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HomeMy WebLinkAboutSoils Report 07.13.2016H -PKU MAR Geotechnlcal Engineering I Engineering Geology Materials Testing 1 Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Parker, Glenwood Springs, and Silverthorne, Colorado SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 72, SPRING RIDGE RESERVE PUD, PHASE 4 TBD HIDDEN VALLEY DRIVE GARFIELD COUNTY, COLORADO PROJECT NO. 16-7-121 JULY 13, 2016 PREPARED FOR: CRAIG WHITLOCK P.O. BOX 1757 GLENWOOD SPRINGS, COLORADO 81602 (sitewvest@msn.com) TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - FIELD EXPLORATION - 2 - SUBSURFACE CONDITIONS - 2 - FOUNDATION BEARING CONDITIONS - 3 - DESIGN RECOMMENDATIONS - 3 - FOUNDATIONS - 3 - FOUNDATION AND RETAINING WALLS - 4 - FLOOR SLABS - 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 FIGURES 4 AND 5 - SWELL -CONSOLIDATION TEST RESULTS TABLE 1- SUMMARY OF LABORATORY TEST RESULTS PURPOSE AND SCOPE OF STUDY This report presents the results of a subsoil study for a proposed residence to be located at Lot 72, Spring Ridge Reserve, TBD Hidden Valley 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 Craig Whitlock dated June 21, 2016. Hepworth-Pawlak Geotechnical, Inc. (now H-P/Kumar) previously performed a preliminary geotechnical study for the subdivision development and reported their findings on June 22, 2004, Job No. 101 126. 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 Building plans for the proposed residence are conceptual. Typical construction in the area consists of one and two story wood frame structures above a basement or crawlspace with an attached garage. Ground floors are typically slab -on -grade. Grading for this type of structure is assumed to be relatively minor with cut depths between about 3 to 9 feet. We assume relatively light foundation loadings, typical of the proposed type of construction. When building location, grading and loading information have been developed, we should be notified to re-evaluate the recommendations presented in this report. -2 - SITE CONDITIONS The property is vacant and vegetated mostly with grass and weeds in the proposed building area. There are scattered stands of scrub oak uphill of the building site. The ground surface slopes down to the west at a grade of about 10 percent in the building site and about 30 to 40 percent uphill to the east. An abandoned irrigation ditch crosses the site just below and parallel to the rear building envelope line where sandstone bedrock crops out. FIELD EXPLORATION The field exploration for the project was conducted on July 1, 2016. Two exploratory borings were drilled at the locations shown on Figure I 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 a 2 inch LD. spoon sampler. The sampler was driven into the subsurface materials 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, below about 6 inches of topsoil, consist of about 4 to 18 feet of sandy silty clay overlying sandstone bedrock. Drilling in the sandstone with auger equipment was difficult due to its hardness and drilling refusal was encountered in the deposit. -3 - Laboratory testing performed on samples obtained from the borings included natural moisture content and density and percent 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, generally indicate low to moderate compressibility under conditions of loading and wetting. A sample tested from Boring 1 at 2' feet showed a minor swell potential when wetted and the sample from Boring 2 at 21/2 feet showed a low collapse potential when wetted. The laboratory testing is summarized in Table 1. No free water was encountered in the borings at the time of drilling or when checked 3 days later and the subsoils were slightly moist. FOUNDATION BEARING CONDITIONS The clay soils have variable low to moderate compressibility or expansion under conditions of loading and wetting. Sandstone bedrock was encountered in Boring 2 at a depth of 4' feet with minor compressibility potential and niay be exposed in the building excavation. Good drainage away from the structure will be critical to the long term performance of the foundation. 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 below topsoil or on bedrock. The soils exposed in the building excavation should be further evaluated to settlement/heave potential and the need for ground treatment to limit foundation movement at the time of construction. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) 4 Footings placed on the undisturbed natural soils or bedrock should be designed for an allowable bearing pressure of 1,500 psf. The exposed soils tend to compress or expand when wetted and there could be on the order of 1 inch or more of post -construction settlement of the foundation if the bearing soils become wet. Care should be taken to reduce the risk of future building distress by providing good surface drainage away from the foundation with roof downspouts where provided that drain well away from the foundation. 2) The footings should have a minimum width of 18 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 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) The topsoil and any loose or disturbed soils should be removed and the footing bearing level extended down to the undisturbed natural 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. Cantilevered retaining structures which are -5 - 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 50 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 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 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 clay soils and 0.50 for sandstone bedrock. 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 -6 - 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 movement risk. 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 and 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 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 Ievel of excavation and at least 1 foot below lowest adjacent finish -7 - 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 11/2 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 covered with filter fabric and 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 IO 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 -8 - 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 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, H-RtKUMAR Louis E. Eller Reviewed by: Steven L. Pawlak, P.E. LEE/ksw M-P*KUMAR APPROXIMATE SCALE 1"=30' LOT 73 6SsoO 6s 6S N N. % 70 6b \ 65;10 N. �\ N. 6 N. N s `570 N N. \ N. N. N. \ N. N. N. \ \ \ \ 64,90] ` �N \\ \� I N. \ N. \ 6480 6470 6460 N. \ LOT 72 \ \ .,Q — 1 1 1 0 -dee 1 BORING 2 I 't 6450 1 _ - . -. I ---t 801.,,N \ I G �N�ELOp 1 1 1` , BORING 1 0 I I 1 16-7-121 HIDDEN VALLEY DRIVE KUMAR Gw:;a hflIC i Li nerarir I Ea,p,n&mi� r� 0e01.),)v py',ar lIa;i Tu55r a f GfoisysnmeaG71 LOT 71 LOCATION OF EXPLORATORY BORINGS Figure 1 a5 LL0 0 5 10 15 20 BORING 1 ELEV.-4= 6454' r 14/12 / WC=6.2 DD=100 13/12 WC=8 7 DD=99 / -200=80 16/12 WC=4 8 DD=115 ✓ f n 23/12 / r • / 50/0 BORING 2 ELEV.= 6454' 15/12 WC=3 8 DD= 106 38/12 WC=34 DD -114 -200=28 5' 10 0 5 10 15 20 25 25 16-7-121 Note: Explanation of symbols is shown on Figure 3 H-P---IKUMAR G,_ca,,,iinicn! E' j r:eedny j Eno r fl ling Gsagv},- t; a'.£rl,ri4 iH,bnp I Enmraur.lc,Ini LOGS OF EXPLORATORY BORINGS aw OCD 0 - Figure 2 LEGEND: 2 TOPSOIL; sandy silt and clay, firm, moist, brown. CLAY (CL); sandy, stiff to very stiff, slightly moist, red, low plasticity. SANDSTONE BEDROCK; weatered to very hard with depth, slightly moist, red. Maroon Formation. 14/12 T NOTES: Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. Drive sample blow count; indicates that 14 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. Practical drilling refusal, 1. Exploratory borings were drilled on July 1, 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 obtained by interpolation between contours shown on the site plan provided. 4. The exploratory boring locations and elevations should be considered accurate only to the degree implied by the method used 5. The fines 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 3 days later. Fluctuation in water level may occur with time 7. Laboratory Testing Results: WC = Water Content (%) DDT Dry Density (pcf) -200 = Percent passing No 200 sieve 16-7-121 H-P-KUMAR riuct1 i i,cli E,IW net/ii1Py Etivr,ve,i,g CAnlouy i.'.veriiYu roving i T_ Iran ;ienlai LEGEND AND NOTES Figure 3 Compression - Expansion % Compression % 1 0 1 2 3 0 1 2 3 4 Moisture Content = 6,2 percent Dry Density = 100 pcf Sample of: Sandy Clay From: Boring 1 at 2 Y2 Feet ,N\ Expe upor wetti nsic ig 11 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 4.8 percent Dry Density -- 115 pcf Sample of: Sandy Silty Clay From: Boring 1 at 10 Feet Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 16-7-121 F -� P Ili MAR SWELL -CONSOLIDATION TEST RESULTS Figure 4 0 •0 c 0 0 0 a 2 E 0 U 3 4 5 6 7 8 Moisture Content 3.8 percent Dry Density 106 pcf Sample or, Sandy Silty Clay From Boring 2 at 2 1/2 Feet i ACI Compression upon wetting J N\ 01 1:0 APPLIED PRESSURE - ksf 10 100 16-7-121 H -P KUMAR SWELL -CONSOLIDATION TEST RESULTS Figure 5 H-P_KUMAR TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 16-7-121 SAMPLE LOCATION NATURAL MOISTURE CONTENT re) NATURAL DRY DENSITY(%) (pci) GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH (PSF) SOIL OR BEDROCK TYPE BORING DEPTH (ft) GRAVEL SAND (%) LIQUID LIMIT (%) PLASTIC INDEX v em 1 21/2 6.2 100 Sandy Clay 5 8.7 99 80 Sandy Silty Clay 10 4.8 115 Sandy Silty Clay 2 2'/z 3.8 106 Sandy Silty Clay 5 3.4 114 28 Weathered Sandstone I