Loading...
HomeMy WebLinkAboutSoils Report 05.25.2016HEPWORTH-PAWLAK GEOTECHNICAL Hepu arch-Pawlak Georcchnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 email. hpgeorgthrgeotech.cora SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 5, PINYON MESA 107 PINYON MESA DRIVE GARFIELD COUNTY, COLORADO JOB NO. 116 157A MAY 25, 2016 PREPARED FOR: JULIAN ULRYCH 226 SOUTH 2ND STREET CARBONDALE, COLORADO 81623 (1 ul rych @ nepine. com) Parker 303-841-7119 ® Colorado Springs 719-633-5562 ® Silverthorne 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 1 - SUBSIDENCE POTENTIAL • - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS ..... - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4 - FOUNDATION AND RETAINING WALLS - 5 - FLOOR SLABS _ 7 - UNDERDRAIN SYSTEM - 7 - SITE GRADING - 8 - SURFACE DRAINAGE - 8 - LIMITATIONS - 9 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS FIGURE 5 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Job No. 116 157A Gtech PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot 5, Pinyon Mesa, 107 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 professional services to you dated May 2, 2016. A field exploration program consisting of exploratory borings was conducted 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 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 will be a one story structure over a walkout basement. Ground floor will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 4 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. SITE CONDITIONS The lot was vacant at the time of our field investigation and the ground surface appeared mostly natural. The lot is situated on a west facing hillside. The slope is moderately steep Job No. 116 157A Ce Ptech -2 -- in the rear, western, portion of the lot and transitions to gently sloping near the street. Several basalt and sandstone cobbles were observed on the ground surface. Vegetation consisted of grass and widely spaced sagebrush with pinyon pine on the upper portion of the lot. 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 5 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 May 10, 2016. Two exploratory borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The borings were advanced with 4 inch diameter continuous flight augers powered by a truck -mounted CME -45B drill rig. The borings were logged by a representative of Hepworth-Pawlak Geotechnical, Inc. Job No. 116 157A Gtech -3 - Samples of the subsoils were taken with a 1% inch I.D. spoon sampler and 2 inch California sampler. The samplers were driven into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This test is similar to the standard penetration test described by ASTM Method D-1586. The penetration resistance values are an indication of the relative density or consistency of the subsoils and hardness of the bedrock. Depths at which the samples were taken and the penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our laboratory for review by the project engineer and testing. SUBSURFACE CONDITIONS Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The subsoils consist of about 1/2 a foot of topsoil overlying 7 to 20 feet of stiff sandy silty clay. Gypsiferous claystone/siltstone was encountered below the clay at depths of 71 to 20 feet down to the bottom of the boring at 26 to 31 feet. Laboratory testing performed on samples obtained from the borings included natural moisture content and gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples, presented on Figures 4 and 5, indicate limited expansion or compressibility when wetted and low to moderate compressibility under additional loading. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils were slightly moist to moist. 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 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 Job No. 116 157A C -tech If r _4_ 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 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 2 inches could occur. Settlement in the event of subsurface wetting will likely cause building distress and mitigation methods such as a deep foundation (such as piles or piers extending down into the bedrock could be used to support the proposed house. If a deep 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 bearing on bedrock or 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 footing areas should be sub -excavated down about 4 feet below design footing grade and the excavated soil replaced compacted back to design bearing level but to a depth of at least 4 feet below footing bearing level. bedrock is encountered before excavation 4 feet below design footing grade, then the excavation can be stopped and design footing grade be re-established with compacted structural fill. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on bedrock a minimum 4 feet of compacted structural fill should be designed for an allowable bearing pressure of 1,200 psf. Based on experience, we expect initial settlement of footings designed and Job No. 116 157A Gtech -5 - constructed as discussed in this section will be about 1 inch or less. Additional settlements of about 1/2 to 11/2 inches could occur if the silt and clay soils below the bearing level become wetted. A'/3 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. 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 silty 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 1/ the fill depth below the footing. 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 Job No. 116 157A Gtech -6 - 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 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 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 Job No. 116 157A Ge Ptech -7 - 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, are suitable to support lightly loaded slab - on -grade construction. 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 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 local area that perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also create a perched condition. We recommend below -grade construction, such as retaining walls, crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. 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 Job No. 116 157A Ge Ptech _8 - 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. 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 11 feet deep. An impervious membrane such as 20 mil PVC 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. SITE GRADING The risk of construction -induced slope instability at the site appears low provided the building is located as planned and cut and fill depths are limited. We assume the cut depths for the basement level will not exceed one level, about 10 to 12 feet. Embankment fills should be compacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should be benched into the portions of the hillside exceeding 20% grade. Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or other means. The risk of slope instability will be increased if seepage is encountered in cuts and flatter slopes may be necessary. If seepage is encountered in permanent cuts, an investigation should be conducted to determine if the seepage will adversely affect the cut stability. This office should review site grading plans for the project prior to construction. 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: Job No. 116 157A Gtech _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 capped with about 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) 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 borings drilled at the locations indicated on Figure 1, the proposed type of construction and our experience in the area. Our services do not include determining the presence, prevention or possibility of mold or other biological contaminants (MOBC) developing in the future. If the client is concerned about MOBC, then a professional in this special field of practice should be consulted. Our findings include interpolation and extrapolation of the subsurface conditions identified at the exploratory borings and variations in the subsurface Job No. 116 157A Gtech - 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, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Tom C Brunner — Staff Engineer Reviewed by: Daniel E. Hardin, P. E. TCB/ksw cc: Patrick W. Stuckey Architects — Patrick Stuckey (stucarch@comcast.net) Job No. 116 157A Gtech APPROXIMATE SCALE 1"=20' 1 1 i i BORING 1 LOT 5 BUILDING SE7B ACK LINE PINYON MESA DRIVE 00 i LOT 4 BORING 2 - a) w Q a> 0 0 5 10 15 20 r— 25 30 35 BORING 1 BORING 2 ' 7/12 WC=5.1 ✓ DD=90 -200=82 ✓ 10/12 WC=10.4 1DD=84 38/12 WC=4.4 DD=109 50/2 28/12 22/6,50/4 10/12 ']WC=13.7 DD=111 , -200=85 17/12 f WC=11.4 DD=108 r r r r r r • r r r� 16/12 WC=7.8 00=102 ' -200=86 r r ' r 16/12 WC=9.4 DD=103 12/12 29/6,50/5 50/1 Note: Explanation of symbols is shown on Figure 3. 0 5 10 15 20 25 30 35 LEGEND: 7 h 7/12 NOTES: TOPSOIL; organic sandy silt and clay, firm, slightly moist, brown. CLAY (CL); sandy, silty, scattered gravel, medium stiff to very stiff, slightly moist to moist, light brown. CLAYSTONE/SILTSTONE; weathered to hard with depth, slightly moist, gypsiferous. Eagle Valley Evaporite. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1586. Drive sample blow count; indicates that 7 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. 1. Exploratory borings were drilled on May 10, 2016 with 4 -inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory borings were not measured and the logs of exploratory borings are drawn to depth. 4. The exploratory boring locations and elevations should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory boring logs represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the borings at the time of drilling. 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 Compression 0 c co a x Lu Compression 0 1 2 3 4 5 6 7 1 0 1 2 Moisture Content = 10.4 percent Dry Density = 84 pcf Sample of: Sandy Silty Clay From: Boring 1 at 5 Feet Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 4.4 Dry Density = 109 Sample of: Sandy Silty Clay From: Boring 1 at 10 Feet percent pcf Expansion upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 116157A I-1 He worth—Pawlak Geotechnical SWELL -CONSOLIDATION TEST RESULTS 100 Figure 4 Compression - Expansion % Compression % 1 0 1 2 0 1 2 3 4 5 6 Moisture Content = 11.4 percent Dry Density = 108 pcf Sample of: Sandy Silty Clay From: Boring 2 at 5 Feet Expansion upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 9.4 percent Dry Density = 103 pcf Sample of: Sandy Silty Clay From: Boring 2 at 15 Feet Compression ��,upon wetting C) 0.1 116 157A 1.0 10 APPLIED PRESSURE - ksf HEPWORTH-PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS 100 Figure 5 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 116157A SAMPLE LOCATION GRADATION 1ATTERBERG LIMITS SOIL OR BEDROCK TYPE BORING NATURAL MOISTURE DEPTH CONTENT (ft) (%) NATURAL DRY DENSITY 1 (Pcf} PERCENT GRAVEL 1 SAND PASSING (%) (%) NO. 200 1 SIEVE LIQUID LIMIT PLASTIC INDEX UNCONFINED COMPRESSIVE ' STRENGTH 1 21/2 5.1 90 1 82 Sandy Silty Clay 5 10.4 84 Sandy Silty Clay 10 4.4 109 Sandy Silty Clay 2 21/2 13.7 111 85 Sandy Silty Clay 5 11.4 108 Sandy Silty Clay 10 7.8 102 86 Sandy Silty Clay 15 9.4 103 Sandy Silty Clay