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HomeMy WebLinkAboutSoils Report 05.30.2006Gtech HEPWORTH - PAWLAI< GEOTECI 1 NICAL May 30, 2006 Peter Toborek P. 0. Box 2962 Basalt, Colorado 81621 1 ly 11 Hilo-1'.ilcl.+l; t ;rtnt.lulu.t'. ti 120 t'.nint\ IL 0.1,1 1 ric[lH'iaal ~I'fIii'.l ulu�1.itI 10n`1 l lk n.: 970-9.1 5-79;,s 1,i' •')il`•t)•1i-6-1i-1 h l .Hit Job No.106 0386 Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 7, Filing 6, Elk Springs Subdivision, 0488 Wood Nymph Lane, Garfield County, Colorado Dear Mr. Toborek: 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 April 28, 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 presented our findings in a report dated February 14, 1997, Job No. 197 617. Proposed Construction: The proposed residence plans are conceptual and we understand that our findings will be considered in the purchase of the lot. Typical construction in the area consists of one and two story log or wood frame construction above a basement, crawlspace or slab -on -grade floor with an attached garage. The site is shown on Figure 1. Ground floors could be slab -on -grade or structural above crawlspace. Cut depths are expected to range between about 2 to 8 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 a rolling upland mesa on the east side of the lower Roaring Fork River Valley. Vegetation consists of grass and weeds with some sage brush in the front part of the lot. The ground surface slope is moderate down to the southwest. There is about 7 feet of elevation difference across the building envelope. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating four exploratory pits at the approximate Locations shown on Figure 1. Pits 3 and 4 were excavated to evaluate the subsoil conditions for utility trenches. The logs of the pits are presented on Figure 2. The subsoils encountered, below about one foot of topsoil, consist of one to two feet of sandy silty clay overlying basalt cobbles and 11,11"1c•a'r 3C3-841-7119 Ltlltlliitit1 Splings 719-633-5i62 /s Sil1'eirhtll'ni.' 970 468 19t6Q -2 - boulders in a sand clay and silt matrix. Gravelly silty to very silty sand was encountered in Pit 1 from 3 to 6 % feet. Results of swell -consolidation testing performed on a relatively undisturbed sample of the sandy clay soils, presented on Figure 3, indicate low to moderate compressibility under conditions of loading and when wetted. 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. The clay and sand soils tend to compress after wetting and there could be about 1 inch or more of post -construction foundation settlement. Footings placed entirely on the basalt rock soils can be designed for an allowable bearing pressure of 2,500 psf and should have relatively low settlement potential. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Utility trenches and deep cut areas may require rock excavating techniques such as chipping or blasting. Loose and 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. Voids created from boulder removal at footing grade should be filled with a structural material such as road base compacted to at least 95 percent of 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 a lateral earth pressure based on an equivalent fluid unit weight of at least 50 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. 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 consist of the on-site soils 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 that local perched groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can Job No. t 06 0386 GeCPte h -3 - 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 of 2 inches. The drain gravel backfill should be at least 1'/2 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 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 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. 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 will 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 ofxeriscape to limit potential wetting of soils below the foundation 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 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 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 Job No.106 0386 Gggtech -4 - 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 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. 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. ElIer Reviewed by: Steven L. Pawlak, P. LEE/ksw 15222Vgah 04; ,: ;l. ,o1;Ats,,.�o FOFI: attachments Figure 1 - La xploratory Pits Figure 2 Logs of Exploratory Pits Figure 3 -- Swell -Consolidation Test Results cc: Chris Striefel Job No.106 [)386 GecDtech LOT 6 • PIT 4 APPROXIMATE SCALE 1"=80' WOOD NYMPH AOgD r-- PIT2 BUILDING ENVELOPE _ PIT1 LOT 7 FILING 6 106 0386 OPEN SPACE HEPWORTH•PAWLAK GEOTECHNICAL. • PIT 3 LOCATION OF EXPLORATORY PITS LOT 8 Figure 1 0 10 LEGEND_ N 1/1 T PIT 1 WC=21.8 DD=92 PIT 2 PIT 3 PIT 4 10 TOPSOIL; sandy silt and clay, organic, moist, dark brown_ CLAY (CL); sandy, silty, stiff, slightly moist to moist, reddish brown. SAND (SM): gravelly, silty to very silty, medium dense, slightly moist to moist, light brown to while, calcareous. BASALT COBBLES AND BOULDERS (GM); in a sandy clay and sit matrix, medium dense, sEghtly moist, light brown 1.0 white, calcareous. 2' Diameter hand driven liner sample. Practical digging refusal with backhoe. NOTES: 1. Exploratory pats were excavated on May 10, 2006 with a Cat 420D 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 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 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 Eme. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) -200 = Percent passing No. 200 sieve 106 0386 HEPWORTH•PAWLAtc GEOTECHNICAL. LOGS OF EXPLORATORY PITS Figure 2 Compression % 0 1 2 3 Moisture Content 21.8 percent Dry Density = 92 pcf Sample of: Sandy Clay From: Pit 1 at 2 Feet No movement upon wetting 0.1 106 0386 H 1.0 HEPWORTH•PAWLAK GEOTECHNICAL 10 100 APPLIED PRESSURE - ksf SWELL -CONSOLIDATION TEST RESULTS Figure 3