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HomeMy WebLinkAboutSoils Report 11.26.2007HI P GeOlteCh HEPWORTH - PAWLAK GEOTECHNICAL Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 email: hpgeo@hpgeotech.com SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED DUPLEX LOT D-23, ASPEN GLEN SUBDIVISION ELK TRACK LANE GARFIELD COUNTY, COLORADO JOB NO. 107 0818 NOVEMBER 26, 2007 PREPARED FOR: JORDAN ARCHITECTURE ATTN: BRAD JORDAN P.O. BOX 1031 GLENWOOD SPRINGS, COLORADO 81602 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 - 2 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION - 3 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 4 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4 - FOUNDATION AND RETAINING WALLS - 5 - FLOOR SLABS - 6 - UNDERDRAIN SYSTEM - 7 - SURFACE DRAINAGE - 7 - LIMITATIONS - 8 - REFERENCES - 9 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURES 4 and 5 - SWELL -CONSOLIDATION TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed duplex residence to be located on Lot D-23, Aspen Glen Subdivision, Elk Track Lane, 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 Jordan Architecture dated October 26, 2007. Chen -Northern, Inc. (1991 and 1993) previously conducted preliminary geotechnical engineering studies for the development and preliminary plat design. A field exploration program consisting of exploratory borings was conducted to obtain information on the subsurface conditions. Samples of the subsoils obtained during the field exploration were tested in the laboratory to determine their classification, compressibility or swell and other engineering characteristics. The results of the field exploration and laboratory testing were analyzed to develop 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 duplex will be a one and two wood frame structure over a partial basement level and partial crawlspace located in the area shown on Figure 1. The attached garage and basement floors will be slab -on -grade. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 8 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. 107 0818 GPis�YeCh -2 - SITE CONDITIONS Lot D-23 is located in the eastern part of the development and was vacant at the time of our field exploration. The lot is off of Elk Track Lane cul-de-sac between the road and the 12th Fairway to the east. The ground surface in the building area is relatively flat with a gentle slope down to the southwest and about 2 to 3 feet of elevation difference. The lot has been disturbed by past grading for the golf course, including a landscape berm and pond at the northeast, rear side of the lot. An open ditch flows along the southern property line. The pond appears to be lined. Vegetation consists of grass and weeds. Scattered cobbles were exposed on the ground surface. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen 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 studies for the subdivision (Chen -Northern, 1991 and 1993), several sinkholes were observed scattered throughout the Aspen Glen development, mainly east of the Roaring Fork River. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River Valley. The closest mapped sinkhole to Lot D-23 is located about 730 feet to the west and the perimeter of a broad subsidence area that contains the sinkhole is about 240 feet from the lot. 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 borings were 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 D-23 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. 107 0818 -3 - FIELD EXPLORATION The field exploration for the project was conducted on November 1, 2007. 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. Samples of the subsoils were taken with 1% inch and 2 inch I.D. spoon samplers. The samplers were driven into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This test is sirriilar 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. 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 foot of topsoil overlying stiff to very stiff, sandy silty clay to depths of 141/2 and 91/2 feet in Borings 1 and 2, respectively. Relatively dense, slightly silty sandy gravel, cobbles and boulders was encountered below the clay soils to the drilled depths of 12 and 151A feet. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit. Laboratory testing performed on samples obtained from the borings included natural moisture content and density. Results of swell -consolidation testing performed on a relatively undisturbed drive sample of the clay soils, presented on Figures 4 and 5, generally indicate low to moderate compressibility under conditions of loading and wetting. The sample from Boring 2 at 5 feet showed a low to moderate expansion potential when wetted under a constant light surcharge. Job No. 107 0818 -4 - No free water was encountered in the borings at the time of drilling or when checked 13 days later and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The upper clay soils have low bearing capacity with variable settlement/heave potential. These soils are typically known to be compressible under loading when wetted and the expansion potential indicated by the laboratory testing can be neglected in the design but the settlement/heave potential of the clay soils should be further evaluated at the time of construction. The underlying gravel and cobble soils have moderate bearing capacity and minor settlement potential. The groundwater level in the area is relatively deep and a basement level should be feasible with little risk of groundwater impacts. We assume that the pond to the northeast and the ditch to the south of the building site are lined and will have no impact on the development. The lined condition and water management plan should be confirmed with the developer. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, we recommend the building be founded with spread footings bearing on the natural soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural soils should be designed for an allowable bearing pressure of 1,500 psf. Footings or piers that bear entirely on the dense gravel and cobble soils can be designed for an allowable bearing pressure of 4,000 psf. Based on experience, we expect initial settlement of footings designed and constructed as discussed in this section will be up to about 1 inch there could be additional differential movements on the order oft to 1 inch if the bearing soils are wetted. 2) The footings should have a minimum width of 18 inches for continuous walls and 2 feet for isolated pads. Job No. 107 0818 Gtech -5- 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 12 feet. 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) Any existing fill, topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the firm natural soils. The exposed soils in footing area should then be moistened and compacted. 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 vegetation, topsoil 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 walls. Job No. 107 0818 GA-Fitech G 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 for the clay soils and 0.5 for the gravel and cobble soils. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 300 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, are suitable to support lightly loaded slab - on -grade construction. The clay soils have variable settlement/heave potential when wetted which could result in some slab movement and distress if the bearing soils become wet. 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 Job No. 107 0818 -7 - 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 area and where clay soils are present 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, 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 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, sump and pump or drywell that outlets in the underlying gravel soils. 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 11/2 feet deep. Where footings bear on the upper clay soils and an underdrain is needed, 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 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. 107 0818 -8- 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 6 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 silt and clay 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 and sprinkler heads should be located at least 5 feet from foundation walls. 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 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 arc not responsible for technical interpretations by others of our information. As the project evolves, we should provide continued consultation and field services during Job No. 107 0818 -9 - 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. Jordy Z. Adamson, Jr., P.E. Reviewed by: Steven L. Pawlak, P.E. JZA/vad REFERENCES Chen -Northern, 1991, Preliminary Geotechnical Engineering Study, Proposed Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated December 20, 1991, Job No. 4 112 92. Chen -Northern, Inc., 1993, Geotechnical Engineering Study for Preliminary Plat Design, Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated May 28, 1993, Job No. 4 112 92. Job No. 107 0818 EXISTING POND --"*":416 APPROXIMATE SCALE 1'-40' GOLF COURSE LOT D26 ELK TRACK LANE 107 0818 LOT D23 1 1 1 I 1 I 1 1 I 1 1 1 1 1 BORING 2 ■ BENCH MARK: ELECTRICAL BOX CONCRETE SLAB; ELEV. = 100.0', ASSUMED. LOT D24 He. worth -Pail Goat e nIcaI LOT D22 LOT D21 LOCATION OF EXPLORATORY BORINGS Figure 1 Elevation - Feet f-- 105 100 95 90 85 BORING 1 ELEV.= 102.5' 8/12 8/12 WC=13.7 DD=100 8/12 WC=16.1 DD=108 26,6,28/2 BORING 2 ELEV.= 99.7' 7/12 WC=7.2 DD=86 16/12 WC=8.3 DD=99 38/6,8/0 Note: Explanation of symbols is shown on Figure 3. 105 100 95 — 90 85 Elevation - Feet 107 0818 Hepworth-Pawlak C °technIcal LOGS OF EXPLORATORY BORINGS Figure 2 LEGEND: 2 Topsoil; sandy silty clay, organics, roots, firm, slightly moist, brown. Clay (CL); silty, sandy, stiff to very stiff, slightly moist, reddish brown. Gravel (GM); silty, sandy, with cobbles and boulders,derise, slightly moist, brown. L 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. 8/12 Drive sample blow count; indicates that 8 blows of a 140 pound hammer falling 30 inches were required to drive the California or SPT sampler 12 inches. T Practical drilling refusal. Depth at which boring had caved when checked on November 14, 2007. NOTES: 1. Exploratory borings were drilled on November 1, 2007 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 measured by instrument level and refer to the Bench Mark shown on Figure 1. 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 or when checked 13 days later. Fluctuation in water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) 107 0818 if-li Hepworth -Pawls v ;vol LEGEND AND NOTES Figure 3 Compression % Compression % 0 1 2 3 4 0 1 2 3 4 Moisture Content = 13.7 percent Dry Density = 100 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 = 16.1 percent Dry Density = 108 pcf Sample of: Sandy Silty Clay From: Boring 1 at 10 Feet / Compression upon '''e wetting \\(\....:."'e i 0.1 1.0 10 APPLIED PRESSURE - ksf 100 107 0818 H Hepworth—Pawlak Geotachnlao1 SWELL -CONSOLIDATION TEST RESULTS Figure 4 0 X w 0 c a w 0 1 2 4 5 6 7 2 1 0 2 .41 0.1 Moisture Content = 7.2 percent Dry Density — 86 pcf Sample of: Sandy Silty Clay From: Boring 2 at 2 Feet Expansion upon wetting 1.0 10 APPLIED PRESSURE - ksf 1 100 Moisture Content = 8.3 percent Dry Density = 99 pcf Sample of: Sandy Silty Clay From: Boring 2 at 5 Feet Expansion upon wetting 0.1 107 0818 1.0 10 APPLIED PRESSURE - ksf H Hepworth—Pawl c tectrnlcof SWELL -CONSOLIDATION TEST RESULTS 100 Figure 5