WQ0013419_Sludge Landfill Sink Hole Work Plan_20221007

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

220568 20221007 sinkhole work plan.docx
ENVIRONMENTAL  CIVIL  GEOTECHNICAL  WATER  COMPLIANCE
WORK PLAN FOR
EVALUATION AND REPAIR OF SINKHOLE IN LANDFILL DAM
SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
October 7, 2022

BACKGROUND

This Work Plan was prepared to address a recent sinkhole development in the embankment dam that
retains Glatfelter’s closed Sludge Landfill (landfill), located in Pisgah Forest, North Carolina.

On Friday, July 8, 2022 Mike Cody and Jack Garren (environmental contractors to Glatfelter) observed a
sinkhole in the dam known as the Sludge Landfill Dam (dam). The dam is located on the Davidson River
Village property formerly used for papermaking and treatment/disposal of papermaking residuals. The
dam retains landfilled wastewater treatment sludge. The sinkhole is located on the landfill side of the
dam, has an estimated diameter of approximately 25 feet, and a depth of approximately 10 feet. Upon
discovering the sinkhole, Cody and Garren installed visual warning tape around the sinkhole’s perimeter
and alerted Davidson River Village (DRV) (owners of the landfill property) of the sinkhole condition and
recommended that DRV restrict access to the landfill and make all authorized landfill site users aware
of the sinkhole and its potential danger.

Figures 1-1 and 1-2 (all figures attached) show the sinkhole as photographed on July 8, 2022. Shortly
after the sinkhole perimeter was secured, Cody contacted Sevee & Maher Engineers, Inc. (SME)
(geotechnical and environmental contractor to Glatfelter) to discuss the sinkhole. Cody also contacted
the North Carolina Department of Environmental Quality (NCDEQ) to arrange a site inspection. Brett
Laverty (NCDEQ) visited the landfill on July 20, 2022 to observe the sinkhole and the dam retaining the
landfilled waste.

Review of existing drawings for the dam (see Figures 1-3, 1-4, and 1-5) indicate that a 48-inch-diameter
reinforced concrete pipe (RCP) and a 30-inch-diameter corrugated metal pipe (CMP) are positioned in
the vicinity of the sinkhole. The 48-inch RCP was constructed on the sideslope of the dam (on the landfill
side) and is now covered by 10 to 70 feet of sludge. The 30-inch CMP, which was constructed across the
base of the dam and outlets into an aerated stabilization basin (ASB) on the opposite side of the dam,
was historically used to drain groundwater discharge (i.e., springs) occurring in the landfill area, leachate
expressed from the sludge, and precipitation runoff/leachate from the landfill surface. The 48-inch RCP
and the 30-inch CMP both connect to a concrete junction box located at the toe of the dam on the

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October 2022
Page 2
landfill side. A 12-inch CMP pipe, which collects water from the base of the landfill also connects to the
junction box.

On July 20, 2022 Mountain Environmental Services, Inc. (MESI) mobilized a remote-controlled robotic
camera to the landfill to video and photograph the interior of the 48-inch and 30-inch diameter pipes.
MESI determined that no more than 10 feet of the RCP and no more than 45 feet of the CMP could be
accessed by the camera. Design drawings indicate the RCP and CMP are approximately 125 feet and
330 feet long, respectively. Figures 1-6 and 1-7 show views of the pipe interiors as photographed on
July 20, 2022.

On July 20, 2022, SME performed an aerial survey of the landfill and dam surface topography. The grass
surface covering the landfill and dam had recently been mowed allowing a vertical resolution of
+/- 6 inches, or less. The purpose of the survey was to visually detect depressions in the surface
topography that could be suggestive of development of other potential sinkholes. The topography
developed from the survey is shown on Figure 1-8. Examination of the topographic contouring for
Figure 1-8 showed no apparent presence of other potential sinkhole locations.

On August 10, 2022, MESI mobilized water-jet pipe cleaning equipment and the robotic camera to the
landfill to clean the 30-inch CMP. The pipe cleaning effort occurred over three consecutive workdays
and was able to clean the first 135 feet of pipe. The cleaning ended when a boulder-like obstruction
filling essentially the whole 30-inch CMP cross-section was encountered. Figure 1-9 shows a view of the
obstruction. The pipe cleaning equipment could not dislodge the obstruction and cleaning was halted.
The pipe cleaning washed considerable soil and rock-like debris from the pipe. Figure 1-10 shows views
of some of the debris, which appears to be a combination of pipe scale and concrete. Figure 1-11 shows
views of the 30-inch CMP flow taken in 2004, 2006, and 2022, and Figure 1-12 shows the soil delta that
was visible at the CMP outfall point on July 12, 2022.

On August 22, 2022, a landscape contractor was on-site to perform scheduled grass mowing of the
landfill cover and dam surface. At approximately 2 p.m., a landscaper noticed brown-colored flow
coming from the 30-inch CMP. Figure 1-13 shows the flow observed by the landscaper. Later that same
day, Jack Garren observed the pipe and noted that the flow had become less. On August 25, 2022, the
flow was observed again, and the flow color had returned to a near-clear condition (see Figure 1-14).

Also, on August 22, 2022, the landscape contractor reported an area of soft ground/shallow standing
water on the landfill cover. The location of the observed soft area is shown on Figure 1-15. Discussions
with the landscaping contractor indicated no soft area in that part of the landfill cover had been noticed
since the landscaping contractor started mowing the cover several years before.

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Attachment 1 includes additional background information relative to the dam and its appurtenances,
and additional discussion of the events described above.

Attachment 2 includes electronic copies of the video of the 30-inch CMP interior taken on July 20, 2022
and August 10, 2022.

CONCEPTUAL MODEL FOR SINKHOLE

Preliminary engineering evaluation of the sinkhole suggests the sinkhole does not present a slope
stability risk in terms of a breach forming through the dam nor present a risk of the landfilled sludge
moving through a possible breach. The evaluation does indicate that control of the water in the 30-inch
CMP needs to be better understood and how future water flow through the 30-inch pipe could affect
long-term drainage and slope stability of the dam.

The conceptual model for the sinkhole is as follows:

• The sinkhole was formed by collapse of, or lateral displacement of, portions of either one or
both of the pipes. Debris of various sizes from the sinkhole entered the 30-inch CMP and/or the
48-inch RCP and caused an obstruction in the pipe. Some of the debris was removed from the
30-inch CMP by cleaning efforts performed on July 20, 2022 and August 10, 2022. Flushing flows
observed on August 22, 2022 and August 24, 2022 also resulted in removing debris from the 30-
inch pipe. An obstruction was encountered in the 30-inch pipe that prohibited pipe cleaning
beyond approximately 135 feet from the 30-inch pipe opening (on the ASB side). The obstruction
apparently has caused debris to accumulate in the CMP (upstream of the obstruction) and the
obstruction appears to have slowed flow through 30-inch CMP. Drainage from the landfill backs
up in the 30-inch CMP as well as in the 12-inch CMP that connects to the same junction box as
does the 48-inch RCP and 30-inch CMP. The water in the 12-inch CMP eventually develops
sufficient pressure to influence the obstruction in the 30-inch CMP and a flushing-flow occurs,
moving more of the sinkhole debris to the ASB. It is uncertain if the piezometric levels in the
dam or landfill are increasing as a consequence of the flow in the 30-inch pipe. It is also uncertain
if the sinkhole or potential future flushing-flows represent slope stability concerns for the dam
overall; however, because the ASB side of the dam appears to be unsaturated (no water
pressures affecting slope stability) and because the sludge exhibits some shear strength (to
resist lateral movement), it is expected that a slope stability failure of the dam in the sinkhole
area and movement of sludge through such a failure is very unlikely.

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Observations that support the conceptual model for the sinkhole are:

• There is no reason to believe the sinkhole is a function of the local geology (i.e., the native
residual soil and underlying bedrock are not considered subject to sinkhole formation);
• Records for the dam and landfill do not identify large void spaces (e.g., buried vessels or
chambers) that were included in the dam construction other than the 48-inch RCP, the 30-inch
CMP, and the 12-inch CMP;
• The landfill was closed with a soil cover in 2009. The soil cover construction included operation
of heavy equipment and relocation of a portion of the second (i.e., the uppermost) dike in the
sinkhole area. No tell-tale signs of the sinkhole were reported when the soil cover was
constructed;
• Several slope stability analyses (SME, 2006; Law, 1976) have been performed for the dam and
retained sludge and have resulted in acceptable factors of safety (i.e., > 1.5) for static conditions,
assuming a phreatic surface in the dam reflective of the landfilled sludge being fully saturated
with a water level near the dam’s crest. Static factors of safety of 1.5 or more are considered
acceptable for dam slope stability;
• Piezometric water levels measured in the dam remained the same or increased small amounts
between the period of May 20, 2022 and September 8, 2022. The landfill area received
approximately 23 inches of rain during that same period. Interpretation of the piezometric
elevations in the dam continue to indicate that much of the dam’s soil mass in the vicinity of the
original dam crest and in the dam portions between the crest and the ASB remain drained and
unsaturated;
• Prior to the sinkhole discovery, no cloudy water had been reported flowing from the 30-inch
CMP since 2019, suggesting that if the sinkhole has been ongoing, the sinkhole progression has
been slow to develop and has not influenced other locations within the dam;
• No water flow has been observed along the exterior of the 30-inch CMP, which suggests the
internal erosion associated with the sinkhole was allowed by an opening into one or both of the
pipes; and
• The shear strength of the sludge (retained by the dam) is adequate to resist rapid downslope
movement of the landfilled sludge in the unlikely event that the sinkhole progresses to become
a breach through the dam.

The conceptual model for the sinkhole condition considers that measures can be implemented to assure
that drainage through the 30-inch CMP continues and that backfilling of the sinkhole with a material of
sufficient shear strength and physical characteristics to resist future erosion can be completed.

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INVESTIGATIONS AND ANALYSES RECOMMENDED TO ACCEPT/REJECT SINKHOLE CONCEPTUAL
MODEL

Acceptance/rejection of the conceptual model for the sinkhole will require collection and analysis of
sludge and dam geotechnical data in the sinkhole area. A summary of the data required is presented
below.

• Conduct additional cleaning and photographic inspection of the 30-inch CMP. The August 22,
2022 flows from the 30-inch CMP likely dislodged the obstruction encountered on August 10,
2022. Efforts will be taken to continue cleaning the 30-inch pipe to the junction box on the
landfill side of the dam. In the event the obstruction in the CMP remains or has changed position
in the 30-inch pipe, the cleaning effort will include steps to reduce the size of the obstruction
and remove it from the pipe.
• Better define the lateral limits of the sinkhole. As shown in Figure 1-2, a buildup of soil and
geosynthetics underlie the two dike(s) that were added to the dam’s crest more than 20 years
ago. The buildup of soil and geosynthetics could be masking the lateral extent of the sinkhole.
To address the extent of the sinkhole, four borings will be drilled in the vicinity of the sinkhole
to detect voids and softness with respect to depth, which could be suggestive of the lateral limits
of the sinkhole. The borings will allow in-situ shear strength testing, penetration testing, and
undisturbed soil sampling (for subsequent laboratory strength evaluation) to be completed with
respect to depth. Multi-level standpipe piezometers will be installed in the borings to better
understand water levels in the sludge and dam in the vicinity of the sinkhole. The results of the
in-situ testing and laboratory testing will be used to perform additional slope stability analyses
for the dam when subjected to various hydraulic conditions.
• Better define the shear strength and water level of the sludge in the vicinity of the sinkhole.
The sludge in the landfill has been capped since 2009. Settlement (i.e., consolidation) of the
sludge has occurred since the landfill was covered. The consolidation has likely resulted in
increased sludge shear strength, which is beneficial relative to dam slope stability. Two borings
will be drilled into the sludge in the vicinity of the sinkhole. The borings will be located
sufficiently away from the toe of the dam (on the landfill side) to minimize any effect the dam
slope could potentially have had on consolidation of the sludge. The borings will be used to
measure the sludge thickness, in-situ shear strength and piezometric conditions.
• Perform dam slope stability analyses relative to the sinkhole. Slope stability of the dam in the
sinkhole area will be evaluated using the water levels measured in the (new and existing)
piezometers and the sludge and soil shear strengths measured in the borings. Attention will be
given to evaluating slope stability of the dam (on the ASB side) with the 30-inch CMP free-
flowing and fully plugged.

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• Prepare a report summarizing the investigations made for the sinkhole. The report will provide
conclusions relative to slope stability of the dam and the basis for those conclusions. Pending
acceptable findings, the report will also make recommendations for backfilling the sinkhole and
mitigation of the 30-inch CMP, if considered to be necessary.

SCHEDULE

Based on discussion with regional pipe cleaning and drilling subcontractors in the vicinity of the landfill,
it is expected that additional cleaning of the pipeline and drilling boreholes in the dam and landfill can
be completed in late fall-early winter 2022. Laboratory testing of soil and sludge samples collected from
the boreholes is expected to require three to four weeks to complete. The evaluation/analysis of the
data with a summary report is expected to be available three to four weeks following completion of the
laboratory testing.

FIGURES

DWG:REV:LMN:CTB:
SME
SEVEE & MAHER
ENGINEERS8/22/2022
FIGURE 1-1
SINKHOLE PHOTOGRAPH #1
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTO TAKEN ON JULY 20, 2022
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DWG:REV:LMN:CTB:
SME
SEVEE & MAHER
ENGINEERS8/22/2022
FIGURE 1-2
SINKHOLE PHOTOGRAPH #2
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTO TAKEN ON JULY 20, 2022
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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.comNASME-STD
FIGURE 1-3
DESIGN RECORD DRAWING #R-200-E
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
8/22/202220220720-UAV-SME-BASE
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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.comNASME-STD
FIGURE 1-4
DESIGN RECORD DRAWING #R-216-E
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
8/22/202220220720-UAV-SME-BASE
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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.com8/22/202220220720-UAV-SME-BASE PLAN-PROF SME-STD
FIGURE 1-5
APPROXIMATE LOCATION OF PIPES
RELATIVE TO SINKHOLE
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
48" DIA INLET PIPE
30" DIA OUTLET PIPE
12" DIA DRAINAGE
PIPE FROM LANDFILL
CONCRETE STRUCTURE
010 20 40 FEET
PLAN
PROFILE
010 20 40 FEET
48" DIA INLET PIPE
30" DIA OUTLET PIPE
12" DIA DRAINAGE
PIPE FROM LANDFILL
CONCRETE STRUCTURE
DIKE NO. 2 (PRE SETTLEMENT)DIKE NO. 1 (PRE SETTLEMENT)
EXISTING GRADE (TYP)
SINKHOLE
SINKHOLE
EXPECTED COLLAPSED SOIL
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.
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DWG:REV:LMN:CTB:
SME
SEVEE & MAHER
ENGINEERS8/22/2022
FIGURE 1-6
INTERIOR OF 48-INCH RCP
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTO TAKEN ON JULY 20, 2022
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DWG:REV:LMN:CTB:
SME
SEVEE & MAHER
ENGINEERS8/22/2022
FIGURE 1-7
INTERIOR OF 30-INCH CMP
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTOS TAKEN ON JULY 20, 2022
30" DIA PIPE INTERIOR FROM PIPE INSPECTION30" DIA PIPE INTERIOR FROM END OF PIPE
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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.com
025 50 100 FEET
EX-TOPO SME-STD
FIGURE 1-8
EXISTING TOPOGRAPHY
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
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.8/22/202220220720-UAV-SME-BASE
NGRID
48" DIA INLET PIPE
30" DIA OUTLET PIPE
12" DIA DRAINAGE
PIPE FROM LANDFILL
CONCRETE STRUCTURE
SINK HOLE
PIPE LENGTH
UNKNOWN
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FIGURE 1-9
VIEW OF OBSTRUCTION IN 30-INCH CMP
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTO TAKEN ON AUGUST 11, 2022OBSTRUCTION IN 30" DIA PIPE
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SEVEE & MAHER
ENGINEERS8/22/2022
FIGURE 1-10
DEBRIS REMOVED FROM 30-INCH CMP
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTO TAKEN ON AUGUST 11, 2022DEBRIS REMOVED FROM 30" DIA PIPE
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SEVEE & MAHER
ENGINEERS8/22/2022
FIGURE 1-11
VIEW OF FLOW 30-INCH CMP
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
WATER FLOW FROM 30" DIA PIPE - JULY 20, 2022
WATER FLOW FROM 30" DIA PIPE - OCTOBER 1, 2004
WATER FLOW FROM 30" DIA PIPE - AUGUST 25, 2006
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SEVEE & MAHER
ENGINEERS8/22/2022
FIGURE 1-12
VIEW OF SOIL DELTA AT 30-INCH CMP
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTOS TAKEN ON JULY 20, 2022
SOIL DELTA AT OUTLET OF 30" DIA PIPEDEBRIS AT OUTLET OF 30" DIA PIPE
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SEVEE & MAHER
ENGINEERS9/16/2022
FIGURE 1-13
VIEW OF FLOW 30-INCH CMP
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTO TAKEN ON AUGUST 22, 2022
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DWG:REV:LMN:CTB:
SME
SEVEE & MAHER
ENGINEERS9/16/2022
FIGURE 1-14
VIEW OF FLOW 30-INCH CMP
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTO TAKEN ON AUGUST 25, 2022
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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.com
025 50 100 FEET
EX-TOPO SME-STD
FIGURE 1-15
SOFT AREA ON LANDFILL SURFACE
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
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.8/22/202220220720-UAV-SME-BASE
NGRID
APPROXIMATE LOCATION
OF OBSERVED SOFT SPOT
ON LANDFILL SURFACE
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FIGURE 1-16
CLOUDY WATER AT 30-INCH OUTFALL
GLATFELTER SLUDGE LANDFILL
PISGAH FOREST, NORTH CAROLINA
SME-STDLMN20220720-UAV-SME-BASE
PHOTO TAKEN ON MARCH 19, 2019
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ATTACHMENT 1

BACKGROUND INFORMATION AND PRELIMINARY OBSERVATIONS

Attachment 1
Page 1
BACKGROUND INFORMATION AND PRELIMINARY OBSERVATIONS

The dam was constructed in 1976. Soil materials used to construct the dam were excavated from a ravine
(area to the north) that now forms the landfill. A report by Law Engineering Testing Company (LETCo.)
dated November 23, 1976, documents the dam’s construction, and includes the following information:

Dam Maximum Height (Main Dam) Approx. 75 ft
Dam Crest Length Approx. 425 ft
Dam Crest Elevation1, 2 2,186 ft
Dam Crest Width 15 ft
Dam Upstream Slope Angle (i.e., ASB
side)
3 Horizontal: 1 Vertical
Dam Downstream Slope Angle (i.e.,
landfill side)
2 Horizontal: 1 Vertical

Note
1. LETCo. Report did not state an Elevation Datum, however 2005 survey
of the dam by Hafler Land Surveying of Brevard, North Carolina
indicates the dam crest is at approximately Elevation 2187 feet using a
1983 NGVD datum
2. The above information does not include two add on dikes constructed
in approximately 1998 and 2000.

To help control seepage through and beneath the dam (from the landfill side), the toe of the dam on the
ASB side includes a blanket drain which collects and discharges seepage before emerging at the dam’s
toe.

In 1998, a 10-foot-high soil dike was added to the top of the dam on the landfill side and in 2000, a second
similar dike was added to the top of the first dike. The dikes were added to increase capacity of the landfill.
Each dike was constructed on a buildup of geosynthetics and stone, which overlaid previously placed
sludge. Examination of Figure 1-2 (see Work Plan) shows exposed geotextile, geogrid, and stone that were
used to support the base of the second dike added to the crest of the dam. Both dikes continue to undergo
settlement due to consolidation of the underlying sludge. Note that a portion of the second dike (i.e., the
2000 dike) was removed when the landfill was covered in 2009.

In 2005, a number of standpipe piezometers were installed in the dam and landfill. Eight of the
piezometers in the dam indicate no or little water is present at the bottom of the piezometer’s screened
interval. Interpretation of the water elevations calculated from the piezometers indicates much of the
dam on the ASB side is unsaturated, possibly due to effective performance of the dam’s toe drain system
or possibly due to lack of free-water on the landfill side of the dam to sustain seepage within the dam.
The water level elevations in the piezometers were measured on August 15, 2022 and September 8, 2022
and compared to each other and also to water level elevations measured on May 20, 2022. Comparison
of the August 15, 2022 and September 8, 2022 elevations showed little water level change in the
piezometers. Comparison of the September 8, 2022 elevations with the May 20, 2022 elevations showed
that many of the water levels in the piezometers went down during summer 2022. Piezometers measuring

Attachment 1
Page 2
water levels with respect to depth within the dam generally indicated presence of a possible phreatic
surface at the base of the dam; whereas several piezometers located within the upper half of the dam
showed several feet of water in or above the piezometer-screened intervals. The piezometric conditions
measured indicate a possible perched water table within a portion of the dam and vertical seepage
gradients in the dam close to unity.

Between the period of May 20, 2022 and August 15, 2022, the landfill area received approximately 15
inches of rain; the landfill area typically receives 67 inches of rain annually.

The landfill was closed in 2009 with a soil cover. The closure construction included regrading most of the
landfill surface to promote drainage of surface water to a catch basin and outlet pipe located near the
southwestern corner of the landfill. Part of the regrading involved removing approximately 6 feet of the
second (uppermost) dike at the approximate sinkhole location.

A 30-inch-diameter CMP and a 48-inch-diameter RCP are located near the center of the dam where the
sinkhole occurred.

• The 30-inch CMP is located at the base of the dam and connects the inward toe of the dam to the
outward toe of the dam. The 30-inch CMP is encased in concrete and is sloped to convey
stormwater and leachate from a concrete junction box located at the toe of the dam on the landfill
side to the ASB on the opposite side of the dam. At its approximate mid-point, the CMP is
estimated to be overlain by approximately 65 feet of compacted soil. The concrete junction box
is buried beneath approximately 70 feet of sludge. Figure 1-11 (see Work Plan) shows various
views of the pipe since 2004.
• The 48-inch RCP pipe is located on the landfill side of the dam and is situated on a concrete slab
that was placed on the face of the dam’s sideslope. The lower end of the 48-inch RCP terminates
in the same concrete junction box as does the 30-inch CMP. The aboveground inlet to the 48-inch
RCP is visible in a stone-lined drainage area that runs perpendicular to the dam crest. The inlet to
the 48-inch RCP occasionally receives precipitation drainage from the surrounding ground
surface. The upper end of the 48-inch RCP is overlain by the two dikes added in 1998 and 2000.
The combined thickness of dike soil and sludge overlying the 48-inch RCP is estimated to range
from approximately 10 to 70 feet.
• A 12-inch-diameter perforated CMP extends from the junction box into the landfill area (below
the sludge) over a distance of more than 400 feet. The 12-inch CMP was part of the original landfill
construction and was installed to collect and divert water occurring in the work area. The collected
water in the 12-inch CMP flows into the junction box and then through the 30-inch CMP to the
ASB.

Attachment 1
Page 3
PRELIMINARY OBSERVATIONS

The following efforts have been made to investigate the sinkhole and it potential cause.

• Design drawings and construction documents have been reviewed and showed the 48-inch RCP,
the 30-inch CMP, and a 12-inch CMP (from the landfill area) all converging into a concrete junction
box, which is located at the toe of the dam on the landfill side. Water enters the junction box from
the 48-inch RCP and the 12-inch CMP and flows out of the junction box through the 30-inch CMP.
Review of the drawings and documents did not reveal any apparent construction flaws with the
dam or its drainage features.
• The July 20, 2022 photography and videoing of the pipe interiors showed that 1) the 48-inch RCP
appears to be plugged with precipitated minerals and debris. The 48-inch RCP also appears to be
disconnected (offset) or collapsed at a point approximately 10 feet below the RCP’s ground
surface opening, and 2) the 30-inch CMP was too full of debris for the camera to advance beyond
45 feet into the pipe (from the ASB side).
• The topographic survey of the dam surface and the sinkhole on July 20, 2022 did not show any
signs of other potential sinkholes in the dam.
• The CMP cleaning and photographing on August 10, 2022 showed 1) the presence of a large
(immovable) obstruction at distance of approximately 135 feet from the pipe’s opening (on the
ASB side), 2) the invert of the CMP appears to have settled several inches at a distance of
approximately 38 feet from the pipe’s opening, and 3) the first 135 feet of the 30-inch CMP (less
approximately 30 feet of water-filled pipe) appears to be in good condition visually and is free of
openings/defects that could allow soil to enter the pipe. The settled portion of the pipe appears
to be near full of water for a distance of approximately 38 to 65 feet, at which point the pipe’s
invert becomes sufficiently sloped to be above the water level in the pipe. The CMP cleaning also
showed that there is no discernable flow occurring along the exterior of the 30-inch CMP. In some
cases, material being removed from the 30-inch CMP (during cleaning) was estimated to be
approximately 8 inches in size or more. The observed pipe flow on August 22, 2022 suggests the
obstruction in the pipe may have been dislodged to allow soil and sludge to be washed through
the 30-inch CMP.

In addition to the recent investigation findings, on one occasion in spring 2019, cloudy water was observed
flowing from the discharge end of the 30-inch CMP, suggesting soil may have been eroding into any one
of three pipes at that time. A photo of cloudy water observed in 2019 is shown on Figure 1-16 to the Work
Plan.

ATTACHMENT 2

PIPE VIDEO INSPECTION

JULY 20, 2022 https://spaces.hightail.com/receive/8WoEjQh8Tv
AUGUST 10, 2022 https://spaces.hightail.com/receive/TEALW7KYk3
AUGUST 11, 2022 https://spaces.hightail.com/receive/xMP7caZhaf