In early March of 2016, the Alaska Department of Transportation (AKDOT) issued a request for proposals (RFP) packet that outlined plans for the reconstruction and improvement for Ted Stevens Anchorage International Airport (ANC) Taxiway Y. These enhancements involved several things including new pavement, upgrade of the entire taxiway’s edge and centerline lighting systems as well as the replacement and/or rehabilitation of an existing storm drain system.
ANC has been active for more than fifty years and has played an integral role in the growth of both Anchorage and the entire state of Alaska. The strategic location of the airport provides unlimited potential for moving goods, services, and infrastructure to be used in the global marketplace, making it among the top five busiest cargo airports in the world. With that in mind, it was important that the improvements to Taxiway Y have as minimal downtime and negative impact to the daily air traffic of the airport as possible.
The Anchorage office of Stantec Consulting Services, Inc was chosen as the engineer of record for the project. One of their challenges was to address the existing 66 inch diameter CMP storm drain which crosses directly under the taxiway and was in need of rehabilitation or replacement due to its deteriorated condition. They determined that a 54” diameter smooth liner pipe would be sufficient for the hydraulic requirements and specified a solid wall HDPE product with 54” outside diameter and 50.48” diameter inside diameter. This approach would not require the shutdown of the taxiway over the deep storm sewer pipe. The excavation and replacement using conventional direct bury techniques would have delayed the pavement and lighting work and caused the taxiway traffic to be disrupted, in addition to being much more expensive than the trenchless method selected.
After bids were received and evaluated, a contract for the work was awarded to a local contractor, Quality Asphalt Paving (QAP) based in Anchorage, AK. QAP is a contractor experienced with multiple relining techniques and products. Upon receiving the award, the contractor investigated other pipe material options and determined that a steel reinforced polyethylene pipe product manufactured by Contech Engineered Solutions would be a better and more viable approach. It allowed for greater burial depths than the specified pipe. The DuroMaxx® SRPE liner pipe was ideal for this solution as it adhered to the both AASHTO MP 20 and ASTM F2562 material specification while the joints complied with ASTM D3212 and would keep the grout from leaking into the pipe during the grouting stages.
The steel reinforced polyethylene (SRPE) liner pipe was manufactured with eighty (80) ksi tensile strength steel reinforcing ribs which provided the inherent strength, while the pressure rated polyethylene (PE) resin provided the durability. This combination of materials resulted in an extraordinarily strong and durable pipe that was a fully structural capable solution, resistant to the effects of temperature and sunlight.
The deteriorating storm drain was under 24’ of cover height. The 54” diameter DuroMaxx SRPE liner pipe weighed just 36 lbs/ft as compared with the 120 lbs/ft for the solid wall product, thereby offering handling advantages as well. Quality Asphalt Paving (QAP) submitted DuroMaxx as an ‘or equal’ and it was approved for use by the ANC project management team.
The 20’ lengths of pipe were manufactured in Ogden, UT and shipped to the Port of Seattle, from where it was transported by ship to the Port of Anchorage and then, ultimately, trucked to the jobsite in July of 2017. Longer lengths are typically used but the 20’ lengths allowed for efficient shipping since multiple shipping modes were used. To aid in the grouting and bracing process, three 2” diameter grout ports per piece of pipe were installed at the Ogden plant. One grout port each was placed in the 12 o’clock, 5 o’clock and 7 o’clock positions of each piece of pipe. The purpose of the grout ports was to allow the contractor to monitor the grout level during grout installation and to hold the liner pipe in place during grout installation. Skid tubes were installed on each piece of pipe to aid in joint alignment and minimize sliplining friction.
Once submittal documents and shop drawings were approved, the contractor went to work. They cleaned, dewatered and inspected the existing host pipe and prepared for the liner pipe to be sliplined directly into the deteriorated storm drain. An important part of the preparation work included laying two continuous and parallel timbers along the invert of the host pipe. These timbers would act as rails, which the liner pipe rested on during the pushing process. The rails insured proper liner pipe alignment and elevation while decreasing the friction between the host pipe and the new pipe.
The contractor started the pipe insertion on a Friday afternoon and finished on Saturday. The liner pipe was pushed uphill through a downstream manhole. Grout tubes of different lengths were connected to the liner pipe and pushed into the host pipe along with the liner pipe. The initial grouting plan consisted of constructing bulkheads at each end of the pipe run between the host pipe and the liner pipe. The grout tubes penetrated the bulkheads, which allowed the grouting sub-contractor to pump grout into the annulus through these different length tubes while being able to monitor the grout levels during the grouting process.
The project was going smoothly until it began to rain prior to the construction of the bulkheads. Drainage water flowed through the annulus between the host pipe and the liner pipe since the bulkheads were not in place. This water caused the liner pipe to lift, and the various grout tubes caused the floating liner pipe to rotate. Once the water subsided, the contractor learned that the grout tubes and grout ports were out of position, and could not be easily placed in the correct position. After consulting with the contractor onsite, Contech recommended they drill new 2” diameter grout ports in the desired positions through the DuroMaxx pipe wall. Doing this would allow the contractor to move forward with their construction plan without having to reinstall the pipe, thus staying on schedule. Additionally, doing so wouldn’t jeopardize the quality of the final system.
Once the new grout ports were drilled in the correct positions, the grout sub-contractor began to grout between the host and liner pipe through a staged approached with multiple lifts. Upward buoyant forces were kept in check by the use of a well-designed buoyancy control system. The grout ports in the 12 o’clock position of each segment of pipe allowed a strut consisting of a steel pipe and bottle jack to penetrate the liner pipe wall and brace against the existing pipe. The bottom of each bottle jack rested on a 4x4 beam running along the invert of the liner pipe, effectively spreading the resisting load along the entire length of liner pipe.
The grout used had a fluid unit weight of 50 pcf. The low unit weight reduced the buoyant forces applied to the liner pipe during the grouting process. Another buoyant force countermeasure strategy used on this project was to place the grout in separate lifts, allowing enough time between lifts to achieve initial set in the grout. The first grout lift was only 8” deep. After the grout was 8” deep at the highest end of the pipe, the contractor filled the remaining void in one lift.
In spite of mother nature causing unexpected site challenges on this project, the adaptability of QAP, Contech and DuroMaxx allowed for creative and smart answers that enabled the team to overcome them. ANC was provided with a rehabilitated storm drain capable of carrying all the loads and hydraulic capacity required without having to trench through the taxiway. The final system provides a 100-year service life, and the trenchless approach allowed the critical path scheduling to be advantageously shifted, allowing the overall construction time to be reduced.