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HomeMy WebLinkAboutGeotechnical Study - Foundation Design & Perc Test 12.05.2018December 5, 2018 Dave Poulsen 193 Red Bluff Vista Glenwood Springs, Colorado 81601 ciavepoul;cnu`L inti H-P�KUMAR Geotechnical Engineering I Engineering Geology Materiels Testing I Environmental 5020 County Road 154 Glenwood Springs, CO 81601 Phone: (970) 945-7988 Fax: (970) 945-8454 Email: hpkglenwood@kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado RECEIVED GARFIELD COUNTY COMMUNITY DEVELOPMENT Project No.18-7-633 Subject: Geotechnical Study for Foundation Design and Percolation Testing, Proposed Residence, Lot 14, Pinyon Peaks Subdivision, Maroon Mesa Road, Garfield County, Colorado Dear Dave, As requested, H-P/Kumar performed a geotechnical study and percolation testing for foundation and septic disposal designs at the subject site. The study was conducted in accordance with our proposal for geotechnical engineering services to you dated October 10, 2018. The data obtained and our recommendations based on the proposed construction and subsurface conditions encountered are presented in this report. Proposed Construction: The proposed residence has not been designed but will be located on the site in the area of Pits 1 and 2, as shown on Figure 1. Ground floors may be structural over crawlspace or slab -on -grade. Cut depths are expected to relatively shallow and range between about 2 to 5 feet. Foundation loadings are assumed to be relatively light and typical of residential type of construction. The septic disposal system is proposed to be located downhill to the south 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 site is located off the north end of the Maroon Mesa Road cul-de-sac in open pinyon forest. The building site slopes down to the south at grades of about 5 to 10 percent. Below the building area, the grade is about 15 percent down to the south. There is a low ridge of Maroon Formation sandstone/siltstone bedrock located just to the north of the proposed building area. -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 locations shown on Figure 1. The logs of the pits are presented on Figure 2. The subsoils encountered, below about 1 foot of root zone, highly weather bedrock, silty sand with rock fragments, consist of weathered Maroon Formation sandstone/siltstone bedrock down to practical digging refusal to the baekhoe at 4 to 4'%z feet. The bedrock bedding is relatively flat. No free water was observed in the pits at the time of excavation and the weathered rock was slightly moist. Foundation Recommendations: Considering the subsurface conditions encountered in the exploratory pits and the nature of the proposed construction, we recommend spread footings placed on the undisturbed natural weathered bedrock designed for an allowable soil bearing pressure of 4,000 psf for support of the proposed residence. Footings should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed bedrock fragments encountered at the foundation bearing level within the excavation should be removed and the footing bearing level extended down to the undisturbed natural weathered bedrock. 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. Shallow, frost -protected footings can also be used. 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 be designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at least 45 pcf for the on -site weathered bedrock (minus 5 inch in size) as backfill. Floor Slabs: The natural on -site weathered bedrock is 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 (3/,-inch screened rock). H-P%KUMAR Project No. 18-7-633 -3- 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 on -site processed soils and rocks devoid of vegetation, topsoil and oversized rock. Underdrain System: Although free water was not encountered during our exploration, it has been our experience where bedrock is shallow 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 (3/-inch screened rock). The drain gravel backfill should be at least 1'h 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 weathered bedrock 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. H-P*KUMAR Project No 18-7-633 -4- 4) Roof downspouts and drains should discharge well beyond the limits of all backfill. Percolation Testing: Percolation tests were conducted on October 18, 2018 to evaluate the feasibility of an infiltration septic disposal system at the site. Two profile pits and adjacent shallow percolation holes were dug at the locations shown on Figure 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. The soils exposed in the percolation holes are similar to those exposed in the Profile Pits shown on Figure 2 and consist of about 1 foot of root zone over weathered Maroon Formation bedrock. Results of a USDA gradation analysis performed on a sample of excavated weathered bedrock (minus 1'/2 inch fraction) obtained from the site are presented on Figure 3. The percolation test results ate presented in Table 1. Based on the shallow bedrock, the tested area is probably not suitable for a conventional infiltration septic disposal system. We expect that a mounded system with a sand bed filter will be needed. An 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 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 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. 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 H-P%KUMAR Project No. 18-7-633 -5- 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, H-PKUMAR Daniel E. Hardin, P. Reviewed by Steven L. Pawlak, P.E. DEH/kac attachments Figure 1 — Location of Exploratory Pits Figure 2 — Logs of Exploratory Pits Figure 3 - USDA Gradation Test Results Table 1 — Percolation Test Results H-P%KUMAR Project No 18-7-633 LOT 15 ciN Neighbor's 0'it Driveway Found /5 Rebor with 1 1/4" Orange Plastic Cop (illegible) S 5722 J3 W, a 93' 50 0 50 100 APPROXIMATE SCALE —FEET LOT 14 Found /5 Rebor with 1 1/4' Orange Plastic Cop L S /15710 S 2724 56" W, 0.66' Edge of Pavement LOT 13 18-7-633 H-P--A5KUMAR LOCATION OF EXPLORATORY PITS Fig. 1 PIT 1 PIT 2 PROFILE PIT 1 PROFILE PIT 2 L__5 LEGENP 0 1- - W - GRAVEL=15 _ - 1SAND=53 SILT=26 EL CLAY=6 0 TOPSOIL; ROOT ZONE, HIGHLY WEATHERED BEDROCK, SILTY SAND WITH ROCK FRAGMENTS, FIRM. MOIST, REDDISH BROWN. f WEATHERED BEDROCK; SANDSTONE/SILTSTONE, HARD, SLIGHTLY MOIST, RED, FLAT BEDDING, MAROON FORMATION. DISTURBED BULK SAMPLE. PRACTICAL DIGGING REFUSAL. NOTES 5 - 1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON OCTOBER 18, 2018. 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 NOT MEASURED AND THE LOGS OF THE EXPLORATORY PITS ARE PLOTTED TO DEPTH. 4. THE EXPLORATORY 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 ENCOUNTERED IN THE PITS AT THE TIME OF DIGGING. PITS WERE BACKFILLED SUBSEQUENT TO SAMPLING. 7. LABORATORY TEST RESULTS: GRAVEL = PERCENT RETAINED ON NO. 10 SIEVE; SAND = PERCENT PASSING NO. 10 SIEVE AND RETAINED ON NO. 325 SIEVE; SILT = PERCENT PASSING NO. 325 SIEVE TO PARTICLE SIZE .002MM; CLAY = PERCENT SMALLER THAN PARTICLE SIZE .002MM. 18-7-633 H-P KUMAR LOGS OF EXPLORATORY PITS Fig. 2 H-PKUMAR TABLE 1 PERCOLATION TEST RESULTS PROJECT NO. 18-7-633 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 -- 28 _____ 28 10 Water added 5 4% '/: 40 41/2 4 '4 4 3% _ '/4 3% 3% 1/2 51/2 5 % 5 4% % 4% 41/2 '/, 41/2 41/4 % P-2 10 Water added 6 5' % 17 51/4 41/2 ' 41/2 3% % 3' 3 3/, 6' 6 ' 6 5'/s 1 51/2 5 '/s 5 4'/. 24 Note: Percolation test holes were hand dug in the bottom of backhoe pits. Percolation tests were conducted on October 18, 2018. The average percolation rates were based on the last three readings of each test.