The URL can be used to link to this page

Home My WebLink AboutSubsoil Study for Foundation Design 05.29.15~ec h HEPWORTH-PAWLAK GEOTECHNICAL SUBSOll. STUDY l-!.pw nnh-P;ml;1k t i.:1•1..-.:h "' I. !n.:. 501l1 C .. 1111 1y Rn.1J I;~ (1k 1111 ..... 1 <;\prui::•. c .,1 ri .... ,1.,;:;1 Pho•ll \ ljLl.\)4'\. j lh~ F.i,. lJj~'-9-l'·S~H •:1n 111: hp·.:~11"?lht't:"•11 di .,;, . FOR FOUNDATION DESIGN PROPOSED RESIDENCE, LOT 275 0207 BLUE HERON VISTA moNBRIDGE DEVELOPMENT GARFIELD COUNTY, COLORADO JOB NO. 113 471J MAY29,2015 PREPARED FOR: ASPEN SIGNATURE HOMES OF moNBRIDGE, LLC ATfN: LLWYD ECCLESTONE P.O. BOX 7628 ASPEN, COLORADO 81612 lccclcst onel'@pblhfl.net Parker 303 -84 l • 7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY .......................................................................... - 1 - BACKGROUND mFORMATION .............................................................................. -1 - PROPOSED CONSTRUCTION .................................................................................. .:-2 - SITE CONDITIONS .................................................................................................... -2 - SUBSIDENCE POTENTIAL ...................................................................................... :-2 - F.IEill EXPLORATION .............................................................................................. -3 - SUBSlJRFACE CONDITIONS ................................................................................... :-3 - ENGINEER1NG ANALYSIS ...................................................................................... :-4 - DESIGN RECOM1vlENDATI0NS ............................................................................... -5 - FOlJNDATIQNS ...................................................................................................... -5 - FOUNDATION AND RETAINING WALLS ........................................................... 6- NONSTRUCTUR,AL FLOOR SLABS ..................................................................... - 7 - UNDERDRAIN' SYSTEM ....................................................................................... :-8 - SURFACE DRAINAGE ....................... : .................................................................. :-8 - LilvfIT A TIONS ............................................................................................................ - 9 - FIGURE 1 -LOCATION OF EXPLORATORY BORINGS FIGURE 2 -LOGS OF EXPLORATORY BORJNGS FIGURE 3 -LEGEND AND NOTES FIGURE 4 -SWELL-CONSOLIDATION TEST RESULTS FIGURE 5 -GRADATION 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 275, Ironbridge Development, 0207 Blue 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 t~ Aspen Signature Homes of Ironbridge, LLC dated April 6, 2015. The current engineering services include a lot specific study using subsurface information collected for previous geotechnical studies at the Ironbridge development an~ site specific subsoil exploration and laboratory testing. A field exploration program consisting of an exploratory boring 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 re<;:ommendations for foundation types, depths and allowable pressures for the current proposed building. This report summarizes the data obtained during this study and presents our conclusions, design recommendations and other geoteclmical engineering considerations based on the propos~ 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, 200'5, 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 pre0ous reports has been considered in the current study of Lot 275 . J ob No. 113 47 11 -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 loadings, location or grading plans chEµige significantly from those described above, we should be notified to re-evaluate the recommendations contained in this report. SITE CONDITIONS The proposed residence is located 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 8 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 with scattered brush. SUBSIDENCE POTENTIAL Eagle Valley Evaporite underlies the project area which is knoWn. to be associated with sinkholes and localized 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 275 did not encounter voids but the alluvial fan depth encountered on Lot 275 was considerably greater than Job No. 113 471J -3 - adjacent lots which could indicate past ground subsidence. In our opinion, the risk of future ground subsidence in the Villas North and Villas South project area 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 Lot 275 development was conducted on May 14, 2015 at the location shown on Figure 1 to evaluate the subsurface conditions. Boring 15 {2005) from our September 14, 2005 subsurface study report was drilled near the middle of Lot 275 as shown on Figure I. 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 Geo technical, 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 ASTMMethod 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 ilie 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 15 (2005) generally consist of a shallow fill depth above sandy 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 31 feet. The natural soils encountered at Boring 1 for the cmTent study were similar alluvial fan dep,osits below about 9 feet of fill material mainly placed in 2006. The fill Job No. 113 4711 • -4 - soils are medium dense and slightly moist to moist, and the underlying natural alluvial fan soils ate loose to medium dense/stiff to medium stiff and generally moist to wet with depth. Drilling with augers in the underlying, coarse river gravel alluvium was difficult due the cobbles and boulders and drilling refusal was encountered in the deposit. Groundwater level was measured in Boring 15 (2005) at a depth of about 27 feet, and cWTently measured in Boring 1 at a depth of about 15~ feet, a rise in the groundwater level of about 18Y2 feet. The Robertson Ditch is an open, earthen channel located about 100 feet west of the lot which could be a source of the groundwater level rise since Boring 15 (2005) was drilled . Laboratory testing performed on samples obtained from the borings included natural moisture content and density and gradation analyses. Results of swell-consolidation testing perfon;ned on a relatively undisturbed drive sample of the silty sand fill soils obtained from Boring l, presented on Figure 4, generally indicate fow to moderate compresSibility under conditions ofloading and when wetted. The laboratory testing for Boring 1 is summarized in Table 1. ENGINEERING ANALYSIS The upper 9 feet of soils encountered in Boring l located at the north side Lot 275 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 projec~ 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 thiclmess of the compressible soils due to potential collapse when wetted, and the potential compression of the wet soils under loading which were encountered with depth. Relatively deep structural fill as encountered in Boring 1 will also have some potential for long tean settlement but should be significantly less than the alluvial fan deposits . Proper grading, drainage and compaction as presented below in the Suiface Drainage sections will help to keep the subsoils dry and reduce the settlement risks. A heavily reinforced structural slab or post- Job No. 11 3 471J -5- tensioned slab foundation desigtied 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. The groundwater level encountered in Boring l drilled for the current study appears unusually high for this area and could be due to leakage of the Robertson Ditch. Season rise in the groundwater level is typically expected to be a few feet or less. You should be aware that excessive ditch leakage may need to be controlled by lining the ditch in the future and should be considered at this time to limit inundation of residential development areas below the ditch. 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 9 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. 1) A conventionally reinforced structural slab or post-tensioned slab placed on about 9 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 l 0 feet but at least half of the slab width whichever is more. Settlement of the foundation is estimated to be about 1 inch based on the long tenn compressibility of the fill. Additional settlement of about 1 inch is estimated if wetting of the debris fan soils were to occur. Settlement from Job No. 113 471J ~h -6- the deep wetting would tend to be uniform across the building area and the settlement potential of the fill section should control the design . 2) The thickened sections of the slab for support of concentrated loads should have a minimum width of 20 inches. 3) The perimeter tum-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 tum-down section should have at least 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 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 fill materials and observe all footing excavations prior to concrete placement for bearing conditions. FOUNDATION AND RETAlNING WALLS Foundation walls and retaining structures (if any) 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 Job No. 113 471J ~ech -7- 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 wan 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 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 mate.rials 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 of0.35. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of300 pcf. The coefficient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factor$ 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 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 Job No. 113 471J -8- 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 sub grade support. Thls material should consist of 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. Requrred 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 wal1s, 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 level 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 Job No. 113 471J -9- should be observed during construction and maintained at all times after the residence has been completed: I) Inundation of the builcling 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 pavement and nonstructural 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. The slope should be at least 6 inches in the first 5 feet in unpaved areas and at least 2~ inches in the first 10 feet in paved areas. Graded swales should have a minimum slope of3%. 4) Roof gutters should be provided with downspouts that discharge at least 5 feet beyond the foundation and preferably into subsurface sQlid drain pipe. 5) Landscaping which requires regular heavy irrigation, such as sod, should be minimized and located at least 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 detennining 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 Job No. 113 471J • -10- 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, SLP/ksw cc: Silich Homes -John Silich (john <@si li chcon struc tion.com) SiJich Homes -Jodi Thimsen (iodi@ silichhom es.co m) Job No. 113 47 1J 113 471J S9sa_ -, \ \ \ APPROX IMATE SCALE 1· ... 20' S9s;- 1-------,_ -------.. ' ' \ ------.\ EXISTING RESIDENCE LOT274 \ I I I I I I I I I I I I I -------, I / / !O/ ~~'S " " \ \ \ BORING15 (2005} 0 PROPOSED RESIDENCE LOT275 --- BLUE HERON VISTA \ ' ' ....... --. ' ....... ' ' 6'966> LOT 276 (VACANT) BORING 1 ' ,_ - -5957 BENCH MARK: MANHOLE AIM; ELEVATION = 5956.31', /lS GIVEN. ~ LOCATION OF EXPLORATORY BORINGS Figure 1 HEPWORTH·PAWU\K GEOTI!:CHNICAL. .. .-' Cl LI.I z ~ a:: ,_ it u a:: LI.I a. I HYDROMETER ANALYSIS I SlEVE ANALYSiS I 7 TIME READINGS U.S. STANDARD SERIES I CLEAR SQUARE OPENINGS O ~~ ~\J.t 15 ~'IN . 60MINl9MIN.4 MIN. 1 MIN. #200 #100 #50 #30 #16 #8 #4 3/B' 3/4' 11/2' 3' 5'6' a· 100 10 90 20 80 30 70 - 40 60 50 50 60 40 70 30 60 20 90 10 0 100 .001 .002 .005 .009 .019 .037 .074 .150 .300 .600 1 .18 2.36 4.75 9.5 19.0 37.6 76.2 152 203 12.5 127 DIAMETER OF PARTICLES IN MIWMETERS QAYlOSllT I F~ I ~ lCON!SE I AfE T COAi& I COBaEi COBBLES 0 % GRAVEL 17 % SAND 25 % SILT AND CLAY 58 % LIQUID LIMIT % PLASTICl1Y INDEX % SAMPLE OF: Sandy Gravelly Slit FROM: Boring 15 at 14 feet 113 471J ~ GRADATION TEST RESULTS Figure . Hepworth-Pawlak Geatec:hnlcal C!l z en Ul ~ a. ..... z w CJ a:: w a. 5 ., HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE1 Job No.113 471J SUMMARY OF LABORATORY TEST RESULTS Lot275 j SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG LIMITS UNCONFINED PERCENT MOISTURE DRY GRAVEL SAND PASSING LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY N0.200 LIMIT INDEX STRENGTH BEDROCK TYPE (%) (%) SIEVE (ft) {%) (pcf] {%) (%) {PSA 1 2 1h 12.9 117 77 Sandy Silt and Clay (Fill) 5 13.6 116 Silty Clayey Sand with Gravel (Fill) 15 9.9 121 18 Silty Sand with Gravel 30 21.2 57 Sandy Silt with Gravel 15 4 14.3 112 Sandy Clayey Silt (2005} 9 14.5 71 Sandy Clayey Silt 14 14.4 112 17 25 58 Sandy Gravelly Silt