WQ0013419_Sludge Landfill Sink Hole Status Report_20230227

4 Blanchard Road, P.O. Box 85A Cumberland, ME 04021
Tel: [phone]  Fax: [phone]
[e-mail]
sme-engineers.com

220568.01 20230227_letter for NCDEQ.docx
ENVIRONMENTAL  CIVIL  GEOTECHNICAL  WATER  COMPLIANCE
February 27, 2023

G. Landon Davidson, P.G.
Regional Supervisor, Water Quality Operations Section
North Carolina Department of Environmental Quality
Asheville Regional Office
2090 U.S. 70 Highway Swannanoa, NC 28778-8211

Subject: Sinkhole at Sludge Landfill

Dear Mr. Davidson,

At request of Glatfelter, this letter was prepared by Sevee & Maher Engineers, Inc (SME) to provide the
North Carolina Department of Environmental Quality (NCDEQ) with a status update for the sinkhole at
Glatfelter’s Sludge Landfill in Pisgah Forest. The sinkhole was first noticed in July 2022.

BACKGROUND

The sinkhole is believed to be associated with leachate drainage piping installed as part of the original
dam (henceforth referred to as “the dam”). The dam forms the downslope end of the Sludge Landfill. As
sludge was placed in the landfill, the height of the dam was first increased about 3 feet (i.e., soil was
added to the dam crest). After the crest increase, the dam height was further increased by construction
of two soil dikes, one on top of the other. The two dikes increased the overall dam height (before
settlement) by approximately 20 feet; the increased height allowed additional sludge to be placed in the
landfill. Both dikes were constructed on soft sludge that was already in-place behind the dam and,
subsequently, both dikes have undergone settlement. Figure 1 shows an aerial view of the exposed
portions of the dam and dikes. Dike #1 is estimated to have been underlain by sludge in the range of 5
to 30 feet thick at the time of construction. Dike #2 is estimated to have been underlain by 30 to 60 feet
of sludge at the time of construction. Both dikes continue to settle into the sludge at a slow, but relatively
constant rate.

The sinkhole is located between the crests for Dike #1 and Dike #2 as shown on Figure 1. Figure 1 also
shows the estimated alignment of the two leachate drainage pipes constructed as part of the dam. The
30-inch pipe (at the base of the dam) was installed to drain leachate from the landfill during and after
operation. The 30-inch pipe is a corrugated metal pipe that was encased in concrete as part of
construction. The 48-inch pipe was installed as a decant pipe to remove leachate from the surface of the
relatively impervious sludge after precipitation. The 48-inch pipe is a reinforced concrete pipe (RCP) that
was installed in sections as the sludge filling in the landfill progressed. The 48-inch pipe was terminated
at the crest of the dam (see Figure 1). The 48-inch and 30-inch pipes connect in a concrete junction box
located at the toe of the dam, on the landfill side. Leachate from the 30-inch pipe flows into an existing
wastewater aeration stabilization basin (ASB). Figure 1 includes a cross-section of the dam and dikes and
includes the approximate locations of the two pipes and the sinkhole. Since the sinkhole was noticed,
the flow from the 30-inch pipe has been estimated to be less than 5 gallons per minute (gpm).

220568.01 20230227_letter for NCDEQ.docx
February 27, 2023
Page 2 of 4

For safety reasons, no personnel or equipment have been allowed within approximately 5 feet of the
sinkhole. Photographs of the sinkhole were taken on July 20, 2022, September 8, 2022, and February 19,
2023. Shortly after the sinkhole was noticed, a pipeline cleaning/inspection contractor attempted to
clean the full length of the 30-inch pipe using water jetting methods. The pipeline cleaning contractor
stopped work after several days, indicating the equipment available was not large enough to remove a
concrete-like obstruction encountered approximately 135 feet into the pipe (as measured from the
discharge end). The pipeline cleaning contractor performed a visual inspection of the cleaned portion of
the 30-inch pipe using closed-circuit television (CCTV) techniques. In general, the cleaned portion of the
pipe appeared in good condition.

In January 2023, a second pipeline cleaning/inspection contractor attempted to clean and inspect the
30-inch pipe. The second contractor provided larger equipment that included very high-pressure jetting
tools capable of reducing obstructions (like concrete) into small pieces for removal. When the second
contractor started work, it was apparent that the 30-inch pipe had essentially become refilled with
material from the sinkhole. Materials similar to those observed near the top of the sinkhole (e.g.,
drainage stone and geogrid, see Figure 2) were present in the material removed from the 30-inch pipe
during the second cleaning. Stone riprap was also encountered in the 30-inch pipe during the second
cleaning and became more frequent as the cleaning progressed inward. The second cleaning extended
approximately 170 feet into the 30-inch pipe before the cleaning contractor indicated it would likely
take several weeks to complete due to the size and amount of riprap encountered. The cleaning
contractor also expressed concern for further sinkhole collapse if the cleaning continued, and the
possible loss of the specially-made cleaning equipment. A CCTV inspection of the second cleaning
showed the 30-inch pipe to be in generally good condition, considering it had been in service since
approximately 1977.

No attempt was made to clean the 48-inch pipe. Visual inspection of the 48-inch pipe (from the open
end of the pipe at the crest of the dam) showed the pipe to be damaged and partially filled with soil-like
material.

DRAINAGE PIPE STATUS

SME believes the 48-inch pipe and 30-inch drainage pipes are not necessary for continued stability of
the dam, the dikes, and/or the landfill. Previous slope stability analyses did not identify either of the
pipes as necessary structural elements relative to dam integrity. Rather, the 48-inch pipe was intended
to serve as a decant pipe for the landfill surface before the dikes were added; as result of the dikes, the
48-inch pipe no longer receives runoff from the landfill surface. The 30-inch pipe, owing to its size, is
thought to have been designed to handle large leachate flows from the landfill during operation as well
as handle some groundwater discharge occurring within the landfill area. A technical document
prepared by Law Engineering in the late 1970s concluded that considerable leachate (i.e., water) from
the landfill must be draining from the sludge into and through the underlying soil and bedrock, given the
low leachate flows observed coming from the 30-inch pipe at that time.

220568.01 20230227_letter for NCDEQ.docx
February 27, 2023
Page 3 of 4
SINKHOLE MITIGATION ACTIVITIES

The following activities are planned to mitigate the sinkhole.

Re-photograph the sinkhole interior. The purpose of the photography will be to evaluate any changed
conditions apparent in the sinkhole, evaluate how those conditions could affect the planned mitigation,
and to allow an estimate of the sinkhole volume to be made.

Drill several borings in the vicinity of the sinkhole and along a portion of the alignment for the 48-inch
pipe. The purpose of the borings will be to evaluate if the sinkhole has a larger below-ground component
that could hinder mitigation of the sinkhole, and to evaluate if other sinkholes might be present along
the alignment for the 48-inch pipe.

Backfill and monitor the sinkhole with clean washed gravel having a size consistent with the gravel
already visibly present in the sinkhole (i.e., estimated No. 78 size washed aggregate). Effort will be made
to drop the gravel into the sinkhole (with an excavator) rather than to push the gravel into the sinkhole.
Dropping the gravel into the sinkhole is expected to result in the gravel being some of the first material
to reach the 30-inch pipe in the event more material is accepted by the 30-inch pipe. The gravel will be
less likely to plug the 30-inch pipe as compared to if the sidewall of the sinkhole collapsed, potentially
causing loose sludge and soil to preferentially enter the 30-inch. The sinkhole will be filled to within 2 to
3 feet of the surrounding ground surface with gravel and then be capped with impermeable soil similar
to that used to cover the Sludge Landfill. The surface of the filled sinkhole will be visually monitored to
determine if settlement is occurring.

Monitor flow from the 30-inch pipe. Flow from the 30-inch pipe will be measured two times per week.
The flow measurements will be used to form an opinion of if conditions are changing in the 30-inch pipe.

Monitor water levels in seven existing piezometers on the dam crest. Water level piezometers were
installed in the dam in 2005 to evaluate dam stability as part of closing the landfill. To date, the
piezometers have showed water levels consistent with those measured prior to the sinkhole. The water
levels will be measured two times per week and then compared to the piezometer level history for the
dam. In the event the water levels suggest changed (i.e., adverse) piezometric conditions within the
dam, then corrective action will be taken to return the piezometric levels in the dam to pre-sinkhole
conditions.

220568.01 20230227_letter for NCDEQ.docx
February 27, 2023
Page 4 of 4
Glatfelter and SME look forward to discussing the sinkhole mitigation with NCDEQ and answering any
questions NCDEQ might have regarding the planned mitigation.
Respectfully,
SEVEE & MAHER ENGINEERS, INC.
Matthew W. Muzzy, P.E.
Principal/Senior Geo-Environmental Engineer
Attachments: Figures
Photos
cc: Brett Laverty (NCDEQ)
Jill Urey (Glatfelter)
Mike Cody (Consultant to Glatfelter)
Jack Garren (Consultant to Glatfelter)

FIGURE

0+00 0+20 0+40 0+60 0+80 1+00 1+20 1+40 1+60 1+80 2+00 2+20 2+40 2+60 2+80 3+00 3+20 3+40 3+60 3+80 4+00 4+20 4+40 4+60 4+80 5+00 5+20 5+40 5+60 5+80 6+00
2100
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0+00 1+00 2+00 3+00 4+00 5+00 6+00
DWG:REV:LMN:CTB:
SME
SEVEE & MAHER
ENGINEERS
ENVIRONMENTAL CIVIL GEOTECHNICAL WATER COMPLIANCE
4 Blanchard Road, PO Box 85A, Cumberland, Maine 04021
Phone [phone] Fax [phone] smemaine.com
SME
SEVEE & MAHER
ENGINEERS
ENVIRONMENTAL CIVIL GEOTECHNICAL WATER COMPLIANCE
4 Blanchard Road, PO Box 85A, Cumberland, Maine 04021
Phone [phone] Fax [phone] smemaine.com2/16/202320220720-UAV-SME-BASE PLAN-PROF SME-STD
FIGURE 1
DAM AND SINKHOLE PLAN
AND CROSS-SECTION
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
48" DIA DECANT PIPE
30" DIA PIPE
JUNCTION BOX
010 20 40 FEET
PLAN
PROFILE
010 20 40 FEET
48" DIA DECANT PIPE
30" DIA OUTLET PIPE
12" DIA DRAINAGE
PIPE FROM LANDFILL
JUNCTION BOX
EXISTING SLOPE
SINKHOLE
SINKHOLE
PIPE LENGTH
UNKNOWN
PIPE LENGTH
UNKNOWN
NOTES:
EXISTING AERIAL IMAGE FROM LOW ALTITUDE AERIAL PHOTOGRAMMETRIC MAPPING
PERFORMED BY SEVEE & MAHER ENGINEERS, INC. (SME) OF CUMBERLAND, MAINE,
DATED JULY 20, 2022. GROUND CONTROL ESTABLISHED USING AEROPOINTS.
N
GRI
D
15' WIDE CREST OF ORIGINAL DAM
12" DIA PIPE FOR
GROUNDWATER DRAINAGE
CREST OF DIKE #2
AFTER SETTLEMENT
RIPRAP DITCH
ESTIMATED LOCATION OF DIKE #1
CREST AFTER SETTLEMENT
PIPE CLEANING STOPPED HERE
1
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1
3
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PHOTOGRAPHS

20230227 photos for letter to NCDEQ.docx
Sevee & Maher Engineers, Inc. 1
February 2023

1. View of sinkhole on July 20, 2022

2. View of Sinkhole taken on September 8, 2022
20230227 photos for letter to NCDEQ.docx
Sevee & Maher Engineers, Inc. 2
February 2023

3. View of Sinkhole taken on February 19, 2023