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Using GCLs and a cellular confinement system, the objective was todesign and construct a centralized deicing pad where fluid could beapplied and collected in a fully controlled manner.
By Daniel Petno and Chris Athanassopoulos
Introduction
The design and construction of a new centralized deicing facility at Cleveland-Hopkins International Airport (CLE) involved several geosynthetic applications, including a plastic-laminated geosynthetic clay liner (GCL), an aggregate-filled cellular confinement system, nonwoven geotextiles, and high-density polyethylene (HDPE) pipe. This article presents a discussion of the project background, design, and construction, with a particular focus on the geosynthetic components. To the authors’ knowledge, this is the first use of either GCLs or geocell materials at an airport deicing facility.
Background
Ice, frost, or snow present significant concerns for airports in temperate and cold-weather climates. Even small amounts of these elements on aircraft surfaces can pose serious safety concerns.
To address this concern, deicing fluids (such as ethylene glycol, propylene glycol, and urea) are typically sprayed on aircraft before takeoff. Historically, airplane deicing operations at CLE have taken place at the individual airline gates (Photo 1). In the past, individual planes were sprayed with deicing fluids while passengers were boarding, with any drippings and overspray collected by vacuum trucks.
For the 2004–2005 deicing season, a total of approximately 1.1 million gallons of concentrated deicing fluid (applied as a 50:50 propylene glycol/water solution) were handled in this manner at CLE. While this deicing approach minimizes departure delays—particularly at a hub airport where many flights depart at one time—it also results in an increased potential for uncontrolled releases of propylene glycol. Although propylene glycol is not as toxic as ethylene glycol, it does have a high biochemical oxygen demand (BOD), driving down dissolved oxygen levels in receiving waters, and potentially affecting aquatic organisms.
In response to past glycol releases into nearby Abram and Silver creeks (both tributaries of the Rocky River, which empties into Lake Erie), the Ohio Environmental Protection Agency (OEPA) mandated compliance with glycol concentration limits at outfalls. The solution was to design and construct a centralized deicing pad at CLE, where deicing fluid could be applied and collected in a fully controlled manner. The City of Cleveland Department of Port Control contracted R.W. Armstrong and Associates Inc. to design a new centralized deicing facility (CDF) and oversee its construction.
Design overview
The CDF at Cleveland-Hopkins covers a total area of 76 acres, which includes Deice Pad 1, Deice Pad 2, associated taxiways, Site A and Site B (see Figure 1). Since the primary deicing operations take place at the two CDF pads and their associated taxiways, these areas were sized to allow deicing fluid to be applied to 8 commercial airplanes at one time (30 planes/hub movement). Site A provides a staging and storage area for deicing applicator equipment, while Site B contains two 2-million-gallon aboveground storage tanks for either onsite recycling or discharge to the Northeast Ohio Regional Sewer District (NEORSD) sanitary system.
As shown in Figure 2, the typical 38-in.-thick CDF pavement cross section consists of (from top to bottom): 16 in. of Portland Cement Concrete; 8 in. of econocrete; 8 in. of crushed aggregate; a hydraulic barrier layer; 6 in. of aggregate-filled cellular confinement system; and existing subgrade.
The cellular confinement system with aggregate infill was placed beneath the GCL to provide drainage, frost protection, and structural support. Nonwoven geotextiles were installed below the cellular confinement system for subgrade separation and on top to cushion the hydraulic barrier above. Nonwoven geotextiles were also incorporated into the underdrain system to provide separation and filtration.
The CDF has a total fluid capture capacity of 5.6 million gallons—4.0 million in two aboveground storage tanks (ASTs) at Site B, and 1.6 million in an underground storage tank (UST) beneath Site A. The underground tank was constructed using precast concrete arch-top structural sections—the first of its kind for this type of application.
A GCL was installed beneath the UST as secondary containment in case of tank leakage, with a separate underdrain system installed between the tank bottom and the GCL. A network of HDPE pipes collects runoff from the pads and transports it by gravity to the UST for retention, then either directly to the stormwater system (clean runoff from the entire airport during the summer), or to the collection system (glycol-laden water during the winter).
Based on the glycol concentrations, runoff is segregated and managed through the use of 5 diversion vaults. One of the vaults diverts the highest concentration flows from the primary glycol application area to an isolated section of the UST. High-concentrate runoff that accumulates in this 150,000-gallon vault is pumped into trucks for further processing, recycling, or off-site disposal. The remaining 4 diversion vaults route runoff to either the stormsewer or to the low-concentration section of the UST.
From here, the glycol-impacted water flows more than 3,500 ft. through a sealed pipe system to a deep wet well. A 10cfs pump station transfers glycol-laden runoff to the aboveground tanks located outside the airfield fence at Site B. From there, fluids are metered into a gravity sanitary sewer line connecting the aboveground tanks to the NEORSD sanitary sewer system.
Inline measurements and telemetry reporting are used to ensure that the NEORSD influent limits (summarized in Table 1) are not exceeded. These limits were established to prevent excessive BOD/COD loads associated with propylene glycol from overwhelming the processing capabilities of the sewer treatment plant.
The specific geosynthetic elements of the CDF design, which are critical to this project’s success, are discussed in the following section.
Geosynthetic elements
Geosynthetic clay liner
Successful completion of the deicing facility project involved contributions from several civil engineering specializations (transportation, structural, hydraulics, and environmental). Since the major design objective is to prevent leakage of deicing fluids, the most critical environmental component of the project is the pavement’s hydraulic barrier layer. A needlepunch-reinforced, plastic-laminated GCL was selected as the hydraulic barrier layer. More than 2.1 million ft.2 of GCL were used to line the deicing pads, associated taxiways, and Site A. The GCL consisted of 0.75 lbs./ft.2 of sodium bentonite encapsulated between woven and nonwoven geotextiles. A 4-mil HDPE plastic geofilm was laminated to the nonwoven geotextile for improved hydraulic performance and chemical resistance. An additional nonwoven geotextile was laminated to the HDPE geofilm to provide increased durability and puncture protection. The GCL was selected for the following reasons:
- Low hydraulic conductivity. The plastic-laminated GCL product is certified by the manufacturer to have a maximum hydraulic conductivity of 5 x 10-10 cm/sec when permeated with distilled water. Because the plastic geofilm is virtually impermeable, the actual hydraulic conductivity of the plastic-laminated GCL is expected to be lower.
- Compatibility with deicing fluids. Past testing performed by the manufacturer (summarized in Figure 3) showed that, when permeated with a 50:50 solution of ethylene glycol, a standard GCL (without any plastic backing) exhibited a long-term hydraulic conductivity of 7.0 x 10-10cm/sec. The manufacturer test results are consistent with past findings in the literature. Petrov et al. (1997) found that GCLs are compatible with nonpolar, miscible organic compounds, provided that the organic concentration is less than 60%.
- Bentonite self-healing characteristics. The potential for punctures was another consideration in selecting a GCL as the hydraulic barrier layer. The swelling and sealing properties of bentonite give a GCL the unique ability to recover from punctures. Studies performed by Shan and Daniel (1991) on GCL samples that were punctured found that the GCL was able to self-heal holes up to a 1-in. diameter, and still maintain a low hydraulic conductivity. This self-sealing ability allowed for rapid installation around penetrations, such as inlet structures, pipes, and manholes.
- Protection against ion exchange. Since the CDF pavement section includes crushed limestone aggregate (AASHTO #57 stone), a plastic laminate was incorporated into the GCL to reduce the risk of ion exchange in the sodium bentonite posed by dissolved calcium leaching from the aggregate.
- Schedule. A major project driver was the schedule—the new deicing facility was required to be operational by autumn of 2006. Due to the amount of time involved in getting funding in place, agreements from the airlines, and receiving OEPA approval of the Permit To Install, the construction duration was compressed to one year.
This aggressive schedule required construction through the winter months of late 2005 and early 2006. After evaluating various materials, the decision was made to install a GCL, as GCLs have been successfully installed in many cold weather settings. Cold weather delays could have pushed the project completion date into 2007, potentially resulting in significant fines from the OEPA. - Simplicity of installation. Another benefit of using a GCL as the hydraulic barrier was ease of installation. GCLs are seamed by overlapping adjacent panels and applying supplemental granular bentonite to the overlap area. In addition, a pneumatic-powered geosynthetic installation device was used to deploy the GCL. The installation device was mounted on a large-capacity tractor, as shown in Photo 2. As the tractor operator drove forward, a ground operator used a control cable to unroll the GCL onto flat panels. Using this equipment, several acres of GCL were deployed daily, accelerating the installation.
Deicing fluid collection system(HDPE pipe)
Another critical environmental component of the CDF design is the liquid collection system. The collection/drainage system consists of more than 57,000 ft. (almost 11 miles) of HDPE pipe and underdrains. (Photo 3). Reinforced concrete pipe (RCP) is typically used for airport drainage applications. However, RCP systems can allow significant infiltration if not installed correctly and can degrade over time. Both possibilities were unacceptable, particularly when repairs would affect airfield operations. HDPE pipe with fusion-welded connections, which is compatible with glycol, was selected instead of RCP, to limit glycol exfiltration and groundwater infiltration potential. The HDPE collection system for all anticipated glycol flows was provided from the inlets, through the diversion valves, and downstream to the disposal facilities. The pipe diameters range from 24–54 in., with motor-actuated gate valves. Because of their size, both the piping and valves were specially fabricated for this project, and required a long lead time. (The lead time and pipe cost unexpectedly escalated in the fall of 2005 due to the impact of Hurricane Katrina on manufacturing plants along the Gulf Coast.)
The collection pipes draining the CDF area were sized to accommodate a 10-year summer storm event. To evaluate expected winter flows and storage requirements, a model was developed using the past 50 years of rainfall data, the existing fleet of aircraft, assumed glycol application rates, and estimates of glycol drippage and overspray. This resulted in sizing the total project storage at 5.6 million gallons—consisting of the ASTs, the UST, and storage in the pipe system. The designers also worked with the HDPE pipe manufacturer to determine the minimum pipe thicknesses needed to withstand the expected loading associated with the pavement and the design aircraft.
Drainage layer/underdrain system
The native soils at CLE consist of glacial tills (primarily silts and clays), with highly variable geotechnical properties. The 38-in.-thick pavement section is designed based on a subgrade California Bearing Ratio (CBR) of 4. Typically, when existing conditions are encountered with CBR values under 4, the subgrade needs to be improved by disking and drying, adding cement or lime treatment, or removing the yielding soil and replacing it with stabilization aggregate.
However, even when adequate subgrade conditions were present during construction, the relatively shallow water table in the vicinity of the site presented an increased likelihood of saturated soils that would further reduce the soil’s strength and bearing capacity. The water table has been found to rise up to within a couple of feet of the ground surface during the wet seasons in northeast Ohio. To address these subgrade limitations, a 6-in. drainage layer was incorporated into the pavement design. The drainage layer was placed directly on subgrade that was sloped to allow groundwater to move up into the drainage layer and flow to the perforated underdrain system that runs along all edges of the airfield pavement.
The intent of the drainage layer and underdrain is to collect shallow groundwater, thus preventing saturated soil conditions, as well as providing structural support and frost heave protection. The underdrain system includes two geosynthetic components: a polyethylene cellular confinement system and nonwoven geotextiles.
In addition to the “standard” groundwater underdrain system addressed above, the CDF also provides a drainage path for the glycol-laden water that makes its way down through the top of the pavement and is blocked by the clay liner. To capture this fluid and keep it from saturating the pavement section over time, a second perforated underdrain system was constructed above the liner. This system is assumed to collect glycol-impacted water for 12 months of the year. Therefore, the outlet pipe could not be tied into the stormdrain system, but flows by gravity to the pump station for disposal to the sanitary sewer. The entire glycol underdrain system is composed of welded HDPE pipe.
Polyethylene cellularconfinement system
As discussed previously, the glacial till soils present on the CLE airfield are subject to great variability. While the soil typically performs well when near optimum moisture levels, saturated soils are not satisfactory for pavement subgrade. This problem was exacerbated when working during the winter months. To bridge some of the softer areas of subgrade, a flexible, 3-dimensional polyethylene cellular confinement system was placed over the subgrade. The cellular confinement system used at CLE is shown in Photo 4—more than 1.3 million ft.2 were used.
Historically, the drainage layer design consisted of combining a poorly graded aggregate (AASHTO #57 stone) with an asphalt or concrete binder. The layer would then be placed on subgrade with an asphalt or concrete paver. After hardening, the drainage course would form a porous and flexible slab. However, since asphalt plants are not in operation during the winter, and cold winter temperatures would prevent cure of cement binder, an aggregate-filled cellular confinement system was selected to bridge the soft spots as the drainage course.
A secondary benefit was that the drainage layer became a structural layer. During design it was determined that the 6-in. drainage layer would improve subgrade soils with a CBR value of 2, up to the required design CBR of 4. The aggregate-filled cellular confinement system produced a stable structural base that distributed loads laterally and reduced subgrade contact pressures. This was not only beneficial in terms of the overall pavement stability, but also in terms of a stable interim construction platform. By spanning weak soils that would otherwise need to be dug up and moisture conditioned, construction was allowed to proceed uninterrupted through the winter months.
Geotextiles
Since the geocellular confinement system (Photo 4) was placed directly on top of fine-grained soils, an 8-oz./yd.2 nonwoven polypropylene geotextile was used as a separation layer between the two distinct soil types. Separation was needed in this application to prevent contamination of the granular infill (and consequently, loss of shear strength) and to prevent punching or migration of the infill material into the subgrade. An additional 8-oz./yd.2 nonwoven geotextile was placed between the top of the aggregate-filled cellular confinement system and the bottom of the GCL for puncture protection.
The puncture risks posed by both the cellular confinement system (below the GCL) and the crushed aggregate concrete subbase (above the GCL) were evaluated during the early stages of construction by building a field test pad. The test pad was constructed using 8-oz./yd.2 nonwoven geotextiles on either side of the aggregate-filled cellular confinement system. Various heavy construction vehicles (loaded dump trucks, graders, etc.) were driven repeatedly across the pad, simulating worst-case construction conditions. The GCL was then exposed and inspected for signs of damage. Since no significant damage was observed in the test pad samples, the 8-oz./yd.2 nonwoven geotextile beneath the GCL and the 3.2-oz./yd.2 nonwoven geotextile laminated to the top of the GCL were deemed adequate for puncture protection. The favorable test pad results indicated that there would be little concern regarding swelling GCL under the rigid pavement or above the underdrain layer. As a constructability issue, the contractor took due care to make sure the GCL did not sit unconfined for a long period such that it would hydrate. As a practical matter, the needlepunch-reinforced GCL proved quite able to withstand the construction process without issue.
Summary
The design and construction of the new centralized deicing facility at Cleveland Hopkins International Airport involved several innovative uses of geosynthetics. In response to historical glycol releases into nearby streams, a new centralized deicing facility was built within one year to capture and control airplane deicing fluids.
The design of the deicing facility had to meet these regulatory requirements, while at the same time minimizing departure delays inherent to deicing operations. Geosynthetics were critical in successful completion of a project with such strict regulatory constraints and customer expectations.
The overall project, which had a budgeted cost of $46.89 million, was constructed within budget and within the EPA-mandated schedule of one year. Because of the tight deadlines, much of the construction activities took place during the winter months in late 2005 and early 2006.
The CLE deicing facility began operations in October 2006 and, despite a couple of technical and communication hiccups, has been operated successfully through its first winter season. The cost of the construction effort was borne by the airlines, and will ultimately be recovered from airline customers, through a passenger-facility charge added to the price of each ticket.
Daniel A. Petno, P.E., is an associate/senior project manager for R.W. Armstrong and Associates in Cleveland, Ohio.Chris Athanassopoulos, P.E., is a technical support engineer for CETCO’s Construction Engineering Group in Arlington Heights, Ill.
This article is a modified, magazine version of a paper presented at the Geosynthetics-2007 Conference and Trade Show in Washington, D.C., January 2007.
References
Colloid Environmental Technologies Co. 2000. Technical Reference 109—GCL Compatibility with Airport Deicing Fluid.
Ohio Environmental Protection Agency. 2004. Report on a Permit to Install Application and Detail Plans for the Cleveland Hopkins International Airport Centralized Deicing Facility.
Petrov, R.J. et al. 1997. Selected Factors Influencing GCL Hydraulic Conductivity, ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol. 123, No. 8, pp. 683–695.
Shan, H.Y., and Daniel, D. 1991. Results of Laboratory Tests on a Geotextile/Bentonite Liner Material, presented at Geosynthetics-1991, Atlanta.
Draft, 2006–2007 Aircraft Anti-icing/De-icing and Discharge Management Plan, Cleveland Hopkins International Airport.