rev. by: SLP DEH/ro TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 PROPOSED DEVELOPMENT ................................... 1 SITE CONDITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 GEOLOGIC SETTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 FIELD EXPLORATION ....................................... 3 SUBSURFACE CONDITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 GEOLOGIC SITE ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 PRELIMINARY DESIGN RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . 5 FOUNDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 BELOW GRADE CONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . 5 FLOOR SLABS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 SURFACE DRAINAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 PAVEMENT SECTION ................................... 6 LIMITATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 FIGURE 1 -LOCATION OF EXPLORATORY BORINGS FIGURE 2 -LOGS OF EXPLORATORY BORINGS FIGURE 3 -LEGEND AND NOTES FIGURE 4 & 5 -GRADATION TEST RESULTS TABLE I -SUMMARY OF LABORATORY TEST RESULTS H·P GEOTECH PURPOSE AND SCOPE OF STUDY This report presents the results of a preliminary geotechnical study for the proposed St. Finnbar Fann Subdivision to be located north of the Roaring Fork River and west of 100 Road, Garfield County, Colorado. The project site is shown on Fig. 1. The purpose of the study was to evaluate the geologic and subsurface conditions and their potential impacts on the project. The study was conducted in accordance with our professional services agreement letter to St. Finnbar Farm Land Company, dated July 13, 1998. A previous soils report for a parcel of land which included the subject site and Preshana Farms to the north, was performed by Lincoln De Vore under Job No. GS-987, dated April 4, 1979. That report was considered in the preparation of our report. A field exploration program consisting of a reconnaissance and exploratory borings was conducted to obtain information on the site and subsurface conditions. Samples of the subsoils obtained during the field exploration were tested in the laboratory to determine their classification and other engineering characteristics. The results of the -field exploration and laboratory testing were analyzed to develop recommendations for project planning and preliminary design. This report sununarizes the data obtained during this study and presents our conclusions and recommendations based on the proposed development and subsurface conditions encountered. PROPOSED DEVELOPMENT The proposed development will consist of a 13 lot subdivision as shown on Fig. 1. Lots range in size from 2 to 13 acres. The development will consist of single family homes. Private driveways will access the building sites with a dedicated road (St. Finnbar Drive) providing access to County Road 100. We assume the residences will be typical of the area and be two to three stories with slab-on-grade or shallow crawlspace. The development will be serviced with sewer tie-in to Ranch at the Roaring Fork and individual wells. H-P GEOTECH - 2 - If development plans change significantly from those described, we should be notified to re-evaluate the recommendations presented in this report. SITE CONDITIONS The St. Finnbar Farm Subdivision is located in the Roaring Fork valley about three miles upstream of Carbondale. The property covers part of the SW 14 of Sec. 31, T. 7 S., R. 87 W. The Roaring Fork River borders the property on the south. The proposed building envelopes are on a nearly level terrace that averages about 6 feet above the modern river channel. A higher terrace that averages about 15 feet above the river borders the property to the north. The valley floor has an average longitudinal slope of about 1 % down to the west. At the time of this study the property was undeveloped except for an old shed and hay fields. Native vegetation consists of cottonwood trees, grass and brush. The lower lying parts of the terrace are wetlands. GEOLOGIC SETTING Regional geologic mapping shows that formation rock in the project area is the Pennsylvania-age Eagle Valley Evaporite (Kirkham and Widmann, 1997). Outcrops are not present on the valley floor in the project area, but outcrops are present on the Roaring Fork valley sides to the north and south. The Eagle Valley Evaporite is made up of gray and tan gypsum, anhydrite and halite with inter bedded siltstone, claystone, shale, and dolomite. The gypsum, anhydrite and halite are soluble in fresh water. The bedding structure in most places is convoluted because of flow deformation in the plastic gypsum, anhydrite, and halite. Subsurface voids and related sinkholes are sometimes present in areas underlain by the Eagle Valley Evaporite because of the solubility of the gypsum, anhydrite, and halite. Evidence of sinkholes was not observed on the property. Holocene and late Pleistocene-age Roaring Fork alluvium is present below the modern river channel and low terrace to the north where the proposed building sites are located. In this reach, the modern river channel is transitional, it changes from a straight pattern upstream of the bridge to braided pattern downstream of the bridge. At H-P GEOTECH - 3 - flood stage the downstream braided section consists of a network of several interconnected channels separated by small island bars. Relict braided channel patterns are still evident on the low terrace to the north of the modern river channel. The low lying areas on the terrace are old abandoned braided channels. The exploratory borings show that the river alluvium on the low terrace is greater that 10 feet deep. It consists of an upper I. 0 to 3. 5 foot thick silty sand that overlies a stratified deposit of rounded gravel, cobbles and boulder in a silty sand matrix. FIELD EXPLORATION The field exploration for the project was conducted on July 20, 1998. Six exploratory borings were drilled at the locations shown on Fig. 1 to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight auger powered by a truck-mountedLongyear BK-51HD drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Samples of the subsoils were taken with a 1 % inch I.D. spoon sampler. The sampler was driven into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This test is similar to the 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, Fig. 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Fig. 2. The subsoils consist of about 1 to l 'h feet of topsoil overlying relatively dense, slightly silty sandy gravel containing cobbles and boulders. Drilling in the dense gravel with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit. In Borings 1 and 3, a thin silty sand layer was encountered below the topsoil at depths of 1 Vz to 31/2 feet. H-P GEOTECH -4- Laboratory testing performed on samples obtained from the borings included natural moisture content and gradation analyses. Results of gradation analyses performed on small diameter drive samples (minus l 1h inch fraction) of the natural coarse granular soils are shown on Figs. 4 and 5. The laboratory testing is summarized in Table I. Free water was encountered in the borings at the time of drilling and when checked 2 days later at depths of 3 to 7 feet. The subsoils were moist to wet. GEOLOGIC SITE ASSESSMENT Geologic factors that should be considered in project planning and development are river flooding, the potential for sinkholes, and earthquake related ground shaking. Parts of the low terrace could be subject to flooding. We believe the proposed building areas on the low terrace are outside the currently active braided stream channel. A hydrologist should evaluate the flood potential in the area and determine if flood mitigation is required at the proposed building sites. Flood velocities should be evaluated to determine if channel stabilization is needed. · The probability of encountering sinkhole related problems at the proposed building site is considered to be low, but the developer and prospective home owners should be aware that the building sites cannot be considered totally sinkhole risk free since the Eagle Valley Evaporite is present in the subsurface. The potential presence of subsurface voids should be considered when planning site-specific foundation studies. If indications of voids or sinkhole related problems are identified by these studies, the problem area should be avoided or the feasibility of engineered mitigation techniques evaluated. Engineered mitigation that can sometimes be used to mitigated sinkhole related problems include: • Void stabilization by grouting or excavation and backfilling • Deep foundation systems • Structural bridging • Mat foundations or other rigid foundation systems H-P GEOTECH -5 - The project area could experience moderately strong earthquake related ground shaking. Modified Mercalli Intensity VI ground shaking should be expected during a reasonable service life for the residences, but the probability for stronger ground shaking is low. Intensity VI ground shaking is felt by most people and causes general alarm, but results in negligible damage to structures of good design and construction. All occupied structures should be designed to w.ithstand moderately strong ground shaking with little or no damage and not to collapse under stronger ground shaking. The region is in the Uniform Building Code, Seismic Risk Zone 1. Based on our current understanding of the earthquake hazard in this part of Colorado, we see no reason to increase the commonly accepted seismic risk zone for the area. PRELIMINARY DESIGN RECOMMENDATIONS The conclusions and recommendations presented below are based on the proposed development, the site reconnaissance, subsurface conditions encountered in the exploratory borings, and our experience in the area. The recommendations are suitable for planning and preliminary design but site specific studies should be conducted for individual lot development. 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 at the building sites. We expect the footings can be sized for an allowable bearing pressure in the range of 3,000 psf to 4,000 psf. The upper sand soils appear loose and may need to be removed and replaced with compacted fill or the bearing level deepened to dense gravel. Foundation walls should be designed to span local anomalies and to resist lateral earth loadings when acting as retaining structures. The footings should have a minimum depth of 36 inches for frost protection. H-P GEOTECH -6- BELOW GRADE CONSTRUCTION Ground water level is shallow throughout the project area. We did not find any borings with an excess hydrostatic pressure caused by a confining upper soil layer. This condition appears to be remote and limited to the low lying flood area which will be avoided with buildings. Due to the shallow water level, it will probably not be practical to protect below grade areas from wetting and hydrostatic pressure buildup by use of an underdrain system. We recommend that slab-on-grade floors be placed near to above existing grade and crawlspaces be as shallow as possible and that basements and deep crawlspaces be avoided. FLOOR SLABS Slab-on-grade construction should be feasible for bearing on the natural soils. There could be some post construction slab settlement. To reduce the effects of some differential movement, floor slabs should be separated from all bearing walls and columns with expansion joints. Floor slab control joints should be used to reduce damage due to shrinkage cracking. A minimum 4 inch thick layer of free-draining gravel should underlie basement level slabs to facilitate drainage. SURFACE DRAINAGE The grading plan for the subdivision should consider runoff through the project and at individual sites. Water should not be allowed to pond next to buildings. Exterior backfill should be well compacted and have a positive slope away from the building for a distance of 10 feet. Roof downspouts and drains should discharge well beyond the limits of all backfill. PAVEMENT SECTION The near surface soils encountered in the borings below the topsoil consists mainly of silty sand and gravel which is a fair to excellent material for support of pavement materials. We recommend the pavement section for the site road consist of 3 inches of asphalt pavement on 6 inches of Class 6 aggregate base course. The subgrade should be evaluated for pavement support at the time of construction. H-P GEOTECH -7- LIMITATIONS This study has been conducted according to generally accepted geotechnical engineering principles and practices in this area at Uris time. We make no warranty either expressed or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the field reconnaissance, review of published geologic reports, the exploratory borings located as shown on Fig. 1, the proposed type of construction and our experience in the area. 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 construction appear different from those described in this report, we should be notified so that re-evaluation of the recommendations may be made. This report has been prepared for the exclusive use by our client for planning and preliminary design purposes. We are not responsible for technical interpretations by others of our information. As the project evolves, we should provide continued consultation, conduct additional evaluations and review and monitor the implementation of our recommendations. 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 geotechnical engineer. Respectfully Submitted, HEPWORTH -PAWLAK GEOTECHNICAL, INC. Daniel E. Hardin, P.E. Reviewed By: Steven L. Pawlak, P.E. DEH/ro cc: High Country Engineering -Attn: Tim Beck H-P GEOTECH - 8 - REFERENCES Kirkham, RM. and Widmann, B.L., 1997, Geology Map of the Carbondale Quadrangle, Garfield County, Colorado: Colorado Geological Survey Open File Report 97-3. H-P GEOTECH 198 483 z ) PROPOSED ST. FINNBAR APPROXIMATE SCALE SUBDIVISION 1" = 400' LOT 7 I\ (co~ \,, ,.,-~ < LOT :J i"r --1 BUILDING ,• ""-". ~~ENVELOPE (LOT 4 \ • ';;:, (TYPICAL) /BORING 41 ~ Le_/,> I I PRESHANA FARM -- HEPWORTH -PAWLAK GEOTECHNICAL, INC. PROPOSED ST. FINNBAR DRIVE LOCATION OF EXPLORATORY BORINGS Fig. 1 0 -" " u. 0 5 -.c -Q. " c 10 a 2 -" -" 0 u. 5 - .c -fr c 10 198 483 BORING 1 ELEV. = 6254' 5/12 32/6.8/0 +""'59 -200=7 BORING 4 ELEV. = 6258' +4-69 -200=7 Ta T-=- T 10.2 -- BORING 2 ELEV. = 6260' BORING 5 ELEV. = 6265' B/6,9/2 51/12 +4-61 -200=6 0 T; - 0,2 - BORING .3 ELEV. = 6265' 3/12 41/8,10/0 .. WC-6.2 -200=12 BORING 6 ELEV. = 6265' 15/8,5/0 Note: Explonotion of •}mbols is shown on Fig . .3. HEPWORTH -PAWLAK GEOTECHNICAL, INC. LOGS OF EXPLORATORY BORINGS 0 -3l u. 5 .c -g. c 10 0 -" " u.. 5 .c -c. " c 10 Fig. 2 LEGEND: TOPSOIL; sandy silt and clay with gravel and cobbles, organic, soft to medium stiff, moist, dark brown. SAND (SM); silty, gravelly, loose to medium dense, moist, brown. GRAVEL (GM-GP); sandy, slightly silty, with cobbles and boulders, large particles are rounded, dense to very dense, very moist ta wet, brown. Drive sample; standard penetration test ( SPT ), 1 J/8-inch l.D. split spoon sample, ASTM D -1586. 5/12 0,2 Drive sample blow count; indicates that 5 blows of a 140-pound hammer falling 30 inches were required to drive· the SPT sampler 12 inches. Free water level in boring and number of days following drilling measurement was mode. Indicates slotted PVC pipe installed in boring to depth shown. T Practical rig refusal. Where shown above bottom of log, indicates that multiple attempts were mode to advance the boring. NOTES: 1. Exploratory borings were drilled on July 20, 1998 with a 4-inch diameter continuous flight power auger. 2. Locations of exploratory borings were approximated based on features shown on the srte plan provided. 3. Elevations of exploratory borings were obtained by Interpolation between contours on the site plan provided. 4. The exploratory boring locations and elevations should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory boring logs represent the approximate boundaries between material t}Pes and transitions may be gradual. 6. Water level readings shown on the logs were made at the time and under the conditions indicated. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content ( % ) +4 = Percent retained on No. 4 sieve. -200 = Percent passing No. 200 sieve. 198 483 HEPWORTH -PAWLAK GEOTECHNICAL, INC. LEGEND AND NOTES Fig. 3 H'l'OROMETER AHAL 'tSIS S1El/E Atw..'l'SIS I I TllE READINGS I U.S. STANOARO SfRIES I Cl.EAR SOOAAE OPE141NGS I 2• HR. 7 HA 45 UIN. 15 UN. 60 MIN.19 MW. • Mitt 1 MIN. ~ 100 00 0 OD • .. ... :s-12•314• n r ,. 5•5• B"a .. 10 BO 2l! '-' "' JO 0 z w en z "' .. .., ~ < "-w .. 5Cl a:: I-I-z w .. z 0 60 w a:: {.) w a:: "-30 70 w 0.. 20 .. 10 9Q 0 100 .001 ,OQ2 .ODS ,009 .019 ·""' .07-4 .1~0 .Joa .100 1.18 2.36 4.75 8.5!2.S 1R.Q J7.5 71U 152 20J DIAMETER OF PARTICLES IN MILLIMETERS 127 a..A.'f 10 SILT I Fll/E I ~lM 1co ..... I ""' il!'f£ """"" I COB&!S GRAVEL 59 % SAND 34 % SILT AND CLAY 7 % LIQUID LIMIT % PLASTICITY INDEX % SAMPLE OF: Slightly Silty Sandy Gravel FROM: Boring 1 ct 5 Feet I H'l'DROUETER ANALYSIS SIEVE ANAl.'rSS I I TIME READINGS I U.S. STANDMD SEJflES I a.fAR SCl\.IARE Of'~INGS I 24 HR. 7 HR 45 MW. 15 MIN. 60 t.IN.19 M'IN. -4 MIN. I MH. 1200 00 0 00 "' • .. 3's"11r314• 11 n• l" 5•s• ,. IOQ 0 .. 10 BO 20 '-' 70 JO 8 z z en 60 40 ~ "' < w 0.. so 50 a:: I-I-z z w .. .. w {.) () a:: a:: w 30 70 w 0.. 0.. 20 ., 10 .. 0 100 ,001 .002 .oos .009 ,019 ·""' .074 .150 .JOO .100 1.18 2.lB "'' a.s12.5 1e.o J7.5 78.2 152 203 DIAMETER OF PARTICLES IN MILLIMETERS ,., Q.AY' TO SILT I flNE I ~UM """'"'' I FlNE "'f" CColRSE I C<>aalEi GRAVEL 69 7. SAND 2.4 % SILT AND CLAY 7 % LIQUID LIMIT % PLASTICITY INDEX 7. SAMPLE OF: Slightly Silty Sandy Gravel FROM: Boring 4 at 2.5 ond 5 Feet, Combined 198 483 HEPWORTH -PAWLAK GRADATION TEST RESULTS Fig. 4 GEOTECHNICAL, INC. C) z Vi UJ <( Q. >--z w u a:: w 0.. I 2! HR. 1 HR 4S WIN, 15 MIN, 100 •• .. 70 60 so .. 30 20 10 0 HYDROMETER ANALYSIS 60 MIN.18 MIN. 4 MIN, 1 MIN, I /200 1100 U.S. STAHOAAD SERIES ,.. po ,,. I ,. ID "" Cl w .. z ~ w a:: 50 >--z w u er w Q. 60 70 80 .. 100 .001 .DOZ ,005 .009 .OHi ,Ol7 .074' .150 .300 .aao 1.18 2.JS 4.75 9..5 12.!i 1Sl.O 37.S 76.2. fS2 203 198 483 DIAMETER OF PARTICLES IN MILLIMETERS CLAY TO sn.r ""' SAND WEO!UM !COARSE I GRAVEL 61 % SAND 33 % SILT AND CLAY 6 LIQUID LIMIT % PLASTICITY INDEX % SAMPLE OF: Slightly Silty Sandy Gravel FROM: Boring 5 at 5 Feet HEPWORTH -PAWLAK GEOTECHNICAL, INC. GRADATION TEST RESULTS ,., % Fig. 5 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE I JOB NO. 198 483 SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL NATURAL GRADATION PERCENT A TTERBERG LIMITS UNCONFINED BORING DEPTH MotSTURE DRY GAAllEl SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR Ueetl CONTENT DENSITY , .. , "" NO. 200 LIMIT INDEX STRENGTH BEDROCK TYPE "" .... SIEVE "" "" IPSFt 1 5 59 34 7 Slightly silty sandy gravel 3 3 y, 6.2 12 Slightly silty sandy gravel 4 2% & 5 69 24 7 Slightly silty sandy (combined! gravel 5 5 61 33 6 Slightly silty sandy gravel