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HomeMy WebLinkAboutSoils Report 06.09.2014• -7fr A HEPWORTH-P/ NLAK GEOTECHNICAL GRANTOR: I r. ^e[ ra'E•€'P.ci ., i'.;E i{i+;i. 1, is AARON M. KIDDER SUBSOIL STUDY FOR FOUNDATION DESIGN STATE OF ) PROPOSED RESIDENCE LOT 12, 'ION MESA SUBDIVISION, FILING 1 COUNTY OF 8099 SAGE MEADOW ROAD GARFIELD COUNTY, COLORADO Acknowledged before me this — day of 2014, by AARON M. KIDDER. Witness my hand and official seal. My Commission expires: JQB 114 176A JUNE 9, 2014 Notary Public PREPARED FOR: KATIE MANN 3 SEVEN OAKS ROAD GLENWOOD SPRINGS, COLORADO 81601 kat tiernaiin226tgniail.cor i 3 F'.IiL r 3k14-84l•71I9 Q Cath ado I?fua. 19611 3'412 o S;I1'eithnm. 970-468-19Z19 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY -1 - PROPOSED CONSTRUCTION r 1 SITE CONDITIONS - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 2 - SUBSIDENCE POTENTIAL - 3 - DESIGN RECOMMENDATIONS r 4 - FOUNDATIONS - 4 - FOUNDATION AND RETAINING WALLS r 5 - FLOOR SLABS - 6 UNDERDRA1N SYSTEM - 7 - SURFACE DRAINAGE r 7 - LIMITATIONS - 8 - FIGURE 1 -LOCATION OF EXPLORATORY BORING FIGURE 2- LOG OF EXPLORATORY BORING FIGURE 3- LEGEND AND NOTES TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 12, Pinyon Mesa Subdivision, Filing 1, 0099 Sage Meadow Road, 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 Katie Mann dated May 15, 2014. We previously performed preliminary geotechnical engineering studies for the subdivision development and presented our findings in reports dated November 11, 2005 and April 10, 2006, Job No. 105 652. An exploratory boring was drilled on the lot to obtain information on the subsurface conditions. Samples of the subsurface materials 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 Development plans for the lot were not available at the time of our study. The residence will generally consist of a 2 story, wood frame structure over crawlspace or basement areas with an attached garage and slab -on -grade floor located within the building envelope shown on Figure 1. Grading for the residence is assumed to be relatively minor with cut depths between about 2 to 8 feet. We assume relatively light foundation loadings, typical of the assumed type of construction. If building location, grading or loading information changes from that described above, we should be notified to re-evaluate the recommendations presented in this report. Job No. 114176A �#z - 2 - SITE CONDITIONS The lot was vacant at the time of our field exploration. The lot is located an the north (uphill) side of Sage Meadow Road as shown on Figure 1. The ground surface is relatively flat with a gentle slope down to the south. There is about 2 to 3 feet of elevation difference across the building envelope. Vegetation consists mainly of sage brush. FIELD EXPLORATION The field exploration for the project was conducted on May 23, 2014. One exploratory boring was drilled on the lot as shown on Figure 1 to evaluate the subsurface conditions. The boring was advanced with flinch diameter continuous flight augers powered by a truck -mounted CME -45B drill rig. The boring was 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 wore driven into the subsurface materials 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 and hardness of the bedrock. Depths at which the samples were taken and the penetration resistance values are shown on the Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS A graphic log of the subsurface profile encountered in the boring is shown on Figure 2. The subsoils, below about g foot of organic topsoil, consist of about 2% feet of sandy clay and silt, and silty sand and gravel with rock fragments to probably cobble size. Siltstonelclaystone bedrock with sandstone zones or layers was encountered in the boring at a depth of about 15 feet down to the maximum explored depth of 41 feet below the lob No. 114176A -3 - existing ground surface. Nearby borings drilled as part of our previous studies encountered similar subsurface profile and typically deeper clay and silt soils. Laboratory testing performed on samples obtained from the boring included natural moisture content and density, percent finer than No. 200 sieve (silt and clay fraction) gradation analysis and liquid and plastic limits. The soils were generally too coarse to obtain undisturbed samples for swell -consolidation testing. The liquid and plastic limits testing indicate the bedrock is low plasticity. The laboratory testing is summarized in Table 1. No free water was encountered in the boring at the time of drilling and the subsoils and bedrock were slightly moist. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa subdivision. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of gypsum and Limestone. There is a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in the area, several sinkholes were observed scattered throughout this part of Garfield County. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities was encountered in the subsurface materials; however, the exploratory boring was 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 12 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. 114 176A -4 - DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, the proposed residence can be founded with spread footings bearing on the natural soils with some risk of differential foundation settlement. The clay and silt soils encountered to a depth on the order of 3 feet are compressible when loaded and wetted and should be removed from below foundation and slab areas of the residence. The design and construction criteria presented below should be observed for a spread footing foundation system. I) Footings placed on the undisturbed natural sand and gravel soils should be designed for an allowable bearing pressure of 1,500 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. The soils could be compressible after wetting under load and there could be some additional post -construction differential foundation settlement of about i4 to 1 inch if the bearing soils are wetted. The bearing conditions should be further evaluated at the time of construction. Precautions should be taken to prevent post -construction wetting of the bearing soils. 2) Footings should have a minimum width of 20 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 14 feet. Foundation walls acting as retaining structures should also be Job No, 114176A N� -5 - designed to, resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) The topsoil, clay and silt soils and loose or disturbed soils should be removed and the footing bearing level extended down to undisturbed natural granular soils. The exposed soils in footing areas should then be moistened and compacted prior to constructing footings. Foundation concrete should contain sulfate resistant cement and be air entrained. 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 residence 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. Backfill should not contain topsoil, vegetation or oversized rock. 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 retaining walls. Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at near optimum moisture content. Backfill 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 Job No. 114176A -6 - 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. The lateral resistance of foundation or retaining wall footings will be a combination of the sliding resistance of the fooling on the foundation materials and passive earth pressure against the side of the footing. Resistance tosliding at the bottoms of the footings can be calculated based on a coefficient of friction of 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 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 ofpassive 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. FLOUR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab - on -grade construction. The upper clay and silt soils are compressible when wetted and preferably should be removed to limit the risk of slab settlement and distress mainly if the subgradc soils are wetted. 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 near optimum moisture content. Required fill can Job No. 114 176A G Loch _7_ consist of the on-site soils or imported granular soils (such as road base) devoid of vegetation, topsoil and oversized rock. UNDEIZDRAIN 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 basement areas, be protected from wetting and hydrostatic pressure buildup by a perimeter underdrain system. The drain system should not be constructed around garage areas or shallow crawlspaces 4 feet or less in depth. Backfill should be adequately compacted and the surface sloped to drain away from the residence, Where provided, the drains should consist of drainpipe 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 1% feet deep. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence 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. Job No 114 176A -8- 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 I2 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 at least 2 feet of the on- site soils to reduce surface water infiltration, 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Sprinkler heads and landscaping which requires regular heavy irrigation, such as sod, should be located at least 10 feet from foundation walls. 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 boring drilled at the location indicated on Figure 1, the assumed 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 extrapolation of the subsurface conditions identified at the exploratory boring 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 Job No. 114 176A -9 - 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. Steven L. Pawlak, P.E. Reviewed by: Daniel E. Har S LP/ksw tn, P.E. lob No. 114 176A cG61Stech APPROXIMATE SCALE 1' 20' LOT 11 14 176A OPEN SPACE LOT 12 BORING 1 • L. BUILDING ENVELOPE r.1 1 LOT 13 SAGE MEADOW ROAD H 31.pworth—Fairlok Geo tech/1[ml LOCATION OF EXPLORATORY BORING Figure 1 25 30 35 40 114176A BORING 1 12/12 WC=10.6 00=67 200=67 23/12 102/10 WC=2,9 -200=24 100/5 WC=6.5 1wL=23 P1=3 86/12 50/1 50/8 NOTE: Explanatlon of symbols Is shown on Figure 3. Gagtech Hera worth—PcrA4k G•atorlrnkal LOG OF EXPLORATORY BORING 0 5 10 _ 15 20 Q. 0 25 30 35 40 1.16 Figure 2 LEGEND; ® TOPSOIL; organic sandy silt and clay, soft, brown. CLAY AND SILT (CL -ML); sandy, stiff, slightly moist, light brown. SAND AND GRAVEL (SM -GM); silty, possible cobbles, medium dense, slightly moist, mixed brown, calareous. SILTSTONE/CLAYSTONE BEDROCK; sandstone layers/zones, hard, slightly moist, Tight brown, Eagle Valley Evaparite. 2 Sn 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 -1586. 23/12 Drive sample blow count; indicates that 23 blows of a 140 pound hammer falling 30 Inches were required to drive the California or SPT sampler 12 inches. NOTES: 1. The exploratory boring was drilled an May 23, 2014 with a 4 -inch diameter continuous flight power auger. 2. Location of the exploratory boring was measured approximately by pacing from features shown on the site plan provided. 1 The exploratory boring elevation was not measured and the log of exploratory boring is drawn to depth. 4. The exploratory boring location and elevation should be considered accurate only to the degree Implied by the method used. 5. The Ilnes between materials shown on the exploratory boring log represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered In the boring at the time of drilling. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pal) -200 = Percent passing No. 200 sieve LL a Liquid Limit (%) PI at Plasticity Index (90 114176A 11 tiipwownt•PAWLAK GLOTicHIU AL LEGEND AND NOTES Figure 3 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS SAMPLE OCATIDN BORING DEPTH Ltd NATURAL NATURAL MOISTURE DRY DENSITY CONTENT GRADATION GRAVEL SAND (%) 1%) ATEERB PERCENT PASSING NO. UQU10 UNIT 200 SIEVE 1 2 10.6 87 67 (Kl 10 2.9 15 & 20 combined 24 6.5 4 23 Job No. 114 176A RG LIMITS UNCONFTNEO SOIL OR BEOROCKTYPE PLASTIC INDEX (%} COMPRESSNE STRENGTH (PSII Sandy Silt and Clay _ Silty Sand and Gravel 3 Siltstone/Claystone