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HomeMy WebLinkAboutSubsoil Study 06.12.06G&Etech HEPWORTH - PAWLAK GEOTECHNICAL June 12, 2006 Carlos Sanchez 1120 West 5th Street Rifle, Colorado 81650 Hepnortl� l'aulal; Geotechnical, Inc. 5020 Counr} Road 15 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 email hpgeoChpgeotech.com Job No.106 0498 Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 47, Four Mile Ranch, Maroon Drive, Garfield County, Colorado. Dear Mr. Sanchez: 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 May 23, 2006. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The proposed residence will be a two story wood frame structure over a basement level Located on the lot in the area shown on Figure 1. The attached garage and basement floors will be slab -on -grade. Cut depths are expected to range between about 3 to 10 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 Ioadings 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 at the time of our field exploration. The ground surface is relatively flat with a moderate slope down to the southwest. There is about 2 to 3 feet of elevation difference across the building area. An old shallow irrigation ditch is located to the west of the building area. Vegetation consists of sagebrush, grass and weeds. Subsidence Potential: Four Mile Ranch is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. 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 immediate area of the subject lot. The pits were relatively shallow, for foundation design only. 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 47 throughout the service life of the building is low and similar to other lots in the area, but the owner should be aware of the potential for sinkhole development. Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 -�- Subsurface Conditions: The subsurface conditions at the site were evaluated by observing two exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 2 feet of topsoil, consist of stiff, slightly sandy to sandy silty clay. Relatively dense, gravel cobbles and boulders in a sandy silt matrix were encountered beneath the clays at depths of 8 and 81/2 feet. Results of swell -consolidation testing performed on relatively undisturbed samples of clay soils, presented on Figure 3, indicate low compressibility under existing moisture conditions and light loading and a minor collapse potential {settlement under constant load} when wetted. The samples showed moderate to relatively high compressibility upon increased loading after wetting. No free water was observed in the pits at the time of excavation and the soils were slightly moist to moist. 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 soil designed for an allowable soil bearing pressure of 1,500 psf for support of the proposed residence. It has been our experience that some of the clay soils in the area can have an expansion potential when wetted. Footings placed on the underlying rocky soils should have relatively low settlement potential. The settlement/heave potential of the subgrade should be further evaluated at the time of construction. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. The topsoil and loose disturbed soils 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 12 feet. Foundation walls acting as retaining structures should be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 55 pcf for the on-site soil as backfill. Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade construction. The clay soils in the area have variable settlement/heave potential when wetted which could result in some slab movement and distress if the bearing soils become 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. 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 less than 50% passing 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 Job No.106 049$ C Ptech consist of the on-site soils or suitable imported granular fill 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 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. 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 oft inches. The drain gravel backfill should be at least 1 % 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. Drying could increase the expansion potential of the clay soils. 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 pavement and walkway areas. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Irrigation sprinkler heads and landscaping which requires regular heavy irrigation, such as sod, should be located at least 5 feet from foundation walls. 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) developing in the future. If the client is concerned about MOBC, then a professional in Job No.106 0498 G ech -4 - this special field of practice should be consulted. Our findings include interpolation and extrapolation of the 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 of the recomrnendations 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 f ll by a representative of the geotechnical engineer. If you have any questions or if we may be of further assistance, please let us know. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Jordy Z. Adamson, Jr., P.E Reviewed by: Steven L. Pawlak, P.E. JZA/ksw attachments Figure 1 — Location of Exploratory Pits Figure 2 — Logs of Exploratory Pits Figure 3 — Swell -Consolidation Test Results Job No. 106 0495 Gtech APPROXIMATE SCALE 1" = 100' LOT 48 OPEN SPACE EASEMENT IRRIGATION 1 EASEMENT I ' OPEN ; SPACE EASEMENT BUILDING SETBACK LOT 47 ! LOT 46 MAROON pRIV PROPERTY BOUNDARY 106 0498 HEPWORTh.PAWLAK GEOTECHNICAL LOCATION OF EXPLORATORY PITS Figure 1 Depth - Feet 0 5 10 LEG END: PIT 1 PIT 2 WC -14.9 DD =106 WC=18.0 DD -93 0 5 10 TOPSOIL; sandy silty clay, organics, roots, firm, slightly moist, dark brown. CLAY (CL); silty, slightly sandy to sandy, stiff, slightly moist to moist, light brown to white, calcareous. GRAVEL, COBBLES AND BOULDERS (GM); up to boulder size in a sandy silt matrix, dense, slightly moist, reddish brown to brown. Basalt fragments. 2" Diameter hand driven liner sample. TPractical backhoe refusal. NOTES: 1. Exploratory pits were excavated on May 26, 2006 with a Caterpillar 416E backhoe. 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. The pits are approximately at the same elevation. 4. The exploratory pit locations should be considered accurate only to the degree implied by the method used. 5. The lines between materials shown on the exploratory pit logs 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 (pot) 106 0498 HEPWORTH-PAWL4K GEOTECHNICAL. LOGS OF EXPLORATORY PITS Figure 2 Compression Compression % 0 1 2 3 4 0 1 2 3 4 5 6 Moisture Content = 14.9 percent Dry Density = 106 pcf Sample of: Slightly Sandy Silty Clay From: Pit 1 at 4.5 Feet Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 L Moisture Content = 18.0 Dry Density = 93 Sample of: Sandy Silty Clay From: Pit 2 at 3 Feet Compression upon wetting percent pcf 0.1 1.0 10 APPLIED PRESSURE - ksf 100 106 0498 H H EPW ORTH•PAW LAK GEOTECHNICAL. SWELL -CONSOLIDATION TEST RESULTS Figure 3