March 10, 2017 SITE AND PROJECT INFORMATION

Transcription

1 March 10, 2017 Derek Hawkes, PE NASH COUNTY 120 W. Washington Street - Suite 2004 Nashville, NC Re: Report of Subsurface Investigation Middlesex Corporate Park, Lot 3 Middlesex, North Carolina GeoTechnologies Project No EA Dear Mr. Hawkes: GeoTechnologies, Inc. has completed the authorized subsurface investigation to evaluate site grading and foundation support considerations for a proposed warehouse/office building which will be constructed on Lot 3 of the Middlesex Corporate Park in Middlesex, North Carolina. Subsurface conditions at the site were investigated by drilling five test borings at the approximate locations shown on the attached Figure 1. Additionally, temporary piezometers were installed in boring B-3 and in the center of the proposed building on Lot 2 (W-1). The test locations were established in the field using a hand held Trimble GPS unit with State Plane coordinates obtained from a provided CAD plan. Elevations were estimated from the provided site plan. As such, the indicated boring locations and elevations should be considered approximate. A summary of the boring coordinates and elevations are presented on the attached Table 1. The borings were advanced to termination depths of about 20 to 25 feet beneath existing site grade using standard penetration test procedures at selected intervals to evaluate the consistency and density of the subsurface soils. The piezometer in W-1 was extended to 34.5 feet. This report presents the findings of the investigation and our recommendations for site grading and foundation support. SITE AND PROJECT INFORMATION The site is located off Middlesex Corporate Parkway in Middlesex, North Carolina. The site is partially wooded and partially cleared and undeveloped. Topography on the site slopes to the northeast with elevations of about 232 to 216 in the building and proposed parking areas. We understand that the proposed project will involve construction of an approximately 60,000 square foot single story warehouse type structure. Detailed structural information was not provided currently. We expect that the structure will be supported on a combination of individual columns and load bearing walls. Column loads are not expected to exceed 100 kips with wall loads of 6 to 9 klf. No site grading plan was provided and we are assuming the new structure will be constructed near existing grade. In addition to the building, parking and driveway areas will be constructed on the north and west side of the building. SUBSURFACE CONDITIONS A generalized subsurface profile prepared from the test boring data is attached to this report as Figure 2 to graphically illustrate subsurface conditions encountered at this site. More detailed descriptions of the

2 Nash County Re: Middlesex Corporate Park, Lot 3 March 10, 2017 Page: 2 conditions encountered at the individual test boring locations are then presented on the attached test boring records. Subsurface conditions on the site are characterized by near surface topsoil which generally extended to depths of 2 to 12 inches below existing grade. The topsoil was underlain by fill which consisted of firm to stiff clay and silt with penetration resistances of 6 to 11 blows per foot (bpf). Underlying the topsoil and fill, the borings encountered residual soils which have formed in place by weathering of the parent bedrock. The residual soils typically consisted of near surface clays that transition to silts with increasing depth. Penetration resistances within the residual soils varied from 4 to 87 bpf. The penetration resistances generally increased with increasing depth. The residual soils were encountered to the boring termination depth of 20.5 feet in boring B-4. In the remainder of the borings, partially weathered rock (PWR) was encountered at depths of 10 to 19.5 feet below existing grade. PWR is defined as material that could be excavated with the soil drilling augers but which exhibited penetration resistances in excess of 100 bpf. The PWR was encountered to about 20 feet, the boring termination depths at locations B-1, B-2 and B-5. Boring B-3 was extended deeper in an attempt to encountered groundwater. That boring encountered auger refusal on harder rock at 24.5 feet below grade. Groundwater was not encountered in the borings at the time of boring completion. An exception was the temporary piezometer on Lot 2 where groundwater was encountered at 23 feet. However, the site is comprised of fine grained soils which are conducive to the development of a temporarily higher perched groundwater condition following periods of inclement weather. Additionally, regional groundwater levels will fluctuate with seasonal and climatic changes and may be different at other times. RECOMMENDATIONS The following recommendations are made based upon a review of the attached test boring data, our understanding of the proposed construction, and past experience with similar projects and subsurface conditions. Should site grading or structural plans change significantly from those now under consideration, we would appreciate being provided with that information so that these recommendations may be confirmed, extended, or modified as necessary. Additionally, should subsurface conditions adverse to those indicated by this report be encountered during construction, those differences should be reported to us for review and comment. Site Grading Considerations. The onsite soils are moisture sensitive and we recommend that earthwork be performed during the typically drier months of May through the end of October. During that time any wet soils can likely be dried by aerating the soils. We recommend that the contractor be required to provide a disc and farm tractor to implement drying operations should the soils be wet of optimum at the time that grading proceeds. Disking and drying is generally effective when the soils are turned approximately every 45 to 90 minutes during favorable drying conditions with a farm tractor pulling an aeration disc. Turning the soils once or twice a day will be ineffective in achieving significant drying. If the site is graded during the months of November through April higher soil moisture contents are expected due to the lower ambient area temperatures and correspondingly lower moisture evaporation rates. During these months disking and drying operations will be ineffective and areas of wet and unstable soils will need to be undercut and replaced with a select off site granular borrow source. The magnitude of required subgrade repair and subsequently the overall site grading costs will be greatly reduced with summer grading between May and the end of October. Site grading should begin with removal of all vegetation and topsoil from those areas designated for the new construction. We anticipate that topsoil thickness will generally be 6 inches or less. However, due to the wooded nature of the site, deeper stripping thickness may be required to remove the root mat. Following removal of these materials, those areas at grade or designed to receive fill should be proofrolled with a partially

3 Nash County Re: Middlesex Corporate Park, Lot 3 March 10, 2017 Page: 3 loaded dump truck or similar piece of equipment in order to identify those areas needing additional repair. Any area which ruts or pumps excessively in the opinion of the engineer should be undercut to firm bearing, or as directed by the engineer. Based on the results of the soil test borings we expect that some repairs will be required at boring B-2 where soft clay was encountered in the upper 4 feet. Additionally, due to the presence of older fill on the site, it is possible that some additional repairs may be required in the upper 2 to 3 feet depending on final grades. The need for repairs can be reduced by grading the site during periods of warmer and dryer weather. The on-site soils, excluding topsoil and any fill with debris or organics, will be suitable for reuse as structural fill provided compaction moisture can be maintained near optimum during placement. We recommend that all fill materials be compacted to a minimum of 95% of the standard Proctor maximum dry density except in the final foot beneath pavements and floor slabs where this requirement should be increased to 98% of the standard Proctor maximum dry density. Additionally, all soils should be moisture conditioned to within 2% of optimum moisture content in order to facilitate compaction operations and to maintain stability of any fill sections. Grading the site during the warmer summer months of the year will greatly facilitate the grading operations and help minimize the repairs due to wet of optimum soils. Difficult Excavation Considerations. As previously discussed, partially weathered rock was encountered on the site at depths as shallow as 10 feet below existing grade. No site grading plan was provided so we do not know how deep cuts may be. Our experience has been that PWR exhibiting resistances of 50 blows per 2 inches of penetration and softer can generally be preloosened with a D-8 dozer drawing a single tooth ripper during general site grading. Material harder than 50 blows per 2 inches of penetration generally requires blasting to remove with conventional equipment. In confined excavation, such as utility trenches, a large track hoe such as a CAT 320 or equivalent equipped with rock teeth can excavate materials having standard penetration resistances in the range of 50 blows per 4 inches to 50 blows per 6 inches. However, the rate of excavation is slow and utility contractors will typically request a trench rock price for excavation of any partially weathered rock materials. Light blasting of these materials will expedite utility installation. Foundation Support Considerations. Following repair of any soft near surface soils or deleterious old fill, the structure can be supported on shallow foundations. Spread footing foundations can be sized for a contact stress of 3,000 psf subject to the restriction that column and wall footings have least dimensions of not less than 24 and 16 inches, respectively. These foundations may bear at nominal depth beneath finished exterior grade except that a minimum embedment of not less than 12 to 18 inches is recommended for frost protection. We expect that column loads of up to 100 kips will exhibit total settlements of less than 1 inch. If higher loads are expected we should be provided with that information, as well as the site grading plan, so that estimated settlements can be re-evaluated. We recommend that all foundation subgrades be carefully inspected by performing hand auger and dynamic hand cone penetrometer (DCP) borings in the foundations. We do recommend that all foundations be protected against inclement weather after excavation to prevent softening of the exposed subgrades. Retaining Wall and Slope Considerations. If any segmental retaining walls are required to provide grade transitions, the on-site silty and clayey soils are not considered well suited for use as backfill material within the reinforced zone for walls over 5 feet in height. An acceptable off site material would be quarry screenings for use in the reinforced zone of the wall. Other sources could be considered; however, testing will need to be performed on the selected materials to verify that they are suitable for this use.

4 Nash County Re: Middlesex Corporate Park, Lot 3 March 10, 2017 Page: 4 We recommend that any fill slopes on the site be constructed at angles of no steeper than 2.5H:1V or flatter in order to provide for long-term stability during the wetter portions of the year. All fill materials placed in slopes should be compacted to a minimum of 95% of the standard Proctor maximum dry density all the way out to the slope face. Additionally, the slope should be quickly vegetated to help minimize erosion of the slope face. Pavement Design Considerations. Neither field nor laboratory CBR tests have been performed on representative samples from this site. However, based on past experience with soils in this area, properly prepared subgrades will exhibit a design subgrade CBR value on the order of 5%. Once a tenant has been identified and site grades are known we recommend that subgrades be tested to determine CBR values for design of pavement sections. The most important factors affecting pavement life in the area of the site are the condition of the subgrade immediately prior to base course stone placement and post-construction drainage. It is recommended that site grades be detailed to promote positive drainage away from pavement areas and that a drainage swale be installed on the uphill side of all pavement areas to intercept and divert perched water which may otherwise occur during the wetter winter months of the year. Since the near surface soils are relatively fine grained, consideration should also be given to installing drains within planter islands and irrigated areas to intercept any perched water which occurs as a result of those open natural areas. Seismic Design Considerations. Based on the results of the test borings and past experience in this area, the subsurface profile is a site class D relative to seismic design considerations. SUMMARY In summary, subsurface conditions are suitable for support of the proposed structure on shallow foundations, designed for a contact pressure of 3,000 psf. Some older fill exists on the site and some near surface repairs should be expected depending on final grades. To help minimize the need for repairs, we recommend to avoid grading during inclement weather or during the winter when soil moisture contents are typically higher. GeoTechnologies, Inc. appreciates the opportunity to be of service on this phase of the project. Please contact us if you have any questions concerning this letter or if we may be of additional service on this or other projects. Sincerely, GeoTechnologies, Inc. DLI/pr-els Attachments L:\ EA\ ea-Sub.docx David L. Israel, P.E. NC Registration No

5 Nash County Re: Middlesex Corporate Park, Lot 3 March 10, 2017 Page: 5 TABLE 1 Summary of Boring Coordinates and Elevations Boring # State Plane Coordinates (ft) Elevation (ft) Northing Easting B B B B B W

6 B-1 B-3 B-2 B-4 B-5 W (' 6(37,& $5($ FIGURE 1

7 Elevation (Feet) GENERALIZED SUBSURFACE PROFILE LEGEND Topsoil B B B B Low Plasticity Silt Low Plasticity Clay Partially Weathered Rock Possible Fill Fill 8 Standard Penetration Resistance Auger Refusal /5" /4.5" /5.5" 50/5" 50/3.5" /4" 206 PROJECT: Middlesex Corporate Park, Lot 3 Middlesex, North Carolina SCALE: As Shown JOB No: EA FIGURE No:2

8 TEST BORING RECORD DEPTH (FT.) DESCRIPTION Topsoil Possible Fill - Firm Brown Sandy SILT w/rocks Hard Orange & Red Silty AY w/rock Fragments ELEVATION PENETRATION (FT.) (BLOWS/FT.) BLOWS PER SIX INCHES Very Hard Purple, Brown & Orange Clayey Fine Sandy Micaceous SILT 11.0 Hard Orangish Brown Fine Sandy Micaceous SILT Partially Weathered Rock - Sampled as Brown Sandy SILT Boring terminated at 19.8' Dry at time of boring /5.5" GTI_MAIN GPJ GTI.GDT 3/10/17 JOB NUMBER BORING NUMBER DATE EA B PAGE 1 OF Wellington Court, Ste 108 Raleigh, NC 27615

9 TEST BORING RECORD DEPTH (FT.) Topsoil & Fill DESCRIPTION Soft Grayish Brown Fine Sandy Clayey SILT Very Stiff Red & Orange Silty AY ELEVATION PENETRATION (FT.) (BLOWS/FT.) BLOWS PER SIX INCHES Very Hard Purple & Orangish Brown Fine Sandy Micaceous SILT Partially Weathered Rock - Sampled as Brown & Purple Sandy SILT /4.5" 20.5 Boring terminated at 20.5' Dry at time of boring /5" GTI_MAIN GPJ GTI.GDT 3/10/17 JOB NUMBER BORING NUMBER DATE EA B PAGE 1 OF Wellington Court, Ste 108 Raleigh, NC 27615

10 TEST BORING RECORD DEPTH (FT.) DESCRIPTION Topsoil Fill - Stiff Brown Sandy Clayey SILT w/rocks, Roots & Glass Hard Orange & Red Silty AY ELEVATION PENETRATION (FT.) (BLOWS/FT.) BLOWS PER SIX INCHES Very Hard Tan, Red & Orange Fine Sandy SILT Partially Weathered Rock - Sampled as Orange & Tan Sandy SILT /5" 24.5 Auger Refusal at 24.5' Dry at time of boring /3.5" 50/4" GTI_MAIN GPJ GTI.GDT 3/10/17 JOB NUMBER BORING NUMBER DATE EA B PAGE 1 OF Wellington Court, Ste 108 Raleigh, NC 27615

11 TEST BORING RECORD DEPTH (FT.) DESCRIPTION Topsoil Fill - Firm Orange & Brown Sandy Silty AY w/rocks Firm Orange Silty AY ELEVATION PENETRATION (FT.) (BLOWS/FT.) BLOWS PER SIX INCHES Very Stiff Reddish Orange Silty AY Very Stiff Gray, Light Brown & Orange SILT Hard Gray, Brown & Purple Fine Sandy Micaceous SILT 16.0 Very Stiff Brown & Purple Fine Sandy Micaceous SILT Boring terminated at 20.5' Dry at time of boring GTI_MAIN GPJ GTI.GDT 3/10/17 JOB NUMBER BORING NUMBER DATE EA B PAGE 1 OF Wellington Court, Ste 108 Raleigh, NC 27615

12 TEST BORING RECORD DEPTH (FT.) DESCRIPTION Topsoil Fill - Stiff Brown & Gray Sandy Silty AY w/rocks Stiff Red & Orangish Brown Silty AY Very Stiff Reddish Orange Silty AY ELEVATION PENETRATION (FT.) (BLOWS/FT.) BLOWS PER SIX INCHES Hard Purple Tan, Brown & Orange Fine Sandy Micaceous SILT 10.0 Partially Weathered Rock - Sampled as Brown, Tan & Purple Sandy Micaceous SILT /4" 50/5.75" 20.2 Boring terminated at 20.2' Dry at time of boring /2.5" GTI_MAIN GPJ GTI.GDT 3/10/17 JOB NUMBER BORING NUMBER DATE EA B PAGE 1 OF Wellington Court, Ste 108 Raleigh, NC 27615