7607_GreatOakMSWLF_LeakLocationWork Plan_Ph1_Cell2_DIN28790_20180201

February 1, 2018
Steve Lamb
SCS Engineers
2520 Whitehall Park Drive, Suite 450
Charlotte, North Carolina 28273
Phone: [phone]
Email: [e-mail]
Subject: Work Plan for Electrical Leak Location Testing of Great Oak Landfill Cell 2
Construction Project
Dear Mr. Lamb,
TRI Environmental, Inc. (TRI) is pleased to provide SCS Engineers (client) with this Work
Plan for an electrical leak location survey of a landfill cell expansion located in Randleman,
North Carolina.
TRI’s leak location / liner integrity survey experience encompasses over 160 million square
feet of installed geomembrane through its Director of Liner Integrity Services, Abigail
Gilson, M.S., P.E. Ms. Gilson has been performing electrical leak location surveys since
2004. Her resume is submitted as an attachment to this Work Plan. TRI’s Statement of
Qualifications along with the resume of a lead field technician is also submitted as an
attachment.
SCOPE OF SERVICES
TRI is proposing to perform an electrical leak location survey of the installed primary
geomembrane after the overlying cover material placement using the dipole method (ASTM
D7007). It is our understanding that the total survey area measures approximately 10 acres.
DIPOLE METHOD
The dipole method is used for geomembrane covered with soil, gravel, water or other porous
or conductive material. A high voltage is applied to the material covering the geomembrane
and the power source is grounded to the semi conductive layer beneath it. A roving dipole is
used to measure and record GPS-based voltage measurements in a grid pattern throughout the
survey area. The voltage potential measurements are observed in real time and recorded by a
data acquisitioner and then graphed as a voltage contour map for post-survey data review. In
the presence of a hole, the current will exit the hole from all directions, creating a bull’s eye
pattern in the electrical voltage potential field. This pattern emerges as a sine wave shape
when the voltage potential measurements are plotted along the length of the survey line. A
leak detection distance test is performed before beginning the survey. This is to ensure that
the peak to peak voltage difference created by the specified minimum hole size measures at
least three times the background noise of the survey area at the proposed measurement
density.
Features such as pipes, wrinkles in the liner, metal objects, and puddles of water can produce
false positive signals by creating anomalies in the flow of current. The dipole operator
Electrical Leak Location Work Plan – Great Oak Landfill Cell 2 Construction
Page 2 of 6
interprets the signals to determine whether they are false, but the location may need to be
excavated if there is any doubt. This survey cannot be performed during rainfall or when the
soil subgrade or cover material is frozen.
The Client will be responsible for preparing the site for the dipole survey and providing
additional support as described below:
• The material covering the geomembrane in the survey area must contain moisture in
order to provide sufficient electrical conductivity. This requirement is determined
during the initial leak detection distance testing before beginning the survey. A
water truck and driver or laborer with a watering backpack may need to be available
for the duration of the survey if the cover material requires watering, or the survey
can be performed after a recent rain event. It is best to add as much moisture as
practical to the cover material during material placement.
• The material covering the geomembrane in the survey area must be isolated from the
surrounding soil outside of the survey area and any grounded objects inside of the
survey area. This is typically achieved by leaving a strip of geomembrane exposed
along the entire perimeter of the survey area, as shown by Figure 1. Any access
roads used to place cover material must be removed before performing the survey, or
a geomembrane rub sheet can be used to bisect the road for electrical isolation while
maintaining truck access, as shown in Figure 2. It is advisable to place a rub sheet at
the location of any access roads regardless before placement of cover material in
order to facilitate removal for the electrical survey. Any inlet and outlet structures
that are grounded (penetrating through the geomembrane) must not be allowed to
touch the cover material in the survey area. The cover material can be placed just
short of such features.
Figure 1: Example of Perimeter Isolation Trench
Electrical Leak Location Work Plan – Great Oak Landfill Cell 2 Construction
Page 3 of 6
Figure 2: Example of Isolated Survey Area Access Road Bisected by Rub Sheet
• The Client shall provide a backhoe or laborer with shovel to uncover potential hole
locations found by TRI survey personnel, as well as to install and remove the
artificial leak on top of the geomembrane.
• The Client shall provide a generator to provide power to the survey equipment. The
generator can be the smallest unit available – a small suitcase Honda 2000 W.
• For testing the primary geomembrane, the equipment must be electrically grounded
to the conductive layer underneath the primary geomembrane. For this project, it is a
GCL. At least two separate electrodes (bare copper wires) should be installed directly
underneath this layer, exiting the anchor trench. Access to these wires must be
provided during the survey of the primary geomembrane. A figure of the
recommended copper wire layout is provided as Figure 3 below.
Electrical Leak Location Work Plan – Great Oak Landfill Cell 2 Construction
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Figure 3: Cell 2 Copper Wire Layout
Electrical Leak Location Work Plan – Great Oak Landfill Cell 2 Construction
Page 5 of 6
A voltage map will be created for the survey area in order to analyze the flow of current in
the cell. An example of voltage mapping is provided in Figure 4.
The colors on the map represent voltage values in order to easily recognize a shift from
positive to negative values. Negative values are represented by red changing to blue with
increasing magnitude and positive values are represented by green changing to yellow with
increasing magnitude. The negative on the top, positive on the bottom polarity of the entire
survey area is created by the current injector electrode. Leak locations are recognized as the
opposite; a positive circular peak directly on top of a negative circular peak, typically
exhibiting a butterfly-like shape. The characteristic leak signal voltage pattern is opposite that
of the current injector electrode, since current is exiting the survey area at current leakage
locations and entering the survey area at the current injector location. The tick marks are
voltage measurement locations and show the general shape of the survey area.
Figure 4: Example of Voltage Mapping
Providing this voltage map as part of TRI’s final reporting is above and beyond the ASTM
standard methodology, but it is TRI’s determination that such a map is crucial to document
that the ELL survey was performed completely and correctly. In addition, TRI’s GPS-based
data acquisition system allows for the survey area grid to be directed by the GPS and not by
string lines placed throughout the survey area manually. Guiding the data acquisition using
string lines requires significant additional support labor and is also imprecise.
Electrical Leak Location Work Plan – Great Oak Landfill Cell 2 Construction
Page 6 of 6
EXECUTION PLAN
Practically all new geomembrane installations can be surveyed at a measurement density of
ten feet by ten feet, as long as the survey area has been properly isolated and prepared. This
allows for a survey speed of up to approximately eight acres per day when a GPS-based data
acquisition system is used. Equipment set up time, data analysis and leak pinpointing will
require additional time. If a decrease in sensitivity is caused by poor site conditions and
subsequently requires a higher measurement density, then the survey could take additional
time. For the approximately 10-acre survey area, the survey should be completed within two
working days, including time for set-up, data analysis and leak pinpointing.
Potential hole locations will be clearly marked by TRI leak location personnel after review of
the three-dimensional voltage map compiled by data collected during the dipole survey. Hole
locations can be pinpointed within a one foot accuracy (depending on the cover material
thickness). TRI personnel will be onsite while the hole locations are excavated to assist in
further pinpointing and excavating the source of the signal created by the hole and also to
verify that no further signal is present after the hole has been isolated from the cover material.
The final report, including hole locations found during the survey, will be submitted within
fourteen days of performing the survey, but likely immediately after survey performance. A
sample Final Report is submitted as an attachment to this Work Plan.
If you have any questions regarding the information provided, please feel free to contact me
at your convenience. I look forward to working with you and the rest of your staff.
Abigail Gilson, P.E.
Director of Liner
Integrity Services,
TRI Environmental,
Inc.
1
Chao, Ming-tai
From:Lamb, Steve <[e-mail]>
Sent:Thursday, March 15, 2018 9:34 AM
To:Chao, Ming-tai; Harrison, Susan
Cc:[e-mail]; [e-mail]; Christopher Robertson
Subject:[External] Leak Location Liner work plan
Attachments:Work Plan_Great Oak Landfill.pdf
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Ming and Susan:

As discussed in last week’s liner pre-con meeting, please find enclosed the Work Plan prepared by our sub consultant,
TRI, for the Cell 2 liner leak location testing.

SCS will notify DEQ prior to conducting the test in case DEQ wants an on-site representative to observe. The current
timeframe for the test is late April.

Thanks,

Steve Lamb, PE
Vice President
SCS Engineers
2520 Whitehall Park Drive, Suite 450
Charlotte, North Carolina 28273
Office – 704.504.3107
Direct - 704-916-1529
Mobile – 704.576.4731