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HomeMy WebLinkAboutSoils Report 06.30.20167 Geotech HEPWORTH-PAWLAK GEOTECHNICAL Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 Email: hpgeo@hpgeotech.com SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT GV5, ASPEN GLEN PRIMROSE LANE GARFIELD COUNTY, COLORADO JOB NO. 116 273A JUNE 30, 2016 PREPARED FOR: GREGORY FRIEDMAN 71 GOLDEN BEAR DRIVE CARBONDALE, COLORADO 81623 (gaf iib!ucrivernet.com) TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION - 3 - SUBSURFACE CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS -4- FOUNDATION AND RETAINING WALLS - 5 - FLOOR SLABS - - 5 - UNDERDRAIN SYSTEM - 6 - SURFACE DRAINAGE - 6 - LIMITATIONS - 7 - REFERENCES - 8 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - GRADATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Job No. 116 273A GecPtech S: R :::� i ::: Vii; PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot GV5, Aspen Glen, Primrose Lane, Garfield County, Colorado. The project site is shown on Figure 1. The purpose of the study was to develop recommendations for the foundation design. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated June 23, 2016. Chen -Northern, Inc. previously conducted geotechnical engineering studies for the Aspen Glen development and presented their findings in reports dated December 20, 1991 and May 28, 1993, Job No. 4 112 92. A field exploration program consisting of exploratory borings was conducted to obtain information on the 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 foundation types, depths and allowable 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 will be one and two story wood frame construction with an attached garage. Ground floor will be structural above a crawlspace or basement level. The garage floor will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 8 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. 116 273A GecPtech SITE CONDITIONS The vacant lot is located in the southwestern part of the subdivision and on the south side of the Roaring Fork River. Vegetation consists of grass and weeds. The ground surface slopes away from Primrose Lane, gently at first but becoming steeper at the northern end of the lot. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone/siltstone and limestone with some massive beds of gypsum. 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 conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous studies in the area, several broad subsidence areas and smaller size sinkholes were observed scattered throughout the Aspen Glen development (Chen -Northern, Inc. 1991 and 1993). These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. The lot is not located within a broad subsidence area and existing sinkholes were not observed in the immediate area of the subject lot. The closest mapped sinkhole is about 1600 feet west of this lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory borings were relatively shallow, for foundation design only. Based on our present knowledge of the subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of future ground subsidence on Lot GV5 throughout the service life of the proposed residence, in our opinion, is low; however, the owner should be made aware of the potential for sinkhole development. If further investigation of possible cavities in the bedrock below the site is desired, we should be contacted. Job No. 116 273A Ge tech 1 FIELD EXPLORATION The field exploration for the project was conducted on June 28, 2016. Two exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight augers powered by a truck -mounted CME -45B 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, Figure 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 Figure 2. The subsoils, below about four inches of topsoil, consist of about 1 to 11/2 feet of sandy silt and clay overlying dense silty sandy gravel with cobbles and small boulders. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit. 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 1'/2 inch fraction) of the coarse granular subsoils are shown on Figure 4. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling. The subsoils were slightly moist to moist. e Job No. 116 273A HPtedh p -4- DESIGN RECOMMENDATIONS FOUNDATION S Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the natural granular soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural granular soils should be designed for an allowable bearing pressure of 3,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. 2) The footings should have a minimum width of 16 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 unsupported length of at least 12 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral earth pressure corresponding to an equivalent fluid unit weight of at least 50 pcf. 5) The topsoil, silt and clay soils and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively undisturbed natural soils. The exposed soils in footing area should then be moistened and compacted. Voids created by boulder removal or sub - Job No. 116 273A Ge&ech c n -5- excavation of silt and clay soils can be backfilled with granular structural fill compacted to at least 98% of standard Proctor density. 6) A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. FOUNDATION AND RETAINING WALLS All foundation and retaining structures should be designed for appropriate hydrostatic and surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The pressures recommended above assume drained conditions behind the Ll walls and a horizontal backfill surface. The buildup of water behind a wall or an upward OF L� IND sloping backfill surface will increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain should be provided to prevent hydrostatic pressure buildup behind walls. Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content near optimum. Backfill placed in pavement and walkway areas should be compacted to at least 95% of 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. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab - on -grade construction. To reduce the effects of some differential movement, floor slabs should be separated from all bearing walls and columns with expansion joints which allow unrestrained vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The requirements for joint spacing and slab Job No. 116 273A 'rtech 4 MP -6- reinforcement 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 basement slabs. This material should consist of minus 2 inch aggregate with at least 50% retained 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 moisture content near optimum. Required fill can consist of the on-site granular soils or a suitable imported gravel 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 groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched condition. We recommend below -grade construction, such as retaining walls and crawlspace areas, be protected from wetting 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 invert level with free -draining granular material. The drain should be placed at each level of excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum 1% to a suitable gravity outlet or drywell. Free -draining granular material used in the underdrain system should contain less 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 backfill should be at least 11/2 feet deep. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: Job No. 116 273A G Ptech r•� f— -7- 1) Inundation of the foundation excavations and underslab areas should be avoided during 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 at least 90% of the 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 minimum slope of 6 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 with about 2 feet of the on- site finer graded soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. LIMITATIONS This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area at this time. We make no warranty either express or implied. The conclusions and recommendations submitted in this report are based upon the data obtained from the exploratory borings drilled at the locations indicated on Figure 1, the proposed type of construction and our experience in 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 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 construction appear different from those described in this report, we should be notified so that re-evaluation of the recommendations may be made. Job No. 116 273A GeStech This 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, we should provide continued consultation and field services during construction to review and monitor the implementation of our recommendations, and to verify that the recommendations 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 fill by a representative of the geotechnical engineer. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Tom C Brunner, Staff Engineer Reviewed by: do . ��ou as / ` Steven L. Pawlak, P. E e �� f ... TCB/ksw fa.aayaea° ;FERENCES Chen -Northern, Inc., 1991, Preliminary Geotechnical Engineering Study, Proposed Aspen Glen Development, Garfield County. Colorado, prepared for Aspen Glen Company, dated December 20, 1991, Job. No. 4 112 92. Chen -Northern, Inc., 1993, Geotechnical Engineering Study firr Preliminary Plat Design, Aspen Glen Development, Garfield County. Colorado, prepared for Aspen Glen Company, dated May 28, 1993, Job. No. 4 112 92. Job No. 116 273A Gtech Oft 0 LOT GV9 LOT GV4 APPROXIMATE SCALE 1"=40' LOT GV7 i BUILDING ENVELOPE LOT GV6 HP GEOTECH JOB NO. 115 569A PRIMROSE LANE tti N L Q 0 ❑ 0 5 10 BORING 1 ELEV.-6054' sch 4 04� 00 69/12 WC=1.7 +4=49 81/11 -200=15 50/3 BORING 2 ELEV.=6059' •eOp o!" 50/6 = -sa 50/5 50/2 0 5 10 15 15 Note: Explanation of symbols is shown on Figure 3. 116 273A LOGS OF EXPLORATORY BORINGS Figure 2 LEGEND: 111 39/12 TOPSOIL; roots, organics, clayey, sandy, very silty, medium stiff, slightly moist to moist, reddish brown. CLAY AND SILT (CL -ML); sandy, slightly gravelly, medium stiff, slightly moist to moist, reddish brown. SAND AND GRAVEL (SM -GM); silty, cobbles and boulders, dense to very dense, slightly moist, brown to reddish brown, rounded rocks. - Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1586. Drive sample blow count; indicates that 39 blows of a 140 pound hammer falling 30 inches were required to drive the SPT sampler 12 inches. Practical Drilling Refusal NOTES: 1. Exploratory borings were drilled on June 28, 2016 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. The logs of exploratory borings are drawn to depth. 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 types and transitions may be gradual. 6. No free water was encountered in the borings at the time of drilling. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve 116 273A HEPWORTH-PAWLAK GEOTECHNICAL LEGEND AND NOTES Figure 3 :10: 10BIaaEMM' HYDROMETER ANALYSIS SIEVE ANALYSIS 4 Hq TIME READINGS U S. STANDARD SERIES 1 CLEAR SQUARE OPENINGS I 0 45 MIN. 15 MIN. 60MIN19MIN.4 MIN 1 MIN. #200 #100 #50 #30 #16 #8 #4 3/8" 3/4" 1 1/2" 3" 5"6" 8" 100 10 20 30 40 50 60 70 80 90 100 .001 .002 .005 .009 .019 .037 074 150 300 .600 1 18 2.36 4 75 9 5 19 0 37.5 76 2 152 203 12.5 127 1. 1 4 1 DIAMETER OF PARTICLES IN MILLIMETERS CLAY TO SILT COBBLES 0 % LIQUID LIMIT SANIT 1 vI GIN 1COVASr GRAVEL 49 % SAND 36 % % SAMPLE OF: Silty Very Sandy Gravel FIT4 1 COARSE_ PLASTICITY INDEX COBBLES SILT AND CLAY 15 % 90 80 70 60 50 40 30 20 10 0 FROM: Boring 1 at 212 and 5 Feet Combined NgWZIMJ3MML HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 116 273A SAMPLE LOCATIONNATURAL GRADATION ATTERE3ERG LIMITS BORING DEPTH (ft) MOISTURE CONTENT (%) NATURAL DRY DENSITY (pct) GRAVEL o (/u) SAND (%) PASPERCENT NO.PLASTICING 200 SIEVE LIQUID LIMIT (%) INDEX (%) UNCONFINED COMPRESSIVE STRENGTH (PSF) SOIL OR BEDROCK TYPE 1 21/z and 5 combined 1.7 49 36 15 Silty Very Sandy Gravel }