20181245 Ver 2_Report of Observation Pit Exploration_20201005

KESSEL ENGINEERING GROUP
582 HENDERSONVILLE ROAD SUITE ONE | ASHEVILLE NC 28803 | P:[828] 277-6351 F:[828] 277-6355
WWW.THEKESSELGROUP.COM

October 2, 2020

Mr. W. Alec Stillwell, P.E.
Stillwell Engineering, P.A.
P.O. Box 838
Sylva, NC 28779

Report of Observation Pit Exploration
Sylva Tractor Supply – Subsurface Stormwater Treatment System
Sylva, North Carolina
KEG Project No. JA20-4003-04

Mr. Stillwell:

Kessel Engineering Group, PLLC (KEG) is pleased to present this report of observation pit exploration
for the proposed subsurface stormwater treatment system at the Sylva Tractor Supply project in Sylva,
North Carolina. The purpose of this exploration was to determine general subsurface conditions in the
area of the proposed subsurface stormwater treatment system to be used by others in the design phase of
the project.

PROJECT INFORMATION
Project information was provided by Mr. W. Alec Stillwell, P.E. during recent telephone conversations
and email correspondence with our Ms. Courtney King, P.E. We have also been provided with Grading
and Stormwater Plan, Sheet C.2, prepared by Stillwell Engineering, P.A. and dated September 18, 2020.
Additional site and subsurface information were gathered during our (on-going) performance of
construction materials testing services at the site (reference KEG Project No. JA20-4003-01) and during
our performance of a subsurface exploration for the (now omitted) bioretention pond (reference KEG
Project No. JA20-4003-04). Project plans are to construct a subsurface stormwater treatment system
below the asphalt paved parking lot to be located on the south side of the project site. We understand that
the bottom of the proposed subsurface stormwater treatment system will have an elevation of 2148.5.

SITE CONDITIONS
At the time of our field exploration, survey stakes (established by others) indicated the location and
current ground surface elevation at each of the four corners of the proposed stormwater system. The
existing ground surface in the proposed construction area consists of existing fill which was reportedly
placed some time ago (and prior to the start of this project) the surface of which has been recently
disturbed during grading operations at the site. Due to recent precipitation events, the ground surface also
contained mud and puddles of standing water.

FIELD EXPLORATION
The site was explored by performing two observation pits (OP-1 and OP-2) at the approximate location as
indicated on the attached Field Exploration Plan. The observation pits were located in the approximate
footprint of the proposed stormwater system. The ground surface elevations at the observation pit
locations were estimated by our Ms. Courtney King, P.E. by referencing the ground surface elevations
noted on the nearby survey stakes. The materials encountered by the observation pits were identified in
the field from cuttings brought to the surface by the excavator. Representative samples of the
Report of Observation Pit Exploration October 2, 2020
Sylva Tractor Supply – Proposed Subsurface Stormwater Treatment System KEG Project No. JA20-4003-04
Sylva, North Carolina

2
encountered materials were collected and transported to the laboratory. In the laboratory, the samples
were examined by a geotechnical engineer and visually classified. In conjunction with our field work, the
observation pits were inspected for groundwater and measurements of the depth to groundwater were
taken. Soil descriptions and groundwater levels are tabulated on the observation pit logs attached to this
report.

LABORATORY TESTING
We performed laboratory testing on a representative sample collect during our field exploration.
Laboratory testing consisted of the determination of the percentage of fine-grained particles present in the
selected sample and the natural moisture content of the sample. The percent by weight passing the
No. 200 sieve is the percentage of fines or the portion of the sample in the silt and clay size range. This
test was conducted in accordance with ASTM D 1140. The moisture content of the soil is the ratio,
expressed as a percentage, of the weight of water in a given mass of soil to the weight of the soil particles.
The natural moisture content was determined in accordance with ASTM D 2216. The representative
sample, collected from the lower portion of OP-1, had 48.9 percent passing the No. 200 sieve and an in-
situ moisture content of 36.9 percent.

SITE GEOLOGY
The project site is located in the Blue Ridge Physiographic Province. The bedrock in this region is a
complex crystalline formation that has been faulted and contorted by past tectonic movements. The rock
has weathered to residual soils which form the mantle for the hillsides and hilltops. The typical residual
soil profile in areas not disturbed by erosion or grading consists of clayey soils near the surface where
weathering is more advanced, underlain by sandy silts and silty sands.

The boundary between soil and rock is not sharply defined and there is often a transitional zone, termed
“partially weathered rock” overlying the parent bedrock. Partially weathered rock (PWR) is defined, for
engineering purposes, as residual material with a standard penetration resistance in excess of 100 blows
per foot. Weathering is facilitated by fractures, joints, and the presence of less resistant rock types.
Consequently, the profile of the partially weathered rock is irregular even over short horizontal distances.
Also, it is not unusual to find lenses and boulders of hard rock and/or zones of partially weathered rock
within the soil mantle, well above the general bedrock level.

Quite often, the upper soils along drainage features and in flood plain areas are water-deposited (alluvial)
materials that have been eroded and washed down from adjacent higher ground. These alluvial soils are
usually soft and compressible, having never been consolidated by pressures in excess of their present
overburden.

SUBSURFACE CONDITIONS
Observation pit OP-1 was performed on September 28, 2020. Observation pit OP-1 encountered fill from
the ground surface to its termination depth of 9½ feet beneath the existing ground surface. The fill
consisted of 3½ feet of silty sands with trace gravel underlain by very moist, slightly clayey silty sands.
The fill generally appeared to decrease in consistency with depth. No groundwater was encountered in
OP-1 at the time of excavation. Observation pit OP-1 was observed the following day and a few inches of
water was observed in the bottom of the hole. An observation well was installed in OP-1 and the pit was
backfilled. Subsequent readings in the observation well indicated the presence of a few inches of water in
the observation well. After review of the soil type encountered in the lower portion of OP-1 and the
recent heavy precipitation events at the site, we concluded that the water in OP-1 could possibly be
attributed to stormwater which infiltrated into the area of the observation pit and become perched (or
trapped) within the fill soil strata.

Report of Observation Pit Exploration October 2, 2020
Sylva Tractor Supply – Proposed Subsurface Stormwater Treatment System KEG Project No. JA20-4003-04
Sylva, North Carolina

3
A second, deeper observation pit was performed on October 1, 2020 to explore the deeper subsurface
conditions at the site for the presence of groundwater. Observation pit OP-2 was performed
approximately 10 feet from OP-1 so that the excavation would be not penetrate the soils recently
disturbed by OP-1. Observation pit OP-2 encountered 13½ feet of very moist fill underlain by wet
alluvium to its termination depth of 14½ feet beneath the existing ground surface. The fill encountered by
OP-2 consisted of near surface silty sands underlain by slightly clayey sandy silts containing some
boulders. The alluvium consisted of clayey silts and rounded cobbles. At the time of excavation,
groundwater was encountered in OP-2 at a depth of 14½ feet. After a stabilization period of 24 hours,
groundwater was measured in OP-2 at a depth of 9½ feet beneath the existing ground surface.
Groundwater levels may fluctuate several feet with seasonal and rainfall variations and water levels in the
nearby water features. Normally, the highest groundwater levels occur in late winter and spring and the
lowest levels occur in late summer and fall.

The above descriptions provide a summary of the subsurface conditions encountered by the observation
pits. The attached logs contain information recorded at each observation pit location. The lines
designating the interfaces between various strata represent approximate boundaries and the transition
between strata may be gradual. Subsurface conditions may vary between observation pit locations.

ANALYSIS AND DESIGN RECOMMENDATIONS
Infiltration Rate
Based on the information gathered from our exploration and our experience with similar subsurface
conditions, we estimate the existing fill soils located in the area of the proposed storm water system will
have an infiltration rate on the order of 0.2 inches/hour. This estimate assumes the storm water system
will be constructed according to the recommendations presented in the following Site Preparation and
Construction Recommendations section of this report.

Subgrade Remediation
The observation pits performed for this exploration encountered soft existing fill soils which generally
appear to decrease in consistency with depth. Soft fill soils will not provide suitable subgrade support for
the subsurface stormwater treatment system. Measures for subgrade remediation should be determined by
the Geotechnical Engineer at the time of construction.

Once the temporary excavation for installation of the proposed stormwater system is extended to final
proposed grade, the temporary excavation bottom should be observed by the Geotechnical Engineer.
Depending on the subsurface conditions encountered in the excavation, we anticipate our
recommendations will likely include undercutting the excavation 12-inches, installing layer(s) of geogrid
reinforcement of Tensar BX110 or equivalent, and backfilling with newly placed engineered fill.
Alternatively, we may recommend undercutting the excavation 12-inches and backfilling the undercut
with a lift of rocky fill. We understand that rocky fill may be generated during excavations required for
the installation of a temporary sediment pond. The installation of rocky fill, if planned, should be
observed and documented by the geotechnical engineer. After the completion of the undercut, the
excavation should be backfilled as soon as possible. We recommend that undercutting and backfilling be
staged during dry weather conditions.

SITE PREPARATION AND CONSTRUCTION RECOMMENDATIONS
De-watering
The observation pits did not encounter groundwater within expected construction depths. If groundwater
is encountered during construction, the engineer should be contacted immediately to review the condition.
Minor seepage from isolated pockets of perched (trapped) water should be expected during temporary
excavations required for installation of the storm drain system. Additionally, the temporary excavation
Report of Observation Pit Exploration October 2, 2020
Sylva Tractor Supply – Proposed Subsurface Stormwater Treatment System KEG Project No. JA20-4003-04
Sylva, North Carolina

4
will be particularly susceptible to deterioration during wet weather conditions. As such, we recommend
that construction be phased during dry weather conditions. The contractor should be prepared to
promptly remove surface water from the construction area. This has been done effectively by means of
gravity ditched and pumping from filtered sumps.

Geogrid Reinforcement (if planned)
Installation of geogrid should be on horizontal ground and not on sloping ground. Geogrid reinforcement
should be laid out in parallel strips and splicing should be minimized. Adjacent layers of geogrid
reinforcement should abut one another; it is not necessary to overlap these layers. If rutting occurs during
the placement or backfilling of geogrid, the geotechnical engineer should be contacted to evaluate the
condition. Tracked equipment should not be operated directly upon the geogrid. A minimum fill
thickness of 6-inches is required prior to operation of tracked vehicles over the geogrid. The turning of
tracked vehicles should be kept to a minimum to prevent tracks from displacing the fill and damaging the
geogrid. Engineered fill should be placed, spread and compacted in such a manner that minimizes the
development of wrinkles in and/or move ment of the geogrid. Installation of geogrid reinforcement should
be observed by an engineering field technician working under the direction of the geotechnical engineer
to confirm that our recommendations are properly interpreted and implemented.

Engineered Fill
Engineered fill used for backfilling the stormwater treatment system should be uniformly compacted in
thin (6-inch to 8-inch loose measure) horizontal lifts to at least 95 percent of the standard Proctor
maximum dry density (ASTM D-698) and within 3 percent of optimum moisture. Based on visual
examination and our experience with similar soils, the existing fill soils do not appear suitable for re-use
as engineered fill due to high moisture contents. In general, soils having a Plasticity Index (PI) greater
than 30 (less than 15 is preferable) should not be used for fill.

Before filling operations begin, representative samples of each proposed fill material should be collected
and tested to determine the compaction and classification characteristics. The maximum dry density and
optimum moisture content should be determined. Once compaction begins, a sufficient number of density
tests should be performed by an engineering technician working under the direction of the geotechnical
engineer to measure the degree of compaction being obtained. Engineered fill should be free of organic
and other deleterious materials. In areas where engineered fill will be placed adjacent to or atop of No. 57
stone, we recommend the installation of a filter fabric such as Mirafi 140N or equivalent.

The surface of engineered fill can deteriorate and lose its support capabilities when exposed to
environmental changes or construction activity. Deterioration can occur from, but is not limited to, the
effects of freezing temperatures, the formation of erosion gullies, exposure to extreme wetting/drying
conditions, long term exposure to natural elements, and rutting/pumping caused by construction traffic.
We recommend that surfaces of the engineered fill that have deteriorated or softened be recompacted
immediately prior to construction of additional engineered fill or pavements.

Slopes
Confined excavations should conform to OSHA regulations. For slopes that are not confined, our
experience suggests that temporary excavation side slopes through the existing soils in the area of
proposed construction should be laid back at a 1.5H:1V (Horizontal to Vertical) slope, or flatter.

Report of Observation Pit Exploration October 2, 2020
Sylva Tractor Supply — Proposed Subsurface Stormwater Treatment System KEG Project No. JA20-4003-04
Sylva, North Carolina
We appreciate the opportunity to offer our professional geotechnical engineering services on this project.
If you have any questions concerning this report, please do not hesitate to contact us.
Sincerely,
KESSEL
Courtney A. �KYng
Senior Engineer
Registered, North
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(NC Firm License No. P-0420)
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�e Nc,'rw Matthew Gibson, P.E.
Protect Engineer
Registered, North Carolina 50361
Attachments: Field Exploration Plan
Observation Pit Logs (OP-1 and OP-2)
Key to Soil Classifications and Consistency Descriptions
Distribution: W. Alec Stillwell, P.E.; Stillwell Engineering, P.A.; [e-mail]
Rob Turner of W.R. Newman and Associates, Inc.; [e-mail]
John Arzonico of W.R. Newman and Associates, Inc.; [e-mail]
5

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KESSEL ENGINEERING GROUP
582 HENDERSONVILLE ROAD SUITE ONE I ASHEVILLE NC 28803 I P: 18281 277-635 1 I F. 18281 277-6355
WWW.THEKF-SSELGROUP.COM

Tan, Slightly Micaceous, Moist, Silty SAND with Trace Gravel (Fill)
Soft, Red, Very Moist, Slightly Clayey, Silty SAND (Fill)
48.9% Passing No. 200 Sieve
Observation pit terminated at 9.5 feet. No groundwater encountered at time of excavation.
Perched water noted at 9' after 24 hours.
END:9-28-20
AFTER 24 HOURS: CAVING>DEPTH TO - WATER> INITIAL:
DATE START:9-28-20
OBSERVATION PIT NO. OP-1
Sheet 1 of 1
OBSERVATION PIT NO. OP-1
PERFORMED BY:Dan Ash w/ WNC Paving
EXCAVATION EQUIPMENT:CAT 320
2
4
6
8
CLIENT:Stillwell Engineering, PA
PROJECT:Sylva Tractor Supply - Stormwater Treatment
LOCATION:Footprint of Proposed System
LOGGED BY:C. King
ELEVATION:2154.5 (feet)
PROJECT NO.:JA20-4003-04
ELEVATION/
DEPTH (FT)
2154
2152
2150
2148
2146
DESCRIPTION SOIL
TYPE
SOIL TEST BORING REVISED 4003-04 TRACTOR SUPPLY - SUBSURFACE STORMWATER TREATMENT SYSTEM.GPJ KESSEL GROUP.GDT 10/2/20
Tan and Brown, Moist, Silty SAND (Fill)
Soft, Red and Dark Gray, Very Moist, Slightly Clayey, Sandy SILT with Some Boulders (Fill)
Soft, Red and Dark Gray, Very Moist, Slightly Clayey, Sandy SILT (Fill)
Dark Gray, Wet, Clayey SILT with Rounded Cobbles (Alluvium)
Observation pit terminated at 14.5 feet. Groundwater encountered at 14.5 feet at time of excavation and 9.5 feet 24
hours after excavation.
END:10-1-20
AFTER 24 HOURS: 9.5 ft CAVING>DEPTH TO - WATER> INITIAL: 14.5 ft
DATE START:10-1-20
OBSERVATION PIT NO. OP-2
Sheet 1 of 1
OBSERVATION PIT NO. OP-2
PERFORMED BY:Jerry with Wayne Smith
EXCAVATION EQUIPMENT:CAT 320
2
4
6
8
10
12
14
CLIENT:Stillwell Engineering, PA
PROJECT:Sylva Tractor Supply - Stormwater Treatment
LOCATION:Footprint of Proposed System
LOGGED BY:C. King
ELEVATION:2154.5 (feet)
PROJECT NO.:JA20-4003-04
ELEVATION/
DEPTH (FT)
2154
2152
2150
2148
2146
2144
2142
2140
DESCRIPTION SOIL
TYPE
SOIL TEST BORING REVISED 4003-04 TRACTOR SUPPLY - SUBSURFACE STORMWATER TREATMENT SYSTEM.GPJ KESSEL GROUP.GDT 10/2/20
Sand
SW
Topsoil
TOPSOIL
KEY TO SOIL CLASSIFICATIONS
KEY TO DRILLING SYMBOLS
Undisturbed Sample
Grab Sample
Split Spoon Sample
Groundwater Table 24 Hours after Completion of Drilling
Groundwater Table at Time of Drilling
Sandy Silt
MLS
Clayey Sand
SC
High Plasticity Clay
CH
Well-graded Gravel
GW
Poorly-graded Gravel
GP
Partially Weathered
Rock
BLDRCBBL
Low Plasticity Clay
CL
Sandy Clay
CLS
Silt
ML
Silty Clay
CL-ML
Silty Sand
SM
Clayey Silt
MH
Bedrock
BEDROCK
Concrete
AS
KEY TO SOIL CLASSIFICATIONS
Particle Size Identification
Boulder: Greater than 300 mm
Cobble: 75 to 300 mm
Gravel:
Coarse - 19 to 75 mm
Fine - 4.75 to 19 mm
Sand:
Coarse - 2 to 75 mm
Medium - 0.425 to 2 mm
Fine - 0.075 to 0.425 mm
Silts & Clay: Less than 0.075 mm