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HomeMy WebLinkAboutSoils Report.pdfI-II: I'WORTH-I'AWLAK (3E0 Ei iN SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE 895 COUNTY ROAD 223 (PETERSON LANE) GARFIELD COUNTY, COLORADO JOB NO. 114 203A JUNE 18, 2014 PREPARED FOR: MS. STEPHANIE GUBE 895 COUNTY ROAD 223 RIFLE, COLORADO 81650 s1cphra alcab.cun► II ;IIH+ e 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 - FLOOR SLABS - 6 - SITE GRADING - 6 - SURFACE DRAINAGE - 6 - LIMITATIONS - - FIGURE 1 - LOCATIONS OF EXPLORATORY BORINGS FIGURE 2- LOGS OF EXPLORATORY BORINGS FIGURE 3 - LEGEND AND NOTES FIGURES 4 & 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 895 County Road 223 (Peterson Lane) east of Rifle, 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 Stephanie Cube dated May 27, 2014. 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 residence will be a 1 1/2 story modular structure. Ground floor will be a structural slab over crawlspace with a slab -on -grade garage floor. Grading for the structure is assumed to be relatively minor with cut depths between about 2 to 4 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. SITE CONDITIONS The site is a relatively flat pad area approximately 900 feet west of County Road 223. The site is accessed from County Road 223 with a gravel drive. The proposed building Job No. 114 203A Ge Gtech -2 - area was previously occupied by a single family residence with a walkout basement level. The previous construction burned and has been removed. The building pad area has reportedly been cut down to the previous walkout basement level, approximately 6 feet of cut depth in areas. The pad area drops off to the north, east and south and is bordered to the west by a relatively steep cut slope rising up to a small butte. Sandstone and claystone bedrock crop out on the small butte to the west. Small stone retaining walls terrace the slope down to the south. The cut surface of the proposed building area consists of sandy clay with gravel and scattered cobbles. Debris from previous construction and stripped soil from the building area were stockpiled on the east edge of the site. An irrigation ditch runs below and south of the property. FIELD EXPLORATION The field exploration for the project was conducted on June 9, 2014. Two exploratory borings were drilled in the proposed building area, as designated by the Client, and approximately as 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 a 2 inch I.D. spoon sampler. The sampler was 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. Due to the cut surface in the proposed building pad area, topsoil was not encountered. Job No. 114 203A Ge Piech -3 - The subsoils in the building pad area consist of about 8 to 17 feet of medium stiff to stiff, silty sandy clay overlying sandstone and claystone bedrock. The bedrock varied from weathered to very hard in consistency. Depth to bedrock apparently varies considerably across the building pad area. Laboratory testing performed on samples obtained from the borings included natural moisture content and density and finer than sand size gradation analysis. Results of swell -consolidation testing performed on relatively undisturbed drive samples of the clay soils, presented on Figures 4 & 5, indicate low to moderate compressibility under conditions of loading and wetting and moderate compressibility under further 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 were slightly moist to moist and the bedrock was relatively dry. FOUNDATION BEARING CONDITIONS The silty sandy clay site soils are suitable for support of the proposed residence on shallow spread footings with some risk of settlement. Any existing fill or loose and disturbed soils should be removed and the foundation level extended down to the natural site soils. Placing the foundation entirely on bedrock could achieve a low settlement risk. 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 site soils. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings placed on the undisturbed natural silty sandy clay soils should be designed for an allowable bearing pressure of 1,500 psf. Based on Job No. 114 203A GE~&ech -4 - experience, we expect initial settlement of footings designed and constructed as discussed in this section will be about l inch or less. There could be about 'A to 1 inch of additional differential settlement under wetted conditions and precautions should be taken to keep the bearing soils dry. 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 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, debris, loose or disturbed soils should be removed and the footing bearing level extended down to the stiff 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 pef for backfill consisting of the on-site clay soils and at least 45 pcf for backfill consisting of imported granular materials. Cantilevered retaining structures which are separate from the structure 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 Job No. 114 203A Geortech • -5 - of an equivalent fluid unit weight of at least 45 pcf for backfill consisting of the on-site soils and at least 40 pcf for backfill consisting of imported granular materials. 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. Backfill should be placed in uniform lifts and compacted to at least 95% of the maximum standard Proctor density at a moisture content slightly above 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 0.40. Passive pressure of compacted backfill against the sides of the footings can be calculated using an equivalent fluid unit weight of 360 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. 114 203A Gtech -6 - FLOOR SLABS The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab - on -grade construction, such as in garage areas with some settlement potential if the bearing soils are wetted. 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. 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. SITE GRADING The risk of construction -induced slope instability at the site appears low provided the building is located as planned and cut and fill depths are limited. Embankment fills (if constructed) should be compacted to at least 95% of the maximum standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be carefully prepared by removing all vegetation and topsoil and compacting to at least 95% of the maximum standard Proctor density. The fill should be benched into the portions of the hillside exceeding 20% grade. A drainage swale should be considered as part of the civil design to route water away from the residence on the uphill side of the site. Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or flatter and protected against erosion by revegetation or other means. SURFACE DRAINAGE Positive surface drainage is an important aspect of the project to prevent wetting of the bearing soils. A foundation drain should not be provided around shallow (less than 4 feet deep) crawlspace areas. Foundation wall backfill should have adequate compaction and Job No. 114 203A Gtech -7 - positive surface slope away from foundation walls. 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. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill and foundation areas. 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 Job No. 114 203A Ge Ptech -8 - 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 GEOTECHNICAL, INC. Ja es A. Parker, P.E., P.G. Reviewed by: Steven L. Pawlak, P.E. JAP/ksw Job No. 1 14 203A Gtech BORING 2 • STEEP SLOPE UP GARAGE PROPOSED MODULAR HOME BORING 1 NOT TO SCALE: ACCESS DRIVE 114 203A Gtech HEPWORTH PAW' AK GEOTECHNIGU LOCATIONS OF EXPLORATORY BORINGS FIGURE 1 FIGURE 2 DEPTH - FEET 0 5 10 15 20 25 BORING 1 ELEV. =100' r— 5/12 WC=13.9 .— 8/12 DD=110 /— -200=68 10/12 / WC=11.4 14/12 DD=120 / 50/3 BORING 2 ELEV.=-101' NOTE: Explanation of symbols is shown on Figure 3. 0 5 10 15 20 25 DEPTH - FEET 114 203A IHEPWGerech t EOTE,CFiNIc AI. LOGS OF EXPLORATORY BORINGS FIGURE 3 LEGEND: —7 4— '04,4 CLAY — CLAY (CL); silty, sandy, medium stif to very stiff, slightly moist to moist, brown. SANDSTONE/CLAYSTONE BEDROCK; weathered to very hard, slightly moist, tan, brown, gray and purple. Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. 5/12 Drive sample blow count; indicates that 5 blows of 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. NOTES: 1. Exploratory borings were drilled on June 9, 2014 with 4 -inch diameter continuous flight power auger. 2. Locations of exploratory borings and the proposed building area were designated in the field by the Client. 3. Elevations of exploratory borings were measured by hand level and are referenced to Boring 1 as elevation 100, assumed'. The boring logs are drawn to depth. 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. 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 114 203A Ilef'wG ech ICAI_ LEGEND AND NOTES COMPRESSION (% ) COMPRESSION (% ) 5 0 1 2 3 4 5 0 Moisture Content = 13.9 percent Dry Density = 110 pcf Sample of: Sandy Silty Clay From Boring 1 at 5 Feet Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE ( ksf ) 100 APPLIED PRESSURE ( ksf ) 100 114 203A Gtech I1EPWOIZTH.PAWI.AK GEOTECHNICAL_ SWELL -CONSOLIDATION TEST RESULTS FIGURE 4 Moisture Content = 11.4 percent Dry Density = 120 pcf Sample of: Slightly Gravelly S Ity Sandy Clay From: Boring 1 at 10 Feet NN0 . Compression upon wetting Ii i 1 i n APPLIED PRESSURE ( ksf ) 100 114 203A Gtech I1EPWOIZTH.PAWI.AK GEOTECHNICAL_ SWELL -CONSOLIDATION TEST RESULTS FIGURE 4 COMPRESSION (% ) 1 2 3 4 5 0 I 1 I I Moisture Content = 13.8 percent Dry Density = 118 pcf Sample of: Silty Sandy Clay From. Boring 2 at 5 Feet ,,—.1111111111111 Compression jupon wetting • • 1 1 i i I 1, 1 1 0 in .,.,, APPLIED PRESSURE (ksf ) 114 203A Gtech I IEPWORTI I-PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS FIGURE:. 5 ECHNICAL, INC. Job No. 114 203A 1- Co CO W O • re ago J < co1 a � = 0 • } O < O. Lu x 0 N SOIL TYPE Sandy Silty Clay Slightly Gravelly Silty Sandy Clay Silty Sandy Clay UNCONFINED COMPRESSIVE STRENGTH (PSF) ATTERBERG LIMITS LIQUID PLASTIC LIMIT INDEX (%) (%) - PERCENT PASSING NO. 200 SIEVE 00 1/400 Z O f I a a N O - ga� , 6 O W a Z m 0 .r 0 .ti 00 ti NATURAL MOISTURE CONTENT (%) i 13.9 ,t • - .-i 00 Cr; 1--i LOCATION DEPTH IR) I5 .-i V'1 W J Ur a Z i a co m .y N