Bringing Light Underground: Denver International Airport North Terminal Sewer Rehabilitation

Background: In direct opposition to the go-go, hustle and bustle atmosphere of the Denver International Airport (DIA), time and deterioration has begun to take a devastating toll on the infrastructure beneath the floors of the terminals. Pipelines that had been operating and draining smoothly up until recently have begun experiencing deterioration and tuberculation. The North Terminal Sewer Rehabilitation Project involved a waste water drain line that services several floor and area drains for this section of the terminal. The project was located beneath the primary terminal of the airport in the tunnels below where the baggage tugs and carts travel for baggage delivery and pick up.

The Problem: The 400′ of 6″ waste line and inside grease interceptor had been backed up for a long period of time while a design for replacement could be completed by engineering firm Burns and McDonnell. After quickly turning over a well planned design for open cut removal and replacement, the job was handed over to FCI Constructors. C&L Water Solutions, Inc. was brought in to perform an initial condition assessment of the lines by utilizing key holes dug by hand in the tunnel floors. C&L determined through CCTV and dye testing that the issue resided in the incorrect plumbing of the sand and oil interceptor. Additionally, the existing 6″ cast iron was deteriorating, but still maintained good line and grade. C&L also determined there was a high degree of tuberculation composed of carbon mineral content that had adhered during periods of slow draining or backups to the inside of the pipe wall. The line is a high use/low use line; during periods of use, the mineral content was introduced into the line and consequently during low flow was allowed to dry and harden. The material was hardened to the likeness of concrete and the opening through the existing pipe had been reduced to less than 3 inches.

The Challenges: Due to the location of the project, no diesel, gasoline or propane equipment is allowed in the tunnels. Additionally, no strong or toxic fumes were tolerated at any point originating from any process of construction. These restrictions required us to find alternative means of access. Equipment lines were run through storm water drains located outside of the structure altogether. In addition to the location challenges, open cut removal and replacement would require removal of life support air handling equipment, ground water pumps, and a tremendous amount of concrete. To complicate matters further, the majority of the excavation would have to be performed by hand, which would come with an astronomical price tag. While the idea of open cut was staggering, nearly as daunting, was determining how best to implement a trenchless solution. Excavations were still required at bend locations, and most of these locations were cleanout wyes that could not be reinstated robotically. Some of these bend locations were located under walls or air handling equipment which added to the complexity of shoring.

The Solution: The first step was to remove the deposits from the existing main to acquire a complete understanding of what the contractors were up against. Many nights were spent utilizing several methods such as citric acid degreasers, non-toxic bathing agents, hot water, jetting with a variety of nozzles, robotic cutting, pigging, and drag scrapping. C&L eventually returned the cast iron pipe to its original inner dimension.

The next step was to propose a solution to DIA to save time and money and really address the issues that were plaguing the system. The contractors proposed re-piping the interceptor and extending the interceptor drain to the exterior wall and tying it into a proposed manhole which would be installed over an existing 12″ sanitary main. They would address the waste line by lining it with a UV cured fiberglass reinforced CIPP liner. The UV method was chosen for the following reasons:

  • Although a PH of 8 was found in the water at various times throughout the project, the fiberglass liner returns excellent results regarding its chemical resistance;
  • The ground at DIA contains very expansive and aggressively mobile soils, therefore the liner had to display a high degree of flexural modulus;
  • Due to the idea that we were lining and reinstating laterals in a pre-existing 6″ line, it was preferred that the liner contain as thin a wall as possible without compromising the structure of the product;
  • The last and perhaps most important aspect is that the liner is cured with UV light which means there is no odor, and the unit can be run off of 220 V 3PH power from the tunnel. 

The laterals were reinstated using robotic cutters that had to be remotely controlled from the exterior of the building; the cutter control cable was run through a storm drain line from the exterior to the interior.

Final Thoughts: Despite the challenges, the tedious nights spent cleaning lines, troubleshooting emergent issues, and having to accomplish project completion while adhering to the site and equipment restrictions, the contractors feel that the client was satisfied with a high quality product that ultimately saved them money and kept disturbance to the facility to a minimum.. Successful collaboration of the right minds and technological methods meant that the challenges were met and overcome. This project has helped solidify the idea that not only is trenchless technology successful; it is fast becoming the preferred method by many clients.