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HomeMy WebLinkAboutSoils Report 01.10.2012io January 10, 2012 ,r TEr HNit:; L David Lubrant david1ubrantra.:live.coiu 338 Stagecoach Trail Durango, Colorado 81301 Subject: Dear David: Job No.11 1 394A Subsoil Study for Foundation Design, Proposed Residence, Lot E7, Aspen Equestrian Estates, 38 Equestrian Way, Garfield County, Colorado As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design of foundations at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to you dated December 29, 201 1. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. We previously performed a subsoil study for development of the Aspen Equestrian Estates Subdivision dated August 8, 1998, Job No. 198 501. Proposed Construction: The proposed residence will be one story wood frame construction with an attached garage located on the site in the area shown on Figure 1. Ground floor will be slab -on -grade. Cut depths are expected to range between about 3 to 4 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 free of snow at the time ofour field work. The lot is flat with a slight slope down to the south. An inactive irrigation ditch is located on the west property line. The ditch bottom is about 3 feet lower than the majority o f the lot. -2 - The site is vegetated with grass and weeds with thick willow brush along the ditch. A wire fence is located on the south property line. The adjoining lot to the east is developed with a two story wood frame house. The lot to the northwest was vacant but had a construction trailer and some building materials on it. Subsidence Potential: Aspen Equestrian Estates is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. Dissolution ofthe gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. Sinkholes associated with dissolution ofthe underlying bedrock have been observed in the Roaring Fork Valley. Sinkholes have not been observed in Aspen Equestrian Estates or in the immediate area of the subject lot. The exploratory pits were shallow, for foundation design only, but evidence of sinkholes were not observed in the pits. Based on our present knowledge of the site, it cannot be said for certain that sinkholes will not develop. In our opinion, the risk of ground subsidence at Lot E7 is low and similar to other lots in the area but the owner should be aware of the potential for sinkhole development. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating four exploratory pits at the approximate locations shown on Figure I. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 1 foot of partly frozen topsoil, consist of h to 5 feet of sandy silty clay overlying relatively dense sandy gravel with cobbles to the maximum pit depth of6h feet. Results of swell - consolidation testing performed on a relatively undisturbed sample of sandy silty clay, presented on Figure 3, indicate moderate compressibility under wetting and loading. Results of a gradation analysis performed on a sample of sandy gravel with cobbles (minus 5 inch fraction) obtained from the site are presented on Figure 4. No free water was observed in the pits at the time of excavation and the soils were moist. The subsoil conditions encountered at this site are similar to those encountered on the adjoining lots. It has been our experience that groundwater in this area can be at about 3 to 4 feet deep during the irrigation season. Job No 1 I 1 394A -3 - Foundation Recommendations: Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, we recommend spread footings placed on the undisturbed natural gravel soils designed for an allowable soil bearing pressure of 2,500 psf for support of the proposed residence. Settlement of footings placed on the gravel soils should be less than l inch. The clay soils tend to compress under Ioad and should be removed from below footing areas. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed soils and existing clay encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural soils. Exterior footings should be provided with adequate cover above their bearing elevations for frost protection. Placement of footings at least 36 inches below the exterior grade is typically used in this area. Continuous foundation walls should be reinforced top and bottom to span local anomalies such as by assuming an unsupported length of at least 10 feet. Foundation walls acting as retaining structures (if any) should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site soil as backfill. Due to the potential for shallow groundwater during irrigation season, we recommend against constructing a crawlspacc. Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade construction. 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 slabs to act as a break for capillary moisture rise. This material should consist of minus 2 inch aggregate with less than 50% passing the No. 4 sieve and less than 2% passing the No. 200 sieve (3/ -inch screened rock). Job No.111 394A -4 - 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 gravel soils or a suitable imported gravel devoid of vegetation, topsoil and oversized rock. Unde(rdrain System: An underdrain system should not be necessary and is —13t--.) n recommended for the proposed slab -on -grade construction. 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 6 inches in the first 10 feet in unpaved areas and a minimum slope of 3 inches in the first 10 feet in pavement and walkway areas. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 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 pits excavated at the locations indicated on Figure 1 and to the depths shown on Figure 2, 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) Job No. I I 1 394A -5 - 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 ofthe subsurface conditions identified at the exploratory pits 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 at once so re-evaluation ofthe recommendations niay 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 ofour information. As the project evolves, we should provide continued consultation and field services during construction to review and monitor the implementation ofour 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 ofthe geotechnical engineer. If you have any questions or if we may be of further assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWL Daniel E. Har n }AL, INC. • ••r. P d.. • din P.E. t X443 4 • a E 90 •• �. `e' �i—s10NAl.SN���`� Reviewed by: RES DEH ijg attachments Figure I — Location of Exploratory Pits Figure 2 -- Logs of Exploratory Pits Figure 3 — Swell -Consolidation Test Results Figure 4 — Gradation Test Results Table I — Summary of Laboratoiy Test Results Job No.1 11394A G tech APPROXIMATE SCALE 1"=30' LOT 8E 44; HP GEOTECH JOB NO. 111348A 4.Y. / / ��� / \i/ /// i / // / / / L BUILDING ENVELOPE / / 64/46.6 TR / / // // / PIT 1 ■ PIT 4 ■ �3 (// APPROXIMATE PROPOSED RESIDENCE LOCATION PIT 2 LOT E7 111 394A HIFPwoR11+PAWIMt G@07eCIQ CAL LOCATION OF EXPLORATORY PITS Figure 1 u_ • 0 5 10 LEGEND: r-� ry ISI PIT1 WC=19.7 00=101 -200=87 PIT 2 • I +4=70 — J -200=4 PIT 3 TOPSOIL; organic sandy silt and clay, moist, dark brown, frozen. CLAY (CL); sandy, silty, medium stiff, moist, brown. PIT 4 GRAVEL (GM -GP); with cobbles and small boulders, sandy, slightly silty, dense, moist, light brown, subrounded rocks. 2" Diameter hand driven finer sample. Disturbed bulk sample. 0 5 10 NOTES: 1. Exploratory pits were excavated on January 3, 2012 with a Bobcat 337 mini -excavator. 2. Locations of exploratory pits were measured approximately by pacing from features shown on the site plan provided. 3. Elevations of exploratory pits were not measured and the logs of exploratory pits are drawn to depth. 4. The exploratory pit 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 pit Togs represent the approximate boundaries between material types and transitions may be gradual. 6. No free water was encountered in the pits at the time of excavating. Fluctuation In water level may occur with time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) +4 = Percent retained on the No. 4 sieve -200 = Percent passing No. 200 sieve 111 394A H HeP Par�ak Ceotaeh2I LOGS OF EXPLORATORY PITS Figure 2 0 1 ae 2 c 0 3 F 8 4 5 Moisture Content = 19.7 percent Dry Density = 101 pcf Passing No. 200 Sieve = 87 percent Sample of: Sandy Silty Clay From: Pit 1 at 1 Y Feet Compression upon wetting 0.1 111 394A 1.0 H HEPWORTH-PAW LAK GROTECHICCAL 14 APPLIED PRESSURE - ksf SWELL -CONSOLIDATION TEST RESULTS 100 Figure 3 14;I•74►1i:7*f1P►IX. 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