Loading...
HomeMy WebLinkAboutSubsoil StudyGtech HEPWORTH-PAWLAK GEOTECHNICAL SUBSOIL STUDY FOR FOUNDATION DESIGN PROPOSED RESIDENCE LOT 48, PINYON MESA CLIFFROSE WAY AND PINYON MESA DRIVE GARFIELD COUNTY, COLORADO JOB NO. 114 038A MARCH 7, 2014 PREPARED FOR: CRAIG RATHBUN C/O THE FLEISHER COMPANY P.O. BOX 3528 BASALT, COLORADO 81621 era igy(a)thelicisltvreo in pain .coin Parker 30 L -S41- 7111) • CI do id(5 191tv; 71041 1.;,(,' • SIIv rihitrn 970-4M-1989 TABLE OF CONTENTS PURPOSE AND SCOPE OF STUDY - 1 - PROPOSED CONSTRUCTION - 1 - SITE CONDITIONS - 2 - SUBSIDENCE POTENTIAL - 2 - FIELD EXPLORATION - 3 - SUBSURFACE CONDITIONS - 3 - FOUNDATION BEARING CONDITIONS - 4 - DESIGN RECOMMENDATIONS - 5 - FOUNDATIONS - 5 - FOUNDATION AND RETAINING WALLS - 6 - FLOOR SLABS 7 - UNDERDRAIN SYSTEM - 8 - SURFACE DRAINAGE 8 - LIMITATIONS - 9 - FIGURE 1 - LOCATION OF EXPLORATORY BORING FIGURE 2 - LOG OF EXPLORATORY BORING FIGURE 3 - LEGEND AND NOTES FIGURE 4 - 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 48, Pinyon Mesa, Cliffrose Way and Pinyon Mesa 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 Rathbun dated February 17, 2014. We previously performed preliminary geotechnical engineering studies for the subdivision development and presented our findings in reports dated November 11, 2005 and April 10, 2006, Job No. 105 652. Additionally, we have conducted geotechnical studies for several homes in the subdivision including those for Lots 43 and 44. A field exploration program consisting of an exploratory boring 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 one and two story, wood frame structure located in the building envelope of the lot as shown on Figure 1. Ground floor will be slab -on -grade in the garage and structural above crawlspace in the living area. Grading for the structure is assumed to be relatively minor with cut and fill depths between about 3 to 6 feet. We assume relatively light foundation loadings, typical of the assumed type of construction. Job No. 114 038A -2 - 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 subdivision is located on a relatively flat topographic bench of alluvial fan deposits above the Roaring Fork River valley and below Spring Valley. The site is located at the base of a steep northwest facing hillside. Vegetation has been removed from the front part of the site apparently during the subdivision development and the building area is vegetated with sparse grass and weeds. The ground surface is relatively flat with a gentle slope down to the west with about 4 feet of elevation difference across the building footprint. A scree field of basalt cobbles and boulders is visible on the hillside to the north and Eagle Valley Evaporite is exposed in County Road 114 cuts above the site. A small hill located to the north of the subject lot consists of Eagle Valley Evaporite based on our previous exploratory borings. SUBSIDENCE POTENTIAL Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Pinyon Mesa Subdivision and was encountered at a depth of 27 feet in the exploratory boring drilled at the subject site. 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. Sinkholes were not observed in the subdivision but geologically young sinkholes are locally present in the evaporite region between Glenwood Springs and Carbondale and we are aware of three relatively recent sinkhole collapses in this area of the Roaring Fork River valley. Based on our current understanding of the evaporite sinkhole process, the areas in western Colorado including the project site, where evaporite is shallow, have the Job No. 114 038A -3 - potential for sinkhole development. The boring drilled at the subject site was relatively shallow, for foundation design, and no voids or cavities were encountered to the drilled depth of 31 feet. The risk of future ground subsidence on Lot 48 throughout the service life of the proposed residence, in our opinion, is low; however, the owner should be 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 February 19, 2014. 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 truckmounted CME -45B drill rig. The boring was logged by a representative of Hepworth Pawlak Geotechnical, Inc. The drill rig access into the lot was limited by the snow cover of at least 2 feet and the boring was drilled about 8 feet off of Pinyon Mesa Drive at the edge of the Iot. 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 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 below about one foot of topsoil (root zone) consist of generally stiff sandy silt and clay overlying very stiff to hard sandy clay. The soils contain scattered gravelly Job No. 114 038A Mach -4 - zones. Medium hard, weathered siltstone bedrock was encountered below the clay at 27 feet down to the boring depth of 31 feet. Laboratory testing performed on samples obtained from the boring included natural moisture content and density and percent finer than No. 200 sieve gradation analyses. Results of swell -consolidation testing performed on relatively undisturbed drive samples, presented on Figure 4, indicate low to high compressibility of the upper silt and clay soil under conditions of loading and wetting with a low collapse potential (settlement under constant load) when wetted. The sample of sandy clay from 15 feet depth showed low to moderate compressibility with a low expansion potential when wetted. The laboratory testing is summarized in Table 1. No free water was encountered in the boring at the time of drilling and the subsoils and bedrock were slightly moist. FOUNDATION BEARING CONDITIONS The subsoils encountered in the boring at shallow foundation depth generally consist of sandy silt and clay which are compressible when wetted under load. The subsurface profile is similar to that encountered in nearby lots of Cliffrose Way. Shallow spread footings placed on the upper compressible soils could have a settlement potential of 2 inches or more and resulting building distress. A possible way to mitigate the building damage risk is to provide a heavily reinforced mat foundation to make the structure more rigid and better able to resist differential settlement. Compacting the bearing soils to a depth of at least 5 feet below shallow footings could be performed to reduce the settlement potential and risk of building distress. Another alternative is a deep foundation system that extends the bearing level down to relatively incompressible bedrock. lithe deep foundation alternative is selected, we should be contacted to provide additional recommendations. Job No. 114 038A Ge&ech -'S - 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 spread footings bearing on at least 5 feet of compacted structural fill. The design and construction criteria presented below should be observed for a spread footing foundation system. 1) Footings or a mat foundation placed on compacted structural fall should be designed for an allowable bearing pressure of 1,500 psf. Based on experience, we expect initial settlements to be less than 1 inch. Additional differential foundation settlement could be up to about 1 inch depending on the depth of any subsurface wetting. Precautions should be taken to prevent post -construction wetting of the bearing soils. 2) The footings should have a minimum width of 20 inches for continuous walls and 2 feet for isolated pads. 3) The mat edges, 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) Foundations should be designed to be relatively rigid with "box like" configuration and isolated footings should be avoided. 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 lateral earth pressures as discussed in the "Foundation and Retaining Walls" section of this report. 5) The topsoil and required soil depth for structural fill should be removed from beneath the building area and to at least 5 feet beyond the building Job No. 114 038A -6 - perimeter. The exposed soils should then be moistened to near optimum and compacted to at least 95% of standard Proctor density. The soils removed from the excavation should be replaced compacted to at least 98% of standard Proctor density within 2 percentage points of optimum moisture content. 6) A representative of the geotechnical engineer should observe the building excavation for bearing conditions and perform field compaction tests at the time of structural fill construction. 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 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 a moisture content near or slightly above optimum. Backfill in pavement and walkway areas should be compacted to at least 95% of the maximum Jab No. 114 038A -7_ standard Proctor density. Care should be taken not to overcompact the backfill or use large equipment near the wall, since this could cause excessive Iateral 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. 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 he compacted to at least 95% of the maximum standard Proctor density at a moisture content near optimum. FLOOR SLABS The natural on-site soils are compressible when wetted and there is a risk of slab settlement and distress if the bearing soils become wetted. We recommend the slabs be supported on compacted structural fill similar to the foundation recommended above. To reduce the bffects of some differential movement, non-structural 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 relatively well graded sand and gravel, such as road base, should be placed beneath slabs for subgrade support. This material should consist Job No. 114 038A -8 - of minus 2 inch aggregate with at least 50% retained on the No. 4 sieve and less than 12% passing the No. 200 sieve. Fill material placed for support of floor slabs above footing bearing level 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 and topsoil. UNDERDRAIN SYSTEM Free water was not encountered during our exploration and an underdrain is not recommended for shallow crawlspace construction or garage slab -on -grade at this site. If a deeper excavation, such as for a basement level is proposed, we should be contacted for additional recommendations. SURFACE DRAINAGE Proper surface grading and drainage is critical to satisfactory performance of the residence. 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. Job No. 114 038A Gmech -9- 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation, such as sod, 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 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 Job No. 114 038A -10 - bearing strata and testing of structural fill by a representative of the geotechnical engineer. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Steven L. Pawlak, P.E. Reviewed by: Daniel E. Hardin, P.E. SLP/ksw cc: Patrick Stuckey (stuarch(comcast.liet) Job No. 114 038A Gtech APPROXIMATE SCALE 1"= 20' GATE POST ID CLIFFROSE WAY 00 w 1 al a \\ BORING 1 \ • 1 \ 1 1\ 1 1 1 I 1 1 e i4_ 1 o cc co \tit \ 1 -z• \ \ a co \ \ F \ BUILDING \ 1 8 1 1 l 1 1 1 /( 1 1 1` 979 980 / / \1 ETBACK LINE 981 co I 03 I 114 038A H Hepworth-Pawfok Geotechnical LOCATION OF EXPLORATORY BORING Figure 1 a3 Y a 0) O 20 0 5 10 15 25 _ 30 BORING 1 ELEV.= 981' i 14/12 • WC=8.2 DD=102 ' 14/12 WC=9,1 DD=101 -200=92 24/12 r WC=7.2 D D=106 r r � ✓ � ✓ 40/12 ✓ WC=5.5 DD=110 ' r -200=73 r � 37/12 ✓ WC=6.8 DD= 103 r J -200=71 42/12 WC=4.0 D O= 115 0 5 10 15 — 20 25 30 35 35 NOTE: Explanation of symbols is shown on Figure 3. LL 0 114 038A 1—I Hepworth—Pawlok Geotechnical LOG OF EXPLORATORY BORING Figure 2 LEGEND: TOPSOIL; organic sandy silt and clay, some gravel, dark brown. SILT AND CLAY (ML -CL); slightly sandy to sandy, scattered gravel, stiff, slightly moist, light brown, slightly calcareous and porous —7 CLAY (CL); sandy, scattered gravel, very stiff to hard, slightly moist, light brown, slightly calcareous, low plasticity. WEATHERED SILTSTONE BEDROCK; medium hard, slightly moist, brown. Eagle Valley Evaporite, 11 Relatively undisturbed drive sample; 2 -inch I.D. California liner sample. 21/12 Drive sample blow count; indicates that 21 blows of a 140 pound hammer falling 30 inches were required to drive the California sampler 12 inches. NOTES: 1. The exploratory boring was drilled on February 19, 2014 with a 4 -inch diameter continuous flight power auger. 2. Location of the exploratory boring was measured approximately by pacing from features shown on the site plan provided on February 28, 2014. 3. The exploratory boring elevation was obtained by interpolation between contours on the site plan provided. 4. The exploratory boring location and elevation 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 transitions may be gradual. 6, No free water was encountered in the boring 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 038A H Hepworth--Pawlak Geotechn[eal LEGEND AND NOTES Figure 3 Compression % Compression - Expansion 0 1 2 3 4 5 1 0 1 2 Moisture Content = 8.2 percent Dry Density = 102 pcf Sample of: Sandy Silty Clay From: Boring 1 at 5 Feet ------------Q..,__._„ °\\.2___ Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 7,2 Dry Density = 108 Sample of: Sandy Clay From: Boring 1 at 15 Feet percent pcf Expansion upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 114 038A I—II Hepworth--Pawlak Geotechnical SWELL -CONSOLIDATION TEST RESULTS Figure 4 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 114 038A SAMPLE LOCATION NATURAL MOISTURE coNTENT i%l NATURAL DRY DENSITY (Pct.) W r GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERGUMITS UNCONFINED COMPRESSIVE STRENGTH (PSI t SOIL OR BEDROCK TYPE BORING DEPTH {ftl GRAVEL (9Z) SAND (%) LIQUID LIMIT (%) PLIC INDEX (%) 1 5 8.2 102 Sandy Silty Clay 10 9.1 101 92 Slightly Sandy Silty Clay 15 7.2 108 Sandy Clay 20 5.5 110 73 Sandy Clay with Gravel 25 6.8 103 71 Sandy Clay 30 4.0 115 Weathered Siltstone I 1