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HomeMy WebLinkAboutSoils ReportHEPWORTH—PAWLAK GEOTECHNICAL 1-4.0,v,srrh-I'::x:111. (J re_luiic,[1, Ing_ 5020 (.4)rin[y Kira. 15.1 (31enkt.Nii1 Gprin ,., 1..ilor:k1.• hi 601 I'Iu r rle. 9711-945 .9,;ti 11v 970..945-8.04 k.111.11: h1 �uov'hilr�.l[r:11.��nr1 SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 4, PINYON MESA PINYON MESA DRIVE GARFIELD COUNTY, COLORADO JOB NO. I15 062A MARCH 12, 2015 PREPARED FOR: SCOTT DILLARD 21 COUNTY ROAD 126 GLENWOOD SPRINGS, COLORADO 81601 scottdiliardrealtor(a)rnail.com) P,Irker 303-841-711 L) C:olor,[41oSpring 719.633.55(12 • Silverrhorne 970-16S-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 PROPOSED CONSTRUCTION .. -1 - SITE CONDITIONS - 2 - SUBSIDENCE POTENTIAL ............................... - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 4 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4 - FOUNDATION AND RETAINING WALLS ... ... ......... ...... .................- 6 - FLOOR SLABS ................................................, - 7 - UNDERDRAIN SYSTEM - 8 - SURFACE DRAINAGE - 8 - LIMITATIONS - 9. - FIGURE 1 - 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 4, Pinyon Mesa, Pinyon Mesa Drive, 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 Scott Dillard, dated February 23, 2015. An exploratory boring was drilled to obtain information on the subsurface conditions. Samples of the subsoils and bedrock 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 reconunendations 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 conceptual at the time of our study and will generally be a two story wood frame structure over a basement or crawlspace likely cut into the uphill slope with an attached garage at the street 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 062A Gmech -2 - SITE CONDITIONS The site was vacant at the time of our field exploration and partially covered with about '/z foot of snow. The site slopes strongly down to the southeast with approximately 6 to 8 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 4 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 February 26, 2015. One exploratory boring was drilled at the Iocation shown on Figure 1 to evaluate the lob No. 115 062A -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-1586. The penetration resistance values are an indication of the relative density or consistency of the subsoils or 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 conditions encountered at the site is shown on Figure 2. The subsoils encountered, below about 1/2 foot of topsoil, consist of about 1I1/2 feet of stiff to hard sandy silt and clay, overlying very hard siltstone/gypsum bedrock (Eagle County Evaporite) down to the drilled depth of 26 feet. Drilling through the bedrock was relatively slow due to the rock hardness. 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 of the upper silt and clay soils, presented on Figures 4 and 5, indicate low compressibility under light loading and existing low moisture content, low collapse potential (settlement under constant load) when wetted, and moderate to high compressibility under increased loading after wetting. The bedrock was too hard to obtain undisturbed samples for swell - consolidation testing. 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. Job No. 115062A ver tech -4 - FOUNDATION BEARING CONDITIONS The sandy silt and clay soils encountered at typical shallow foundation depth tend to settle when they become wetted. A shallow foundation placed on the silt and day soils will have a 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, if the bearing soils become wet, will mainly be related to the depth and extent of subsurface wetting. We expect that initial settlements will be less than 1 inch. If wetting of the shallow soils occurs, additional settlements of 1 to 11/2 inches could occur. Settlement in the event of subsurface wetting will likely cause building distress and mitigation methods such as extending to bearing level down onto bedrock or removing the upper Iayer of sandy silt and clay and replacing with properly compacted structural fill should be uscd to reduce the settlement potential. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, the building should be founded with spread footings bearing on the bedrock with a low risk of settlement. Alternately, the building can be founded on compacted structural fill with a low 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 and shallow basement footing areas should be sub -excavated down about 6 to 8 feet below existing ground surface and the excavated soil replaced compacted back to design bearing level but to a depth of at least 4 feet below footing bearing level. Job No. 115 062A -5 - The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the bedrock should be designed for an allowable bearing pressure of 2,000 psf. Based on experience, we expect settlement of footings bearing on the bedrock to be about 1 inch or Less and essentially occur during construction. 2) Footings placed on a minimum 4 feet of compacted structural fill for the garage and shallow basement areas 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 to 1 inches could occur if the silt and clay soils below the bearing level become wetted. A'/ increase in the allowable bearing pressure can be taken for toe pressure of eccentrically loaded (retaining wall) footings. 3) The footings should have a minimum width of 20 inches for continuous walls and 2 feet for isolated pads. 4) 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. 5) 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. 6) 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% Job No. 115 062A -6 - of optimum moisture content. The structural fill should extend laterally beyond the footing edges equal to at least'/ the fill depth below the footing. 7) 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. 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 foundation wall backfill should be expected, even if the Job No. 115 062A -7 - 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 Ioaded 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 free draining gravel should be placed beneath basement slabs for drainage and 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 Iess than 2% passing the No. 200 sieve. Job No. 115 062A Gmech -8 - 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, oversized rock and gypsum. UNDERDRAIN SYSTEM Although free water was not encountered during our exploration, it has been our experience in the area and where bedrock is shallow 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 gravity outlet or 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 Ph feet deep. An impervious membrane such as a 20 mil PVC liner 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 after the residence has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. Job No. 115 O62A Gem ech -9_ 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 1, the general 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 boring and variations in the subsurface conditions may not become evident until excavation is performed. If conditions encountered during Job No. 115062A G t -10 - 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, 1NC. Steven L. Pawlak, P.E. Reviewed by: Daniel E. Hardin, P.E. SLP/ljf Joh No. 115 062A APPROXIMATE SCALE 1" = 20' 115 062A 1000 - LOT 5 995_` 005--�� _- 1005 B lU LDING SETBACK LINE -71 1 LOT 4 • \ BORING 1 1-1 Hepworth—Pollak Geotadmtooe 1 PINYON MESA DRIVE LOT 3 LOCATION OF EXPLORATORY BORING 1 Figure 1 BORING 1 ELEV.= 99T 0 _ 0 5 10 15 20 25 13/12 WC=82 O 0=96 -200=83 14/12 WC=6.9 D D=105 -200=77 24/6,50/5 WC=8.9 DD=104 50/5 WC= 6.2 D D=100 50/4 50/3 5 10 15 20 25 30 30 NOTE: Explanation of symbols is shown an Figure 3. 115 062A GecPtech HU'wORTI#PAWLAA GEOTECHNICAL LOG OF EXPLORATORY BORING Figure 2 LEGEND: ® TOPSOIL; organic sandy silt and clay, brown. G SILT AND CLAY (ML -CL); sandy, stiff to very stiff/hard with depth, slightly moist, light brown. SILTSTONE/GYPSUM BEDROCK; very hard, slightly moist, Tight brownlwhfte. Eagle Valley Evaporite. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. 14/12 Drive sample blow count; indicates that 14 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 February 26, 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 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 fog 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 062A H Hepworth—Pawlok Ceotechnfcel LEGEND AND NOTES Figure 3 Compression % 0 1 2 3 4 5 6 8 9 L_ Moisture Content = 8.2 percent Dry Densely = 96 pcf Sample of: Sandy Silt and Clay From: Boring 1 at 21/2 Feet 01 1.0 APPLIED PRESSURE - ksf Compression upon wetting 10 100 115 062AGgstech Hepworth—Pawlak Geotechnical SWELL -CONSOLIDATION TEST RESULTS Figure 4 Compression % Compression % 0 1 2 3 4 0 1 2 3 4 5 Moisture Content = 6.9 percent Dry Density = 105 pct Sample of: Sandy Silt and Clay From: Boring 1 at 5 Feet Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 8.9 percent Dry Density = 104 pcf Sample of; Sandy Silt and Clay From: Boring 1 at 10 Feet Compression upon wetting 0.1 115 062A 1.0 10 APPLIED PRESSURE - ksf 1.1 Hepworkh—Pawlak Geotechnical SWELL -CONSOLIDATION TEST RESULTS 100 Figure 5 Job No.115 062A ST RESULTS UNCONFINED COMPRESSIVE SOIL OR STRENGTH BEDROCK TYPE (PSF) Sandy Silt and Clay Sandy Silt and Clay Sandy Silt and CIay Siltstone • ATTERBERG LIMITS LIQUID PLASTIC LIMIT INDEX (°/n) (1) w0 ZN> co w t*) ao t.....n n 1 GRADATION 0 Q e co J Lu a e 0 JCC rC = CC61 0 4 p W n Z 0 1- ,-I ,-1 -I lif IX Q _ Z - Za0 N 00 C1 Q Gt 00 N O• Z o a 0 O J = a c c =C In o- 111k-i'lPI-E 0 Z_ 0 co