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HomeMy WebLinkAboutSoils Report 01.27.2020IC+A Kumar & Associates, Inc.® Geotechnical and Materials Engineers and Environmental Scientists An Employee Owned Company 5020 County Road 154 Glenwood Springs, CO 81601 phone: (970) 945-7988 fax: (970) 945-8454 email: kaglenwood@kumarusa.com www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 294, IRONBRIDGE DEVELOPMENT RIVER BEND WAY GARFIELD COUNTY, COLORADO PROJECT NO. 18-7-255.01 JANUARY 27, 2020 PREPARED FOR: KENDRICK DEVELOPMENT, LLC ATTN: WES COLE 430 IRONBRIDGE DRIVE GLENWOOD SPRINGS, COLORADO 81601 wcole(2 ironbrid2ectub.com 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 NONSTRUCTURAL FLOOR SLABS - 6 - UNDERDRAIN SYSTEM - 7 - SURFACE DRAINAGE - 7 - LIMITATIONS - 8 - FIGURE 1 - LOCATION OF EXPLORATORY BORING FIGURE 2 - LOG OF EXPLORATORY BORING FIGURE 3 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS Kumar & Associates, Inc. ® Project No. 18-7-255.01 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot 294, Ironbridge Development, River Bend Way, 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 Kendrick Development, LLC dated January 22, 2020. Hepworth-Pawlak Geotechnical (now Kumar & Associates) previously performed a preliminary geotechnical study for the Ironbridge Villas and presented the findings in a report dated September 14, 2005, Job No. 105 115-6. On April 11, 2018, H-P/Kumar (aka Kumar & Associates) drilled an exploratory boring 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 Design level plans were not available for this subsoil study. We understand the proposed structure on Lot 294 will be a single story, 1,800 ft2 residence. Cut depths are expected to range between about 2 to 3 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. If building conditions or foundation loadings are significantly different from those described above, we should be notified to re-evaluate the recommendations presented in this report. SITE CONDITIONS The site was vacant and vegetated with grass and weeds at the time of our field exploration. The site was covered with about 'A to 1 foot of snow at the time of this report. The terrain is Kumar & Associates, Inc. ® Project No. 18-7-255.01 -2 - relatively flat with an estimated 1 foot of elevation difference across the building envelope. A two tiered-small-.b1ock-retaini.ag_waItand-single=family-residences_are-to-the-west,-single family residences are to the north, south, and east, and River Bend Way and the Roaring Fork River are to the east. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge 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. A sinkhole opened in the cart storage parking lot located east of the Pro Shop and west of the Villas North parcel in January 2005. Other irregular bedrock conditions have been identified in the affordable housing site located to the northwest of the Villas North parcel. Irregular surface features that could indicate an unusual risk of future ground subsidence were not observed in the Villas North parcel, but localized variable depths of debris fan soils and bedrock quality encountered by the previous September 14, 2005 geotechnical study in the Villas North development area could be the result of past subsidence. The subsurface exploration performed west of the proposed residence on Lot 294 did not encounter voids but the alluvial fan depth encountered was considerably greater than encountered on nearby lots which could indicate past ground subsidence. In our opinion, the risk of future ground subsidence on Lot 294 throughout the service life of the proposed residence is low and similar to other areas of the Roaring Fork River valley where there have not been indications of ground subsidence, but 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 April 11, 2018. One exploratory boring was drilled at the location shown on Figure 1 to evaluate the 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 HP Kumar (aka Kumar & Associates). Kumar & Associates, Inc. ® Project No. 18-7-255.01 3 Samples of the subsoils were taken with 1% inch and 2 inch I.D. spoon samplers. The samplers wereddriven-into the-subsoils-aocariaus-depths-with-blows_from-a_L4a-pound-hamcner fa.iiing3f 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. 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 consist of about 4 feet of medium dense, silty clayey sand and gravel fill overlying about 15 feet of stiff, sandy silt and clay, underlain by dense, slightly silty sandy gravel and cobbles with probable boulders. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles and probable boulders and drilling refusal was encountered in the deposit. Laboratory testing performed on samples obtained from the boring included natural moisture content and density, finer than sand size gradation analyses and unconfined compressive strength. Results of swell -consolidation testing performed on a relatively undisturbed drive sample of the silt and clay soil, presented on Figure 3, indicate low to moderate compressibility under conditions of loading and wetting. The laboratory testing is summarized in Table 1. No free water was encountered in the boring at the time of drilling and the subsoils were slightly moist to moist with depth. FOUNDATION BEARING CONDITIONS Approximately 4 feet of fill from overlot grading was encountered in our boring. The residence to the south, Lot 293, was constructed with an 8 to 9 -foot deep basement level. Backfill used for the basement may be encountered during foundation excavations for the proposed residence on Lot 294. The natural silt and clay soils encountered below the fill possess a relatively low bearing capacity and a low to moderate risk of settlement. The amount of settlement will depend on the thickness of the compressible soils due to potential collapse when wetted, and potential compression of the underlying soils after wetting. Properly compacted structural fill will also Kumar & Associates, Inc. ® Project No. 18-7-255.01 4 have some potential for long term settlement but should be considerably less than the underlying natural-silt_and_claxsail.. Sources..otwetting_includeeirrigatian,--surface water runo£1=and-utility line leaks. A heavily reinforced structural slab or post -tensioned slab foundation designed for significant differential settlements is recommended for the building support. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory boring and the nature of the proposed construction, we recommend the building be founded with a heavily reinforced structural slab foundation bearing on about 3 feet of compacted structural fill. A post -tensioned slab foundation could also be used. The design and construction criteria presented below should be observed for a shallow foundation system. 1) A heavily reinforced structural slab placed on about 3 feet of structural fill should be designed for an allowable bearing pressure of 1,500 psf. A post -tensioned slab if used should be designed for a wetted distance of 10 feet but at least half of the slab width, whichever is more. Based on experience, we expect initial settlement of the slab foundation designed and constructed as discussed in this section will be about 1 inch or less. Additional settlement could occur if the bearing soils were to become wetted. The magnitude of the additional settlement would depend on the depth and extent of wetting but may be on the order of 1 to 1'/2 inches. 2) The thickened sections of the slab for support of concentrated loads should have a minimum width of 20 inches. 3) The perimeter turn -down section of the slab should be provided with adequate soil cover above the bearing elevation for frost protection. Placement of foundations at least 36 inches below exterior grade is typically used in this area. If a frost protected foundation is used, the perimeter turn -down section should have at least 18 inches of soil cover. 4) The foundation should be constructed in a "box -like" configuration rather than with irregular extensions which can settle differentially to the main building area. Kumar & Associates, Inc. ® Project No. 18-7-255.01 -5 - The foundation walls, where provided, should be heavily reinforced top and hottom-to.span_local-anomalies..such- hy_assuming.an. unsupported.length of .t least 14 feet. Foundation walls acting as retaining structures (if any) should also be designed to resist lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) The organic root zone and any loose or disturbed soils should be removed. Additional structural fill placed below the slab bearing level should be compacted to at least 98% of the maximum standard Proctor density at a moisture content near optimum and can consist of the onsite or similar soils. 6) A representative of the geotechnical engineer should evaluate the compaction of fill materials 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 Kumar & Associates, Inc. ® Project No. 18-7-255.01 -6 this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall backf i 1 l. -should -be expected, even -i -the-mateeialas-placed.correctly,_and_could.-result,in..distress to facilities constructed on the backfill. Backfill should not contain organics, debris or rock larger than about 6 inches. 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 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. NONSTRUCTURAL FLOOR SLABS Compacted structural fill can be used to support lightly loaded slab -on -grade construction separate from the building foundation. The fill soils can be compressible when wetted and result in some post -construction settlement. To reduce the effects of some differential movement, slabs -on -grade should be separated from the building to 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 well -graded sand and gravel, such as road base, should be placed beneath slabs for support. 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. 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. Kumar & Associates, Inc. ® Project No. 18-7-255.01 7 UNDERDRAIN SYSTEM It is our understanding that the finished floor elevation at the lowest level of the proposed residence will be at or above the surrounding grade. Therefore, a foundation drain system is not required. 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, be protected from wetting and hydrostatic pressure buildup by an underdrain system. If finished floor elevation of the proposed residence has a floor level below the surrounding grade, we should be contacted to provide recommendations for an underdrain system. All earth retaining structures should be properly drained. SURFACE DRAINAGE Providing proper surface grading and drainage is important to limiting wetting of bearing soils and potential building distress. 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 6 inches in the first 5 feet in unpaved areas and a minimum slope of 21/2 inches in the first 10 feet in paved areas. 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. Kumar & Associates, Inc. ® Project No. 18-7-255.01 8 LIMITATIONS 1. 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 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 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, Kumar & Associates, inc. Shane J. Robat, P.E. Reviewed by: Steven L. Paw ak, SJR/kac Kumar Project No. 1 LOT 295 (VACANT) BORING 1 ■ LOT 294 Fl 7r-r'" LOT/293 - -- (EXISTING RESIDENCE WITH BASEMENT) 10 0 is 20 APPROXIMATE SCALE—FEET 18-7-255.01 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1 BORING 1 LEGEND 0 5 — 10 — 15 20 25 0 01 1 33/12 10/12 WC=9.4 DD=102 — 200=78 9/12 WC=7.7 DD=112 10/12 WC=13.5 DD=111 — 200=81 UC=2,600 56/12 7 / FILL: SILTY CLAYEY SAND AND GRAVEL, MEDIUM DENSE, SLIGHTLY MOIST, BROWN. SILT AND CLAY (ML—CL); SANDY, STIFF, SLIGHTLY MOIST TO MOIST WITH DEPTH, BROWN. GRAVEL AND COBBLES (GM—GP); SLIGHTLY SILTY, SANDY, PROBABLE BOULDERS, DENSE, SLIGHTLY MOIST, BROWN, ROUNDED ROCK. DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE. 33/12 DRIVE SAMPLE SLOW COUNT. INDICATES THAT 33 BLOWS OF A 140—POUND HAMMER FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES. I PRACTICAL AUGER DRILLING REFUSAL. NOTES 1. THE EXPLORATORY BORING WAS DRILLED ON APRIL 11, 2018 WITH A 4—INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER. 2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED APPROXIMATELY BY PACING. 3. THE ELEVATION OF THE EXPLORATORY BORING WAS NOT MEASURED AND THE LOG OF THE EXPLORATORY BORING IS PLOTTED TO DEPTH. 4. THE EXPLORATORY BORING LOCATION 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 THE TRANSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORING AT THE TIME OF DRILLING. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (%) (ASTM D 2216); DD = DRY DENSITY (pcf) (ASTM D 2216); —200 = PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140); UC = UNCONFINED COMPRESSIVE STRENGTH (ASTM D 2166). 18-7-255.01 Kumar & Associates LOG OF EXPLORATORY BORING Fig. 2 1 4 �r. CONSOLIDATION - SWELL 1 0 1 — 2 — 3 —4 10 APPLIED PRESSURE - KSF 10 1 DO 18-7-255.01 Kumar & Associates SWELL -CONSOLIDATION TEST RESULTS Fig. 3 SAMPLE OF: Sandy Silt and Cfay `FRO : ar ng OTO' WC = 7.7 %, DD = 112 pcf NO MOVEMENT UPON WETTING Mode lest melte an only t the pro Iniad. Th. 1e117nq npoct YwC not be ropmduced, i ce I In lull, without the written approval of Kumon end NeonI tw, Inc. Swell ccordantlon WuUc performed In accordance with ASiY 0-4M& 10 APPLIED PRESSURE - KSF 10 1 DO 18-7-255.01 Kumar & Associates SWELL -CONSOLIDATION TEST RESULTS Fig. 3 Kumar & Associates, Inc."' Geotechnical and Materials Engineers and Environmental Scientists TABLE 1 SUMMARY OF LABORATORY TEST RESULTS SAMPLE LOCATION NATURAL MOISTURE I CONTENT (%) NATURAL DRY DENSITY (pcf) GRADATION PERCENT PASSING NO. SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH (Dsfl + SOIL TYPE BORING DEPTH (ft) GRAVEL (%) SAND (%)200 LIQUID LIMIT (%) PLASTIC INDEX (%) 1 5 9.4 102 78 Sandy Silt and Clay 10 7.7 112 Sandy Silt anti Clay 15 13.5 111 81 2,600 Sandy Silt and Clay