In 2009, the North Carolina Asheville Regional Airport Authority (Airport), with partners Progress Energy Carolinas Inc. and Charah Inc., began development of the Westside Development Fill Project (Westside Project), a long-term infrastructure strategy located in the southwest quadrant of the Airport’s property. The project included phased construction of a developable pad for general aviation and commercial use, a new taxiway running parallel to the existing runway, and a major expansion of the existing runway.
The Airport’s Westside Project encompasses over 53 acres of partially wooded land. When the partners determined that it would require nearly 2 million cubic yards of suitable engineered fill material to construct a developable pad for both general aviation and commercial use (Figure 1), coal combustion products (CCPs) emerged as an appropriate solution because of the potential cost savings over conventional fill materials.
|1. Coal byproducts get new life. Aerial view of Asheville Regional Airport Authority’s Westside Project, which made use of coal combustion products. Courtesy: Charah Inc.|
Many Design Challenges
Charah began development of the Westside Project by completing an environmental assessment and an environmental due diligence audit. As part of this audit, Charah characterized the physical and engineering properties of the proposed CCPs by referencing the American Society for Testing and Materials (ASTM) Standard’s Guide for Design and Construction of Coal Ash Structural Fills E 2277-03.
Charah also investigated the geologic and hydrogeologic conditions within the 53-acre area with borings and the installation of groundwater wells and piezometers. Subsequently, it obtained the information necessary to characterize the sub-terrain and input data required by the Environmental Protection Agency’s (EPA’s) Industrial Waste Management Evaluation Model to model the groundwater. Additionally, the company surveyed for and delineated any preexisting environmental resources (including jurisdictional streams, wetlands, and cultural resources).
As development of the project continued, Charah incorporated local, state, and federal permitting procedures for project design and construction; coordinated the design and implementation of erosion, sediment, and pollution prevention controls and activities; and followed the testing, engineering, and construction practices for CCP engineered fill projects.
When the environmental assessment was completed, engineering began on this state-of-the-art engineered fill project. The project features environmentally conscious controls such as a comprehensive liner and cap system, drainage collection system, and storm water management system.
State regulations do not require use of a comprehensive liner system or an high-density polyethylene (HDPE) cap liner when using CCPs as an engineered fill; however, Charah and Progress Energy considered the application of these design elements environmentally responsible and the only method suitable for this project. Progress Energy’s project manager, Rob Reynolds, noted, “As a group, Progress Energy, Charah, and the Asheville Airport agreed to utilize only state-of-the-art designs, products, and materials for this project. We worked hard to design and construct the first-of-its-kind, next-generation facility utilizing the latest in environmentally sound engineering.”
The Westside project was engineered with a layered bottom and cap liner system. The bottom liner system includes a compacted in situ soil subgrade overlain with a bentonite geocomposite clay liner (GCL) and a 60-mil HPDE liner. Both the GCL and HDPE liners act as a barrier layer preventing any CCP material or related moisture to pass through the comprehensive liner system.
To convey any contact water generated on top of the HDPE to the drainage collection system, Charah proposed the use of a geocomposite drainage liner placed on top of the 60-mil HDPE liner. The drainage collection and conveyance system consisted of a perforated 8-inch HDPE pipe encapsulated by #57 washed stone wrapped in geotextile fabric. This system will convey any generated contact water from the CCP fill to grit chambers. Collected water is recycled within the CCP active working area as dust control or transported to and treated by an approved wastewater treatment facility.
The engineered fill needed for the Westside Project was acquired by excavating CCPs from the ash basin at Progress Energy’s Asheville Plant in Skyland, N.C., approximately 1.5 miles from the airport. CCPs are carefully excavated using long-reach excavators and loaded into tri-axle dump trucks. The CCPs are then placed in a decant stockpile adjacent to the ash basin so that an acceptable level of moisture can be achieved. When the moisture content drops to the desired level, the CCPs are transported to the project site for placement and compaction as an engineered fill.
Excavating ash from the Asheville power plant’s ash storage basin also provides additional wet ash storage volume for the 376-MW plant.
Upon completion of CCP placement, a 30-mil HDPE cap liner is used to encapsulate the material. In addition to the HDPE cap liner, a minimum of 6 feet of compacted soil is placed across the CCP fill limits at a compaction rate of at least 95% percent modified Proctor. This specification meets Federal Aviation Administration fill placement requirements for the development of utilities, aviation facilities, and infrastructure (Figure 2).
|2. Multi-layer construction. This cross section shows the complexity of the engineered fill design. Source: Charah Inc.|
Airport director Lew Bleiweis observed, “In addition to the fill requirements, the site presented a drainage challenge, which involved the preservation of the existing runway storm water drainage patterns, while still allowing the engineered fill material to be placed. Significant drainage improvements were constructed along the eastern side of the engineered fill project to re-direct runoff around the work area and continue to maintain storm water drainage as needed to maintain safe maneuvering, takeoff, and landing of airplanes.”
CCPs have been beneficially utilized for airport construction projects in Pennsylvania, Texas, and Wisconsin. However, this project is unique in one very important way. According to Scott Sewell, Charah’s vice president of operations, “This project is the first of its kind, being an ‘encapsulated’ engineered fill meeting stringent protective features as found in the EPA’s proposed rules for the reuse of coal combustion products.”
Construction Under Way
Construction of the Westside Project commenced in August 2010 (Figure 3). During all phases of construction, Charah relied on a third-party construction quality assurance (CQA) firm to ensure that construction and environmental controls met the strict environmental provisions set by the jurisdictional agencies, the Airport, Progress Energy, and Charah.
|3. Construction progress. This photo shows the HPDE liner being installed. Courtesy: Charah Inc.|
The first construction steps were to clear, grub, and excavate existing soils and establish the design subgrade surface. Next, Charah began installing the bottom liner and drainage collection systems.
Afterward, decanted CCPs were hauled by tri-axle dump trucks equipped with tarped beds from Progress Energy’s Asheville Plant. Charah utilized dozers to spread the CCPs in uniform even lifts to meet the specified elevation tolerance of ±0.25 feet. Compaction of the CCP material was achieved using a vibratory smooth drum roller making passes as needed to achieve the compaction requirement of 95%, based on the modified Proctor compaction test. Compaction was confirmed as part of the CQA program.
Development of the project will require nearly 2 million cubic yards of CCPs to establish the rough grade elevations across the site. The use of CCPs as an alternative to traditional fill materials (such as soil and rock) provided the airport with an environmentally safe and economically valuable opportunity. Cost savings to the airport from using CCPs instead of conventional fill are estimated at nearly $16 million dollars.
As Bleiweis explained, “The partnership between the Airport, Progress Energy, and Charah was beneficial for all of us. Progress Energy found an environmentally sound and economically prudent way to reuse the coal combustion by-product, and the Airport saved millions of dollars in costs that traditional fill methods would have required.”
Charles Price, Charah’s president and CEO, said that “the uniqueness of this project not only lies in the site’s characteristics and design, but also in the partnership between the Airport, Progress Energy, and Charah. The [Westside Project] is a great example of how CCP engineered fills should be constructed.”
The project, expected to be completed in 2014, will ultimately create more than 15 acres of aeronautical land use.
—Contributed by Bobby Raia (firstname.lastname@example.org), project controls manager for Charah Inc.