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Home My WebLink AboutSoils Report 07.29.2016APR 12 ZO1T H-PKUMAR Geotechnical Engineering 1 Engineering Geology Materials Testing 1 Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com July 29, 2016 Rodger Eshelman 7278 County Road 100 Carbondale, Colorado 81623 (rbeshelman @ gmai l.com) Office Locations: Parker, Glenwood Springs, and Silverthorne, Colorado Job No.16-7-218 Subject: Subsoil Study for Foundation Design, Proposed ADU, Lot 7, Cotton Hollow, Garfield County, Colorado Dear Mr. Eshelman: 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 July 14, 2016. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The proposed ADU will be one story wood frame construction above a walkout basement level. Ground floor will be slab -on -grade. Cut depths are expected to range between about 4 to 9 feet. Foundation loadings for this type of construction are assumed to be relatively light and typical of the proposed 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: The property is vacant of structures and accessed by an existing dirt drive. An existing well is located at the end of the drive. Vegetation consists of a pinion and juniper forest with brush, grass and weeds. The site is located on a north facing hillside below an upland rolling mesa. Topography in the building area is steeply sloping down to the northeast at a grade of about 20 to 25 percent. A dry drainage swale is located about 60 feet north of the building area. Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two exploratory pits in the building area and one profile it in the septic field area at the approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 11/2 to 2 feet of topsoil, consist of sandy silty clay with scattered basalt gravel and cobbles. Results of swell -consolidation testing performed on relatively undisturbed samples of the clay, presented on Figures 3 and 4, indicate low to moderate compressibility -2 - under Tight loading and a low to high collapsible potential (settlement under constant Toad) when wetted. The samples were moderately to highly compressible under increased loading after wetting. Results of a gradation analysis performed on a sample of silt loam (minus 34 inch fraction) obtained from the profile pit are presented on Figure 3. 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, below topsoil, designed for an allowable soil bearing pressure of 1,500 psf for support of the proposed ADU. 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 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 50 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. 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 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 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 H -Pt KUMAR -3 - excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum l% 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 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 wail 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 ADU 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 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 ADU. 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 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. 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 H -P KUMAR -4 - 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. ouis Eller Reviewed by: Daniel E. Hardin, P.E. LEE/ksw attachments cki] 4 CIGF,:4' G. /y4, G,..2.„ 1t 24443 ZV Figure 1 — Location of Exploratory Pits 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 /6490 1 1 1 6480 164701 1 r r 1 6460 1i \ 1 6450 i \ 1 / 16440 1 r1 / J \ 1 /! 1 1 \ r \ r \ 1 1\ 1 \ \ 1 i \ 6434 \ ° 1 1 \ 64Y�1 640E EXISTING WELL �° c3 / 1 .' 01 if ii\ b.�' r +— oso I y ' ' 1 \ \ �� • PIT 1 r s� 1 \ 1 1 1 4 I ii f' • PIT 2 1 c 1 1 A. ry0 \ Ii r`'' 4b N. PROFILE PIT �w��-app-� 1 1 \ \ • 11p1V\G / i0~ !i ` 4. \ \ -,- �� i a` Ii \� 1 I boo 2i co tui 1 i r C7 i sr i 30 0 30 60 SCALE -FEET 045 16-7-218 H-P_KUMAR J LOT 7 COTTON HOLLOW LOCATION OF EXPLORATORY PITS Fig. 1 —10 LEGEND PIT 1 EL. 6417' WC=8.3 D0=69 PIT 2 EL. 6408' WC=8.9 D0=89 PROFILE PIT EL. 6402' Z -j 4+=1 1-200=82 r r 0 — 5 — 10— TOPSOIL; ORGANIC SANDY SILTY CLAY, FIRM, SLIGHTLY MOIST TO MOIST, DARK BROWN. r _I CLAY (CL); SILTY, SANDY, SCATTERED BASALT GRAVEL AND COBBLES, MEDIUM STIFF TO STIFF, SLIGHTLY MOIST TO MOIST, BROWN, CALCAREOUS, LOW TO MEDIUM PLASTICITY. HAND DRIVEN LINER SAMPLE. DISTURBED BULK SAMPLE. NOTES 1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE PRIOR TO OUR ARRIVAL ON JULY 21, 2016. 2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED. 3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE APPROXIMATED FROM CONTOURS ON THE SITE PLAN. 4. THE EXPLORATORY AND ELEVATIONS PIT LOCATIONS 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 THE TRANSITIONS MAY BE GRADUAL. 6. GROUNDWATER WAS NOT OBSERVED IN THE PITS AT THE TIME OF OUR SITE VISIT. 7. LABORATORY TEST RESULTS: WC = WATER CONTENT (%) (ASTM D 2216); DD = DRY DENSITY (pcf) (ASTM D 2216); -200 = PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140). 16-7-218 H -P KUMAR LOGS OF EXPLORATORY PITS Fig. 2 2 0 — 2 — 4 —10 z 0 0 --12 J 0 N Z 0 C-) —14 —16 —18 —20 SAMPLE OF: Sandy Silty Clay FROM: Pit 1 0 6.5' WC = 8.3 %, DO = 69 pcf Thum [Mt eurfia wet b M. .4 0 i1M d. The l.�•.e.pl YY. NL Mhrrt u. 110p1n ypeval WIMP NW 1rwqkillf. YC accasImme �0-s6.4. ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING 16-7-218 1.0 APPLIED PRESSURE - KSF H -P= KUMAR 10 SWELL -CONSOLIDATION TEST RESULT Io, Fig. 3 x -2 -3 w - 4 z 0 a c -5 J 0 2 0 U -6 - 7 - 8 sol h. nrroi..ey1 F M111. eros 1..Wilt el rmx .d of Kuno" .w M ooMtr4 Ix. sw uni reww SAMPLE OF: Sandy Silty Clay FROM: Pit 2 0 4.5' WC = 8.9 %, DO = 89 pcf ADDITIONAL COMPRESSION UNDER CONSTANT PRESSURE DUE TO WETTING 16-7-218 1.0 APPLIED PRESSURE — 1(ST H -P KUMAR 10 SWELL -CONSOLIDATION TEST RESULT 400 Fig. 4 PERCENT RETAINED HYDROMETER ANAL' SI$ SIEVE AN TIME READINGS U S STANDARD SERIES ALY15 2p4q W�{pp CLEAR SQUARE OPENINGS 0 45 MIN. 15 MIN 60MIN19MIN.4 MIN.1 111325 0140 060 035 •18 010 04 318' 314' 1 1/2 3' 5'8' 8' 100 10 20 30 40 50 60 70 80 90 100 001 .002 005 009 .019 ,045 ,106 .025 500 1.00 2.00 4 75 9 5 19 0 37.5 76.2 152 203 CLAY 1 SLT DIAMETER OF PARTICLES IN MILLIMETERS V NM 1 FINE 1 WADI -14 1Cqur,E QW6e Swot 1 LEE PIP -4 VR 16-7-218 GRAVEL 2 % SAND 32 % SILT 48 % CLAY 18 % USDA SOIL TYPE: Silt Loam FROM: Prohle Pit at 3 to 5 feet H-Pt-KUMAR USDA GRADATION TEST RESULTS 90 80 70 60 50 40 30 20 10 0 PERCENT PASSING Fig. 5 ri-r ICUIVIAI< TABLE 1 SUMMARY OF LABORATORY TEST RESULTS Job No. 16-7-218 SAMPLE LOCATION NATURAL NATURAL. DRY DENSITY (pcf} GRADATION PERCENT PASSING NO. 200 SIEVE USDA SOIL TEXTURE 1 PIT DEPTH (ft) MOISTURE CONTENT (9/0) GRAVEL (%) A. SAND (%) GRAVEL (%) SAND (%} SILT (%) CLAY (%) SOIL TYPE 1 6' 8.3 69 Sandy Silty Clay 3 4' 8.9 89 Sandy Silty Clay Profile Pit 3 to 5 2 29 51. 18 Silt Loam