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HomeMy WebLinkAboutSoils Report 08.31.2015HEPWORTH-PAWLAK GEOTECHNICAL Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glcnvkotx1 Springs, Colorado 81601 Phone: 970.945.7988 Pax.970-945.8454 email hpg u@hpxeorcch corn SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 79, SPRING RIDGE RESERVE ELK RIDGE DRIVE GARFIELD COUNTY, COLORADO JOB NO. 115 393A AUGUST 31, 2015 PREPARED FOR: JOHN STONE P.O. BOX 3097 GLENWOOD SPRINGS, COLORADO 81602 (istone.ams @gmail.com) Parker 301-841-7119 • Colorado Springs 719-633-5562 • Silverthome 970-468-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 1 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 2 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 3 - FOUNDATIONS - 3 - FOUNDATION AND RETAINING WALLS - 4 - FLOORSLABS -6- UNDERDRAIN SYSTEM - 6 - SURFACE DRAINAGE - 7 - LIMITATIONS - 7 - FIGURE 1 - LOCATION OF EXPLORATORY BORINGS FIGURE 2 - LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Job No. i 15 393A Ggrytech PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 79, Springridge Reserve, Elk Ridge 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 John Stone dated August 24, 2015. 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 Plans for the proposed residence had not been developed at the time of our study. In general, we understand that the building will be located in the rear of the building envelope and be cut into the hillside with a walkout lower level. We assume relatively light foundation loadings typical of the general building construction. When specific building location, grading and loading information have been developed, we should be notified to re-evaluate the recommendations presented in this report. SITE CONDITIONS The site was vacant at the time of our field exploration and vegetation in the proposed building area consisted of grass. The site is moderately to strongly sloping down to the Joh No. 115 393A Gpstech -2 - southwest with on the order of 6 to 8 feet of elevation difference across the assumed building area. An irrigation ditch runs along the hillside above the proposed building area, but was dry during the time of our field exploration. There are scattered cobbles on the ground surface that increase in frequency higher on the hillside. FIELD EXPLORATION The field exploration for the project was conducted on August 25, 2015. 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 11/4 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 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 were variable and below about 1 to 2 feet of topsoil consist of 3' feet of stiff, sandy silt and clay at Boring 1. Below the topsoil at Boring 2 and at about 51/2 feet at Boring 1, relatively dense, about 2 to 5 feet of silty sandy rock fragments from gravel to cobble size were encountered. Underlying the rock fragment soils at depths of 3 to 11 feet, hard to very hard, siltstone/sandstone bedrock was encountered down to the drilled depths of about 15 to 25 feet. Job No 115 393A Ge�bech 3 Laboratory testing performed on samples obtained from the borings included natural moisture content, density, and finer than sand size gradation analyses. Results of swell - consolidation testing performed on a relatively undisturbed drive sample of the silt and clay soil, presented on Figure 4, indicate low compressibility under existing low moisture and light loading conditions and low collapse (settlement under constant load) when wetted. The sample showed high compressibility under additional loading after wetting. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling and the subsoils and bedrock were slightly moist. FOUNDATION BEARING CONDITIONS The upper silt and clay soils found in Boring 1 to a depth of 51 feet possess relatively low bearing capacity. Where encountered, this soil should be removed and the excavation extended down to the natural granular soil or bedrock. 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 granular soils and bedrock. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural granular soils and bedrock should be designed for an allowable bearing pressure of 2,000 psf. Footings placed entirely on the firm bedrock can be designed for an Job No. 115 393A G SteCh -4 - allowable bearing pressure of 4,000 psf. Based on experience, we expect settlement of footings designed and constructed as discussed in this section will be about 1 inch or less. 2) The footings should have a minimum width of 16 inches for continuous walls and 2 feet for isolated pads. 3) Exterior footings and footings beneath unheated areas should be provided with adequate soil cover above their bearing elevation for frost protection. Placement of foundations at least 36 inches below exterior grade is typically used in this area. 4) Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 10 feet. Foundation walls acting as retaining structures should also be designed to resist lateral earth pressure as presented in the "Foundation and Retaining Wall" section of this report. 5) The topsoil, silt and clay soils and any loose or disturbed soils should be removed and the footing bearing level extended down to the relatively dense natural granular soils or bedrock. 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 or well -broken bedrock. 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 Job No 115393A Ge -5- 45 pcf for backfill consisting of the on-site soils. Backfill should not contain organics or rock larger than about 6 inches. 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 near optimum moisture content. 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 material is placed correctly, and could result in distress to facilities constructed on the backfill. The Iateral 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 0.45. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The coefficient of friction and passive pressure values recommended above assume ultimate soil strength. Suitable factors of safety should be included in the design to limit the strain which will occur at the ultimate strength, particularly in the case 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. Job No 115 391A Geiztech -6 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 or well -broken bedrock 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 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, 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 I% to a suitable gravity outlet. Free -draining granular material used in the underdrain system should contain less than 2% passing the No. 200 Joh No. 115 391A Gevaech -7 - sieve, less than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at least 1' feet deep. SURFACE DRAINAGE The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. 2) Exterior backfill should be adjusted to near optimum moisture and compacted to at least 95% of the maximum standard Proctor density in pavement and slab areas and to at least 90% of the maximum standard Proctor density in landscape areas. 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 at least 2 feet of the on- site soils to reduce surface water infiltration. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation 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 Job No, 115 393A Ggritech -8 - 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 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 G :!. ,,� CAL, INC. p, "X 1622,2 :* :39/v/i r Steven L. Pawlak F.E. Reviewed by: Daniel E. Hardin P.E. SLP/ksw Job No 115393A Gerxrytech LOT 81 LOT 80 1 1 LOT 79 ----------- RR1GATjON DITCH ! ` BORING 2 • r ! 1 1 1 BORING 1 ! • ! BUILDING ENVELOPE L BENCH MARK: GROUND IN CENTER OF ROAD_ O MIDDLE OF LOT; ELEV. • . 100.0, ASSUMED. J LOT 78 APPROXIMATE SCALE 1' 60" ELK RIDGE DRIVE 115 393A HEPWORTH-PAWLAK GEQTECHMGAL LOCATION OF EXPLORATORY BORINGS Figure 1 Elevation - Feet 120 105 100 95 90 85 80 115 393A BORING 1 ELEV. 108 3 i 11/12 WC 55 DD 97 38/12 WC 27 • `` DD 124 • -200 58 88/9 • It. 50;'0 50.0 BORING 2 ELEV. 1151 90 3 5C:'1 50. :JCJ. O Note• Explanation of symbols is shown on Figure 3 Hepworth—Pawlak Geotechnical LOGS OF EXPLORATORY BORINGS 120 105 100 95 90 85 80 Elevation - Feet Figure 2 LEGEND: ® TOPSOIL; organic sandy silt and clay, brown, root zone. 2 38/12 NOTES: SILT AND CLAY (ML -CL); sandy, stiff, slightly moist, red. GRAVEL AND COBBLES (GM); silty, rock fragments, medium dense, slightly moist, red. Probable weathered bedrock with depth. SILTSTONE/SANDSTONE BEDROCK; very hard, slightly moist, red, steep bedding dip. Maroon Formation. 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 38 blows of a 140 pound hammer failing 30 inches were required to drive the California or SPT sampler 12 inches. 1. Exploratory borings were drilled on August 25, 2015 with 4 -inch diameter continuous flight power auger. 2. Locations of exploratory borings were measured approx mately 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 fogs 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 115 393A Hepworth—Pawlak Geotechnical LEGEND AND NOTES Figure 3 0 1 2 ae 3 0 0, c o. 4 E 0 0 5 6 7 8 Moisture Content ,,, 5.5 percent Dry Density 97 pcf Samp'e of: Sandy Silt and Clay From: Boring 1 at 2 Feet Compression upon wetting 0.1 1.0 APPLIED PRESSURE r ksf 10 100 115 393A Hepworth—Pawlak Geotechnical SWELL -CONSOLIDATION TEST RESULTS Figure 4 Job No.115 393A U N Z J O J u, < w U_ IX Z 1- x CO U w w ~ i- } O CC w O o,a wcc Jm0 aI--J a u_ ' o cc cc o a a 2 w 0 2 u] SOIL OR BEDROCK TYPE Sandy Silt and Clay Silt and Clay with Rock Fragments UNCONFINED COMPRESSIVE STRENGTH (PSF) ATTERBERG LIMITS PLASTIC INDEX (%) LIQUID LIMIT (%) PERCENT PASSING NO. 200 SIEVE III GRADATION a z < s. CO GRAVEL (%) NATURAL DRY DENSITY (pct] v NATURAL MOISTURE CONTENT (%) tri VI N N 11 SAMPLE LOCATION 1 DEPTH (ft) N V1 BORING 1