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HomeMy WebLinkAboutSubsoil StudyJuly 29, 1999 Hepworth-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 hpgeo @ hpgeotech.com Terrell and Patty Tankersley c/o American Overseas Pet. Ltd. (RUMBAI) P.O. Box 770249 Houston, Texas 77215-0249 Job No. 199 512 Subject: Subsoil Study for Foundation Design and Percolation Testing, Proposed Residence, Parcel 1, Simmons Subdivision, Canyon Creek Area, Garfield County, Colorado Dear Mr. & Mrs. Tankersley: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for foundation design at the subject site. Percolation testing was also conducted to evaluate the infiltration characteristics of the subsoils for design of the on-site septic disposal system. The study was conducted in general accordance with our proposal for geotechnical engineering services to you dated June 17, 1999. The data obtained and our recommendations based on' the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The proposed construction includes a single family residence over a walkout basement and a guest cottage, located on the site as shown on Fig. 1. Cut depths are expected to be up to about 8 to 10 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. An septic disposal system is proposed to be located northeast of the proposed main residence as shown on Fig. 1. 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 southwesterly facing hillside near the confluence of Canyon Creek and East Canyon Creek. The ground surface in the building areas is relatively flat with a strong slope down generally the south. Beyond the west, south and east sides of the building sites the slope becomes steep down to East Canyon Creek. There are several shallow abandoned irrigation ditches on the site. Vegetation is mostly grass and weeds with scrub oak and brush on the steep slopes. Terrell and Patty Tankersley July 29, 1999 Page 2 Cobbles and boulders up to several feet in diameter were observed on the ground surface. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits in the proposedbuilding areas and one profile pit in the proposed septic disposal site at the locations shown on Fig. 1. The logs of the pits are presented on Fig. 2. The subsoils encountered, below about. 11/2 to 2 feet of topsoil, consist of slightly silty sandy gravel and cobbles with boulders. Some of the boulders observed in the pits were up to about 5 feet in diameter. Results of a gradation analysis performed on a sample of the gravel (minus 5 inch fraction) obtained from the site are presented on Fig. 3. No free water was observed in the pits at the time of excavation and the subsoils 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 gravel and cobble soil designed for an allowable bearing pressure of 3,000 psf for support of the proposed buildings. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. All topsoil and loose or 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 granular soils. Voids created by boulder removal should be backfilled with compacted gravel or 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 10 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 45 pcf for the on-site soil, excluding vegetation, topsoil and oversized rock, as backfill. Floor Slabs: The natural granular soils, exclusive of topsoil, are suitable to support lightly to moderately 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 H -P GEOTECH Terrell and Patty Tankersley July 29, 1999 Page 3 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 granular soils devoid of vegetation, topsoil and oversized rocks, 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 also 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 buildings have 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 H -P GEOTECH h Terrell and Patty Tankersley July 29, 1999 Page 4 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 buildings should be sloped to drain away from the foundation in all directions. We recommend a minimum slope of 6 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. Percolation Testing: Percolation tests were conducted on July 8, 1999 to evaluate the feasibility of an infiltration septic disposal system at the site. One profile pit and three percolation holes were excavated at the locations shown on Fig. 1. The test holes (nominal 12 inch diameter by 12 inch deep) were hand dug at the bottom of shallow backhoe pits and soaked with water one day prior to testing. The. soils exposed in the percolation holes are similar to those exposed in the Profile Pit shown on Fig. 2 and consist of sandy gravel and cobbles with boulders. The percolation test results, presented in Table I, indicate average percolation rates at the test sites of from 8 to 20 minutes per inch. Based on the subsurface conditions encountered and the percolation test results, the tested area should be suitable for a conventional infiltration septic disposal system. 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 expressed 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 Fig. 1 and to the depths shown on Fig. 2, the proposed type of construction and our experience in the area. 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. H -P GEOTECH Terrell and Patty Tankersley July 29, 1999 Page 5 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. Sincerely, HEPWORTH - PAWLAK GEOTECHNICAL, INC. David A. Young, P.E. Reviewed By: Daniel E. Hardin, P.E. DAY/ksm attachments H -P GEOTECH LEGEND ■ EXPLORATORY PIT 0 PERCOLATION TEST HOLE APPROXIMATE SCALE 1" = 120' PROPOSED RESIDENCE 'ft o EAST C,qN PROPOSED ,_ 5680 GUEST 5670 COTTAGE i ,, 5690 / i r 5700 5710 5720 ;730 / rEXISTING, r PIT 2 PIT 1 WELL i P 1 ♦ PROFILE PIT ♦ P 3 P 2 a /Q or SEPTIC DIS - SIT PROPERTY BOUNDARY 1 199 512I HEPWORTH — PAWLAK I LOCATION OF EXPLORATORY PITS I Fig. 1 GEOTECHNICAL, INC. PIT 1 PIT 2 PROFILE PIT ELEV. = 5716' ELEV. = 5708' ELEV. = 5710' 0 o• ° ? Q Q• •OA° a Q• °. °e' yes o:0 5 a • a Q; -O• • •* b. -O• o°; I +4=66 a ° e J -200=7- g .J GUEST MAIN SEPTIC 10 HOUSE RESIDENCE SITE LEGEND: rlTOPSOIL; silty sand with gravel, cobbles and boulders, medium dense, slightly moist, brown, with roots and organics. oma'* GRAVEL, COBBLES, AND BOULDERS (GP—GM); sandy, slightly silty, dense to very dense, slightly moist, brown, primarily subrounded to rounded `rocks to about 4 or 5 feet in diameter. Disturbed bulk sample. _J —fir Practical digging refusal with backhoe. NOTES: 1. Exploratory pits were excavated on July 7, 1999 with a backhoe. 2. Locations of exploratory pits were measured approximately by pacing from features on the site plan provided. 3. Elevations of exploratory pits were approximated by contours on the site plan provided. Pits logs 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. Fluctuations in water level may occur with time. 7. Laboratory Testing Results: +4 = Percent retained on No. 4 sieve —200 = Percent passing No. 200 sieve 1199 512 I HEPWORTH - PAWLAK I LOGS OF EXPLORATORY PITS I Fig. 2 I GEOTECHNICAL, INC. HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE I PERCOLATION TEST RESULTS JOB NO. 199 512 HOLE NO. HOLE DEPTH (INCHES) LENGTH OF INTERVAL (MIN) WATER DEPTH AT START OF INTERVAL (INCHES) WATER DEPTH AT END OF INTERVAL (INCHES) DROP IN WATER LEVEL (INCHES) AVERAGE PERCOLATION RATE (MINJINCH) P-1 46 10 9 71/2 1 1/2 20 7 1/2 7 1/2 7 6 114 3/4 61/4 5 1/2 314 5 1/2 43/4 3/4 43/4 41/4 1/2 41/4 33/4 1/2 33/4 31/4 1/2 3 1/4 23/4 1/2 2 314 21/4 1/2 21/4 1 13/4 1/2 P-2 38 10 water added 63/4 5 1/2 1 1/4 20 5 1/2 43/4 3/4 4 3/4 4 3/4 4 3 1/2 1/2 3 1/2 3 1/2 7 6 1 6 5 1/2 1/2 5 1/2 5 1/2 5 41/2 1/2 41/2 4 1/2 4 3 112 1/2 P-3 52 10 water added water added 9 1/2 43/4 43/4 8 43/4 21/2 21/4 8 53/4 21/4 53/4 3 1/2 21/4 3 1/2 1 114 21/4 8 6 1/2 1 1/2 61/2 5 1 1/2 5 31/2 1 1/2 3 1/2 2 112 1 2 112 1 1 1/2 NOTE: Percolation holes were hand dug in bottom of shallow backhoe pits and pre-soaked on July 7, 1999. Percolation tests were performed on July 8, 1999 by H -P Geotech. The average percolation rates were based on the last three readings of each test.