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HomeMy WebLinkAboutSoils Report.pdfech HEPWORTH-PAWLAK GEOTECHNICAL September 25, 2012 Jordan Architecture Attn: Brad Jordan P.O. Box 1031 Glenwood Springs, Colorado 81602 .ordanarchitectt7a westoffice net Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 email: hpgeo@l-mgeotech.com Job No.112 295A Subject: Subsoil Study for Foundation Design, Proposed Residence, 1866 Elk Springs Drive, Lot 82, Filing 7, Elk Springs, Garfield County, Colorado Dear Mr. Jordan: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design of foundations at the subject site. The study was conducted in general accordance with our agreement for geotechnical engineering services to Jordan Architecture dated August 29, 2012. 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, Job No. 197 617. Proposed Construction: The proposed residence will be one and two story wood frame construction above a crawlspace with an attached garage and located on the site as shown on Figure 1. Garage 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 subject site and adjoining lots are undeveloped. The property is located on an upland rolling mesa. Vegetation consists of sage brush, grass and weeds. The ground surface slopes down to the northwest at about 4 to 10 percent through the building area. A dry drainage is located along the north property line. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating three 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 one foot of topsoil, consist of sandy clay of various thickness overlying basalt cobbles and boulders in a sandy silt and clay matrix. Results of swell -consolidation testing performed Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989 -2 - on relatively undisturbed samples of the sandy clay, presented on Figures 3, 4 and 5, indicate low compressibility under existing low moisture conditions and light loading and a moderate collapse potential (settlement under constant load) when wetted. Pit 3 was excavated due to the difference in the sandy clay thickness encountered between Pits 1 and 2. No free water was observed in the pits at the time of excavation and the soils were slightly 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 basalt rock soils designed for an allowable soil bearing pressure of 2,000 psf for support of the proposed residence. As an alternative to lowering the bearing level, the subexcavated depth of clay soils could be backfilled with compacted structural fill. Footings should be a minimum width of 18 inches for continuous walls and 24 inches for columns. Utility trenches and deep cut areas where the basalt rock is shallow may require rock excavating techniques such as chipping or blasting. The sandy clay 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 basalt rock soils. Structural fill and voids created from boulder removal at footing grade should be filled with a structural material such as road base compacted to at least 98 percent of standard Proctor density at a moisture content near optimum or with concrete. 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, excluding rock larger than about 6 inches. Floor Slabs: The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab -on -grade construction with some risk of settlement if the sandy clay 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 relatively well graded sand and gravel (such as road base) should be placed beneath interior slabs for support. This material should consist of minus 2 inch aggregate with less than 50% passing the No. 4 sieve and less than 12% passing the No. 200 sieve. 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 and crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrain system. Job No.112 295A rgtech -3 - 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 11/2 feet deep. An impervious membrane such as 30 mil PVC should be placed beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils. 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. Free -draining wall backfill should be capped with at Least 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 should be located at least 10 feet from the building. Consideration should be given to the use of xeriscape 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.112 295A HGE(Ptech -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, HEPWOR - PAWLAK GEOTECHNICAL, INC. Louis E. Eller Reviewed by: Steven L. Pawlak, P.E. 16222 LEE/ksw h ‘3Z/'0,eo/J q � attachments Figure 1 — Loc ° , ::.. T loratory Pits Figure 2 — Logs of Exploratory Pits Figures 3, 4 and 5 — Swell -Consolidation Test Results Table 1 — Summary of Laboratory Test Results Job No.112 295A Cie Ptech PIT 1 ELEV.= 6914.5' PIT 2 ELEV.= 6914' PIT 3 ELEV.= 6914' WC=9.7 DD=99 WC=9.3 DD=80 WC=11.3 DD=81 -200=81 l� 10 LEGEND: IZITOPSOIL; organic sandy silt and clay, firm, slightly moist, dark brown. CLAY (CL); sandy, silty, medium stiff, slightly moist, reddish brown, porous, calcareous zones. BASALT COBBLES AND BOULDERS (GC); in a sandy silt and clay matrix, dense, slightly moist, light brown. calcareous. 2" Diameter hand driven liner sample. Practical digging refusal with backhoe. NOTES: 1. Exploratory pits were excavated on September 6, 2012 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 obtained by interpolation between contours shown on the site plan provided. 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 time, 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pcf) -200 = Percent passing No. 200 sieve 112 295A HEP WO AyyLA hh LOGS OF EXPLORATORY PITS Figure 2 Compression % 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Moisture Content = 9.3 percent Dry Density = 80 pcf Sample of: Silty Clay, Calcareous From: Pit 1 at 3 Feet Compression upon wetting 0.1 112 295A 1.0 APPLIED PRESSURE - ksf 10 100 Ge~c��t+,ech Hepworth—Pawlok Geotechnical 1 SWELL-CONSOLIDATION TEST RESULTS I Figure 3 Compression 0 1 2 3 4 5 6 7 8 9 10 11 12 Moisture Content = 113 percent Dry Density = 81 pcf Sample of: Silty Clay From: Pit 1 at 6 Feet Compression upon wetting 0.1 1.0 112 295A I 10 APPLIED PRESSURE - ksf 100 Ge&ech Hepworth—Pawlak Geotechnical 1 SWELL-CONSOLIDATION TEST RESULTS I Figure 4 0 1 0 2 c 0 co 2 3 IE 0 0 4 5 6 7 Moisture Content = 9.7 percent Dry Density = 99 pcf Sample of: Silty Clay From: Pit 2 at 1 Foot 11111 1 1111 i Compression upon wetting I I 0 100 0.1 1.0 10 APPLIED PRESSURE - ksf 112 295A1-ip�� � c�eCyll@Cii Hepworth—Pawlak Geotechnical SWELL -CONSOLIDATION TEST RESULTS Figure 5 Job No. 112 295A CU 2 QH U H >- 2 O O 0 '"i a gWo J m Ca 0_ LL 0 0 < r w 2 S v) SOIL OR BEDROCK TYPE 1 Silty Clay II Silty Clay II - UNCONFINED COMPRESSIVE STRENGTH (PSF) j ATTERBERG LIMITS U z 0- .9 or. as o PERCENT PASSING NO. 200 SIEVE I 84 P Q 0 ., GRAVEL (%) NATURAL DRY DENSITY (Pcf) 08 I 00 rn T - NATURAL MOISTURE CONTENT (%) M CT M ri r x 0 cP 0 3 CI — wJ as Q a ti ' 12