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HomeMy WebLinkAboutSoils Study for Foundation Design & Perc Test 05.24.2001May 24, 2001 Beulah Wilson Estate Attn: Virginia Sterrett 6235 County Road 109 Carbondale, Colorado 81623 Hepwartls-Pawlak Geotechnical, Inc. 5020 County Road 154 Glenwood Springs, Colorado 81601 Phone: 970-945-7988 Fax: 970-945-8454 hpgeo@hpgeotech.com Job No. 101 338C Subject: Subsoil Study for Foundation Design and Percolation Test, Proposed Residence, Lot C, Beulah Wilson Subdivision Exemption, County Road 109, Garfield County, Colorado Dear Ms. Sterrett: As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study and -percolation test for foundation and septic disposal designs at the subject site. The study was conducted in accordance with our agreement for geotechnical engineering services to the Beulah Wilson Estate dated May 3, 2001. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Evaluation of potential geologic hazard impacts on the site are beyond the scope of this study. Proposed Construction: Plans for the residence are conceptual at this time and the report was prepared for purchase of the lot. One to two story wood frame construction above a basement or crawlspace is typical of the area. Cut depths are expected to range between about 3 to 8 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed type of construction. The septic disposal system is assumed to be located downhill and northeast of the building area. 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. Subsidence Potential: Lot C is underlain by Pennsylvania Age Eagle Valley Evaporite bedrock. The evaporite contains gypsum deposits. Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can produce areas of localized subsidence. Sinkholes were not observed in the immediate area of the subject lot. The exploration pits were relatively shallow, for foundation design only. Based on our present knowledge of the site, it cannot be said for certain that sinkholes will not Beulah Wilson Estate May 24, 2001 Page 2 develop. In our opinion, the risk of ground subsidence at Lot C is low but the owner should be aware of the potential for sinkhole development. Site Conditions: Lot C is located on the west side of County Road 109 and bordered to the west by a very steep east -facing slope. The ground surface in the building area is relatively flat with a gentle slope down to a large irrigation ditch which crosses the lot below the bu' ding area. A low area created by filling the County Road is located between the irrigation ditch and the County Road. The building area is vegetated with sage brush, grass and weeds. The remaining part of the site and steep hillside are vegetated with a pinion and juniper forest. There is an existing well near the middle of the lot. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits in the building area and one profile pit in the septic disposal area at the approximate locations shown on Fig. 1. The logs of the pits are presented on Fig. 2. The subsoils encountered in the proposed building area, below about 2 feet of topsoil, consist of sandy silty clay with occasional gravelly layers to the maximum depth explored, 13 feet. Results of swell -consolidation testing performed on relatively undisturbed samples of the sandy clay, presented on Figs. 3 and 4, indicate low compressibility under existing moisture conditions and light loading and a low collapse potential (settlement under constant load) when wetted. ' The samples were moderately to highly compressible under increased loading after wetting. 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 1,200 psf for support of the proposed residences. The soils tend to compress after wetting and there could be some post -construction foundation settlement. Footings should be a minimum width of 18 inches for continuous walls and 2 feet for columns. Loose and disturbed soils and existing fill encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural soils. 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 H -P GEOTECH Beulah Wilson Estate May 24, 2001 Page 3 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 backf ll. 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 cob,rnns 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 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 maxinnim size of 2 inches. The drain gravel backfill should be at least 11/2 feet deep. An impervious membrane such as 20 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: H -P GEOTECH Beulah Wilson Estate May 24, 2001 Page 4 1) Inundation of the foundation excavations and underslab areas should be avoided during construction. Drying could increase the expansion potential of the soils. 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 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 should be located at least 10 feet from the building Consideration should be given to the use of xeriscape to limit potential wetting due to irrigation. . Percolation Testing: Percolation tests were conducted on May 9, 2001 to evaluate the feasibility of an infiltration septic disposal system at the site. One profile pit and three percolation holes were dug 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 were 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 11/2 feet of topsoil and 41 feet of sandy silty clay overlying silty sand gravel to the pit depth, 8 feet. The percolation test results are presented in Table 11. 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, the proposed type of constriction and our experience in the area. Our findings include interpolation and extrapolation of the subsurface H -P GEOTECH 3 Beulah Wilson Estate May 24, 2001 Page 5 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 verify that the recommendations have been appropriately interpreted. Significant design changes may require additional analysis or modifications to the recommendations presented herein. We recoiniuend 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. Louis E. Eller Reviewed by: Daniel E. Hardin, P.E. LEE/ksw attachments H -P GEOTECH APPROXIMATE SCALE 1" = 60' L 1 PIT 2 ■ PIT 1 PARCEL B BENCH MARK GROUND AT PROPERTY CORNER; ELEV. = 100.0', ASSUMED. P 3 P 2 A A• PROFILE P 1 PIT A PARCEL C PARCEL D EXISTING IRRIGATION DITCH 1 1 0 0 2. 1 73 0 c 0 (o 101 338C HEP WORTH-PAWLAK GEOTECHNICAL, INC. LOCATION OF EXPLORATORY PITS AND PERCOLATION TEST HOLES Fig. 1 0 5 Le - C• L o 10 15' LEGEND: ti 1/a PIT 1 ELEV.= 108.6' We -12.2 00=104 -200=45 LLr3 P1.41 WC -12.5 00=109 PIT 2 PROFILE PIT ELEV.= 111.2 ELEV.= 106.5' WC -13.4 DD -00 WC -15.0 . 00=101 -200-50 TOPSOIL; sandy silt and clay. organic. firm, moist, dark brown 5 10 15 — CLAY (CL);silty. sandy with occasional gravelly zones and scattered cobbles, stiff, slightly moist to moist. brown. GRAVEL (GM); silty. sandy, dense to very dense. moist. Tight brown, subrounded rock. Hand driven liner sample. Disturbed bulk sample. _J NOTES: 1. Exploratory pits were excavated on May 8, 2001 with a backhoe. 2. locations of exploratory plts were measured approximately by pacing from features on the site plan provided. 3. Elevations of exploratory pits were measured by instrument level and refer to the Bench Mark shown shown on Fig. 1. Loge 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: WC = Water Content (X ) —200 = Percent passing No. 200 sieve DD = Dry Density ( pcf ) LL = Liquid Limit ( % ) +4 = Percent retained on No. 4 sieve Pi = Plasticity Index ( % ) 101 338C HEPWORTH—PAWLAK GEOTECHNICAL. INC. LOGS OF EXPLORATORY PITS Depth — Feet Fig: 2 Moisture Content = 12.5 percent Dry Density = 109 pcf Sample of: Silty Sandy Clay From: Pit 1 at 4.5 Feet 'Compression upon wetting • • i .-.. I , ...• .. w (7°A \ IL , e...,........>„..."Yer.......>:„...........). 1 ' I 1 I I O r tV M1 11'1 td % UOf889JdWO3 n o) CONSOLIDATION TEST RESUL O C.) co M 0 �1! r• 1) l 1 HEPWORTH—PAWLAK SWELL CONSOLIDATION TEST RESULTS I GEOTECHNICAL, INC_ 0 1 2 4 5 6 Moisture Content = 13.4 percent Dry Density = 99 pcf Sample of: Silty Sandy Clay From: Pit 2 at 4.5 Feet Compression upon wetting 411, r 0.1 1.0 10 APPLIED PRESSURE — ksf 100 • 101 338C Fig. 4 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE I SUMMARY OF LABORATORY TEST RESULTS JOB NO. 101 3380 SAMPLE LOCATION' NATURAL MOISTURE CONTENT (961 NATURAL DRY DENSITY IPm1 GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBBRG LIMITS UNCONFINED COMPRESSIVE STRENGTH (PSFI SOIL OR BEDROCK TYPE PIC DEPTH (feet) GRAVEL (%1 SAND (961 LIQUID LIMIT (96I PLASTIC INDEX (%) 1 2% 12.2 104 45 23 ' 8 Very Clayey Sand 4%2 12.5 109 Silty Sandy Clay 2 434 13.4 99 Silty Sandy Clay 7 15.0 101 50 r Silty Sand and Clay • HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE II PERCOLATION TEST RESULTS JOB NO. 101 338C 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 (MIN./INCH) P-1 32 15 9 8 • 1 45 8 7 A % 7 34 6 /4 %4 6% 61 % 61/4 5% 74 534 5 '/4 5 4 % 14 4 /4 4 1: 14 P-2 36 15 10 8 /4 1 14 30 8 I4 8 % 8 7 14 74 714 6% • 1/a 6 14 6 '/4 1/4 614 5344 14 53. 514 / 5 14 4 /4 P-3 40 15 12 11 - 1 30 11 10'/4 % 10 Y2 93'4 % 9 /4 9 14 3 9 1/4 9 14 9 83 1/a 83 8 14 8 7 1z 14 Note: Percolation test holes were hand dug and soaked on May 8, 2001. Percolation testing was performed on May 9, 2001. The average percolation rate was based on the last three readings of each test.