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HomeMy WebLinkAboutSubsoil Study 07.29.13U HEPWORTH - PAWLAK July 29, 2013 GEOTECHNICAL Chris Olson 310 White Horse Drive New Castle, Colorado 81647 (Cjo lson50@grnai l.co m) 1 ICrL1'V+rlf14',IlI'I,tR (.I C`.I(l'll6lllt:Il. h._. 5020 Colin] Iv.l.l.l 154 'LIIL I1\v & ISI Springs, L, ,s I Ni Phomc: 970-945-796,5 FiN: 970.945-8454 eriz,liL hrgt-oTthrgelucih.eunt Job No.113 161A Subject: Subsoil Study for Foundation Design and Percolation Test, Proposed Residence, Lot 2, Sillivan Subdivision Exemption, County Road 335, Garfield County, Colorado Dear Mr. Olson: 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 you dated May 17, 2013. 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 an the site is beyond the scope of this study. 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 Iight and typical of the proposed type of construction. The septic disposal system is proposed to be located east 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. Site Conditions: The property is vacant of structures and consists of 3.358 acres located between Country Road 335 and the Colorado River. Vegetation consists of grass and weeds with scattered brush and scrub oak trees. The ground surface is relatively flat with a slight slope down to the northwest. Parker 303-841-7119 (:olov iLlo Springs 719-633-5562 0 Silverthorne 970-468-1989 -2 - Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating one exploratory pit in the building area and one profile pit in the septic disposal area at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 6 inches of topsoil, consist of slightly gravelly clayey silty sand. Results of swell -consolidation testing performed on relatively undisturbed samples of clayey sand, presented on Figures 3 and 4, indicate low compressibility under existing moisture conditions and light loading and a low to moderate collapse potential when wetted. The samples were highly compressible under additional load after wetting. Results of a gradation analysis performed on a sample of slightly gravelly clayey silty sand (minus 3 inch fraction) obtained from the site are presented on Figure 5. The laboratory test results are summarized in Table I. 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 residence. The soils tend to compress after wetting and there could be some post -construction foundation settlement. The amount of settlement would be related to the depth of compressible soils and extent of wetting. Additional differential settlements of I to 3 inches are possible if the bearing soils become wetted. Potential sources of wetting include utility line breaks, lawn irrigation and surface drainage. Precautions as described in this report should be taken to reduce the risk of wetting the bearing soils. Keeping the bearing soils relatively dry will be critical to the long-term performance of the foundation. if the risk of settlement is not acceptable to the owner, a deep foundation bearing on underlying less compressible material may be possible. Borings drilled deeper on the site would be required to provide recommendations for a deep foundation. Footings should be a minimum width of 20 inches for continuous walls and 30 inches for columns. 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. 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 S0 pcf for the on-site soil as backfill. A representative of the geotechnical engineer should observe all footing excavations prior to concrete placement to evaluate bearing conditions. Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade construction. The soils tend to compress after wetting and there Job No, l 13161 A GgEtec i -3 - could be some post -construction slab settlement if the subsoils 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 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 ofthe 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 and crawlspace 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 or sump and pump. 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 % 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: 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. Job No.113 161A GtZtech -4- 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 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. 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 building caused by irrigation. Percolation Testing: A profile pit and three percolation test holes were excavated on May 20, 2013 at the locations shown on Figure 1. The subsoils exposed in the Profile Pit consisted of about 6 inches of topsoil overlying clayey silty sand with gravel to the bottom pit depth of 8 feet. The results of a gradation analysis and hydrometer performed on a sample of clayey silty sand (minus 3 inch fraction) obtained from the site are presented on Figure 5. The sample tested has an USDA Soil Texture Classification of Loamy Sand/Sandy Loam. No free water or evidence of a seasonal perched water table was observed in the pit and the soils were slightly moist to moist. Percolation test holes were hand dug and soaked with water on May 22, 2013. Percolation testing was conducted on May 23, 2013, by a representative of Hepworth - Pawlak Geotechnical, Inc. The percolation rates varied from 12 minutes per inch to 15 minutes per inch with an average of 14 minutes per inch. The percolation test results are summarized on Table 2. Based on the subsurface conditions encountered and the percolation test results, the tested area should be suitable for an on site waste disposal system. A professional civil engineer should design the 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 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, 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. Job No.113 161A Gaited -1 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. Respectfully Submitted, HEPWORTH - PAWLAK GEOTECHNICAL, INC. Louis E. Eller Reviewed by: Daniel E. Hardin, P.E. LEE/ljg attachments: Figure 1 — Locations of Exploratory Pits and Percolation Test Holes Figure 2 — Logs of Exploratory Pits Figures 3 and 4 - Swell -consolidation Test Results Figure 5 — USDA Gradation Test Results Table 1 -- Summary of Laboratory Testing Table 2 — Percolation Test Results Job No.113 I6I A ?7'29' W 00.00' x APPROXIMATE SCALE 1"=60' COLORADO RIVER LOT 2 N Gt� fJ ° AREA LESS THAN 40% SLOPE BETWEEN THE TOP OF BANK AND THE CENTERLINE OF ROAD (ro CONTINUOUS ACRE -1-/—) 25.0' 0 0 1 1 Q PROPOSED RESIDENCE ' ■ PIT 1 P-3 P-2 D 0 ■ AP -1 PROFILE PIT 0O0[M ROA© - 113 161A LZtech HEPWORTH-PAWLAK GEOTECHNICAL LOCATIONS OF EXPLORATORY PITS AND PERCOLATION TEST HOLES FIGURE 1 i1) ri a a) O 0 5 10 LEGEND: _J NOTES: PIT 1 PROFILE PIT WC=12.8 DD=87 WC=10.1 DD=90 -200=40 [sG=8=68-J =18 =6 0 5 10 TOPSOIL; organic sandy silt and clay, firm, moist, dark brown. SAND (SC); clayey, silty, slightly gravelly, sandy clay layers, medium dense, slightly moist to moist, brown 2" Diameter hand driven liner sample. Disturbed bulk sample. 1. Exploratory pits were excavated on May 20, 2013 with a 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 time. 7. Laboratory Testing Results: WC = Water Content (%) DD = Dry Density (pct) �fl = Par -pont ratainarl nn fha Mn A Depth - Feet -200 = Percent passing No. 200 sieve G = Gravel fraction per USDA Classification (%) S = Sand fraction per USDA Classification (%) M = Silt fraction per USDA Classification (%) C = Clay fraction per USDA Classification (%) 113 161A Gtech HEPWORTH-PAWLAK GEOTECHNICAL LOGS OF EXPLORATORY PITS FIGURE 2 Compression % 1 1 1 1 (0 W 0) Moisture Content = 12.8 percent Dry Density = 87 pcf Sample of: Very Silty Sandy Clay From: Pit 1 at 3 Feet Compression upon -------wetting • • • 0.1 1.0 10 100 APPLIED PRESSURE - ksf 113 161A Gtech SWELL -CONSOLIDATION TEST RESULTS FIGURE 3 H EPWORTH•PAWLAK GEOTECHNICAL Compression % C0 Co ti! t7l 01 W N j O O Moisture Content = 10.1 percent Dry Density = 90 pcf Sample of: Very Silty Clayey Sand From: Pit 1 at 6 Feet • • ....'*Compression upon -wetting • • 0.1 1.0 10 100 APPLIED PRESSURE - kst 161A 1-113 Ptech SWELL -CONSOLIDATION TEST RESULTS FIGURE 4 HEPWORTH-PAWLAK GEOTECHNICAL RCE T RETAI HYDROMETER ANALYSIS 24 R. 7 HR TIME READINGS U.S. STANDARD SERIES 0 45 mIN.15 MIN. 60MIN19MIN.4 MIN. 1 MIN. #200 #100 #50 #30 #16 #8 SIEVE ANALYSIS CLEAR SQUARE OPENINGS #4 3/8' 314' 1 112° 3" 5'6" 8" 100 10 20 30 40 50 60 70 80 90 90 80 70 60 50 40 30 20 10 100 - 0 .001 .002 .005 .009 .019 .037 .074 .150 .300 .600 1.18 2.36 4.75 9.5 19.0 37.5 76.2 152 203 12.5 127 DIAMETER OF PARTICLES IN MILLIMETERS CLAY SILT v. FINE 1 FINE 1 MEDIUM 1 COARSE IV, COARSE CRAEFl SMALL 1 MEDIUM 1 LARGE COBBLES GRAVEL 8 % SAND 68 % USDA SOIL TYPE: Loamy Sand/Sandy Loam SILT 18 % CLAY 6 % FROM: Pit 2 at 3 to 5 Feet 113 161A HEPWORTH-PAWLAK GEOTECHNICAL USDA GRADATION TEST RESULTS FIGURE 5 HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 113 161A SAMPLE LOCATION NATURAL MOISTURE CONTENT Ch) NATURAL DRY DENSITYNO. (PCr) GRADATION PERCENT PASSING 200 SIEVE ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH I (PSF) SOIL OR BEDROCK TYPE PIT DEPTH (ft) GRAVEL OS) SAND �) LIQUID LIMIT (%) PLASTIC INDEX CYO 1 3 12.8 87 Very Silty Sandy Clay 6 10.1 90 40 Very Silty Clayey Sand Profile Pit 3 to 5 6 59 35 Sandy Loam/Loamy Sand HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 2 PERCOLATION TEST RESULTS JOB NO. 113 161A 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) -7 AVERAGE PERCOLATION RATE (MIN./INCH) P 1 48 15 Water added 8 51/4 2' 13 5'''A 3% 1% 3'h 2% 1 6'4 4% 2 43 3% 1% 3% 2% 1 2% 1 1% P 2 45 15 Water added 9 6 3 15 6 4 2 4 3 1 7 5'/, 1% 5% 4 1% 4 2% 1% 2% 1' 1 1% 3 1 P 3 52 15 Water added 8 5% 2% 15 5'%z 4 1% 4 3 1 7 5% 114 5% 4 1% 4 2% 1' 2% 1% 1 1% % 1 Note: Percolation test holes were hand dug in the bottom of backhoe pits and soaked on May 22, 2013. Percolation tests were conducted on May 23, 2013. The average percolation rates were based on the last two readings of each test.