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HomeMy WebLinkAboutLot 5 Geotech report.pdf H P Hepworth-Pawlak Geotechnical,Inc. 5020 County Road 154 e _)rtecI-•i Glenwood Springs,Colorado 81601 Phone:970-945-7988 HEPWORTH-PAWLAK GEOTECHNICAL Fax:970-945-8454 email:hpgeo@hpgeotech.com PRELIMINARY SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 5, ELK MEADOWS SUBDIVISION 1632 BUFFEHR CREEK ROAD VAIL, COLORADO JOB NO. 114 086A JULY 11, 2014 • PREPARED FOR: ELK MEADOWS DEVELOPMENT, LLC ATTN: SHARON COHN 141 E.MEADOW DRIVE, SUITE 211 VAIL, COLORADO 81657 (sharon(a,solarisvail.com) • Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - • SITE CONDITIONS - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4- FOUNDATION AND RETAINING WALLS - 5 - FLOORSLABS - 6 - UNDERDRAIN SYSTEM - 7 - SITE GRADING - 7 - SURFACE DRAINAGE - g - LIMITATIONS - 8 - FIGURE 1 - LOCATIONS OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Job No. 114 086Atech PURPOSE AND SCOPE OF STUDY This report presents the results of a preliminary subsoil study for a proposed residence to be located on Lot 5, Elk Creek Meadows Subdivision, 1632 Buffehr Creek Road, Vail, 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 as part of our proposal for geotechnical engineering services to Elk Creek Development, LLC dated March 24, 2014. We previously performed a preliminary subsoil study for Lots 1 through 3 at the subdivision and presented our findings in a report dated April 18, 2014, Job No. 114 086A. Potential geologic hazards at the site have been addressed by others and are beyond the scope of this report. 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, compressibility or swell 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 A single family residence is planned on the lot shown on Figure 1. The residence will be two story wood frame structure with the lower level retaining cut of the hillside slope. Ground floors 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. Job No. 114 086A Gtech - 2 - When building location, grading and foundation loading information have been developed, we should be notified to re-evaluate the recommendations presented in this report. SITE CONDITIONS The lot is vacant and the ground surface appeared mostly natural. There is some fill along the north side of the site from construction of Buffehr Creek Road. The terrain is strongly sloping down to the northwest towards Buffehr Creek Road. Slope grades are estimated at about 5%to 7%. Elevation difference across the assumed building area is estimated at about 5 to 6 feet. Vegetation consists of thick grass with aspen trees on the valley side slopes. There are several scattered boulders on the ground surface. FIELD EXPLORATION The field exploration for the project was conducted on June 12, 2014. 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 1% inch and 2 inch I.D. spoon samplers. The samplers were 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. Job No. 114 086A GecgPtech - 3 - SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils encountered, below about 2 to 3 feet of organic topsoil, consisted of medium stiff, very sandy silty clay with gravel that occasionally graded to very clayey silty sand underlain at depths from about 10 to 14 feet by loose to medium dense, silty gravely sand. The silty gravely sand soils extended down to the depth drilled of 26 feet in Boring 1 and to a depth of about 16 feet in Boring 2 where relatively dense, silty sandy gravel with cobbles and possible boulders was encountered. The silty sand soils became more gravelly with depth. Drilling in the dense granular soils with auger equipment was difficult at times due to the cobbles and possible boulders. Laboratory testing performed on samples obtained from the borings included natural moisture content and density, and percent finer than sand size gradation analyses. Results of swell-consolidation testing performed on a relatively undisturbed drive sample of the silty sand soils, presented on Figure 4, indicate generally moderate compressibility under conditions of loading and wetting with a low hydro-compression potential. The clay soil samples were disturbed due to their medium stiffness and higher moister contents and were not tested for swell-consolidation. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling or when checked 1 day later and the subsoils were very moist to moist becoming slightly moist with depth in Boring 2. FOUNDATION BEARING CONDITIONS At assumed excavation depths for the residences, we expect the subgrade soils will consist of the very sandy silty clay soils that are considered moderately compressible. Lightly loaded spread footings bearing on these soils should be feasible for foundation support of the building with some risk of settlement. The risk of settlement is due primarily to the compressible nature of the very sandy silty clay soils. Extending the Job No. 114 086A ettech - 4 - footings down the bear entirely on the sand and gravel soils would provide a lower risk foundation. DESIGN RECOMMENDATIONS FOUNDATIONS 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 soils with some risk of settlement. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural soils should be designed for an allowable bearing pressure of 1,200 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 to 11/2 inches for the assumed light loadings. We should review the settlement potential when foundation loadings are available and make recommendations to mitigate the settlement if needed. 2) The footings should have a minimum width of 18 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 48 inches below exterior grade is typically used in this area. 4) Continuous foundation walls should be heavily reinforced top and bottom to span local anomalies and better withstand the effects of some differential settlement such as by assuming an unsupported length of at least 14 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. Job No. 114 086A Gegtech - 5 - 5) The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the firm natural soils. The exposed soils in footing area should then be adjusted to near optimum moisture content and compacted. Unstable subgrade conditions will need to be corrected prior to the footing construction. 6) A representative of the geotechnical engineer should observe 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 least 55 pcf for backfill consisting of the on-site soils. Cantilevered retaining structures which are separate from the main buildings and can be expected to deflect sufficiently to mobilize the full active earth pressure condition should 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. The backfill should not contain topsoil or oversized rocks. 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 walls and a horizontal backfill surface. The buildup of water behind a wall or an upward 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(SPD) at a moisture content near optimum. Backfill in pavement and walkway areas should be compacted to at least 95% SPD. 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 Job No. 114 086A Gtech - 6 - should be expected, even if the material is placed correctly, and could result in distress to facilities constructed on the backfill. Use of a select granular import material such as road base and increasing compaction to at least 98% SPD could be done to reduce the settlement potential. 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 footings can be calculated based on a coefficient of friction of 0.35. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 375 pcf. The coefficient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factors of safety 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 against the sides of the footings to resist lateral loads should be a suitable granular material compacted to at least 95%of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab- on-grade construction. There could be some slab settlement due to the compressible nature of the clay soils. 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 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 level slabs to facilitate drainage. 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 Job No. 114 086A c; tech - 7 - consist of the on-site soils devoid of topsoil and oversized rocks, or a suitable granular material such as road base can be imported. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in mountainous areas and where clay soils are present that 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 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 sump and pump. 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 and covered by filter fabric such as Mirafi 140N. SITE GRADING The risk of construction-induced slope instability at the site appears low provided the building is located as planned and cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about 10 feet. Embankment fills should be limited to about 8 to 10 feet deep and be compacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil and compacting to at least 95%of the maximum standard Proctor density. The fill should be benched into the portions of the site exceeding 20% grade. Job No. 114 086A CSE Ptech - 8 - Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or other means. The risk of slope instability will be increased if seepage is encountered in cuts and flatter slopes may be necessary. If seepage is encountered in permanent cuts, an investigation should be conducted to determine if the seepage will adversely affect the cut stability. SURFACE DRAINAGE Positive surface drainage is an important aspect of the project. The following drainage precautions should be observed during construction and maintained at all times after the building has been completed: 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 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 with filter fabric such as Mirafi 140N and 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. 5) Landscaping which requires regular heavy irrigation should be located at least 5 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 make no warranty either Job No. 114 086A Gtech - 9 - 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. 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 - PAW e . GEOTECHNICAL, INC. .08tt ata ogee . °em �? aw .:. shy David A. Young, P.E. • e ts ' =•g '`-216 Q tee# 9kSi '"a �, DAY/ljg f* -, MAL1d cc: Elk Meadows Development—Brian Redinger(brian@solarisvail.com) Job No. 114 086A ec h / O APPROXIMATE SCALE 1" = 30' BORING 1 • Q I z Lu Y I w W I Q r LU C IL = J W H LL LOT 5 m 1632 BUFFEHR CREEK ROAD BORING 2 • 114 086A ~ Ch LOCATION OF EXPLORATORY BORINGS Figure 1 Hepworth—Pawlak Geotechnical BORING 1 BORING 2 ELEV.= 100' ELEV.= 96' 0 0 _ /. ... - �i I 4/12 3/12 — / rl / 5 ' 11/12 ' 4/12 5 — ' / WC=19.1 DD=79 — / / / ' -200=50 — / / — / — 10 ' 4/12 C.;E 6/12 10 — / WC=17.4 6 - DD=95 ' -200=45 — — u_ 15 8/12 ".�'''� 10/12 15 — L : WC=10.8 1';: WC=15.9 — - p. DD=108 c6 DD=112 CD — — a p a *. -200=33 _ ":;1X 20 1 p 13/12 4 73/12 2C) iT f 2523/12 25 30 30 LOT 5 Note: Explanation of symbols is shown on Figure 3. H 114 086A C7Q Ch LOGS OF EXPLORATORY BORINGS Figure 2 HEPWORTH-PAWLAK GEOTECHNICAL LEGEND: ... TOPSOIL; organic sandy silty clay, soft, very moist, dark brown. N . CLAY(CL); silty, very sandy to occasionally very clayey silty sand, medium stiff/loose, moist to very moist, L red-brown, low plasticity, calcareous. 7 SAND (SM); silty, gravelly, loose to medium dense, moist, red-brown. GRAVEL (GM);with cobbles, sandy, silty, dense, slightly moist, brown. Relatively undisturbed drive sample; 2-inch I.D. California liner sample. x■ Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample,ASTM D-1586. 4/12 Drive sample blow count; indicates that 4 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. NOTES: 1. Exploratory borings were drilled on June 12, 2014 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 approximated by hand level and refer to the ground surface at Boring 1 as assumed elevation = 100'. Boring logs 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 or when checked 1 day later. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content(%) DD = Dry Density(pcf) -200 = Percent passing No. 200 sieve 114 086A1-iP,,� �D@Vd ch LEGEND AND NOTES Figure 3 HEPWORTH-HAWLAK GEOTECHNICAL Moisture Content = 10.8 percent Dry Density = 108 pcf Sample of: Silty Sand From: Boring 1 at 15 Feet 0 1 Compression ° upon 0 wetting co 2 a. E 0 v 3 4 0.1 1.0 10 100 APPLIED PRESSURE-ksf 114 086A GegtleCh SWELL-CONSOLIDATION TEST RESULTS Figure 4 Hepworth—Pawlak Geotechnical V a b 3 co q >, o cks ct o wa U V. rx V. 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