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HomeMy WebLinkAboutSoils Report 03.30.2015Gtech 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 278 BLUE HERON VISTA IRONBRIDGE DEVELOPMENT GARFIELD COUNTY, COLORADO JOB NO. 113 471I MARCH 30, 2015 PREPARED FOR: ASPEN SIGNATURE HOMES OF IRONBRIDGE, LLC ATTN: LLWYD ECCLESTONE P.O. BOX 7628 ASPEN, COLORADO 81612 Iecclestone(a pblhfl.net TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - BACKGROUND INFORMATION - I - PROPOSED CONSTRUCTION - 2 SITE CONDITIONS - 2 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION - 3 - SUBSURFACE CONDITIONS - 3 - ENGINEERING ANALYSIS - 4 - DESIGN RECOMUMENDATIONS - 5 FOUNDATIONS - 5 - FOUNDATION AND RETAINING WALLS - 6 - NONSTRUCTURAL FLOOR SLABS -7- UNDERDRAIN SYSTEM - 8 - SURFACE DRAINAGE - 8 - LIMITATIONS - 9 - FIGURE 1 - LOCATION 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 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a residence to be located on Lot 278, Ironbridge Development, BIue Heron Vista, 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 proposal for geotechnical engineering services to Aspen Signature Homes of Ironbridge, LLC dated March 18, 2015. The current engineering services consist of a lot specific study using subsurface information collected for previous geotechnical studies at the Ironbridge development. A field exploration program consisting of exploratory borings was previously 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 previous field exploration and laboratory testing were analyzed to develop recommendations for foundation types, depths and allowable pressures of the current proposed building. 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. BACKGROUND INFORMATION The proposed residence is located in the existing Ironbridge subdivision development. Hepworth Pawlak Geotechnical previously conducted subsurface exploration and geotechnical evaluation for development of Villas North and Villas South parcels, Job No. 105 115-6, report dated September 14, 2005, and performed observation and testing services during the infrastructure construction, Job No. 106 0367, between April 2006 and April 2007. Additional subsurface exploration, laboratory testing and geotechnical evaluation was conducted for the current proposed residence construction throughout the Villas parcels, Job No. 113 471A, report dated February 28, 2014. The information provided in these previous reports has been considered in the current study of Lot 278. Job No. 113 4711 GeHeDDtech -2 - PROPOSED CONSTRUCTION The proposed residence will be a two story, wood frame structure with structural slab foundation and no basement or crawlspace, and located as shown on Figure 1. A post - tensioned slab foundation is expected at this time. Grading for the structure is assumed to be relatively minor with cut and fill depths on the order of a few feet or Less. We assume relatively light foundation loadings, typical of the proposed type of construction. If building Ioadings, location or grading plans change significantly from those described ahnve we should be notified to ro evaluate the recommendations cunilained in this report. SITE CONDITIONS The proposed residence is Iocated in the west -central part of the Villas North parcel. The natural terrain prior to development in 2006 sloped down to the east at about 5% grade. The subdivision in this area was elevated by filling on the order of 10 to 15 feet above the original ground surface to create a relatively level building site off of Blue Heron Vista and slightly downhill of the Robertson Ditch to the west of the lot. Vegetation consists of grass and weeds. SUBSIDENCE POTENTIAL Eagle Valley Evaporite underlies the project area which is known to be associated with sinkholes and Iocalized ground subsidence in the Roaring Fork River valley. A sinkhole opened in the cart storage parking lot located east of the Pro Shop and west of the Villas North parcel in January 2005. Other irregular bedrock conditions have been identified in the affordable housing site located to the northwest of the Villas North parcel. Irregular surface features were not observed in the Villas North development area that could indicate an unusual risk of future ground subsidence, but localized variable depths of the debris fan soils encountered by the previous September 14, 2005 geotechnical study in the Villas North development area could be the result of past subsidence. The subsurface exploration performed in the area of the proposed residence on Lot 278 did not encounter voids or subsurface irregularities indicative of sinkhole development. In our opinion, the Job No. 113 4711 Ge-I'cr eech -3 - risk of future ground subsidence in the Villas North and Villas South project arca is low and similar to other areas of the Roaring Fork River valley where there have not been indications of ground subsidence. FIELD EXPLORATION The field exploration for current planned development of Villas North and Villas South parcels was conducted between December 24, 2013 and January 2, 2014. Boring 5 (2014) was drilled adjacent to Lot 278 at the location shown on Figure 1 to evaluate the subsurface conditions. Boring 5 (2005) from our September 14, 2005 subsurface study report was drilled near the middle part of Lot 278 as shown on Figure 1. 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. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered in the proposed residence area are shown on Figure 2. The subsoils encountered in Boring 5 (2005) consist of a shallow fill depth above sandy clayey silt and very silty sand with gravel (alluvial fan deposits) overlying dense, sandy gravel and cobble soils (river gravel alluvium) at a depth of about 14 feet. The natural soils encountered at Boring 5 (2014) were similar alluvial fan Job Na 113 4711 -4 - deposits below about 12 feet of fill material mainly placed in 2006. The fill soils are medium dense and slightly moist to moist, and the underlying natural alluvial fan soils are loose to medium dense/stiff and generally moist. Drilling in the underlying, coarse river gravel alluvium with augers was difficult due 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 density and finer than sand size gradation analyses. Results of swell -consolidation testing performed on a relatively undisturbed drive sample of the natural sandy silt soils obtained from Boring 5 (2005), presented on Figure 4, indicate low compressibility under light loading and a minor collapse potential (settlement under constant load) when wetted and moderate compressibility under additional Ioading. The laboratory testing is summarized in Table 1. ENGINEERING ANALYSIS The upper 12 feet of soils encountered in Boring 5 (2014) Iocated adjacent to Lot 278 consist of fill place mainly in 2006 as part of the subdivision development. The field penetration tests and laboratory tests performed for the study, and review of the field density tests performed during the fill construction indicate the structural fill was placed and compacted to the project specified 95% of standard Proctor density. Debris fan soils which tend to collapse (settle under constant load) when wetted were encountered below the fill. The amount of settlement will depend on the thickness of the compressible soils and their wetted depth. Relatively deep structural fill as encountered in Boring 5 (2014) will also have some potential for long term settlement but usually significantly less than the alluvial fan deposits. Proper grading, drainage and compaction as presented below in the Surface Drainage sections will help to keep the subsoils dry and reduce the settlement risks. A heavily reinforced structural slab or post -tensioned slab foundation designed for significant differential settlements is recommended for the building support. As an alternative, a deep foundation that extends down into the underlying dense, river gravel alluvium could be used to reduce the building settlement risk. Job No. 113 4711 GeiPtech 5 - 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 a heavily reinforced structural slab or post -tensioned slab foundation bearing on about 12 feet of compacted structural fill. If a deep foundation system is considered for building support, we should be contacted for additional recommendations. The design and construction criteria presented below should be observed for a spread footing foundation system. I) A conventionally reinforced structural slab or post -tensioned slab placed on about 12 feet or more of compacted structural fill should be designed for an allowable bearing pressure of 1,500 psf. The post -tensioned slab placed on structural fill should be designed for a wetted distance of 10 feet but at least half of the slab width whichever is more. Settlement of the foundation is estimated to be about 1 to 11/2 inches based on the long term compressibility of the fill. Additional settlement of about 2 inches is estimated if deep wetting of the debris fan soils were to occur. Settlement from the deep wetting would tend to be uniform across the building/development area and the settlement potential of the fill section should control the design. 2) The thickened sections of the slab for support of concentrated Ioads should have a minimum width of 20 inches. 3) The perimeter turn -down section of the slab should be provided with adequate soil cover above the bearing elevation for frost protection. Placement of foundations at least 36 inches below exterior grade is typically used in this area. If a frost protected foundation is used, the perimeter turn -down section should have at Ieast 18 inches of soil cover. 4) The foundation should be constructed in a "box -like" configuration rather than with irregular extensions which can settle differentially to the main Job No. 113 4711 GecGtech -6-_ building area. The foundation walls, where provided, should be heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 14 feet. Foundation walls acting as retaining structures (if any) should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) The root zone and any loose or disturbed soils should be removed. Additional structural fill placed below the slab bearing level should be compacted to at least 98% of the maximum standard Proctor density within 2 percentage points of optimum moisture content. 6) A representative of the geotechnical engineer should evaluate the compaction of the filI materials and observe all footing excavations prior to concrete placement for 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 50 pcf for backfill consisting of the on-site soils. Cantilevered retaining structures which are separate from the 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 40 pcf for backfill consisting of the on-site soils. 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. lob No. 113 4711 Gag -tech -7 Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density near optimum moisture content. 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 retaining 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 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 300 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 Iateral loads should be compacted to at least 95% of the maximum standard Proctor density at near optimum moisture content. NONSTRUCTURAL FLOOR SLABS Compacted structural fill can be used to support lightly Loaded slabs -on -grade separate from the building foundation. The fill soils, can be compressible when wetted and result in some post -construction settlement. To reduce the effects of some differential movement, nonstructural floor slabs should be separated from buildings to allow for 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 relatively well graded sand and gravel, such as road base, should be placed beneath slabs as subgrade support. This material should consist of Job No. 1134711 Meeh - S - minus 2 inch aggregate with at least 50% retained on the No. 4 sieve and less than 12% 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 near optimum moisture content. Required fill can consist of the on-site soils devoid of vegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM It is our understanding the finished floor elevation at the lowest level of the proposed residence will be at or above the surrounding grade. Therefore, a foundation drain system is not required. 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 basement areas (if provided), be protected from wetting and hydrostatic pressure buildup by an underdrain and wall drain system. If the finished floor elevation of the proposed residence has a floor Ievel below the surrounding grade, we should be contacted to provide recommendations for an underdrain system. All earth retaining structures should be properly drained. SURFACE DRAINAGE Precautions to prevent wetting of the bearing soils, such as proper backfill construction, positive backfill slopes, restricting landscape irrigation and use of roof gutters need to be taken to help limit settlement and building distress. The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation of the building structural slab foundation excavations 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 Job No. 113 4711 Ge Ptech -9 - pavement and nonstructural slab areas and to at least 90% of the maximum standard Proctor density in Iandscape areas. 3) The ground surface surrounding the exterior of the building should be sloped to drain away from the foundation in all directions. The slope should be at least 6 inches in the first 5 feet in unpaved areas and at least 21/2 inches in the first 10 feet in paved areas. Graded swales should have a minimum slope of 3%. 4) Roof gutters should be provided with downspouts that discharge at least 5 feet beyond the foundation and preferably into subsurface solid drain pipe. 5) Landscaping which requires regular heavy irrigation, such as sod, should be minimized and located at Ieast 10 feet from the building foundation. Consideration should be given to use of xeriscape to reduce the potential for wetting of soils below the building caused by irrigation. 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. 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 Job No. 1134711 Gtech -10 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 GEO' HNICAL, INC. Steven L. Pawlak, P.E Reviewed by: Daniel E. Hardin, P.E. SLP/ksw cc: SiIich Homes — John Silich (jt�hu4a silichconstructiull.conlI) Silich Homes — Jodi Thimsen (pdie)silichconstructioli.coin) Silich Homes — Dave Guthrie (dight•ieOsilichconstiuclion.e n) Job No. 113 471i - Ggstech BLUE HERON VISTA 5959 BORING 5 (2014) • LOT 279 5960 — — LOT 278 • BORING 5 (2005) PROPOSED / RESIDENCE / I I s959 i I ! I APPROXIMATE SCALE 1' 20 5958 5959 5960 1 5960 LOT 277 113 4711 H HEPWORTH•PAWLAK GEOTECHNICAL LOCATION OF EXPLORATORY BORINGS Figure 1 Elevation - Feet - 5960 5955 - 5950 5945 - 5940 5935 5930 BORING 5 (2014) ELEV. = 5960' LOT 279 PROPOSED FLOOR LEVEL = 5960.5' (LOT 278) 32/12 27/12 WC=11.9 DD=117 9/12 WC=13.5 DD=100 -200=71 8/12 WC=10.8 DD=103 -200=47 BORING 5 (2005) ELEV.= 5950' LOT 278 11/12 16/12 WC=9.3 DD 100 -200- 54 17/12 Note: Explanation of symbols is shown on Figure 3. 5960 5955 5950 5945 5940 5935 5930 Elevation - Feet 113 4711 Gertech HEPWORTH-PAWLAK GEOTECHNICAL LOGS OF EXPLORATORY BORINGS Figure 2 [*I 27112 T FILL; mixed clayey silt, sand and gravel with cobbles, medium dense, slightly moist to moist, constructed mostly in 2006. SILT (ML); sandy to very sandyslightly clayey, gravelly layersstiff, moist, light brown to brown, slightly calcareous. SAND AND SILT (SM -ML); scattered gravel, loose to medium dense, slightly moist to moist, light brown. GRAVEL, COBBLES AND BOULDERS (GM -GP)• slightly silty, dense, slightly moist, light brown. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1586. Drive sample blow count; indicates that 27 b'ows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. Depth boring had caved when checked 3 or less days following drilling. Practical drilling refusal. NOTES: 1. Boring 5 (2014) on Lot 279 was drilled in December 2013 and the Boring 5 (2005) on Lot 278 was drilled in July 2005. The exploratory borings were drilled 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 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 3 days or less following drilling. 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 113 4711 HEPWORTH•PAWLAK GEOTECHNICAL LEGEND AND NOTES Figure 3 Compression % 0 1 2 3 Moisture Content _ 13.5 percent Dry Density = 100 pcf Sample of: Sandy Clayey Silt From: Boring 5 (2014) at 15 Feet, Lot 279 0.1 1.0 APPLIED PRESSURE - kst 10 100 113 4711 I-1 HEPWORTH-PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS Figure 4 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 113 4711 SAMPLE LOCATION NATURAL MOISTURE' � � 1%) fix -F) GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG UMfTS UNCONFINED COMPRESSIVE `'TREE IPSF) SOIL OR BEDROCK TYPE BORING DEPTHDRYDENSITY (ft) GRAVEL I%) SAND I%) UQUIDUMIT 1%1 PLA571C MCC( (%) 5 (2005) 81/2 9.3 100 54 Silt and Sand with Gravel 5 (2014) 10 11.9 117 Silty Clayey Sand (Fill) 15 13.5 100 71 Sandy Clayey Silt 20 10.8 103 47 Very Silty Clayey Sand