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HomeMy WebLinkAboutSoils Report 10.22.2019I ltm& tlissrre, Irk.® +� 6 eotechnicafuar and Materials Engineers arid 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.coin 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 C5, ASPEN GLEN THUNDERSTORM AND RIVER'S BEND GARFIELD COUNTY, COLORADO PROJECT NO. 19-7-597 OCTOBER 22, 2019 PREPARED FOR: LINDA CROTEAU 18 BUFFALO CARBONDALE, COLORADO 81623 tindacroteau[acon3cast.net 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 - 3 - DESIGN RECOMMENDATIONS - 4 - FOUNDATIONS - 4 - FOUNDATION AND RETAINING WALLS - 5 - FLOOR SLABS - 6 - UNDERDRAIN SYSTEM - 7 - SURFACE DRAINAGE - 7 - LIMITATIONS - 8 - 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 TABLE 1- SUMMARY OF LABORATORY TEST RESULTS Kumar & Associates, Inc. Project No. 19-7-597 PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located on Lot C5, Aspen Glen, Thunderstorm and River's Bend, 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 Linda Croteau dated October 7, 2019. Chen -Northern, Inc. previously conducted a preliminary geotechnical study for preliminary plat design under their Job No. 4 112 92, dated December 20, 1991 and May 28, 1993. 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 and we understand our findings will be considered in the purchase of the lot. For the purpose of our study, we assume the residence will be a 1 and 2 -story structure above crawlspace or basement with a garage at the main level. Floors could be slab -on -grade or structural above crawlspace. Grading for the structure is assumed to be relatively minor with cut depths between about 3 to 10 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. Kumar & Associates, Inc. ® Project No. 19-7-597 -2 - SITE CONDITIONS The lot is located on the southeast corner of Thunderstorm and River's Bend as shown on Figure 1. The ground surface is relatively flat and nearly level to gently sloping across the lot. The ground surface appears to have been graded and possibly stripped of topsoil. Vegetation consisted of grass and weeds. 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. Several sinkholes were observed by Chen -Northern scattered throughout the Aspen Glen property during the subdivision development. These sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River valley. The lot is not located within a broad subsidence area and sinkholes were not observed in the immediate area of the subject lot. The closest mapped sinkhole within a broad subsidence area is located about 800 feet north 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 C5 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 October 15, 2019. 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 - Kumar & Associates, Inc.Project No. 19-7-597 -3 - mounted CME -45B drill rig. The borings were logged by a representative of Kumar & Associates. 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 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 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, below about one foot of root zone, consist of about 8 feet of medium stiff to stiff, silty sandy clay overlying dense, slightly silty sandy gravel and cobbles with boulders. Drilling in the coarse 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 and finer than sand size gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples of the clay soils, presented on Figures 4 and 5, indicate low to moderate compressibility under light loading and existing moisture condition and low expansion or minor collapse potential (settlement under constant load) when wetted. The samples showed moderate to 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. The soil moisture was low in Boring 1 and much higher in Boring 2. FOUNDATION BEARING CONDITIONS The soils encountered at proposed excavation depths consist of low bearing clay with variable expansion/compression potential mainly when wetted. It has been our experience the clay soils in this area are prone to compression rather than expansion upon wetting. Lightly loaded spread Kumar & Associates, Inc. Project No. 19-7-597 natural clay soils with a risk of settlement as described below, -4 - footings placed on the natural soils can be used for building support with a risk of settlement mainly if the bearing soils become wetted. Compacted structural fill could be used below shallow footings such for the garage and crawlspace areas to help mitigate the settlement potential. Structural fill should consist of low permeable soils such as the onsite clay to limit penetration of water to the lower natural soils. DESIGN RECOMMENDATIONS FOUNDATIONS Considering the subsurface conditions encountered in the exploratory borings and the nature of the proposed construction, the building can be founded with spread footings bearing on the If the risk of settlement and building distress is not acceptable, a deep foundation extending down to the dense gravel and cobble soils should be used. 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. 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 settlement of around 1/2 to 1 inch is possible if the underlying soils are wetted and would likely be differential between shallow and deeper bearing levels and the slightly moist to moist soil areas. 2) The footings should have a minimum width of 20 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. Foundation walls acting as retaining structures should also be designed to resist Kumar & Associates, Inc. Project No. 19.7.597 -5 - lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) The organic soils and loose or disturbed soils should be removed down to the firm natural clay soils. The exposed soils in footing area should then be evaluated for expansion/compression potential and the need for mitigation such as removal and placement of structural fill. Structural fill placed below footing areas should extend horizontally out from the edge of the footing to a distance equal to at least Y2 the depth of fill below the footing and be compacted to at least 98% of standard Proctor density at near optimum moisture content. 6) A representative of the geotechnical engineer should evaluate structural fill for compaction and observe all footing excavations for bearing conditions prior to concrete placement. 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 (if any) 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. 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. Kumar & Associates, Inc. Project No. 19-7.597 -6 - 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. Backfill consisting of a granular soil such as road base and compaction to at least 98% standard Proctor density could be used to reduce the settlement risk. 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 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, can be used to support lightly loaded slab -on -grade construction with a risk of movement mainly if the bearing soils are wetted similar to that described above for footings. 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. Kumar & Associates, Inc. Project No. 19-7.597 -7 - We recommend vapor retarders conform to at least the minimum requirements of ASTM E1745 Class C material. Certain floor types are more sensitive to water vapor transmission than others. For floor slabs bearing on angular gravel or where flooring system sensitive to water vapor transmission are utilized, we recommend a vapor barrier be utilized conforming to the minimum requirements of ASTM E1745 Class A material. The vapor retarder should be installed in accordance with the manufacturers' recommendations and ASTM E1643. 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, non -perforated sump or drywell based in the underlying gravel and cobble deposit. 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. 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. SURFACE DRAINAGE Proper grading and drainage will be very important to keeping the bearing soils dry and limiting the building settlement and potential 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. Kumar & Associates, Inc.'" Project No. 19-7-597 -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 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. 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 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 Kumar & Associates, Inc.'' Project No. 19-7-597 -9 - 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, in Steven L. Pawl Reviewed by: Daniel E. Hardin, P.E. SLP/kac Kumar & Associates, Inc. Project No. 19-7-597 LOT C6 20 0 20 40 APPROXIMATE SCALE -FEET LOT C1 i i i i • • I \ \\ •` i \ �. f I BORING 1 I L // f / // e. r �/ // � / . 1 / // L / / . // / I I \ • • \ • • LOT C5 \ \ \•` I \ I i \ \\ s` BORING 2 \ \\ \ \ • 1 I / \ •• ,� LOT C4 13th GREEN / 19-7-597 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1 O 01 0 - 5 - 10 BORING 1 18/12 WC=6.0 DD=110 14/12 WC=6.9 DD=107 -200=85 50/4 BORING 2 5/12 WC=14.6 DD=99 -200=86 UC=2,700 5/12 WC=19.5 DD=102 50/5 0 5 10 — 15 15 20 20 19-7-597 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2 LEGEND f ROOT ZONE; ORGANIC SANDY SILT AND CLAY, RED—BROWN. CLAY (CL); SILTY, SANDY, SLIGHTLY MOIST AND STIFF IN BORING 1, MOIST AND MEDIUM STIFF IN BORING 2, RED. GRAVEL AND COBBLES (GM—GP); SLIGHTLY SILTY, SANDY, BOULDERS, DENSE, SLIGHTLY MOIST, BROWN, ROUNDED ROCK. DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE. DRIVE SAMPLE, 1 3/8—INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST. 18/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 18 BLOWS OF A 140—POUND HAMMER FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES. t PRACTICAL AUGER REFUSAL. NOTES 1. THE EXPLORATORY BORINGS WERE DRILLED ON OCTOBER 15, 2019 WITH A 4—INCH—DIAMETER CONTINUOUS—FLIGHT POWER AUGER. 2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE NOT MEASURED AND THE LOGS OF THE EXPLORATORY BORINGS ARE PLOTTED TO DEPTH. 4. THE EXPLORATORY BORING LOCATIONS 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 THE TRANSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (%) (ASTM D2216); DD = DRY DENSITY (pcf) (ASTM D2216); —200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140); UC = UNCONFINED COMPRESSIVE STRENGTH (psf) (ASTM D 2166). 19-7-597 Kumar & Associates LEGEND AND NOTES Fig. 3 2 1 .. 0 CONSOLIDATION - SWELL —1 —2 'fh. $ lest emit. ,ppy only to the edmpp. tooted. The toeing report oholl not be reproduced. eecopt In full, without the written epprovW of Ifumm end Iheneloler. Inc. S7lei Conrakdateon lee!'wg performed in otterdonne with ASTIR p-4546. SAMPLE OF: Sandy Silty Clay FROM: Boring 1 ® 2.5' WC = 6.0 %, DD = 110 pcf EXPANSION UNDER CONSTANT PRESSURE UPON WETTING 1.0 APPLIED PRESSURE - KSF 10 100 19-7-597 Kumar & Associates SWELL—CONSOLIDATION TEST RESULT Fig. 4 F CONSOLIDATION - SWELL 1 —2 — 3 — 4 —5 —6 —7 Theea Lut resulte appy only to 11. 'ample tested. The telling report eboM not be reproduced, enePt in tub, wbhout the written approval of Kumar and Aesoctotes, Inc. Swell Gcneeildelion testing performed in occardonca Witt, ASV D -43t6 SAMPLE OF: Sandy Silty Clay FROM: Boring 2 ® 5' WC = 19.5 %, DD = 102 pcf ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING 1.0 APPLIED PRESSURE — KSF 10 19-7-597 Kumar & Associates SWELL—CONSOLIDATION TEST RESULT Fig. 5 '� I ni Associates, Is E Q GeotOehniCal and Materials Engineers and Environmental Scientists TABLE 1 SUMMARY OF LABORATORY TEST RESULTS ProEect No. 19 -7 - SAMPLE LOCATION NATURAL MOISTURE CONTENT (%) NATURAL DRY DENSITY (.v. (Peg GRADATION 1 PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH (psf} SOIL TYPE BORING DEPTH (ft) GRAVEL , (/o) SAND o (/u) LIQUID LIMIT (%) PLASTIC INDEX !a (%) 1 2'/2 6.0 110 Sandy Silty Clay 5 6.9 107 85 Sandy Silty Clay 2 2'/2 14.6 99 86 2,700 Sandy Silty Clay 5 19.5 102 Sandy Silty Clay I