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HomeMy WebLinkAboutSoils Report 02.12.2015SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 7, PINYON MESA SAGE MEADOW ROAD GARFIELD COUNTY, COLORADO JOB NO. 115 011A FEBRUARY 12, 2015 PREPARED FOR: JULIAN HARDAKER 41 SAGE MEADOW ROAD GLENWOOD SPRINGS, COLORADO 81601 julian:ir tltchesha‘hepmt.con: TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY ........................ - I - PROPOSED CONSTRUCTION ................................... : 1- SITE CONDITIONS ..,.... - 2 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 4 - DESIGN RECOMMENDATIONS .., . r 4 - FOUNDATIONS -4- FOUNDATION AND RETAINING WALLS 6 - FLOOR SLABS - 7 - UNDERDRAIN SYSTEM - 8 - SURFACE DRAINAGE ........ - 8 - LIMITATIONS - 9 - FIGURE I - LOCATION OF EXPLORATORY BORING FIGURE 2 - LOG OF EXPLORATORY BORING FIGURE 3 - LEGEND AND NOTES FIGURES 4 AND 5 - 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 proposed residence to be located on Lot 7, Pinyon Mesa, 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 Julian Hardaker, dated January I4, 2015. An exploratory boring was drilled 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 The proposed residence design was preliminary at the time of our study and will generally be a two story wood frame structure over a basement with an attached garage at the main level. The basement and garage floors will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 5 to 12 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. If building loadings, Location or grading plans change significantly from those described above, we should be notified to re-evaluate the recommendations contained in this report. Job No. 115 011A Geegtech -2 - SIT E CONDITIONS The site was vacant at the time of our field exploration and covered with about h to 1 foot of snow. The site slopes strongly down to the southwest with on the order of four feet of elevation difference across the assumed building area. Vegetation consists of grass and weeds with scattered sage brush, SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa development. 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 portions of the lot. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. During previous work in the area, sinkholes have been observed scattered throughout the lower 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 7 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, FIELD EXPLORATION The field exploration for the project was conducted on January 15, 2015. One exploratory boring was drilled at the location shown on Figure 1 to evaluate the Job No. 115011A Gateth -3 - subsurface conditions. The boring was advanced with 4 inch 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 a 2 -inch I.D. spoon sampler. The sampler was 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-2586. 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 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 conditions encountered at the site is shown on Figure 2. The subsoils encountered, below a thin root zone and topsoil, consist of about 20 feet of stiff, slightly calcareous sandy silt and clay with scattered gravel underlain by very stiff, sandy clay with gravel down to the drilled depth of 31 feet. It has been our experience in this area that the very stiff clay is underlain by siltstone/claystone bedrock at depth on the order of 35 to 40 feet. Laboratory testing performed on samples obtained from the boring included natural moisture content and density and finer than sand size gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 and 5, indicate low compressibility under light loading and existing low moisture content, low collapse potential (settlement under constant load) in the upper silt and clay soils and low expansion potential in the underlying very stiff clay soils when wetted, and moderate to high compressibility under increased Ioading after wetting. The sample from 10 feet showed an expansion potential when wetted which appears to be an anomaly in the predominantly compressible silt and clay soil layer common to this area of the subdivision. The Iaboratory testing is summarized in Table 1. Job No. 115 011A Gm 4 No free water was encountered in the boring at the time of drilling and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The sandy silt and clay soils encountered at typical shallow foundation depth tend to settle when they become wetted. The lower compressibility potential of the underlying very stiff, sandy day soils may also impact a shallow foundation if deep wetting were to occur but the risk appears low. A shallow foundation placed on the upper silt and clay soils will have a high risk of settlement if the soils become wetted and care should be taken in the surface and subsurface drainage around the house to prevent the soils from becoming wet. It will be critical to the long term performance of the structure that the recommendations for surface drainage and subsurface drainage contained in this report be followed. The amount of settlement, lithe bearing soils become wet, will mainly be related to the depth and extent of subsurface wetting. We expect that initial settlements under light foundation Ioading will be less than 1 inch. If wetting of the shallow soils occurs, additional settlements of 2 to 3 inches could occur. Settlement in the event of subsurface wetting will likely cause building distress and mitigation methods such as deep compaction, a deep foundation (such as piles or piers extending down roughly 40 feet below existing ground surface) or a heavily reinforced mat foundation, on the order of 2 feet thick, and designed by the structural engineer should be used to support the proposed house. If a deep foundation or mat foundation is desired, we should be contacted to provide further design recommendations. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, the building can be founded with spread footings Job No. 115 011 A G� -5 - bearing on compacted structural fill with a risk of settlement, mainly if the bearing soils become wetted, and provided the risk is acceptable to the owner. Control of surface and subsurface runoff will be critical to the long-term performance of a shallow spread footing foundation system. The garage footing areas should be sub -excavated down about 8 to 10 feet below existing ground surface and the excavated soil replaced compacted back to design bearing level but to a depth of at least 6 feet below footing bearing level. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on a minimum 6 feet of compacted structural fill for the garage and at least 3 feet of compacted structural fill for the basement level of the residence should be designed for an allowable bearing pressure of 1,200 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. Additional settlements of about 1 to Ph inches could occur if the silt and clay soils below the bearing level become wetted. A h increase in the allowable bearing pressure can be taken for toe pressure of eccentrically loaded footings. 2) The footings should have a minimum width of 24 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 heavily reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 14 feet. The foundation should be configured in a "box like" shape to help resist differential movements. 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. 115 011 A Gtech -6- 5) The topsoil and any loose or disturbed soils should be removed below the building area. The exposed soils in footing areas after sub -excavation to design grades should then be moistened and compacted. Structural fill should consist of low permeable soil (such as the on-site sandy silt and clay soils) compacted to at least 98% standard Proctor density within 2% of optimum moisture content. The structural fill should extend laterally beyond the footing edges equal to about %h the fill depth below the footing. 6) A representative of the geotechnical engineer should evaluate the structural fill as it is placed for compaction and 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 fine-grained 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 fine-grained 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. Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum standard Proctor density at a moisture content near optimum. Ball in pavement and Job No. 115 011A Gegritech -7 - walkway areas should be compacted to at least 95% of the maximum standard Proctor density. Care should be taken not to overcompact the backfill or use large equipment near the wall, since this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall backfill should be expected, even if the material is placed correctly, and could result in distress to facilities constructed on the backfill. 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 325 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 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, can be used to support lightly loaded slab - on -grade construction with settlement risk similar to that described above for foundations in the event of wetting of the subgrade 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 relatively well graded sand and gravel such as road base should be placed beneath interior slabs to limit capillary moisture rise. This 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. Job No. 115 01 I A G eCfi All fill materials for support of floor slabs should be compacted to at least 95% of maximum standard Proctor density at a moisture content near optimum. Required fill can consist of the on-site soils devoid of vegetation, topsoil and oversized rock. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in the area that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched condition. We recommend below -grade construction, such as retaining walls and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. An underdrain should not be placed around shallow footing depth structures such as the garage area and crawlspace, if provided. 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 an interior sump of solid casing. 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 PA feet deep. An impervious membrane such as a 20 mil PVC Iiner should be placed beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. SURFACE DRAINAGE It will be critical to the building performance to keep the bearing soils dry. The following drainage precautions should be observed during construction and maintained at all times rifler the residence has been completed: Job No. 115 011 A Ga1ech -9- 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 covered with filter fabric and 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. Natural vegetation lined drainage swales should have a minimum slope of 3%. 5) Landscaping which requires regular heavy irrigation 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 I, 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 Job No. 115 oI I A -10 - 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 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. Reviewed by: Daniel E. Hardin, P.E. S LP/ksw Job No. 115 01 1A Ge Ptech LOT 6 7008. rq 7006 1006 7009 7070 1004--,� 1003 - 1002 1001 1000 999-_ LOT7 �l i r-,___ !\ • �� BORING 1 j I I ! I , 1 L ! tI SAGE MEADOW ROAD LOT 8 APPROXIMATE SCALE 1'= 30` 115 011A M ilspwarth—Pawldc Geoiach'nlcat LOCATION OF EXPLORATORY BORING Figure 1 0 - 5 - 10 15 20 25 30 BORING 1 ELEV.= 1003' 33/12 15/12 WC=7.7 DD=107 -200=78 29/12 WC=8.0 DD=114 24/12 WC=8.4 DD=97 32/12 WC=6.3 DD=108 -200=79 21/12 35/12 WC=7.8 DD=128 5 10 15 20 25 30 35 35 . 115 011A NOTE: Explanation of symbols is shown on Figure 3. H Hepworth—Pawlak Geotechnl aI LOG OF EXPLORATORY BORING Figure 2 m LL 0. 0) 0 LEGEND: ® TOPSOIL; organic sandy silty clay, firm, moist, dark brown. L SILT AND CLAY (ML -CL); sandy, gravelly in upper 5 feet, stiff to very stiff, slightly moist, brown, slightly calcareous. CLAY (CL); silty, sandy, slightly gravelly. very stiff to hard, slightly moist, brown, low to medium plasticity. 11 Relatively undisturbed drive sample; 2 -Inch I.D. California liner sample. 33/12 Drive sample blow count; indicates that 33 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. NOTES: 1. The exploratory boring was drilled on January 15, 2015 with a 4 -inch diameter continuous flight power auger. 2. The exploratory boring location was measured approximately by pacing from features shown on the site plan provided. 3. The exploratory boring elevation was obtained by interpolation between contours shown on the site plan provided. 4. The exploratory boring location and elevation should be considered accurate only to the degree implied by the method used. 5. The lines 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 (pcf) -200 = Percent passing No. 200 sieve 115 D11A 1%I Hepworth—Pawlok Geotechnical LEGEND AND NOTES Figure 3 Compression - Expansion % Compression % 3 2 1 0 1 2 0 1 2 3 4 5 6 7 8 Moisture Content = 8.0 percent Dry Density = 114 pcf Sample of: Silty Sandy Clay From: Boring 1 at 10 Feet Expansion upon wetting 0.1 1.0 10 APPLIED PRESSURE - Icsf 100 --o Moisture Content = 8.4 Dry Density = 97 Sample of. Sandy Silt and Clay From: Boring 1 at 15 Feet 0.1 11 Compression upon wetting percent pcf 1.0 10 APPLIED PRESSURE - ksf 100 115 011A 1-I Hepworth—Powiok Geotechnical SWELL -CONSOLIDATION TEST RESULTS Figure 4 t a 0 1 co Q 0 2 Moisture Content = 7.8 percent Dry Density = 126 pcf Sample of: Sandy Clay From: Boring 1 at 30 Feet Expansion upon wetting 0.1 1.0 APPLIED PRESSURE - ksf 10 100 115011A N Flepworth—Pawlak Geotechnical SWELL -CONSOLIDATION TEST RESULTS Figure 5 Job No. lis 011A 0.1 U 00 00 Sandy Silt and Clay ATTERBERG LIMITS z dO z o z U Z N FIRa ,� W 4 N X L7 J W Pg <00 rd 3 U D 0 m .'d m e 0 0 C Z C N z 0 v m 0 0 m 0 0 0 m ' A development agreement will be required for any project for which the Applicant is requesting a vested rights period longer than 3 years. 2 An im • rovements a ■ reement ma be re • uired for an • ro'ect for which • ublic im • rovements are necessa . A CO0 A CO A V d OI a 01 f' A f' W d N d s A 0 A CC0 A 02 0VV la , 4-106 A , (31 A . A - 0 rD m n ,0 A N c, G., Variance Administrative Interpretation Administrative Interpretation Appeal. Waiver of Standards Accommodation, Federal Fair Housing Act Code Text Amendment Call -Up to the BOCC Rezoning Develop. 100 -Year Floodplain Variance Location and Extent Review Vacation of a County Road or Public ROW Development in 100 -Year Floodplain Minor Temporary Housing Facility Amendment to an Approved LUCP -- -- i i;.; ran Administrative Review Limited Impact Review Major Impact Review CI Ilia SIUI. c c. c c c ■ D e c c t e c c c c General t c c Vicinity e e D Site Application Materials 17d p - Map 0 re Plan 0 0 11 11 wall E.. -i. -.. -. M. -1U Statement of request and response to standards. — _ _ _ Cla 0- m N 0 7 0a � m = Q m oa Z 4 7 Statement of request and response to standards. Demonstration of general conformance with the Comp. Plan and compliance with any applicable IGAs. Application materials as determined by Director. Legal description of property. 1 1 See Section 4-301 for additional submittal requirements. 1 Amendment Description, subject to Minor or Substantial ,Modification determination oar section 4-108. p op .I ... II e III igi c ... e c t e t c 1 c c_andscape c e 13 t- a c Grading Impact Rezoning Statement Development Improvements Traffic WaterSuppfylDistribution Wastewater Floodplain a G. rr. O 3 .i N 9p; 0< o and Drainage Plan m Plan -n Analysis 0 Justification Report = of Appeal — Agreement' Agreement 2 T. Study r Plan E Man./treat Plan Z Analysis 0 a -- D 75 13 �' n} o 0 WI c cr ¢' ca E. et) N 1 Submitted with companion application.