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HomeMy WebLinkAboutSubsoil Study for Foundation Design 05.25.2006Il/lay 25,2006 Frank Kroening Job No.106 0463 Subject:Subsoil Study for Foundation Design, Proposed Residence, Lot 4, Filing 6, Elk Springs Subdivision, 'Wood Nymph Road, Garflreld County, Colorado Dear Mr. Kroening: 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 i|l4ay 12,2006. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Hepworth-Pawlak Geotechnical, Inc., previously performed a preliminary geotechnical study for Filings 6 through 9, Elk Springs (formerly Los Amigos Ranch PUD) and reported our findings on February 14, 1997,JobNo. 197 617. Proposed Construction: The proposed residence plans are conceptual at this time and this report was prepared for purchase of the lot. We assume that the residence will be a one to two story wood frame structure above a basement level. The site is shown on Figure 1. Cut depths are expected to range between about 2 to 9 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the assumed 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 is located on an upland mesa on the east side of the lower Roaring Fork Valley. Vegetation consists of pinion and juniper trees with sage brush, grass and weeds. The ground surface slopes moderately down to the west. Numsrous basalt cobbles and boulders aÍe visible on the ground surface. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating 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 Il2 foot of topsoil, consist of 1 to 2 feet of sandy silty clay overlying basalt cobbles and boulders in a silt and sand matrix. Refusal to backhoe digging was encountered at a depth of 3 feet .) in both pits. No free water was observed in the pits at the time of excavation and the soils were slightly moist. X'oundation 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 o{.2J99-Sf for support of the proposed residence. The matrix soils tend to compress after wffi'g and there could be some post-construction foundation settlement. Footings should be a minimum width of 16 inches for continuous walls andZ feet for columns. Utility trenches and deep cut areas may require rock excavating techniques such as chipping or blasting. Our experience in the area is that excavation with conventional excavators is usually possible to at least 2 feefbelow the refusal encountered with our backhoe. Excavation below 5 feet will probably require rock excavation such as chipping or blasting. We expect groundwater level will be significantly lower than expected basement excavation depths at this site. Loose and disturbed soils encountered atthe foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural soils. Voids created from boulder removal at footing grade should be filled with a structural material such as road base compacted to 95 percent standard Proctor density at a moisture content near optimum. 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 should be designed to resist alateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site soil as backfill. tr'loor 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 difierential 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 50o/o passing the No. 4 sieve and less than2o/o passing the No. 200 sieve. All fill materials for support of floor slabs should be compacted to at least 95Yo 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. JobNo.106 0463 cåFtecrr -J- 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 lsvel 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 lo/o to a suitable gravity outlet. Free-draining granular material used in the underdrain system should contain less than 2o/o passing the No. 200 sieve, less than 50olo passing the No. 4 sieve and have a maximum size o12 inches. The drain gravel backfill should be at least llzfeetdeep. Surface Drainage: The following drainage precautions should be observed during construction and maintained at all times after the residence has been completed: 1) Inundation ofthe 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 95Yo of the maximum standard Proctor density in pavement and slab areas and to at least 90o/o of the maximum standard Proctor density in landscape areas. Free-draining wall backfill should be capped with about 2 feet of the on-site, finer graded soils to reduce surface water infiltration. 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. A swale may be needed uphill to direct surface runoff around the residence. 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. 5) Landscaping which requires regular heavy irrigation and lawn sprinkler heads should be located at least 5 feet from the building. Consideration should be given to the use of xeriscape to limit potential wetting of soils below the foundation caused by inigation. Limitations: This study has been conducted in accordance with generally accepted geotechnical engineering principles and practices in this area atthis 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 JobNo.l06 0463 cå&ecrr -4- locations indicated on Figure I 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 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 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 veri$' 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. If you have any questions or if we may be of further assistance, please let us know Respectfully Submitted, HEPWORTH . PAWLAK GEOTECHNICAL, INC. Louis E. Eller Reviewed by: Daniel E. Hardin, P.E LEE/kmm attachments Figure 1 - Location of Exploratory Pits Figure 2 - Logs of Exploratory Pits JobNo.106 0463 eåFtecrr APPROXIMATE SCALÊ 1": B0' WOOD NYMPH ROAD \ \ \ \ì I I I I \ \PlTl ¡ \ \I I\ \I I\ \PtT 2a I I -J \ \I\I LOT 5 LOT 4 FILING 6 LOT 3 OPEN SPACE 1 06 0463 HEP\¡VoRÏH-PAWLAK GEorEcHNrcaL LOCATION OF EXPLORATORY PITS Figure 1 PIT 1 PIT 2 0 0 -J o)(¡) LL I _c o_oô 5 5 oo LL I E o_ c)o '10 10 LEGEND: TOPSOIL; sandy silt and clay, organic, moist, dark brown CLAY (CL); silty, sandy, stiff, slightly moist to moist, brown. BASALT COBBLES AND BOULDERS (GM-GP); in a silt and sand matrix, dense, slightly moist to moist, light brown to white, calcareous. F I 2" Diameter hand driven liner sample. I I J Disturbed bulk sample Practical digging refusal with backhoe NOTES: 1. Exploratory pits were excavated on May 19, 2006 with a Case 580 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. 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 1 06 0463 & HEP\IVoRTH-PAWLAK GEoTECHNICAL LOGS OF EXPLORATORY PITS Figure 2