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Home My WebLink AboutSoils Report 10.19.2005C�tech HEPWORTH -PAWLAK GEOTECHNICAL October 19, 2005 Steve Russell 7207 Elk Lane Basalt, Colorado 81621 Hepnurth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970.945-7988 Fax:970-945-8454 email: hpgeoehpgeutech.com Job No.105 881 Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 51, Filing 5, Elk Springs Subdivision, 361 Monarch Road, Garfield County, Colorado Dear Mr. Russell: 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 October 3, 2005. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: Building plans for the proposed residence are conceptual and this report was prepared for sale of the property. Typical construction in the area consists of one and two story wood frame structures above basement, crawlspace or slab -on -grade. Cut depths are expected to range between about 3 to 9 feet. Foundation loadings for this type of construction are 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: Lot 51 is located on the east side of Monarch Road. Vegetation consists of sage brush, grass and weeds with scattered pinon trees. The ground surface is moderately sloping down to the southwest at a grade of about 6 percent with about 10 feet of elevation difference across the building envelope. Scattered basalt cobbles are exposed 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 1/2 foot of topsoil were variable and consisted of basalt cobbles and boulders in a silty sand matrix in Pit 1 and silty sand with gravel in Pit 2. Results of swell -consolidation testing performed on a relatively undisturbed sample of the silty sand from Pit 2 at 6 feet, presented on Figure 3, indicate moderate compressibility under loading and wetting and a low collapse potential (settlement under constant load) when wetted. Results of a gradation analysis performed on a sample of silty sandy gravel with cobbles (minus 5 inch fraction) obtained from Pit 1 at 4 thru 6 feet are presented on Figure 4. No free _7_ 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 silty sand and matrix soils tend to compress after wetting and there could be some post -construction foundation settlement. The settlement could be differential for footings which transition between the silty sand and gravel soils. Footings should be a minimum width of 18 inches for continuous walls and 2 feet for cobimns. 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. Excavations in the large basalt boulders may require rock excavating techniques such as blasting or chipping. Voids created from boulder removal at footing grade should be filled with a structural material such as concrete or road base compacted to 100% 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 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 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 dranage 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 mountainous areas 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. Job No.I05 881 0. 0 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, Iess than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The drain gravel back -fill should be at Least l % 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. 2) Exterior bacic 111 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 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 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 report, we should be notified at once so re-evaluation of the recommendations may be made. T_1. 1T_ 1/1L DOS 0 0 -4- 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 t us know. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Reviewed by: ��y111�uliii!lJIJ �U• Daniel E. Hardin, P. 24443 LEE/lcsw e s+0i�i4Ar C;ke attachments Figure 1— Location of Exploratory Pits Figure 2 — Logs of Exploratory Pits Figure 3 — Swell -Consolidation Test Results Figure 4 — Gradation Test Results APPROXIMATE SCALE 1' = 60' LOT 50 LOT 49 MONARCH ROAD 7060 1 \ N. N \ N. 1 \ N. �� ,7060 N. 1 -----\--1 7050 ` PIT 2\ii 1 ` \ i \ \ 1 \ 1 ! 1 1 \1 1 \ ENVELOPE / 1 1 f I 1 1\ BUILDING lI I 1 1 \ \ \\N. N. \ 1 7050 LOT 64 1 1 LOT 65 LOCATION OF IXCPLORATORY PITS Figure 1 PIT 2 ELEV.= 7054' ID 03 Q. a) 0 5 10 LEGEND: r-� 151 PIT1 ELEV.= 7052' WC=41 2 DD=72 +4=48 — -a-200=28 TOPSOIL; sandy silt and clay, organic, firm, moist, dark brown. WC--10.6 DD=73 0 5 10 SAND (SM); silty, slightly gravelly, occasional cobbles, loose to medium dense, slightly moist to moist, Tight brown, highly calcareous. BASALT COBBLES AND BOULDERS (GM); in a silty sand matrix, dense, slightly moist to moist, Tight brown, highly calcareous. 2" Diameter hand driven liner sample. Disturbed bulk sample. TPractical digging refusal In basalt boulders with backhoe. NOTES: 1. Exploratory pits were excavated on October 11, 2005 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 and checked by instrument level. 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. Fiuctuaiion 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 105 881 GeTorbEtCh Hapeath-Pa,ldc ceotechnicar LOGS OF EXPLORATORY PITS Figure 2 Compression % 0 1 2 3 4 5 6 7 6 9 Moisture Content = 10.6 percent Dry Density = 73 pcf Sample of: Silty Sand From: Pit 2 at 6 Feet T Compression upon wetting r 0.1 1.0 APPLIED PRESSURE - ksf 10 100 105 881 Gatech lieogrorth—Pailak Geotechnical SWELL -CONSOLIDATION TEST RESULTS Figure 3 ! J:t;[.lPILliflaii I►I21! 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