Civil Geotechnical Surveying

Transcription

1 Civil Geotechnical Surveying Mr. David Burnett Cabarrus County Schools 4425 Old Airport Road Charlotte, North Carolina May 16, 2017 Reference: Geotechnical Engineering Evaluation Future PLC Site Concord, Cabarrus County, North Carolina CESI Project Number Mr. Burnett: Concord Engineering & Surveying, Inc. (CESI) has performed a geotechnical engineering evaluation of the above referenced site in accordance with our Proposal P00 dated April 21, On May 9, 2017, CESI's Geotechnical Engineer, Edward S. Cummings III, PE observed 10 test pit excavations on the future PLC Site located in an undeveloped area southwest of the existing Winkler Middle School facility. The test pits were performed to evaluate subsurface soil conditions to determine the presence of topsoil, plow zone, unsuitable soils, shallow rock and groundwater. FINDINGS The site was accessed through a metal gate southwest of Winkler Middle School. The site is predominately over grown fields with woods. The procedures used by CESI for field sampling and testing are in general accordance with established geotechnical engineering practice. Ten test pits were excavated using a CAT 315 CL track hoe at the locations shown on the enclosed Test Pit Excavation Location Plan. Soil strata in each test pit was observed, identified and recorded by our geotechnical engineer. Topsoil was not present at any of the test pit locations. However, soils disturbed from past agricultural practices called plow zone soils were present 8 to 16 deep at the surface of test pit locations TP-2 through TP-10. Fill soil consisting of fine sandy SILT (ML) measuring 10 thick was present at the surface of TP-1. Beneath the fill soil we aggregate base course stone used as a base for 45 Spring Street, SW P.O. Box 268 Concord, N.C N.C. License Number C-0263

2 construction activities during middle school construction. The aggregate base course stone was between 10 and 19 below existing grade. Washed in soils called alluvial soils or alluvium were present beneath the plow zone soils at test pit TP-5 The alluvium was between 15 and 30 below present grade. These alluvial soils consisted of wet grayish brown fine sandy SILT (ML) with roots. Weathered in place, residual soils were beneath the plow zone soils, alluvial soils and fill soils. The residual soils consisted of FAT CLAY (CH), ELASTIC SILT (MH), fine sandy SILT (ML) and silty fine SAND (SM). The Natural Resources Conservation Service (NRCS) has listed the site as having the following primary soils series: Enon and Iredell. Based on NRCS published documents, these soil series can have FAT CLAY (CH) between 6 and 28 below existing grade. However, our test pit excavations revealed FAT CLAY (CH) and ELASTIC SILT (MH) to depths between 8 and 42 below ground surface. FAT CLAY (CH) and ELASTIC SILT (MH) have a high shrink swell potential and are unsuitable for direct footing, slab and pavement support. Table 1: Summary of Test Pit Data Test Pit No. TP - 1 TP - 2 TP - 3 TP - 4 TP - 5 TP - 6 TP - 7 TP - 8 TP - 9 TP-10 Topsoil Depth (Inches below existing Ground Surface) Plow Zone (Inches below Existing Ground Surface) Alluvium or Fill (Inches below Existing Ground Surface) 0-10 fill soil aggregate base course fill Fat CLAY (CH) Elastic SILT (MH) (Inches below Existing Ground Surface) Trackhoe Refusal (Inches below Existing Ground Surface) (MH) (CH) 60 and (CH) (CH) (MH) (MH) (MH) 36 and (CH) (MH) (MH) to depths explored Future PLC Site - Geotechnical Engineering Evaluation May 16, 2017 CESI Project Number Page 2

3 Groundwater: Groundwater was in test pit TP-5 in the alluvial soils at 30 below existing grade. The elevation of the groundwater may vary depending upon seasonal factors such as precipitation. Rock: Track hoe refusal was in test pits TP-1 through TP-9 between 30 and 127 below existing ground. TP-10 was terminated at 72 due to heavy rain. See the attached Depth to Track Hoe Refusal Plan. RECOMMENDATIONS Topsoil, root zones, stumps, organics, plow zone, alluvium, fill soils, FAT CLAY (CH) and ELASTIC SILT (MH) and any other unsuitable materials should be stripped beyond the proposed structural and pavement envelopes. In fill sections, the area should be stripped an additional width to accommodate a minimum 1:1 structural fill slope plus 5 feet. Please see the attached Anticipated Stripping / Undercut Depth Plan. After stripping and undercutting, we recommend that the areas to provide support be carefully evaluated for the presence of soft surficial soils by proofrolling with a 25-ton, four-wheeled, rubber-tired roller, a loaded dumptruck, or similar approved equipment. The proofroll operation should be carefully monitored by our Engineer. Areas that wave, rut, or deflect excessively and continue to do so after several passes of the proofroller may need to be undercut to stiffer soils. The undercut areas should be backfilled in thin lifts with approved, compacted fill materials. Due to the presence of fine grained soils on the site, it is imperative that positive surface drainage be maintained during grading operations to prevent water from ponding on the surface. The surface should be rolled smooth to enhance drainage if precipitation is expected. Subgrades damaged by construction equipment should be immediately repaired to avoid further degradation in adjacent areas and to help prevent water ponding. The presence of unsuitable soils at the surface as well as track hoe refusal rock at variable depths should be taken into consideration during civil site design. Fill Material and Placement Recommended criteria for soil fill characteristics and compaction procedures are listed below. The project design documents should include the following recommendations to address proper placement and compaction of project fill materials. We do not recommend the use of soils of high plasticity, with a PI greater than 25 as structural fill. Earthwork operations should not begin until representative borrow soil samples are collected and tested (allow 3 to 4 days for sampling and testing). Earth Fill Materials General guidelines for project fill should control properties such as Plasticity Index (PI), gradation, and organic content. The use of the following USCS soil Future PLC Site - Geotechnical Engineering Evaluation May 16, 2017 CESI Project Number Page 3

4 types, as defined by ASTM D 2487, should be satisfactory for use as project fill: GW, GP, GM, GC, SW, SP, SM, SC, ML, MH (provided the PI is 25 percent or less for MH soils), or combinations thereof. Organic content should be no greater than 5 percent by weight, and no large roots should be allowed. Additionally, maximum particle sizes should be limited to 4 inches or less. Compaction Recommendations One standard Proctor compaction test and one Atterberg limits test for each soil type used as project fill. Gradation tests may be necessary and should be performed at the Project Geotechnical Engineer s discretion. Maximum loose lift thickness 8 inches. Compaction requirements 95 percent of the maximum dry density to a depth of 2 feet below subgrade, and 100 percent within the upper 2 feet as determined by the standard Proctor compaction test. Soil moisture content at time of compaction within plus 3 percent to minus 3 percent of the optimum moisture content. One density test every 2,500 square feet for each lift or two tests per lift, whichever is greater (for preliminary planning only; the test frequency should be determined by our geotechnical engineering staff). Trench fill areas one density test every 75 linear feet at vertical intervals of 2 feet or less. The construction should be monitored by our Construction Materials Testing Engineer. Our Construction Materials Testing Engineer s (or technician) duties should include observation of proofrolling activities, density testing of the soil backfill and density testing of the aggregate base course stone. Field observations, monitoring, and quality assurance testing during earthwork and pavement construction are an extension of this analysis. The building foundations should be sized for a maximum net allowable bearing pressure not to exceed 3,000 pounds per square foot. The exploration findings indicate the structures may be supported by shallow spread footings bearing on stiff residual soil or newly placed structural fill soil. All footings should bear at a minimum depth of 12 inches below exterior grades for frost protection. Based on our site observations and geotechnical findings, difficult excavation due to rock should be anticipated during grading and utility installation operations. As a result, CESI recommends that Rock be defined as the following: General Mass Rock Excavation: Any material which cannot be excavated with a single-tooth ripper drawn by a crawler tractor having a draw bar pull rated at not less than 56,000 pounds (Caterpillar D8K or equivalent) or excavated by a trackhoe having a bucket curling force rated at not less than 33,000 pounds (Caterpillar 315C or equivalent). Future PLC Site - Geotechnical Engineering Evaluation May 16, 2017 CESI Project Number Page 4

5 Trench Rock Excavation: Any material which cannot be excavated with a backhoe having a bucket curling force rated at not less than 33,000 pounds (Caterpillar 315C or equivalent). If these rock excavation conditions are, CESI should be notified for verification and quantification. Excavation of trench rock will then require fracturing using jack hammering or blasting. FOLLOW-UP SERVICES Field observations, monitoring, and quality assurance testing during earthwork, foundation installation and building construction are an extension of the geotechnical design. As a result, we recommend the following: Verification of removal of topsoil, plow zone soils, alluvium, fill soils, FAT CLAY (CH), and ELASTIC SILT (MH); Documentation and quantification of track hoe refusal rock before removal; Proofroll observations of areas to receive fill soils; Density testing during fill soil placement; Density testing for utility trench backfill; Foundation bearing evaluation within excavated footings; Reinforced concrete testing and reinforcing steel inspections; Structural masonry and structural steel inspections; Special inspections required by International Building Code with North Carolina amendments; Proofroll observations within the pavement areas prior to aggregate and asphalt placement. SUMMARY Based on our site observations and findings, once the topsoil, plow zone soils, alluvium, fill soils, FAT CLAY (CH) and ELASTIC SILT (MH) are stripped, the site grading contractor will encounter silty fine SAND (SM) and fine sandy SILT (ML) suitable for the anticipated school construction. We anticipate civil site design can be accomplished to minimize exposure due to the presence of unsuitable soil and track hoe refusal rock. Future PLC Site - Geotechnical Engineering Evaluation May 16, 2017 CESI Project Number Page 5

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7 NORTH 45 Spring Street SW PO Box 268 Concord, North Carolina, Phone: (704) Fax: (704) NC License C-0263 TEST PIT EXCAVATION LOCATION PLAN PROPOSED PLC SITE Concord, North Carolina DATE: DRAWN BY: JGE PROJECT #: SCALE: NTS DRAWING NO.: 1

8 NORTH 45 Spring Street SW PO Box 268 Concord, North Carolina, Phone: (704) Fax: (704) NC License C-0263 ANTICIPATED STRIPPING / UNDERCUT DEPTH PLAN PROPOSED PLC SITE Concord, North Carolina DATE: DRAWN BY: JGE PROJECT #: SCALE: NTS DRAWING NO.: 2

9 NORTH 45 Spring Street SW PO Box 268 Concord, North Carolina, Phone: (704) Fax: (704) NC License C-0263 DEPTH TO TRACK HOE REFUSAL PLAN PROPOSED PLC SITE Concord, North Carolina DATE: DRAWN BY: JGE PROJECT #: SCALE: NTS DRAWING NO.: 3

10 RECORD OF TEST PIT EXCAVATION PROJECT: Future PLC Site at Winkler Middle School EXCAVATOR: CAT 315 CL CLIENT: Cabarrus County Schools DATE: May 9, 2017 CESI REPRESENTATIVE: Edward S. Cummings III P.E. CESI JOB NO: TEST PIT NO. DEPTH (in.) From To TP FILL: Dark brown fine sandy SILT (ML) with roots DESCRIPTION FILL: Aggregate base course from original construction trailer pad RESIDUUM: Yellowish brown ELASTIC SILT (MH) Speckled dark gray and yellowish brown silty fine to coarse SAND (SM) with hard rock fragments, very hard excavation at 27, track hoe refusal on 36 TP PLOW ZONE: Moist dark brown fine sandy SILT (ML) with roots 8 32 RESIDUUM: Moist yellowish brown FAT CLAY (CH) Speckled yellowish brown and white fine sandy SILT (ML) Speckled yellowish brown and white silty fine SAND (SM) with 1 to 7 soft rock fragments, hard excavation from 48 to 84, track hoe refusal on boulder east 60, track hoe refusal on boulder north 84 TP PLOW ZONE: Moist dark brown fine sandy SILT (ML) with roots RESIDUUM: Moist yellowish brown FAT CLAY (CH) Yellowish brown and white silty fine SAND (SM), hard excavation from 42 to 96, track hoe refusal on 96

11 RECORD OF TEST PIT EXCAVATION PROJECT: Future PLC Site at Winkler Middle School EXCAVATOR: CAT 315 CL CLIENT: Cabarrus County Schools DATE: May 9, 2017 CESI REPRESENTATIVE: Edward S. Cummings III P.E. CESI JOB NO: TEST PIT NO. DEPTH (in.) From To DESCRIPTION TP PLOW ZONE: Moist brown fine sandy SILT (ML) with roots 8 24 RESIDUUM: Yellowish brown FAT CLAY (CH) Yellowish brown silty fine SAND (SM), hard excavation from 24 to 48, track hoe refusal on 48 TP PLOW ZONE: Moist dark reddish brown fine sandy SILT (ML) with roots ALLUVIUM: Wet grayish brown fine sandy SILT (ML) with roots, water at RESIDUUM: Yellowish brown ELASTIC SILT (MH) with 1/2 to 1 rock fragments Dark yellowish brown fine sandy SILT (ML) Dark yellowish brown silty fine SAND (SM) with soft 1/2 to 7 rock fragments, hard excavation from 60 to 80, track hoe refusal on rock@ 80 TP PLOW ZONE: Moist dark reddish brown fine sandy SILT (ML) with roots 8 20 RESIDUUM: Reddish yellow ELASTIC SILT (MH) Speckled dark gray and reddish yellow silty fine SAND (SM) with soft 1 to 8 soft to hard rock fragments, hard excavation 20 to 30, track hoe 30

12 RECORD OF TEST PIT EXCAVATION PROJECT: Future PLC Site at Winkler Middle School EXCAVATOR: CAT 315 CL CLIENT: Cabarrus County Schools DATE: May 9, 2017 CESI REPRESENTATIVE: Edward S. Cummings III P.E. CESI JOB NO: TEST PIT NO. DEPTH (in.) From To DESCRIPTION TP PLOW ZONE: Moist dark reddish brown fine sandy SILT (ML) with roots 9 16 RESIDUUM: Moist reddish yellow ELASTIC SILT (MH) Speckled dark gray and white silty fine SAND (SM) with soft to hard 1/2 to 4 rock fragments, massive 36 on northeast wall of excavation, track hoe refusal on 78 on southwest wall of excavation TP PLOW ZONE: Moist red fine sandy SILT (ML) with roots 8 16 RESIDUUM: Reddish yellow fine sandy SILT (ML), rock vein in center of pit running north to south, over rock vein was 8 yellow FAT CLAY (CH) Speckled reddish yellow and black silty fine SAND (SM) Speckled gray and white silty fine SAND (SM), hard excavation from 48 to 90, trackhoe refusal at 72 rock vein in center of pit TP PLOW ZONE: Moist reddish brown fine sandy SILT (ML) with roots 8 24 RESIDUUM: Moist yellowish brown ELASTIC SILT (MH) Moist yellowish brown and gray fine sandy SILT (ML) Moist gray silty fine SAND (SM), hard excavation from 60 to 127, track hoe 127

13 RECORD OF TEST PIT EXCAVATION PROJECT: Future PLC Site at Winkler Middle School EXCAVATOR: CAT 315 CL CLIENT: Cabarrus County Schools DATE: May 9, 2017 CESI REPRESENTATIVE: Edward S. Cummings III P.E. CESI JOB NO: TEST PIT NO. DEPTH (in.) From To DESCRIPTION TP PLOW ZONE: Moist dark reddish brown fine sandy SILT (ML) with roots RESIDUUM: Moist reddish yellow ELASTIC SILT (MH) Speckled dark gray and reddish yellow silty fine SAND (SM) very hard excavation from 24 to 72