Preliminary soil study results of the Jack Bennett Road property

Posted Monday, May 7, 2007

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Pittsboro, NC - The preliminary subsurface exploration findings are due to be discussed on the May 9th Board of Education meeting. The meeting is to be held in the central office board room and is scheduled to begin at 5:00 p.m.

Mr. Randy L. Drumheller
Construction Manager
Chatham County Schools
P.O. Box 128
Pittsboro, North Carolina 273 12

Re: Preliminary Subsurface Exploration Services
Jack Bennett Road I Herndon Creek Property
New High School Chatham County
Chatham County, North Carolina
Tai Job No. 07-1 22-1

Dear Mr. Drumheller:

In accordance with your authorization, we have completed a preliminary subsurface exploration program for the referenced project. This program consisted of the drilling of twenty-seven (27) soil borings, classification of soil samples recovered, engineering services including a site visit, the analyses of subsurface conditions and the preparation of this report.

FIELD EXPLORATION
The field exploration program consisted of the drilling of twenty-seven (27) soil borings (B-1 thru B-27) by a CME 55 drill rig to a maximum depth of 40.0 feet below the existing ground surface. Standard Penetration Tests (ASTM D-1586) were performed at every 2.5-foot interval to 10.0 feet deep followed by one at every 5.0 feet interval. The approximate locations and logs of the twenty-seven (27) borings are shown on the attached "Boring Location Plan" and "Boring Logs." The borings were located by Steven M. Puckett, PLS.

Due to excessive muddy conditions limiting access by the ATV drill rig, two borings, B-22 and B-24 were advanced by hand auger probing to a depth of 10.0 feet below the ground surface using a 4-inch diameter hand auger. Dynamic Cone Penetration (DCP) tests were performed at 2.5-foot intervals utilizing a 15-lb steel ring weight falling 20-inches to drive a 1.5-inch diameter rod with a 45-degree cone in three, 1-314-inch increments. Nc values were determined by averaging the last two DCP values.

SURFACE AND GEOLOGIC CONDITIONS
The undeveloped project site is located on the western edge of the Triassic Basin with likely presence of Diabase dikes and sills. Based on a USGS 7.5 minute topographic map, the site appears to vary in surface elevation from 3 10 MSL to 230 MSL along Herndon Creek. Jordan lake is located about 1/4 mile east of the project site, with permanent pool elevation of 216.0 MSL, according to the Army Corps of Engineers website.

Wet soil conditions were observed over about 1/3 of the site, resulting in 1-foot to 3-foot ruts from the drill rig. This was indicative of perched or trapped water in soft, plastic clays. Diabase Dike boulders were observed near the eastern property boundary. We received reports of Diabase dike boulders from adjacent property construction teams.

SUBSURFACE CONDITIONS
Based on the results of the field exploration program, the subsurface materials of the project site can be described as consisting of the following units of soil:

1. Topsoil: The surface material consists of a 3-inch to 6-inch thick layer of topsoil in all twenty-seven (27) borings. Borings B-8 and 18 contained over 12 inches of topsoil. Root mat extends to a deeper depth.

2. Fill: Fill associated with the existing gravel drive was encountered in boring B-26 to a depth of 5.3 feet. This poor engineering quality material exists over a relatively small area.

3. Plastic, Soft to Firm Silty, Clayey Soil: This unit of material consists of poor engineering quality plastic clays and plastic silts. It was encountered in eleven borings to a depth of 2.5 to 7.9 feet deep, with N-values ranging from 2 to 7, and was often wet. This unit of soil also contained some loose sand layers. This unit of soil was formed by the in-place weathering of the parent Triassic and Diabase rock.

4. Sandy-Silty-Clayey Soil: This unit of soil is also natural, residual soil derived from the inplace weathering of the parent rock. It contains stiff to hard, sandy, silty soils of moderate to good engineering quality. It extends down to the weathered rock layer.

5. Diabase: Formed from intrusive dikes and sills, this unit of soil is encountered sporadically throughout the site. Diabase formations are typically wet, contain plastic clays and boulders that vary in size from 1.0 to 10.0 feet in diameter. This unit of soil
represents a small portion of the site subsoil, but has the potential to generate moderate quantities of unsuitable soil and boulders in cut excavations.

6. Weathered Rock: The predominant subsurface material at the project site is the Weathered Triassic Siltstone, which was encountered below the above soil overburden in twenty (20) borings. Defined by material having a N-Value of 50 bpf or more, the depth to this very hard material occurs as shallow as 2.8 feet in boring B-9, and as deep as 12.6 feet in boring B-12. This unit of soil is of good engineering quality.

7. Partially Weathered Rock/Boulders: Some of the weathered rock becomes much harder with depth and is classified as partially weathered rock or boulders. For this project, the partially weathered rock is defined as very hard, rock-like material having an N value of 50 blows over one inch or less. It was encountered in two (2) borings, B-6 and B-16 at depths of 16.0 and 25.0 feet below the ground surface. The difference between weathered rock and partially weathered rock can be very small and may vary erratically from place to place and in depths.

8. Auger Refusal: Rock, represented by auger refusal, was not encountered in any of the borings. However, hard rock ledges and/or large boulders likely exist at shallower depths between borings.

9. Groundwater: Perched or trapped groundwater was encountered in two (2) borings, B-1 and B-23, at depths under 5.0 feet deep. This represents perched or trapped groundwater over the relatively low permeability Triassic weathered rock. The overburden soils were wet in the low, plastic clayey areas during the field exploration.

The subsurface materials encountered in the borings are presented in the attached "Summary of Subsurface Materials".

PROPOSED PROJECT
The project site is in the preliminary design phase. Based on our conversations with you, we understand that the proposed project site will be developed for a high school. No grading plan, building layout, or existing site topography was available. We assume that the proposed building will be located near the highest existing site elevation, and that about 10.0 feet of cut and fill will be required to grade the site.

RECOMMENDATIONS

Engineering Behavior of Soils
The site subsurface soils are sensitive to surface water damage. Grading operations performed in the wet, winter months of November through March will be difficult, time consuming and expensive.

Excavation
All normal size earth-moving equipment can easily move the materials above the weathered rock layer. Normally, a large dozer such as CAT D-8, or equal, will be required to break up the hard soil and weathered rock in an open, mass grading operation. Blasting will be required to remove the partially weathered rock and hard rock.

Slow excavation in trenches with a CAT 330 track mounted hydraulic excavator equipped with rock teeth will be required to remove the hard soil and weathered rock. Blasting will be require in trench excavation to remove some of the weathered rock and the hard rock.

At this time, we anticipate encountering a small quantity of rock excavation. However, we do anticipate encountering substantial amounts of hard soil and weathered rock. The appropriate definition of rock in the project specifications will be important. We would like to re-address this issue afier a grading plan becomes available, after specifications become available, and after additional borings are performed.

Cut and Fill
In dry seasons, most of the dry, non plastic, natural, residual soil and the weathered rock will provide suitable borrow materials. The natural soil and weathered rock will become unsuitable in cold, winter months. The existing soft to firm plastic clays and silts will not be suitable for re-use as structural fill.

Unsuitable Soils
The quantity of undercut that will be required will vary depending on many factors, and will require field engineering judgment to determine the exact locations and depths.

The civil design should address the presence of a substantial amount of unsuitable soil that will be encountered. We recommend planning on encountering an average of 18 to 36 inches of unsuitable soil in structural areas (parking, building and utilities), depending on the final site layout. The final grades should consider this depth of unsuitable soil, and a spoil area should be designated.

Foundation
It appears that the proposed new building should be supported on a shallow foundation system designed for 4,000 psf bearing capacity. The footings should be supported on the existing, non-plastic soil or new controlled fill. Buildings located in areas of deep, sofi clay will require undercut.

Seismic Classification
Buildings situated in areas with less than 12.0 feet of fill will likely be able to use a site seismic class C, according to the 2000 International Building Code.

CLOSURE
It has been a pleasure to work with you and your design team on the preliminary phase of this project. Please call us if you have any questions or if we can offer any additional services during the design phase of this project. A more detailed report should be issued after site-specific borings are performed for each building following determination of the final layout.

Very Truly Yours,
Tai and Associates, PLLC