HomeMy WebLinkAboutWQ0000020_Corrective Action Plan_20200306 (3)DUKE
ENERGY
March 6, 2020
Mr. Brett Laverty
North Carolina Department of Environmental Quality
Asheville Regional Office
2090 US 70 Highway
Swannanoa, NC 28778
410 S. Wilmington Street
Raleigh, NC 27601
Mailing Address
Mail Code NC 15
Raleigh, NC 27601
919-546-7863
Subject: Response to North Carolina Department of Environmental Quality Site Assessment
Review letter dated August 12, 2019 — Asheville Airport Area 1 Structural Fill
Dear Mr. Laverty:
Duke Energy is in receipt of the above -referenced letter from the North Carolina Department of
Environmental Quality (NCDEQ), which requested that a permanent cap system be developed to "abate
the infiltration of groundwater and/or precipitation into Area 1" and "to abate or control the.... seepage
water." Duke Energy partnered with Geosyntec Consultants of NC, P.C. (Geosyntec) to prepare the
enclosed Design Report - Permanent Cap System for Area 1 structural fill (Area 1). The proposed
permanent cap system will:
Abate or control seepage from Area 1 through the installation of a permanent cap system to
reduce and abate infiltration from precipitation into the CCR, which will lower the interstitial
water elevation over time;
• Achieve long-term slop stability factors of safety equal to or above 1.5; and
• Improve and actively manage stormwater runoff volume from Area 1.
Upon NCDEQ concurrence with this proposed cap system, Duke Energy will work with the Greater
Asheville Regional Airport Authority to obtain required access to the property and install the permanent
cap system. Note that the Greater Asheville Regional Airport Authority has conveyed that they are not in
agreement with the cap design through a February 24, 2020 letter to Duke Energy; that letter and Duke
Energy's response are also attached. If you have any questions or need any clarification regarding the
information provided, feel free to contact me at iohn.toepfer@duke-enersy.com or at 919-546-7863 at
your convenience.
Res ullyRPE.i'
ted,
J n Toepfe
Lead Engineer, Duke Energy EHS CCP
Waste & Groundwater Programs
cc: Ms. Jessica Bednarcik —Duke Energy
Mr. Michael Reisman — Greater Asheville Regional Airport Authority
Mr. James McNash — Geosyntec
March 6, 2020
Mr. Laverty Letter
enc: Design Report Permanent Cap System, Asheville Regional Airport — Area 1 Structural Fill, Rev. 0,
Geosyntec Consultants of NC, P.C., March 2020
Greater Asheville Regional Airport Authority letter, February 24, 2020
Duke Energy response to Greater Asheville Regional Airport Authority, March 5, 2020
sh
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R E G I Oeville NAL AIRPORT
Take the easy way out.
February 24th, 2020
Duke Energy
Mr. William M. Harrison, P.E.
CCP Engineering — Mgr Programmatic Engineering
400 S. Tryon Street
Charlotte, NC 28202
RE: Response to Duke Energy Letter Dated February 15th, 2020
Dear Mr. Harrison:
We are in receipt of your letter noted above which was in response to the Greater
Asheville Regional Airport Authority's (GARAA's) comments on the most current
proposed plans for Area 1. The following information and details are provided in
response to your letter.
It is important to reiterate that the intent of the airport in permitting CCB material
to be placed in any location on the airport, including Area 1, was specifically to
improve those areas in order to permit new aeronautical and non -aeronautical
development. Without the creation of land for future development, there was no
purpose in the airport accepting the placement of this material on its property.
2. The initial project which took place during the 2006-2009 timeframe transformed
approximately 14 acres of land for development. As a result of more recent
issues of course with the stability of the Area 1 slope, the NCDEQ has mandated
permanent improvements to this area.
3. Duke Energy presented its preliminary plans to GARAA on November 1st, 2018,
through a meeting between Duke Energy and Airport staff. At that time, the
stated intent was for Duke Energy to strip a certain amount of topsoil from Area
1, install a cap liner system, and then replace the top soil. GARAA staff were
generally in agreement with this plan, as it maintained the airports ability to
develop this acreage in the future without significant additional site
improvements, based on our understanding that the finished grade would be
generally level, while accounting for proper surface runoff.
4. There was no further communication from Duke Energy on this matter until
October 18th, 2019, when Duke Energy met with airport staff. No new
information was provided concerning design at this meeting, but rather it was an
opportunity for new personnel involved with the project for Duke Energy to meet
61 Terminal Drive, Suite 1 • Fletcher, NC 28732 • 828-684-2226 • flyavl.com
Mr. William M. Harrison, P.E.
February 24th, 2020
Page 2
airport staff and discuss the project in general.
5. Duke Energy transmitted proposed 90 percent complete plans to airport staff on
January 19th, 2020, in preparation for a meeting on January 23rd, 2020. At this
time Duke requested a short turn around time for airport review which was not
feasible given airport priorities. The plans provided at this time to the airport
were somewhat consistent with the preliminary plan discussed in November
2018, but this was the first that airport staff became aware of Duke Energy's
alternative plan for how it intended to accommodate surface runoff.
It was noted by Geonsyntec in this meeting that the proposed 90 percent plans
were designed to abate and control seepage at the northern Area 1 slope. No
consideration for the ability of the airport to develop the site was indicated,
presented, or identified in the discussion, or in the plans.
Comments with questions and concerns from the airport were transmitted to
Duke Energy on February 11th, 2020. Subsequently, Duke Energy responded
with its February 15th letter.
6. The response to the airport's comments on the 90 percent plans in Duke
Energy's February 15th, 2020 letter somewhat minimized the airports concerns
over most of the issues raised. Of significant importance however, is the design
for the trapezoidal/herringbone ditches, and how that design, along with the
overall surface elevations, leaves the airport with property that is incapable of
being developed as planned, without significant additional site work by the
airport, or a developer in the future.
7. Duke Energy's February 151h letter identifies that only 2 feet of soil cap would be
provided at the centerline of the herringbone ditches, increasing to six feet at the
high points. However, six feet of equal cover would be required over the entire
site for future development to occur. It further seems to indicate that at the time
such modifications take place, such changes would need to be designed by a
registered engineer with knowledge of geomembrane liner requirements. This
statement is interpreted to indicate that Duke Energy would not participate in the
cost of this design modification or the site improvements needed, but that it
would be the responsibility of the airport and/or the developer.
8. Duke Energy's February 15th, 2020 letter also responded to the airports query
about addressing the removal of existing water trapped above the bottom liner.
The response to this query was inadequate, as it only commented on the
reduction of future infiltration but did not address the existing situation which
would still be present.
Mr. William M. Harrison, P.E.
February 24t", 2020
Page 3
It is understood that Duke Energy is required to submit its final design to NCDEQ no
later than March 2nd, 2020. While it took Duke Energy approximately 14 months to
submit plans for review to the airport, it was not reasonable to expect the extremely
short turn around requested, with so little time left before the submittal deadline. The
current submittal would leave the airport with the requirement to make significant
investment on its own to be able to accommodate any future development, which is
unacceptable and inconsistent with Duke Energy's contractual obligations. The GARAA
therefore objects to the design submitted by Duke Energy and requests a re -design of
the corrections needed to the site that satisfactorily meet the needs and requirements of
all agencies involved.
Sincerely,
Michael A. Reisman, A.A.E.
Deputy Executive Director
Cc: Jessica Bednarcik
Grady Shields
Landon Davidson
If' DUKE
14"ENERGY,
Mr. Lew S. Bleiweis
Executive Director
Asheville Regional Airport
61 Terminal Drive, Suite 1
Fletcher, NC 28732
Dear Mr. Bleiweis:
Jessica L. Bednarcik, Vice President
400 south Tryon street, S►06K
Chadotte, NC 28202
7041382.8768
March 5, 2020
We have received Mr. Reisman's letter dated February 24, 2020, stating the Greater
Asheville Regional Airport Authority (GARAA) objects to the 90% proposed design for
capping the Area 1 structural fill. This plan was submitted to meet the requirement for a
remedial strategy that abates the infiltration of stormwater and/or precipitation into the
Area 1 CCP fill as required by NCDEQ DWR in its letter dated August 12, 2019
(Corrective Action #1). We acknowledge the GARAA's concerns regarding possible
impacts of the design's method for handling stormwater and final grading on potential
future development of the area, but Duke Energy simply cannot develop a design that
anticipates an unknown future state. Should GARAA decide to develop the area, some
degree of stormwater planning and final grading design revisions will be required by any
engineering firm GARAA would choose, but with no immediate plan for site
development, we believe further review and modification would jeopardize the
commitment Duke Energy made to provide this strategy to DWR in March 2020. To be
clear, although Duke Energy has taken all reasonable steps to accommodate GARAA
as a courtesy, Duke Energy's primary consideration is to address the issues DWR
raised in NOV-2018-DV-0101—a notice of violation on which GARAA is also named —
and subsequent correspondence.
Additionally, GARAA is concerned that these plans as currently proposed could require
additional investment by GARAA to accommodate future development, implying that
Duke Energy in some way is contractually obligated for all or some of these costs,
specifically future design and permitting. The only agreement between Duke Energy
and GARAA is the Access Agreement, under which Duke Energy's limited obligations,
except for those related to certain groundwater -related functions, cease with expiration
of the structural permit on August 31, 2020. As Duke Energy has previously explained,
Duke Energy (1) does not own the property on which GARAA's structural fill is located;
(ii) does not own the coal combustion products used to construct the fill (ownership of
Page 2of2
March 2, 2020
the coal combustion products transferred to Charah at such time they were loaded by
Charah or its contractor onto its trucks); (iii) did not design or build the fill; and (iv) does
not operate the fill. In light of the foregoing, with the exception of the limited offer in the
following paragraph, Duke Energy will not play any part in the planning, design, or
implementation of projects concerning future development of GARAA's structural fill,
including Area 1—functions that are purely within the purview of GARAA, the sole
sponsor, owner, and independent governing body of the Asheville Regional Airport.
To meet the commitments made to DWR for the remedial strategy, Duke Energy will be
submitting the plans for Area 1 to DWR, noting GARAA's objection as proposed. Duke
Energy believes this is the most prudent course of action to timely satisfy DWR's
concerns over fill stability and impacts to surface waters. The 90% design plans
reviewed with GARAA on January 23, 2020 were not available earlier due to requests
for additional information from DWR that were communicated to Duke Energy and
GARAA in DWR's letter to Duke Energy and GARAA dated August 12, 2019. These
additional requests required further evaluation by Duke Energy's engineering contractor
to satisfy additional DWR concerns identified and incorporated into the design as
needed. These considerations were discussed with GARAA during a meeting with
GARAA on October 18, 2019. Other than the conceptual diagram for potential
expansion of Area 1, including the extension of Wright Brothers Way, Duke Energy did
not receive any specific design information from GARAA regarding how the impact of
future development needs could be incorporated.
Should DWR require revisions to the plan, Duke Energy will make reasonable efforts to
consider GARAA's reasonable future development needs, provided GARAA makes
those needs known to Duke Energy in a timely fashion. It is our hope to come to an
acceptable design with DWR, and obtain GARAA's concurrence to implement prior to
the Area 1 permit expiring in August. In closing, we call your attention to the Access
Agreement's Right of Access provision, which commits GARAA to provide Duke Energy
"with timely access to those areas within the airport necessary for ... [Duke Energy] to
fulfill its monitoring, inspection and any other obligations under the Structural Fill
Permit..."
Cc: Michael Reisman, Greater Asheville Regional Airport Authority
Michael Kafka, Duke Energy
Landon Davidson, NCDEQ
Prepared for
('DUKE
ENERGY
PROGRESS
Duke Energy Progress, LLC
400 South Tryon Street
Charlotte, North Carolina 28202
DESIGN REPORT
PERMANENT CAP SYSTEM
Revision 0
Asheville Regional Airport — Area 1 Structural Fill
Fletcher, North Carolina
Prepared by
Geosynte&
consultants
Geosyntec Consultants of NC, Y.C.
Geosyntec Consultants of NC, PC
1300 South Mint Street, Suite 300
Charlotte, North Carolina 28203
License No. C-3500
N Cq///'i/ Project No. GC6463
yO�oF E s s/o 2 % March 2020
a SEAL —
044112 —
'GMEE- James D. McNash, P.E.
S�D M� Car na Registration No. 044112
Date: 6 March 2020
ARA – Area 1 Structural Fill
Design Report – Permanent Cap System, Rev. 0
GC6463/ARA_Permanent_Cap_Design_Report i March 2020
LIST OF ACRONYMS AND ABBREVIATIONS
Acronym/Abbreviation Definition
ARA Asheville Regional Airport
Area 1 Area 1 Structural Fill
CCP Coal Combustion Products
Charah Charah, Inc.
Duke Energy Duke Energy Progress, LLC.
DWR Division of Water Resources
FAA Federal Aviation Administration
FS Factor of Safety
ft Foot/Feet
fps feet per second
GCL Geosynthetic Clay Liner
Geosyntec Geosyntec Consultants of North Carolina, PC
gpd Gallons per day
H:V horizontal to vertical
HELP Hydrologic Evaluation of Landfill Performance
hr Hour
in. Inches
LLDPE linear-low density polyethylene
MDC Minimum Design Criteria
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Acronym/Abbreviation Definition
NC North Carolina
NCAC North Carolina Administrative Code
NCDEQ North Carolina Department of Environmental Quality
NOV Notice of Violation
RCP Reinforced Concrete Pipe
Report Design Report – Permanent Cap System
SCM Stormwater Control Measures
SHWT Seasonal High Water Table
SCS Soil Conservation Service
USACE United States Army Corps of Engineers
yr Year
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TABLE OF CONTENTS
1. Introduction ..........................................................................................................................1
1.1 Site Background ..........................................................................................................1
1.2 Project Background.....................................................................................................2
1.3 Report Purpose and Organization ...............................................................................3
2. Permanent Cap System ........................................................................................................4
2.1 Overview .....................................................................................................................4
2.1.1 Permanent Cap System Layers ......................................................................4
2.1.2 Internal Drainage ...........................................................................................5
2.1.3 Surface Water Management System Design .................................................6
3. Auxiliary Civil Design Features ..........................................................................................7
3.1 Erosion and Sediment Control ....................................................................................7
3.2 Surface Water Ponds ...................................................................................................7
3.3 Wildlife Barrier Fence ................................................................................................7
4. Design Criteria And Assumptions .......................................................................................9
4.1 Permanent Cap Design Infiltration .............................................................................9
4.2 Permanent Cap System Internal Drainage Layer ........................................................9
4.3 Slope Stability .............................................................................................................9
4.4 Settlement .................................................................................................................10
4.5 Stormwater Management Design and Surface Water Ponds ....................................10
4.5.1 Open Channel Conveyances ........................................................................10
4.5.2 Channel Lining Material ..............................................................................11
4.5.3 Outlet Protection ..........................................................................................11
4.5.4 Dry Ponds and Sediment Basins ..................................................................11
4.6 General Design Assumptions....................................................................................13
5. Design Calculations ...........................................................................................................15
5.1 Infiltration Analysis ..................................................................................................15
5.2 Slope Stability Analysis ............................................................................................15
5.3 Stormwater Design Calculations ..............................................................................16
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6. Design Documents .............................................................................................................17
6.1 Design Drawings.......................................................................................................17
6.2 Technical Specifications ...........................................................................................17
6.3 Construction Quality Assurance Plan .......................................................................17
7. Conclusions and Limitations..............................................................................................18
7.1 Conclusions ...............................................................................................................18
7.2 Limitations ................................................................................................................18
8. References ..........................................................................................................................19
LIST OF FIGURES
Figure 1. Site Location Map
Figure 2. Surface Water Sampling Locations
LIST OF APPENDICES
Appendix A Permanent Cap System Infiltration Analysis
Appendix B Permanent Cap System Slope Stability Analysis
Appendix C Surface Water Management System Calculation Package
Appendix D Design Drawings
Appendix E Technical Specifications
Appendix F Construction Quality Assurance Plan
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1. INTRODUCTION
1.1 Site Background
The Area 1 Structural Fill (Area 1) is situated near the northeastern Asheville Regional Airport
(ARA) property boundary (Figure 1) and was constructed pursuant to a contract between Charah,
Inc. (Charah) and the property owner, ARA Authority, to expand airport operations. Area 1 is
owned, operated, and maintained by the ARA Authority. The Area 1 footprint formerly consisted
of a topographic valley prior to construction and contained a historical stream channel that flowed
northward from the property. The historical stream channel traverses a residential area situated on
the northern property boundary adjacent to Area 1 before discharge into the French Broad River.
Area 1 was constructed by filling the topographic valley with compacted coal combustion products
(CCP) purchased from Duke Energy Progress’s (Duke Energy) Asheville Steam Electric Plant by
Charah. The historical stream channel was re-routed upgradient of Area 1 within a 54-inch (in.)
diameter reinforced concrete pipe (RCP) and a concrete junction box was installed to transition
from the 54-in. to a 60-in. diameter RCP bedded with drainage aggregate beneath the Area 1
footprint. The historical stream channel branch west of the 60-in. diameter RCP was also filled
with drainage aggregate and covered. The 60-in. diameter RCP flows into a concrete junction box
with a 15-in. diameter RCP with headwall which discharges a base flow to the northwest and a 60-
in. diameter RCP with headwall that discharges to the north. Construction photographs indicate
that the 15-in. diameter RCP outlet pipe was installed with a lower invert elevation than the 60-in.
diameter RCP. Soil backfill was placed from the RCP spring line to 2 feet (ft) above the top of
pipe.
As-built drawings prepared by Vaughan Engineering [2010] on behalf of Charah indicate that Area
1 was constructed with a geosynthetic clay liner (GCL) base liner and a soil cap system. Soil
backfill was compacted on the GCL above and within 50 ft of the 60-in. diameter RCP. The 60-
in. diameter RCP divides Area 1 into western and eastern components (termed “west cell” and
“east cell” herein); thus, the Area 1 north slope is divided into east and west cells as bisected by
the approximately 100-ft wide compacted soil corridor (referred to herein as the “RCP Soil
Corridor”). CCP were beneficially used as structural fill within the remainder of Area 1. The soil
cap system was constructed with approximately 6-ft and 2-ft thick soil layers on the top deck and
side slopes, respectively. Access to the base of the structural fill slope is achieved via a gravel
access road situated outside a security fence maintained by the ARA Authority.
Duke Energy provides routine inspections and water quality monitoring as required by the North
Carolina Department of Environmental Quality (NCDEQ) Division of Water Resources (DWR)
Permit No. (WQ0000020) (Permit).The ARA Authority provides routine maintenance for the Area
1 soil cap. Surface water quality monitoring occurs at surface water monitoring locations SW2-
A1, SW3-A1, SW4B-A1, SW6-A1, and SW8-A1 quarterly. Surface water sampling locations
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SW2-A1 and SW4B-A1 are located within the junction box upgradient of the 60-in. diameter RCP
and downstream of the 15-in. diameter RCP; while, SW9-A1 is located at the end of the 15-in.
diameter RCP headwall. Surface water sampling location SW3-A1 is located at the concrete
headwall immediately before the existing stream channel exists the ARA property boundary.
Meanwhile, surface water sampling locations SW6-A1 and SW8-A1 are located downstream of
the former sediment basin northeast of the east cell and at a seep located at the northwest corner
of the east fill respectively. Current and historical surface water sampling locations are shown on
Figure 2.
Duke Energy installed two temporary stormwater diversion channels with down drains to re-direct
surface water runoff away from the northern slope of the Area 1 east cell in response to NCDEQ-
mandated corrective action measures in 2018. The ARA Authority acknowledged responsibility
for the corrective actions measures associated with the 60-in. diameter RCP and developed a repair
plan for the RCP after a third party evaluation of the RCP’s structural condition. Sealing of the
cracks observed within the RCP was completed by the ARA Authority in December 2019. In
addition, the ARA Authority excavated sediment and vegetation from the former sediment pond
northwest of Area 1, created a series of riprap lined diversion channels, and the west cell slope toe
was subsequently surfaced with riprap. As-built surveys and documents were not available at the
time of this Report.
1.2 Project Background
Duke Energy identified wet areas and a small slough1 in the soil cap with an isolated seep that
contained negligible amounts of CCP at the base of the east cell of the Area 1 north slope during
inspection activities at ARA on 7 September 2017. Duke Energy promptly notified Mr. Brett
Laverty, P.G. with the NCDEQ DWR) on 7 September 2017, as required by the Permit.
Regulatory inspections of the Area 1 north slope were conducted on 7 and 15 September 2017.
NCDEQ subsequently issued short-term requirements with submittals at 7-day, 30-day, and 90-
day intervals to Duke Energy via email correspondence on 15 September 2017. The short-term
requirements were formalized within a Notice of Violation (NOV), NOV-2017-PC-0616, dated 17
November 2017. Subsequently, Duke Energy submitted the Engineering Analysis Report
[Geosyntec, 2017] to respond to the short-term requirements, which included available slope
monitoring data, construction history, interstitial water levels, and slope stability analysis results.
NCDEQ DWR provided a response to the Engineering Analysis Report on 30 April 2018, which
required several corrective actions that included: a soil cap permeability evaluation, GCL
performance assessment, and additional interstitial water elevation measurements. Duke Energy
submitted these assessments as attachments to the Conceptual Hydrogeologic Model Report
1 Referred to as a breach by NCDEQ in a 15 September 2017 email to Duke Energy.
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[Geosyntec, 2019] that describes the interstitial and groundwater conditions based on available
information. Subsequently, NCDEQ responded with Site Assessment Review letter [NCDEQ,
2019], dated 12 August 2019 to Duke Energy. NCDEQ indicated in the Site Assessment Review
letter that permanent cap system development shall be designed to “abate the infiltration of
groundwater and/or precipitation into Area 1” and “to abate or control the . . . seepage water”.
Duke Energy indicated that a permanent cap design for Area 1 would be submitted to NCDEQ for
review and comment by March 2020. Geosyntec Consultants of North Carolina, PC (Geosyntec)
was retained by Duke Energy to prepare a permanent cap system design for Area 1. As such, this
Design Report (Report) provides the design criteria and assumptions, methodology, and
engineering calculations that support the Area 1 permanent cap design.
1.3 Report Purpose and Organization
This Report was prepared under the responsible charge of Mr. James D. McNash, P.E.(NC) and
reviewed by Dr. Victor M. Damasceno, Ph.D., P.E.(NC), both with Geosyntec. Professional
engineer certification of this Report is provided on the cover sheet. This Report is organized as
follows:
Section 2 – Permanent Cap System describes the general configuration and features of the
proposed permanent cap system.
Section 3 – Auxiliary Civil Design Features describes the civil design features that support
the construction and performance objectives of the permanent cap system design.
Section 4 – Design Criteria and Assumptions identifies the design criteria, constraints, and
assumptions to develop the permanent cap system design.
Section 5 – Design Calculations describes the design calculations to demonstrate that the
permanent cap system design will achieve project objectives.
Section 6 – Design Documents introduces the design documents that support material
procurement, installation, and certification of the permanent cap system.
Section 7 – Conclusions and Limitations presents the conclusions and limitations of this
Report.
Section 8 – References cites the reports, guidance, and other documents referenced within
this Report.
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2. PERMANENT CAP SYSTEM
2.1 Overview
The proposed permanent cap system consists of a composite soil-geomembrane liner system
designed to prevent and to minimize surface water infiltration into Area 1, manage surface water,
and to limit hydraulic head upon the geomembrane liner. The following subsections describe each
design component of the proposed permanent cap system.
2.1.1 Permanent Cap System Layers
The permanent cap system layer design was developed separately for the top deck and side slope
conditions, respectively. As such, the permanent cap system top deck design consists of the
following layers (from top to bottom):
0.5-ft thick vegetative soil layer;
1.5-ft to 5.5-ft thick protective soil layer;
a drainage geocomposite layer (in drainage corridors);
40-mil thick linear-low density polyethylene (LLDPE) geomembrane; and
2-ft thick prepared subgrade layer.
Since the existing Area 1 top deck soil cap contains approximately 6-ft of lower permeability soil
[Vaughan Engineering, 2010], Geosyntec assumed that 3 to 4 ft of existing soil would be stripped,
segregated, and stockpiled for use in the vegetative soil and protective soil layers. Additional soil
to achieve the 6-ft thicknesses in areas above the geomembrane liner component would be sourced
from offsite. The protective soil layer thickness varies as the top deck grading strategy (Section
2.1.3) limits the protective soil thickness at the top deck perimeter to achieve minimum desired
slopes. Geosyntec developed the top deck grading plan under the assumption that future
development of Area 1 would require the drainage herringbone areas to be filled to provide a more
level surface for pavements and foundations, and a minimum of 6 ft to 7 ft of separation between
the ground surface and the geomembrane liner component would be maintained. The minimum
separation for future development should be established based on actual development
requirements.
The permanent cap system on the side slopes was designed with the following layers:
0.5-ft thick vegetative soil layer;
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1.5-ft thick protective soil layer;
a drainage geocomposite layer;
40-mil thick LLDPE geomembrane; and
a compacted subgrade that consists of CCP or existing soil cap material.
As-built drawings indicate that the soil cap side slopes contain approximately 2-ft of soil cover
over CCP. For the proposed permanent cap system, the existing soil cap material will be stripped
and stockpiled for use in the vegetative soil and protective soil layers and the geomembrane will
be placed in contact with remaining soil or CCP. The proposed drainage geocomposite consists
of a drainage geonet with a non-woven geotextile separator heat bonded to each side and was
selected to allow infiltrated water to rapidly drain from the side slope geomembrane. The non-
woven geotextile provides separation from fine grained materials and additional cushioning
between soil layers and the geomembrane.
The vegetative soil and protective soil layer thicknesses for each configuration were selected based
on their design purpose. The 2-ft thick layer above the geomembrane on the side slopes was
selected to protect the geomembrane liner and maintain a minimum level of veneer stability. The
permanent cap top deck slopes are relatively shallow; therefore, veneer stability is not considered
a likely failure mechanism in that area. The 6-ft thick combined vegetative soil and protective soil
layers were selected to provide a minimum separation between the future development activities
generally proposed by the ARA Authority. However, the required separation should be established
at the time of future development.
2.1.2 Internal Drainage
The permanent cap system was designed to shed surface water and to drain water infiltrated within
the cap to minimize potential ponding atop the geomembrane liner, which in turn will minimize
migration of water into Area 1. As such, the subgrade on which the geomembrane is placed was
designed to follow existing grades and promoting northward drainage at one to three percent
slopes. However, a geocomposite drainage layer will be placed above the geomembrane liner at
the bottom of each herringbone to drain infiltrated water along the herringbone pattern center line
to provide a shorter drainage path to the side slopes. The proposed side slopes comprise a drainage
geocomposite layer to promote water removal from the geomembrane and limit moisture
accumulation within the permanent cap soil layers. The drainage geocomposite layer will daylight
into an aggregate toe drain wrapped in geotextile, which is anticipated to permit free drainage from
the geocomposite.
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2.1.3 Surface Water Management System Design
The existing soil cap system is graded such that approximately 66 percent of the area drains
towards the north slope at an average slope of two percent; while, approximately 34 percent of the
area drains southward towards Wright Brothers Way. Visual observations indicate shallow
undulations within the existing topography and the existing wildlife fence mow strip impede runoff
and promote infiltration due to the shallow top deck slopes and long maximum drainage paths
(~800 ft).
The proposed surface water management system design was developed to limit drainage lengths
within the permanent cap system footprint and provide steeper drainage slopes compared to
existing conditions. As such, the northern portion of the permanent cap system was designed with
a ridge and valley (or a herringbone) configuration that limits the drainage lengths along the
herringbone centerline to 100-ft where the surface flow is anticipated to transition from sheet flow
to channelized flow. When stormwater is routed eastward from Area 1, the stormwater is conveyed
down the side slopes and through a perimeter drainage channel into a stormwater pond located
immediately northeast of Area 1. When stormwater is routed westward, the stormwater is
channelized within a shallow trapezoidal shaped channel designed to convey stormwater while
allowing maintenance activities by the ARA Authority. The shallow channel routes stormwater
into a stormwater pond located at the northwest corner of the structural fill. Stormwater south of
the top deck high point is routed into an existing stormwater channel towards Wright Brothers
Way.
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3. AUXILIARY CIVIL DESIGN FEATURES
3.1 Erosion and Sediment Control
During Area 1 construction, three sediment control basins were formed and utilized to manage
sediment during Area 1 construction. One sediment basin is located at each of the north toe of the
west fill and the northwest and northeast corners of the east fill. These sediment features are
roughly intact and are proposed to be redeveloped, designed, and graded to manage the stormwater
runoff and sediment from the proposed permanent cap system. Erosion and sediment control
design and feature design will be provided under separate cover.
3.2 Surface Water Ponds
The permanent cap system was designed to route stormwater runoff from the northern portion of
the west fill through a stormwater basin at the northwest corner of Area 1. An underground stream
daylights at the northwest toe of Area 1 and this flow and stormwater eroded the existing perimeter
pond dike structure. As such, a rock drain is proposed to convey the base stream flow beneath the
pond; while an outlet control structure and perimeter berm is prosed to retain the stormwater runoff
prior to release into the existing stream.
Modifications to the former sediment basin located at the northwest corner of the east fill are not
proposed; however, stormwater from the east fill will be routed into the former sediment basin
located at the northeast corner. The basin will be regraded and structure installed to slowly release
runoff through a proposed channel towards the northwest pond and out to the existing unnamed
tributary. Each pond was designed to be a dry pond and to release stormwater within 48 hours
pursuant to Federal Aviation Administration (FAA) regulations as not to attract nuisance wildlife.
3.3 Wildlife Barrier Fence
The existing wildlife barrier fence that separates non-aeronautical areas from aeronautical areas at
the ARA will be removed and replaced once final grades are established. The purpose of the
relocation is to:
construct a near-watertight seal around each fence post;
limit damage to any installed geomembrane during development of the Area 1 by the ARA
Authority; and
locate the bottom of the fence posts above the proposed geomembrane liner, whenever
possible.
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The proposed wildlife barrier fence will be consistent with the existing fence and contain 8-in.
diameter wooden posts, approximately 10-ft above grade, and a concrete mow strip. Each fence
post will be installed in accordance with the specifications identified by ARA Authority utilized
elsewhere at the Site and terminate approximately 36-in. below ground surface. In permanent cap
system areas where fence posts extend below the geomembrane liner, a pipe boot will be installed
around each fence post footer, welded to the geomembrane liner, and clamped with a neoprene
seal to the fence post above ground surface. The concrete mow strip will subsequently be installed
in accordance with the drawings and around each pipe boot or post.
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4. DESIGN CRITERIA AND ASSUMPTIONS
The purpose of this section is to describe the design criteria and assumptions for which the
permanent cap system was designed. For instance, applicable design criteria include but are not
limited to: (i) reduction of infiltration into the Area 1 structural fill; (ii) global and veneer slope
stability; and (iii) minimization of water pressure or head accumulation within the geocomposite
drainage layers.
4.1 Permanent Cap Design Infiltration
The permanent cap system was designed with a 40-mil LLDPE geomembrane liner, vegetative
soil and protective soil layers, and vegetation to minimize infiltration into to the Area 1 structural
fill. Thus, the permanent cap system design criterion is to effectively “abate” the infiltration from
precipitation into Area 1 CCP.
4.2 Permanent Cap System Internal Drainage Layer
The geocomposite drainage layer on the perimeter side slopes and other select locations provides
the permanent cap system’s internal drainage layer to convey infiltrated water that is not removed
from surface water runoff or evapotranspiration. The geocomposite drainage layer limits the
hydraulic head accumulation above the geomembrane liner and provides several benefits:
reduced infiltration through liner defects;
reduced surface maintenance as water pressure is released within protective cover soils that
results in less moisture accumulation along the side slope toes and less sloughing; and
addition cushioning between the geomembrane and surficial soils.
The geocomposite drainage layer was designed to convey infiltrated water above the
geomembrane liner to stormwater features and was positioned at side slopes and within surface
water drainage corridors. The geocomposite drainage layer was sized to convey the water
infiltrated along the side slopes within the geocomposite material thickness.
4.3 Slope Stability
The permanent cap system side slopes were designed to achieve a minimum target factor of safety
(FS) equal to 1.5 or greater, which was selected pursuant to the North Carolina Administrative
Code (NCAC) – Title 15A Subchapter 2K – Dam Safety rules for slopes under normal, long-term
loading conditions. The Title 15A Subchapter 2K criteria are consistent with the recommendations
set forth by the United States Army Corps of Engineers (USACE) [2003], which are considered
the standard of practice.
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4.4 Settlement
To construct the proposed permanent cap system, 2 feet (side slopes) to 4 feet (top deck) of the
existing soil cap will be removed to place the geomembrane liner. Afterwards, 2 feet (side slopes)
to 7 feet (top deck) of combined vegetative and protective cover soils will be installed over the
geomembrane which will result in a net increase of up to 3 feet of soil cover over the in-place CCP
in select areas. Compacted CCP is relatively incompressible, has been in-place since 2010, and is
not expected to settle since minimal, additional soil will be added to Area 1. As such, Geosyntec
did not evaluate the settlement of top deck features.
4.5 Stormwater Management Design and Surface Water Ponds
The permanent cap system stormwater management design is prepared based on the criteria
identified from the following technical documents as well as best engineering practices from
industry technical literature:
Buncombe County, NC Code of Ordinances, Division 3 – Stormwater Plan Submittal
[Buncombe County, 2019]
Buncombe County, NC. Stormwater Management Plan Review Checklist [Buncombe
County, 2013]
Buncombe County, NC. Checklist and Guidelines for the Preparation of Erosion and
Sedimentation Control Plans [Buncombe County, 2006]
Federal Aviation Administration (FAA) Airport Drainage Design Advisory Circular [FAA,
2013]
North Carolina Department of Environmental Quality (NCDEQ) Erosion and Sediment
Control Planning and Design Manual (Erosion and Sediment Control Manual) [NCDEQ,
2013]
NCDEQ Stormwater Design Manual (NCDEQ Manual) [NCDEQ, 2017]
Specific design criteria for each component of the stormwater management system are described
below.
4.5.1 Open Channel Conveyances
Open channel conveyances shall be designed based on criteria from Section 6.21 (Permanent
Diversions) from NCDEQ [2013], which selects channel design criteria based on the area required
to be protected. For “major structures, homes, main school buildings, and high capacity roads”
which require a high level of protection, permanent diversions shall be designed to convey the
100-year (yr), 24-hour (hr) storm event. Buncombe County design criteria state that calculations
for open channel conveyances be presented for the 25-yr storm event, at a minimum [Buncombe
County, 2006]. As such, open channels were designed and analyzed for the 100-yr, 24-hr storm
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event without overtopping and for the 25-yr, 24-hr storm event with a minimum of 0.5-ft of
freeboard.
4.5.2 Channel Lining Material
Channels with computed flow velocities less than five feet per second (fps) will be grass-lined and
have 3 horizontal to 1 vertical (3H:1V) side-slopes or flatter to aid in the establishment and
maintenance of vegetation in accordance with Section 6.30 from NCDEQ [2013].
Channels where computed flow velocities that exceed five fps during the 100-yr, 24-hr storm
event, will be lined with an appropriate lining material to resist erosive forces and will be
constructed in accordance with best practices and manufacturer recommendations. The
appropriate lining material shall be selected based on permissible (or applied) shear stress
methodology, where the critical shear stress is to be greater than the permissible shear stress for
the 100-yr, 24-hr storm event.
4.5.3 Outlet Protection
Outlet protection shall be designed in accordance with criteria from the NCDEQ [2013] and
Buncombe County [2006]. NCDEQ requires outlet protection to be designed for the greater of the
10-year storm event or design discharge of the water conveyance structure. Buncombe County
requires the design for the 25-year storm event; however, conveyance features were designed to
convey the 100-yr, 24-hr storm event; therefore, outlet protection shall be designed for the same
rainfall event.
4.5.4 Dry Ponds and Sediment Basins
The stormwater ponds located north of the Area 1 permanent cap system will be designed as dry
ponds and outfitted as sedimentation basins during construction. NCDEQ [2017] requires
stormwater control measures (SCMs) for high density projects “to be designed, constructed, and
maintained so that the project achieves either ‘runoff treatment’ or ‘runoff volume match’. Two
sediment basins that will be converted into dry ponds (West and East Pond) were selected as the
to achieve runoff treatment and meet the design requirements set forth in Stormwater Manual and
Erosion Control Manual. The criteria for the design of a dry pond from NCDEQ [2017] are as
follows:
Separation from the Seasonal High-Water Table (SHWT) (MDC 1) – “The lowest point of
the dry pond shall be a minimum of six (6) inches above the SHWT.”
Temporary Pool Depth (MDC 2) - “The maximum depth of the temporary pool shall be 10
feet.”
Uniform Grading and Positive Drainage (MDC 3) – “The bottom of the dry pond shall be
graded uniformly to flow toward the outlet structure without low or high spots other than
an optional low flow channel.”
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Location of Inlet(s) and Outlet (MDC 4) – “The inlet(s) and outlet shall be located in a
manner that avoids short circuiting.” In the event that site conditions prohibit a long,
narrow pond, baffles may be used to length the stormwater flow path. The baffles should
extend to at least the temporary pool elevation.
Pretreatment (MDC 5) – “Pretreatment devices shall be provided to settle sediment and
prevent erosion. Pretreatment devices may include measures such as gravel verges, filter
strips, grassed swales, and forebays.”
Drawdown Time (MDC 6) – “The design volume shall draw down between two and five
days.”
Protection of the Receiving Stream (MDC 7) – “The dry pond shall discharge the runoff
from the one-year, 24-hour storm in a manner that minimizes hydrologic impacts to the
receiving channel.”
Outlet (MDC 8) – “The dry pond shall include a small permanent pool near the outlet
orifice to reduce clogging and keep floating debris away from the orifice. A screen or other
device shall be provided to prevent large debris from entering the outlet system.”
Vegetation (MDC 9) – “The dam structure, including the front and back embankment
slopes shall be planted with non-clumping turf grass, and trees and woody shrubs shall not
be allowed.”
The criteria for the design of a sediment basin from NCDEQ [2013] are as follows:
Primary Spillway – Shall be a Riser or Barrel Pipe.
Maximum Drainage Area – Shall be no more than 100 acres.
Minimum Sediment Storage Volume – Shall be 1,800 cubic feet per acre of disturbed area.
Minimum Surface Area – Shall be 435 square feet per cubic foot per second of the 10-year
storm design inflow.
Length to Width Ratio – The length to width ratio shall be no less than 2:1 and no greater
than 6:1.
Minimum Depth – The minimum depth shall be no less than 2 feet.
Dewatering Mechanism – A skimmer shall be attached at the bottom of the riser or a
flashboard shall be used.
Dewatering Time – The minimum dewatering time shall be no less than 48 hours.
The FAA requires that all water retention structures (i.e., ponds) be designed to discharge water
generated from storm events within 48 hours [FAA, 2013]. As such, the FAA requirement was
selected in lieu of NCDEQ MDC 6, as the dry ponds are to be designed as not to attract nuisance
animals.
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NCDEQ [2017] also provides additional recommendations such as the use of an emergency
spillway to reduce the potential for embankment failure during large storm events. Emergency
spillways for the East and West Ponds were designed based on similar criteria for Sediment Basins,
for when each pond would function as a temporary sedimentation basin during construction
activities. From the NCDEQ [2013], emergency spillways shall be constructed in undisturbed soil
(not fill) and contain a trapezoidal cross-section with 3H:1V side slopes or flatter. The control
section of the spillway shall be straight and at least 20 ft long; convey the 10-yr, 24-hr storm event
(less any reduction due to flow in the principal spillway) and contain at least one foot of freeboard
above the design flow depth.
Buncombe County [2006] requires conveyance structures to convey the 25-yr, 24-hr storm event.
However, the emergency spillway systems were conservatively designed to convey flows for the
100-yr, 24-hr storm event (consistent with other stormwater features) without consideration of the
principal spillway and to provide a minimum 0.5 ft of freeboard.
4.6 General Design Assumptions
The permanent cap design was prepared based on assumptions developed from available as-built
drawings, interstitial and groundwater level measurements, and supplementary data and
considerations from NCDEQ and ARA Authority. These criteria and assumptions include:
As-built drawings [Vaughan Engineering, 2010] depict the approximate configuration the
existing GCL that underlies the CCP within Area 1. The as-built drawings depict that the
GCL is situated beneath the starter dike structures and at perimeter elevations depicted
within the drawings.
Available interstitial, groundwater, and GCL as-built elevation information indicates that
the GCL separates interstitial water within Area 1 from underlying groundwater
[Geosyntec, 2019; Vaughan Engineering, 2010]. As such, the permanent cap system was
designed to abate infiltration from precipitation only. The permanent cap system design
was prepared assuming the GCL was installed under industry standards and with a
construction quality assurance program.
The existing security fence will be removed and replaced during permanent cap system
construction. The grades around the replaced fence will be maintained such that runoff is
facilitated and impounded water is not retained on the permanent cap system.
The ARA Authority or its contractor will provide additional structural fill to raise areas of
the permanent cap system to grades established during future development activities.
Stormwater features were not designed to accommodate future development (i.e.,
pavements and grading) as development plans were not available at the time of this Report.
Moreover, the purpose for designing and installing the permanent cap system is to address
the issues identified in the NOV discussed in Section 1.2 and to meet the objectives listed
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in Section 7.1 of this Report. Any future development activities contemplated by owner
and operator of Area 1 must be designed and sealed by a North Carolina-licensed
professional engineer.
The existing groundwater upwelling or seep, surface water sampling location SW10-A1,
at the northwest corner of Area 1 may be derived from former historical stream channel,
and was rerouted beneath the stormwater pond within a rock drain.
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5. DESIGN CALCULATIONS
5.1 Infiltration Analysis
The Hydrologic Evaluation of Landfill Performance (HELP) model Version 3.07 [Schroeder, et.
al., 1994a, 1994b] was used to estimate infiltration into the structural fill. The HELP model is a
quasi-two-dimensional water balance computer program used to evaluate the water movement and
retention through the waste, cap system, and liner system components. The model, along with
site-specific weather data and design information, was utilized to estimate runoff,
evapotranspiration, drainage, and infiltration.
Geosyntec evaluated the infiltration through side slope and top deck areas for the existing soil cap
and proposed permanent cap system. In each case, calculations were performed for a
representative 1-acre acre area and infiltration at the CCP boundary was selected and scaled
upwards based on the representative area for each case. Appendix A presents the evaluation cases,
input parameters, and results of the analysis. For the 22.7-acre structural fill, the computed
infiltration into Area 1 CCP was reduced from approximately 22,397.1 gallons per day (gpd) to
13.9 gpd. However, simplifications associated with the HELP model likely underpredict
infiltration under current (pre-permanent cap) conditions due to localized undulations and
depressions within the current topography and the existing security fence mow strip. The model
likely overpredicts infiltration under proposed conditions since channelized flow and the
geocomposite drainage strips are not considered. The infiltration analysis for proposed conditions
was developed assuming a high-quality installation with construction quality assurance oversight
to minimize defects within the installation.
The infiltration analysis was also utilized to evaluate the hydraulic head within the geocomposite
side slopes for the peak daily infiltration into the drainage layer. Giroud et. al [2004] was applied
within Appendix A to demonstrate that the hydraulic head will remain within the geocomposite
drainage layer during the peak daily infiltration.
5.2 Slope Stability Analysis
Global slope stability analyses were performed using Spencer’s method [Spencer, 1973], as
implemented in the computer program SLIDE®, version 6.039 [Rocscience, 2016]. Spencer’s
method satisfies vertical and horizontal force and moment equilibrium, and is considered more
rigorous than other methods such as the simplified Janbu method [Janbu, 1973] and Bishop’s
method [Bishop, 1955]. SLIDE® generates potential circular slip surfaces, calculates the FS for
each of these surfaces, and identifies the most critical slip surface with the lowest calculated FS.
The veneer slope stability of the permanent cap system was evaluated using the method proposed
by Giroud et al. [1995] for geosynthetic-soil layered systems. Appendix B provides the
methodology, critical cross sections, and engineering properties for each material layer to perform
slope stability analysis, which demonstrates that the calculated FS’s exceed 1.5 for global, veneer,
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and base sliding mechanisms for current interstitial water levels and anticipated future water levels
within the fill.
5.3 Stormwater Design Calculations
The surface water runoff hydrographs, channel capacity, and dry pond routing for each drainage
area were calculated using hydrologic and hydraulic procedures presented in the Urban Hydrology
for Small Wetlands Technical Release 55 (TR-55) [Soil Conservation Service (SCS), 1986];
NCDEQ Erosion Control Manual [NCDEQ, 2013]; Manning’s kinematic equation; channel shear
stresses, and other recognized engineering procedures encoded in HydroCADTM (HydroCAD)
software [HydroCAD, 2016]. Appendix C provides the methodology, conveyance feature layout,
and design land use conditions selected to design the stormwater conveyance features such that
the FAA and local and state requirements are achieved.
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6. DESIGN DOCUMENTS
6.1 Design Drawings
Design drawings that support the construction of the permanent cap system and auxiliary design
features at ARA are provided as Appendix D. The design drawings include subgrade and final
grading plans, stormwater pond grading plans, cross sections, and construction details for a
construction to complete design objectives.
6.2 Technical Specifications
The permanent cap system for Area 1 will be constructed with quality materials and industry
proven construction practices. Technical specifications that support quality construction activities
for the permanent cap system are provided within Appendix E of this Report.
6.3 Construction Quality Assurance Plan
The permanent cap system will be installed with good construction practices and under a stringent
CQA program to minimize defects in the installed geomembrane liner. The CQA Plan applicable
for the permanent cap system is provided in Appendix F of this Report.
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7. CONCLUSIONS AND LIMITATIONS
7.1 Conclusions
As demonstrated within this Report, the Area 1 permanent cap system design meets the following
objectives:
Abate or control seepage from Area 1 through the installation of a permanent cap system
to reduce and abate infiltration from precipitation into the CCP, which is anticipated to
lower interstitial water elevations over time;
Achieve long-term, slope stability factors of safety equal to or above 1.5;
Improve and actively manage stormwater runoff volume from Area 1 prior to flow from
the property boundary; and
Provide drawings, technical specifications, and CQA plans to ensure a high-quality
installation.
Geosyntec notes that the permanent cap system design was intended to achieve the above
objectives only. Modifications to the permanent cap system (i.e., revisions to grading due to
unknown future development activities) must be designed and sealed by a professional engineer
licensed in the state of North Carolina.
7.2 Limitations
The permanent cap system design and supporting calculations, specifications, and drawings
documented within this Report were prepared based on assumptions presented in Section 4.7 and
intended solely to construct a permanent cap system for Area 1 at the Asheville Regional Airport.
Use of the design documents presented herein for other sites or purposes at ARA is not authorized
or certified within this document.
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8. REFERENCES
Bishop, A. (1955), “The Use of the Slip Circle in the Stability Analysis of Slopes,” Géotechnique,
Volume 5, No. 1, Jan 1955, pp. 7-17.
Buncombe County, NC, 2006. Checklist and Guidelines for the Preparation of Erosion and
Sedimentation Control Plans.
Buncombe County, NC, 2013. Stormwater Management Plan Review Checklist.
Buncombe County, NC, 2019. Code of Ordinances, Division 3 – Stormwater Plan Submittal. As
Accessed on 21 January 2019.
Federal Aviation Administration, (2013). Airport Drainage Design – Advisory Circular. U.S.
Department of Transportation Agency, Federal Aviation Administration. Washington,
D.C.
Geosyntec (2017). “Engineering Analysis Report Asheville Regional Airport – Area 1 Structural
Fill Asheville, North Carolina.” December 2017.
Geosyntec 2019. “Hydrogeologic Conceptual Model Report”, prepared for Duke Energy, January
2019.
Giroud, J.P., Bachus, R.C. and Bonaparte, R. (1995), “Influence of Water Flow on the Stability of
Geosynthetic-Soil Layered Systems on Slopes” Geosynthetics International, Vol. 2, No. 6,
pp. 1149-1180, January 1995.
Giroud, J.P., Zhao, A., Tomlinson, H.M., and Zornberg, J.G. (2004), "Liquid Flow Equations for
Drainage Systems Composed of Two Layers Including a Geocomposite", Geosynthetics
International, Vo. 11, No. 1.Koerner, R.M. (1998), “Designing with Geosynthetics”,
Fourth Edition.
HydroCADTM Software Solutions, LLC., 2016. HydroCADTM Stormwater Modeling System,
Version 10.00-22. Chocorua, New Hampshire.
Janbu, N. (1973), “Slope Stability Computations,” Embankment Dam Engineering, Casagrande
Memorial Volume, R. C. Hirschfield and S. J. Poulos, Eds., John Wiley, New York, 1973,
pp. 47-86.
NCDEQ, 2013. Erosion and Sediment Control Planning and Design Manual. Raleigh, North
Carolina.
NCDEQ, 2017. Stormwater Design Manual. Raleigh, North Carolina.
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NCDEQ, 2019. Site Assessment Review. Asheville Airport CCP Structural Fill – Area 1, 12 August
2019.
Rocscience (2016). SLIDE v6.0. Rocscience Inc., Toronto, Ontario.
Schroeder, P. R., Aziz, N. M., Lloyd, C. M. and Zappi, P. A, 1994a. “The Hydrologic
Evaluation of Landfill Performance (HELP) model: User’s Guide for Version 3”,
EPA/600/R-94/168a, September 1994, U.S. Environmental Protection Agency Office of
Research and Development, Washington, DC.
Schroeder, P.R., Dozier, T.S., Zappi, P.A., McEnroe, B.M., Sjostrom, J.W., and Peyton, R. L.
1994b. “The Hydrologic Evaluation of Landfill Performance (HELP) Model: Engineering
Documentation for Version 3”, EPA/600/R-94/168b, September 1994, U.S.
Environmental Protection Agency Office of Research and Development, Washington,
DC.
Spencer, E., 1967. “A Method of Analysis of the Stability of Embankments Assuming Parallel
Inter-Slice Forces.”, Geotechnique, Vol. 17, No. 1, pp. 11-26.
Soil Conservation Service (SCS), 1986. Urban Hydrology for Small Watersheds, Technical
Release 55 (TR-55), 2nd Ed., United States Department of Agriculture, Soil Conservation
Service. Washington, D.C.
United States Army Corp. of Engineers (2003), “Engineering and Design: Slope Stability Engineer
Manual”, Manual No. 1110-2-1902, 31 October 2003.
Vaughan Engineering (2010), “Charah – Asheville Regional Airport Coal Combustion Product
Engineered Fill, March 2010 Update, “As Built” Surface Development as of December 29,
2009”, 8 March 2010.
FIGURES
Asheville RegionalAirport
French Broad River
§¨¦26
§¨¦26
§¨¦26
Area 1 - East Fill
Area 1 - West Fill 60" dia meter RCP
French Broad River
SITE LOCATION MAP
Asheville R egional AirportFletcher, North Carolina
Figure
1
\\Charlotte-01\Data\GIS\Projects\D\Duke\Asheville Airport\mxds\Cap Design Report JHOBARTCHARLOTTE, NC MARCH 2020
1,000 0 1,000 2,000500 Feet
³
Notes:1. Ser vice Layer Credits: Source: Esri, DigitalGlobe, GeoEye,Earthstar Geographics, CNES/Airbus DS, USDA, U SGS,AeroGRID , IGN, and the GIS User Community.2. Parcel boundaries obtained from Buncombe County GIS websiteon 20 December 2017.
Legend
Area 1 Sto rm wate r Network
Area 1
Parcel Bo unda ry