REPORT OF PRELIMINARY GEOTECHNICAL EXPLORATION ENKA INTERMEDIATE SCHOOL Sand Hill Road Candler, North Carolina Prepared For: BUNCOMBE COUNTY SCHOOLS Prepared By: AMEC ENVIRONMENT & INFRASTRUCTURE, INC. 1308 Patton Avenue Asheville, North Carolina 28806 AMEC Project No. 6252130078 July 24, 2013
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 TABLE OF CONTENTS 1.0 PROJECT INFORMATION... 1 1.1 Site Information... 1 1.2 Project Information... 1 1.3 Site Geology... 2 2.0 SUBSURFACE EXPLORATION... 2 2.1 Preliminary Geotechnical Exploration... 2 2.2 Environmental Services... 3 3.0 SUBSURFACE CONDITIONS... 3 4.0 PRELIMINARY SEISMIC SITE CLASSIFICATION... 6 5.0 GERAL DISCUSSION OF SITE AND SUBSURFACE CONDITIONS... 6 5.1 Existing Fill... 6 5.2 Shallow Rock and Boulders... 7 6.0 PRELIMINARY FOUNDATION RECOMMENDATIONS... 7 7.0 GERAL CONSTRUCTION RECOMMENDATIONS... 8 7.1 Site Preparation and Erosion Protection... 8 7.2 Excavation Conditions... 8 7.3 Groundwater Control... 8 7.4 Surface Water Control and Moisture Sensitive Soils... 9 7.5 Cut and Fill Slopes... 9 7.6 Suitability of Site Soils for Use as Structural Fill... 9 8.0 QUALIFICATIONS... 9 Attachments Figure 1 Topographic Site Location Map Figure 2 Boring Location Plan Key to Symbols and Descriptions Soil Test Boring Records (18) 2
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 1.0 PROJECT INFORMATION 1.1 Site Information Project and site information was obtained from site visits and conversations with you. Based on this information, we understand that Buncombe County Schools is considering the site located at the intersection of Sardis Road and Sand Hill Road in Candler, North Carolina (Figure 1) for construction of a new Intermediate School. The property is bound by Asheville Commerce Parkway (partially constructed) and Hominy Creek to the north, Sand Hill Road to the east/southeast, undeveloped commercial land to the south (separating the site from Sardis Road), and overhead power lines and existing industrial facilities to the west. The site is currently vacant and partially overgrown with vegetation. There is evidence of past earthwork activity at the site including areas where soil has been cut (in the east/northeast portions of the site) and areas where fill has been placed (northwest portion of the site and for construction of the roadway along the northern border of the site). Rock outcrops are present at the ground surface within the topographically high area near the center of the south side of the site. A pile of boulders, likely removed during excavation of borrow material from the east/northeastern portion of the site, is also located in the topographically high area of the site. A pile of asphalt and concrete debris is located in the topographically low area of the site (in the northwest portion of the site). Existing sewer and stormwater utility lines were observed to be present on the north boundary of the site, extending east from Asheville Commerce Parkway. Several dirt roadways are present throughout the site. A culvert and fill soil was placed to establish an access road near the northwest corner of the site. The fill over and around this culvert is currently unstable and has slid into the adjacent drainage feature on both sides of the culvert. Evidence of erosion is present throughout the site. Erosion gullies (some more than 4 feet deep) are present along the dirt access road on the west side of the site and along the slope at the western border of the site. 1.2 Project Information The layout of the site and the location of the school building are still being considered at this time. However, we understand that the building will likely be located near the center of the site and that the playing fields will be located on the western portion of the site (near the overhead power lines). The proposed traffic pattern is still being considered at this time; however, we understand that the site will connect with Asheville Commerce Parkway and Sand Hill Road. Buncombe County Schools plans to utilize the prototype building design that was used for their two other recently constructed intermediate schools. This building consists of a two-story structural masonry building supported by shallow foundation and grade-slabs. 1
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 1.3 Site Geology The project site is located in the Blue Ridge Physiographic Province. The bedrock in this province is a complex mixture of igneous, sedimentary and metamorphic rock that has been repeatedly squeezed, fractured, faulted and distorted by past tectonic movements. The virgin soils encountered in this area are the residual product of in-place weathering of rock, which was similar to the rock presently underlying the site. In areas not altered by erosion or disturbed by the activities of development, the typical residual soil profile consists of clayey soils near the surface, where soil weathering is more advanced, underlain by sandy silts and silty sands. The less weathered soils exhibit relict features of the parent rock, including foliation patterns and joints. The boundary between soil and rock is not sharply defined. This transitional zone termed "partially weathered rock" (PWR) is normally found overlying the parent bedrock. Partially weathered rock is defined, for engineering purposes, as residual material with standard penetration resistance values in excess of 100 blows per foot. Fractures, joints, and the presence of less resistant rock types facilitate weathering. Consequently, the profile of the partially weathered rock and hard rock is quite irregular and erratic, even over short horizontal distances. Also, it is not unusual to find lenses and boulders of hard rock and zones of partially weathered rock within the soil mantle, well above the general bedrock level. 2.0 SUBSURFACE EXPLORATION 2.1 Preliminary Geotechnical Exploration For this preliminary exploration, AMEC drilled eighteen soil test borings at the approximate locations shown on Figure 2. Eleven soil test borings were drilled in a predetermined grid pattern across the site. Seven additional boring locations were selected in order to explore the area where shallow rock is present. The boring locations were selected and marked in the field by AMEC. The as-drilled locations were later surveyed by McGill Associates, PA and will be shown on the topographical site survey drawing (which was not yet available at the time of this report). An existing topographical survey prepared by Ed Holmes and Associates, PA (and spot checked by McGill) was used during this evaluation of site and subsurface conditions and is shown on the attached Boring Location Plan (Figure 2). The boring locations and ground surface elevations at the boring locations were provided by McGill. The soil test borings were drilled by mechanically twisting hollow-stem augers into the soil. Soil sampling and penetration testing were performed in general accordance with ASTM D1586. At assigned intervals, soil samples were obtained with a standard 1.4-inch I.D., 2-inch O.D. split-spoon sampler. The sampler was first seated 6 inches to penetrate any loose cuttings, and then driven an additional 12 inches with blows of a 140-pound hammer falling 30 inches. The number of hammer blows required to drive the sampler the final 12 inches was recorded and is designated the "N- 2
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 Value" or "penetration resistance". The N-Value, when properly evaluated, is an index to soil strength and foundation supporting capability. Representative portions of split spoon samples were sealed in glass jars and returned to our laboratory where they were visually classified by a geotechnical engineer. Depth to groundwater was measured in borings B-1 through B-11 at the time of drilling and after a period of 24 hours. The boreholes were backfilled with auger cuttings after being sounded for groundwater. Borings B-12 through B-18 were backfilled upon completion; therefore, 24-hour groundwater levels were not measured (the primary intent of these borings was to determine the presence and depth of rock). 2.2 Environmental Services A Phase I Environmental Site Assessment including regulatory file reviews and limited environmental sampling was also conducted. In order to collect groundwater samples for environmental testing, temporary sampling points were installed within five of the boreholes during our preliminary exploration. The temporary sampling points were removed and the boreholes were backfilled with grout after collection of the groundwater samples. The results of our environmental services were presented under separate cover. 3.0 SUBSURFACE CONDITIONS The following descriptions provide a general summary of the subsurface conditions encountered. The Test Boring Records represent our interpretation of the field drilling logs based on engineering examination of the field samples. The lines designating the interfaces between various strata represent approximate boundaries and the transition between strata may be gradual. It should be noted that subsurface conditions will vary between the boring locations. In general, the soil test borings encountered fill, alluvium, residuum, partially weathered rock, and auger refusal material/rock as described below and tabulated in Table 1. Topsoil: Topsoil was not encountered at the ground surface at the test boring locations. Fill / Alluvium: Fill and alluvial soils were encountered in Borings B-1 and B-2 which are located within the access road along the north border of the site. Fill soil was likely placed over alluvial soil to create the existing roadway and during installation of the existing underground utilities. These soils were sampled as firm to stiff sandy and clayey Silt, very loose to very firm silty and clayey Sands, and dense Gravel. The fill / alluvium was encountered to depths of 22 feet in boring B-1 and 16 feet in boring B-2. N-values obtained in the fill and alluvium ranged from 3 to 32 pbf. The higher N-values were likely caused by rock pieces. 3
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 Fill and alluvium where also encountered in boring B-3. Fill sampled as firm to very stiff sandy silt and loose to firm silty sand was encountered to a depth of 27 feet. Alluvial soil sampled as loose sand with plastic silts and clayey plastic silt was encountered from 27 feet deep to the boring termination depth of 35 feet. N-values in this fill ranged from 6 to 19 bpf. Fill: Fill soil was also encountered in boring B-5. Fill sampled as loose silty sand was encountered to a depth of 3 feet and soft sandy silt was sampled from depths of 3 to 8 feet. Topsoil was sampled within the fill in the 6 to 7.5 feet sample; therefore, this area of the site was not likely stripped prior to the placement of fill. Residuum: Residual soil was encountered below the fill and alluvial soils in borings B-1, B-2, and B-5 and at the ground surface in the remainder of the borings. In general, the residual soil was sampled as loose to very dense silty sand and firm to very stiff sandy silt. Partially Weathered Rock and Auger Refusal: Partially weathered rock and auger refusal was encountered at the approximate depths and elevations shown in Table 1, ranging from as shallow as 1 foot (in boring B-12) to as deep as 29 feet (in boring B-1). Groundwater: Borings B-1 through B-11 were sounded for presence of groundwater at the time of boring and after a stabilization period of at least 24 hours and the results were recorded on the Test Boring Records. The stabilized groundwater levels in borings B-1, B-2, B-3, B-6, and B-11 were measured within the temporary sampling points. Borings B-12 through B-18 were backfilled upon completion; therefore, 24-hour groundwater levels were not recorded. A summary of the encountered groundwater levels is included in Table 1. 4
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 Boring No. B-1 B-2 B-3 Fill and Alluvium 0 22 (2063-2041) 0 16 (2047-2031) 0 35 (2080-2045) B-4 B-5 0 8 (2097-2089) B-6 B-7 B-8 B-9 B-10 B-11 B-12 B-13 B-14 B-15 Table 1. Summary of Subsurface Conditions Strata Depths and Elevations, feet Residual Soil 22 27 (2041-2036) 16 23 (2031-2024) 0 25 (2095-2070) 8 25 (2089-2072) 0 60 (2109-2049) 0 25 (2094-2069) 0 25 (2098-2073) 0 14 (2106-2092) 0 25 (2105-2080) 0 35 (2097-2062) 0 2 (2102-2100) 0-8 (2099-91) 12-20 (2087-79) 0 7 (2105 2098) 0-5.5 (2100-2094) 12-17 (2088-2083) B-16 0 6.5 (2105-2098.5) B-17 0 1 (2104 2103) B-18 0 14.8 (2108-2093.1) Partially Weathered Rock 27 29 (2036-2034) 23 23.3 (2024.3-2024) 14 16 (2092-2090) 2 2.5 (2100-2099.5) 8 12 (2091 2087) 7 12 (2098 2093) 5.5-12 (2094-88) 17-18.2(2083-81) 6.5 13 (2098.5-2092) Boring Termination or Auger Refusal AR @ 29 (AR @ 2034) AR @ 23.3 AR @ 2024 BT @ 35 (BT @ 2045) BT @ 25 (BT @ 2070) BT @ 25 (BT @ 2072) BT @ 60 (BT @ 2049) BT @ 25 (BT @ 2069) BT @ 25 (BT @ 2073) AR @ 16 (AR @ 2090) BT @ 25 (BT @ 2080) BT @ 35 (BT @ 2062) AR @ 2.5 (AR@2099.5) AR @ 20 (AR @ 2079) AR @ 12 (AR @ 2093) AR @ 18.2 (AR @ 2081.4) AR @ 13 (AR @ 2092) AR @ 1 (AR @ 2103) AR @ 14.8 (AR @ 2093.1) PWR Partially Weathered Rock BT Boring Termination without auger refusal. AR Auger Refusal Groundwater level readings were taken at least 24 hrs after drilling unless otherwise noted. TOB Time of Boring Not Encountered Groundwater 15.8 (2046.8) 8.5 (2038.8) 17.7 (2061.1) 20 @ TOB (2075@TOB) 41.1 (2067.4) 9 (2085) 9 (2089) 28.6 (2086.2) 19.5 @ TOB (2079.5@TOB) 18.2 (2081.4) 5
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 4.0 PRELIMINARY SEISMIC SITE CLASSIFICATION Seismic site class is calculated based on correlation of the average N-value with soil shear wave velocity for the top 100 feet below the proposed building elevations in accordance with Section 16 of the North Carolina Building Code. Our experience in the site area has shown that it is reasonable to assume that material with standard penetration resistances (N-values) equal to or greater than 100 blows per foot will exist below auger refusal, where auger refusal is encountered below a layer of increasing harder residual soil. Because the location and finish floor elevation of the new Intermediate School building has not been selected at this time, a seismic site class cannot be assigned. However, if the building is located near the center of the site, where minimal fill/alluvial soil was encountered, a Seismic Site Class of C or D will be available. For preliminary design purposes, a Seismic Site Class of D may be assigned. Subsurface conditions similar to those encountered in boring B-3 will warrant additional exploration and consideration prior to assigning a Seismic Site Class. 5.0 GERAL DISCUSSION OF SITE AND SUBSURFACE CONDITIONS In general, the site and subsurface conditions are favorable for construction of a new Intermediate School. Two items identified during this exploration that could affect the ease and cost of construction are the existing fill and the shallow rock and boulders. 5.1 Existing Fill Remedial site repair may be necessary in areas where non-engineered fill and alluvium is present. The existing fill present along the roadway at the northern border of the site (as sampled in borings B-1 and B-2) will need to be reworked in order to provide positive support for asphalt-paved roadways and parking areas. The subsurface conditions along the roadway will likely vary and the amount of reworking necessary will depend on the time of year and amount of construction traffic in this area. Recommendations for reworking the subgrade should be provided during construction based on the actual site conditions but may include recommendations to remove and recompact two to three feet of soil below the proposed roadway subgrade elevation or placement of geogrid reinforcement. The fill soil in this area of the site includes sandy and clayey silts which are highly sensitive to moisture and may be difficult to work with during rainy times of the year. The existing fill encountered in boring B-3 is suitable for positive support of the proposed ballfields. However, approximately 5.5 feet of sandy silt was encountered at the ground surface. In general, silty soils are generally not free draining and therefore ponding of rainwater and surface water may occur if the ground surface is not sloped to promote drainage. 6
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 5.2 Shallow Rock and Boulders The cluster of borings located near the center of the site show that the location and depth of rock varies greatly within short horizontal distances. The rock present at the ground surface is thought to be boulders within the soil matrix and not continuous bedrock. A pile of boulder is also present in this area of the site. It is likely that these boulders were removed from the soil during previous excavation of borrow from the site and placed in the area where excavation was determined to be the most difficult due to the presence of rock/boulders. During mass grading activities, boulders present within the soil matrix may be able to be removed with large excavation equipment with ripping tools and/or pneumatic hammers. However, continuous bedrock and large boulders present above the proposed site elevations and within trenches for underground utilities and building foundation excavations will likely require blasting for removal. 6.0 PRELIMINARY FOUNDATION RECOMMENDATIONS Based on the existing topography of the site, we anticipate that both cut and fill will be required to establish a level site for construction of the proposed Intermediate School building. Based on the available boring data and our experience with similar soil conditions, the subsurface conditions are favorable for support of placement of compacted structural fill and/or shallow foundation support of the proposed Intermediate School building (up to two stories tall). The residual soils present in the center and eastern portions of the site are suitable for support of maximum allowable bearing pressures in the range of 3,000 to 5,000 psf. Foundations bearing in structural fill (compacted to at least 95 percent of the standard Proctor maximum dry density) or a combination of residual soil and compacted fill may be designed for an allowable bearing pressure of 3,000 psf. A final design foundation bearing pressure can be provided after site layout plans and elevations have been selected and the subsurface conditions confirmed by additional exploration within the selected construction areas, if required. If the proposed building is located in the area of boring B-5, some reworking of the existing fill will be required prior to placement of structural fill or construction of shallow foundations. Depending on final site grades, this could involve complete removal and replacement of this fill. Recommendations for construction of shallow foundations can be provided during the design phase of this project. 7
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 7.0 GERAL CONSTRUCTION RECOMMENDATIONS 7.1 Site Preparation and Erosion Protection In general, vegetation and surface soils containing organic matter or other deleterious materials should be stripped and removed from the proposed construction limits. The asphalt and concrete debris located in the northwest portion of the site should be removed from the construction area. At this time, evidence of on-going erosion, such as deep erosion gullies and washed-out fill slopes, is present at the site. Assuming that construction of the proposed Intermediate School building and campus will not begin immediately; we recommend that erosion control measures be installed at the site in order to avoid degradation of otherwise suitable site subgrade. Erosion control measures should include stabilizing currently unstable areas of the site (around the culvert on the north side of the site), filling existing erosion control gullies, and establishing ground cover/grass. 7.2 Excavation Conditions As discussed in Section 5.2, boulders present within the soil matrix may be able to be removed with large excavation equipment with ripping tools and/or pneumatic hammers. However, continuous bedrock and large boulders present above the proposed site elevations and within trenches for underground utilities and building foundation excavations will likely require blasting for removal. In general, partially weathered rock with N-values ranging from 50 blows per 6 inches to 50 blows per 3 inches can often be excavated with bulldozers or powerful tractor-drawn rippers without blasting, although often with difficulty. Much can depend on the quality of the equipment and the experience of the operators, as well as the nature of the material being excavated (i.e., presence and direction of more weathered seams, bedding planes, etc.). Partially weathered rock harder than 50 blows per 2 inches generally requires blasting for efficient excavation. Excavation below refusal depths in the borings will require blasting for removal of the refusal materials. 7.3 Groundwater Control Groundwater was encountered at depths as shallow as 8.5 feet below the existing ground surface elevations. The encountered groundwater levels (summarized in Table 1) should be compared to the proposed site grades (when available) to determine the extent of groundwater control that will be required during construction. 7.4 Surface Water Control and Moisture Sensitive Soils Sandy and clayey silts were encountered near the existing ground surface in our soil test borings. This type of soil is sensitive to moisture changes and will require additional time to dry and rework if the moisture content is much above optimum. In order to avoid degradation of otherwise suitable 8
Enka Intermediate School Report of Preliminary Geotechnical Exploration July 24, 2013 AMEC Project No. 6252130078.01 soil subgrade, it will be important to maintain control of surface water at the site. This has been accomplished on past projects by sloping the ground surface to promote drainage and rolling the ground surface at the end of each day with a smooth drum roller. 7.5 Cut and Fill Slopes Cut slopes may be constructed as steep as 2:1 (horizontal:vertical). Fill slopes should be constructed on a suitable firm foundation. Where normal slope maintenance is desired, we recommend fill slopes be constructed at 2.5:1 (horizontal:vertical), or flatter. It has been our experience with soils similar to those encountered at the site, that permanent slopes constructed steeper than 2:1 (horizontal:vertical), may exhibit surficial erosion and/or sloughing during periods of heavy rain or prolonged rainfall. All slopes should be seeded and mulched as soon as practical to prevent surface erosion. Permanent slopes constructed at 3:1 (horizontal:vertical) or flatter would be desirable for mowing. 7.6 Suitability of Site Soils for Use as Structural Fill Based on our experience with soil similar to that encountered at the site, the on-site soil should be suitable for use as compacted structural fill. In general, soils containing more than 5 percent (by weight) fibrous organic materials or having a Plasticity Index (PI) greater than 30 (less than 20 is recommended for soils within 5 feet of the finished floor elevation) should not be used for fill. The sandy and clayey silts present at the site are sensitive to moisture changes and will require additional time to dry and rework if the moisture content is much above optimum. Partially weathered rock material may be used for structural fill provided it can be broken down by the excavation and compaction equipment into particles with maximum dimensions of 6 inches. 8.0 QUALIFICATION OF REPORT This geotechnical exploration was preliminary in nature and does not attempt to represent the subsurface conditions between the boring locations. The recommendations provided in this preliminary report should be confirmed or amended as appropriate during the final design phase of this project. The assessment of site environmental conditions for the presence of pollutants in the soil, rock, or ground water of the site is beyond the scope of this exploration. 9
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