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HomeMy WebLinkAboutSoils Report for Foundation Design & Perc Test 08.28.2014--"Ttew`.fir-•c x HEPWORTH-PAWL 11' G EOTECHN ICA! August 28, 2014 Brian Welder 3125 Maroon Creek Road Aspen, Colorado 81611 (bwelderric earthlink.net) Job No.114 327A Subject: Subsoil Study for Foundation Design and Percolation Test, Proposed Residence, Lot 20, Mountain Springs Ranch, Hidden Glen, Garfield County, Colorado Dear Mr. Welder: 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 August 8, 2014. 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: The proposed residence will be one and two story log construction above a walkout basement and located in the lower part of the lot roughly as shown on Figure I . Ground floors could be structural above crawlspace or slab -on -grade. Cut depths are expected to range between about 4 to 10 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 proposed to be located east and downhill of the residence. 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 vacant lot is located in the southwest part of the subdivision on Hidden Glen. The building site is located in a small open area of grass and weeds surrounded by stands of scrub oak, brush and evergreen trees. The ground surface slopes moderately down to the east. Scattered basalt boulders are visible on the ground surface. Pm i 100-S-11 7119 • i.ill01Th Srrin..:, 719_6; -•,-;-362 • ;IR erthomc -2 - Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits in the building area and two profile pits in the septic disposal area at the approximate designated locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 1 to 21/2 feet of topsoil, consist of very stiff to hard sandy silty clay with basalt boulders. Digging through the hard clay and boulders was difficult and practical refusal was encountered at Pit 2 and Profile Pit 1. Results of swell -consolidation testing performed on relatively undisturbed samples of the sandy silty clay, presented on Figure 3, indicate low compressibility under existing moisture conditions and light loading and variable low expansion potential or compressibility when wetted. Results of a USDA gradation analysis performed on a sample of sandy silty clay (Loam) (minus 3 inch fraction) obtained from Profile Pit 2 are presented on Figure 4. The laboratory test results are summarized in Table 1. No free water was observed in the pits at the time of excavation and the soils were slightly moist. Foundation Recommendations: The very stiff to hard clay soils are typically expansive when wetted which could result in differential movement of lightly loaded footings. Considering the subsoil conditions encountered in the exploratory pits and the nature of the proposed construction, spread footings placed on the undisturbed natural soil designed for an allowable soil bearing pressure of 2,500 psf and minimum dead load pressure of 800 psf can be used with a risk of movement. The soils tend to heave when wetted and there could be post -construction foundation movements on the order of 1 to 2 inches. Placing a minimum 3 feet of structural fill such as road base below the footing would help to mitigate the movement potential. The expansion potential of the clay soils should be further evaluated at the time of construction. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. The topsoil and loose disturbed soils encountered at the foundation bearing level within the excavation should be removed to expose the undisturbed natural soils. Voids created by boulder removal should be backfilled with structural fill. 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 60 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. Compaction of structural fill should be to at least 98% of standard Proctor density. Floor Slabs; The natural clay soils, below the topsoil, can be used to support lightly loaded slab -on -grade construction with a high risk of heave it the bearing soils are wetted. Use of a crawlspace is recommended to minimize potential floor movement. If slab -on - lob No.114 327A Gecrtech -3 - grade is used, a minimum 3 feet of road base is recommended to reduce the heave potential. To reduce the effects of some differential movement, floor slabs where used 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 should consist of imported granular 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 and where there arc clay soils 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 maximum size of 2 inches. The drain gravel backfill should be at least 1% 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 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 Job No.I 14 327A GecD'tech -4 - 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 irrigation should be located at least 5 feet from the building and beyond any foundation wall backfill, Percolation Testing: Two profile pits and three percolation test holes were excavated on August 13, 2014 at the locations shown on Figure 1. The subsoils exposed in the profile pits consisted of about 1 foot of topsoil overlying sandy silty clay with boulders to the bottom pit depths of 5%2 and 8 feet. The results of a gradation analysis performed on a sample of sandy silty clay with gravel (minus 3 inch fraction) obtained from Profile Pit 2 are presented on Figure 4. The sample tested has an USDA Soil Texture Classification of Loam. No free water or evidence of a seasonal perched water table was observed in the pits and the soils were slightly moist to moist. Percolation test holes were hand dug and soaked with water on August 13, 2014. Percolation testing was conducted on August 14, 2014 by a representative of Hepworth - Pawlak Geotechnical, Inc, The percolation rates varied from 107 minutes per inch to 320 minutes per inch with an average of 196 minutes per inch. The rates are slower than typical of a conventional infiltration disposal system. The percolation test results arc summarized on Table 1. 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 -evacuation 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 Job No. l 14 327A - J - 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 CII 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. L uis E. Eller Reviewed by: Steven L. Pawlak, P.E. LEE/ksw attachments Figure I - Location of Exploratory Pits and Percolation Test Holes Figure 2 - Logs of Exploratory Pits Figure 3 - SweIl-consolidation Test Results Figure 4 - USDA Gradation Test Results Table 1 - Summary of Laboratory Test Results Table 2 - Percolation Test Results Job No.114 327A GLIZtech APPROXIMATE SCALE 1' = 300' 114 327A i LOT 20 PIT 1 • PIT 2 PROFILE 11114 PROFILE ILPIT 1 1 PIT pa P2 H Hepworth—Pawlak Geotechnical LOT 19 TO MOUNTAIN SPRINGS ROAD LEGEND: ■ EXPLORATORY PIT A PERCOLATION TEST HOLE LOCATION OF EXPLORATORY PITS Figure 1 L a 0 0 5 10 LEGEND. PIT 1 PIT 2 PROFILE PIT 1 PROFILE PIT 2 WC= 15.7 DD=107 WC=14.5 OD= 109 / / nTOPSOIL; organic sandy slit and clay, firm, slightly moist to moist, dark brown. 6 • 0 GRAVEL =10% - - SAND=20% SILT=43% 5 CLAY -27% 10 CLAY (CL); with basalt boulders, sandy, silty, very stiff to hard, slightly moist, brown, medum plasteciry, blocky. 2" Diameter hand driven I ner sample. Disturbed bulk sample. TPractical digging refusal on boulders. NOTES: 1. Exploratory pits were excavated on August 13, 2014 with a Cat 4168 backhoe. 2. The exploratory pits were located in the building area designated by the client. 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 fines 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) 114 327A H Hepworth—Pawlak Geolechnlcal LOGS OF EXPLORATORY PITS Figure 2 Compression - Expansion °4 Compression % 1 0 1 2 0 1 2 3 4 5 0. 1.0 10 APPLIED PRESSURE - ksf 100 Moisture Content = 15.7 percent Dry Density = 107 pcf Sample af: Sandy Clay From: Pit 1 at 6 X Feet 2_1.-----r- Compression upon wetting Expansion upon wetting u 0. 1.0 10 APPLIED PRESSURE - ksf 100 0.1 1.0 10 APPLIED PRESSURE - ksf 100 114 327A HEPWORTH•PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS Figure Moisture Content = 14.5 percent Dry Density = 109 pcf Sample of: Sandy Silty Clay From: Pit 2 at 4 Y2 Feet 2_1.-----r- Compression upon wetting 0.1 1.0 10 APPLIED PRESSURE - ksf 100 114 327A HEPWORTH•PAWLAK GEOTECHNICAL SWELL -CONSOLIDATION TEST RESULTS Figure .Z�3�►1r.»/_1I2Ii HYDROMETER ANALYSIS i I SIEVE ANALYSIS I TIME READINGS U.S STANDARD SERIES 1 CLEAR SQUARE OPENINGS 1 24 KR. 7 HR 1 MIN. p 45 MIN.15MlN.60MINI9MIN,4 MIN. #325 ; 140 #60 #35 X18 #10 #4 3/8' 3/4' 1 112 3' 5'6' 8. 100 wowy, , --lam-- w—.—aaawl a --t.! — w_. .-- a—r—a--!ii—a—ai—a—aaaa-- —a. ......_ a—�—Imama--l.�� a— r— A_ aa--.l—a— —.tea--.— ..ailaaar.rr—l—ate!—.—a—a—Ali—!-ala—. —_-- a— w�a—a--! 1—!aa_a—s!... 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WE WWI lIEW IWMI aa_ P—ww -- .001 .002 .005 .009 .019 .045 .105 .025 .500 1.00 2.00 4.75 9.5 19.0 37.5 762 152 203 CIAY sur DIAMETER OF PAR1]CLES IN MILLIMETERS 114 327A SAND oRAvfl I V . F R 1 1 MENAI lcomse A oma t- s 1 MECUM I u& GRAVEL 10 % SAND 20 % USDA SOIL TYPE: Loam H Hepworth—Pawlak Geotechnlcol SILT 43 % CLAY 27 % 80 70 60 50 40 30 20 10 0 FROM: Profile Pit 2 at 4 to 4 y2 Feet USDA GRADATION TEST RESULTS 24:• ► ►r Figure 4 Job No. 114 327A F. 1--1 6 z U >- V 0 J m o_ LL 1 O } O w X X Cfi A7TERBERG LIMITS IA v Sandy Silty Clay 0 0 ti r J C7 0 0 �--1 0. J 2 g N f2 Z00 z U SAMPLE LOCATION t. H Q! trk 1- [Y c a N O -44 fie HEPWORTH-PAWLAK GEOTECHNICAL, INC. TABLE 2 PERCOLATION TEST RESULTS HOLE NO. HOLE DEPTH LENGTH OF (INCHES) INTERVAL (MIN) P1 P2 P3 39 33 33 20 20 20 WATER DEPTH AT START OF INTERVAL (INCHES) 7 JOB NO. 114 327A WATER DEPTH AT END OF INTERVAL (INCHES) 6 3/4 DROP IN WATER LEVEL (INCHES) 6 3/4 61/2 6 1/2 6 1/2 6 1/2 6 1/4 6 1/4 6 1/4 6 1/2 6 5 3/4 5 1/2 5 1/2 5 1/4 5 1/8 5 5 6 1/2 61/2 6 1/4 6 1/4 6 1/4 6 1/4 6 5 3/4 5 1/2 5 1/2 5 1/4 5 1/8 5 4 3/4 4 7/8 1/4 1/4 0 0 1/4 0 0 0 1/2 1/4 1/4 0 1/4 1/8 1/8 1/4 1/8 4 7/8 4 3/4 1/8 4 3/4 4 3/4 0 4 3/4 4 3/4 0 4 3/4 4 1/2 1/4 41/2 4 1/2 0 4 1/2 4 3/8 1/8 4 3/8 4 1/4 1/8 AVERAGE PERCOLATION RATE (MIN./INCH) 320 107 160 Note: Percolation test holes were hand dug in the bottom of backhoe pits and soaked on August 13, 2014. Percolation tests were conducted on August 14, 2014. The average percolation rates were based on the last four readings of each test. 1