In 2015, Montgomery County, Maryland identified a culvert under Germantown Road that was severely deteriorated and in need of repair or replacement. The existing 96-inch culvert was located under 26’ of cover and included a major modular block retaining wall situated over the upstream end. The depth of cover and the retaining wall, combined with the fact that the road above was heavily trafficked and difficult to detour made a replacement solution highly problematic. There were also several utilities passing through the fill above the culvert. Notwithstanding the excessive capital expense required, a normal replacement approach would have been highly disruptive to local residents and commuters.
Once a reline solution was determined to be the most viable option, the initial challenge was to determine if the culvert truly could be relined. The total lineal footage was 470 feet with a 3-degree vertical elbow located 102’ from the outlet end. It was also critical that the hydraulics checked, including the appropriate passage of the design flows, while a manageable level of outlet velocities was attained. The county decided the best solution was to engage an outside engineer to design and detail a full reline solution. The county also determined a competitive bid approach would provide the most economical construction cost after learning that relining techniques for culverts of this size had become widely known to area contractors.
Ultimately, the consulting engineer, The Wilson T. Ballard Company, decided the best design approach would be a relining solution that incorporated an 84-inch diameter, 12 gage, aluminized type 2 (ALT2) spiral rib corrugated metal pipe (CMP). This solution would provide the desired Manning’s “n” of 0.012 while also extending the service life of the existing host pipe by an estimated 100 years. Aluminum coated steel for CMP was introduced in the mid 1980s and has proven to provide an extensive service life improvement compared to galvanized steel for CMP. This improvement is commonly a factor of three times longer when in the appropriate environment.
During the project scoping and evaluation process, representatives from Contech Engineered Solutions visited the site to help assess the existing culvert conditions. Timber supports were aged and appeared to have been installed during the original construction. This caused some concern as they suspected that the soil prism above the pipe could be variable from a soil arching standpoint. The corroded invert also showed full corrosion failure resulting in inward, rotational movement of the side wall which meant that an applied coating was out of the question and that movement or shifting of the host pipe was possible. Either way, a fully structural rehabilitation method was needed.
The low bidder for the project, Concrete General, Incorporated of Gaithersburg, MD, mobilized their crew and began the preliminary work to prepare the host pipe for relining. During this initial site preparation, a 20’ long section of the existing culvert shifted downward. Workers were inside that section at the time and the soil arch sheared. As a result of this unfortunate movement in the culvert, the safety of the workers inside of the structure became a much higher risk, and the selected 84-inch pipe would no longer cleanly fit into the host pipe.
At this point, the County needed to reconsider the viable options at their disposal. Given the previously mentioned design considerations, open cut was still out of the question. Instead, they decided to stabilize the pipe with invert paving and box beam frames to temporarily address the structural concerns, allowing them time to determine another reline option.
After some consideration, they selected Aluminum Tunnel Liner Plate manufactured by Contech Engineered Solutions as the best option. Tunnel Liner Plate offered several advantages over other options. The individual sections can be assembled inside the host pipe and would allow for tunneling through the host pipe if needed. It also provided a safe working environment for the workers as they progressed through the length of the structure. The structural design of the new Tunnel Liner Plate pipe adhered to the AASHTO LRFD design methodology for tunnel liners. The aluminum alloy material is extremely durable. It features a marine grade, 5052 aluminum alloy that has been used in tough culvert environments since the mid 1960’s. The heaviest plate weighed only 19 pounds which allowed for easy handling and assembly and quick hand tunneling within the culvert. Each section of Tunnel Liner Plate is 18” in width, along the centerline of the new pipe. The new pipe rings were made up by 5 individual plates that feature high efficiency longitudinal seams with offsets in the plate ends. The staggered bolt pattern in the longitudinal seams provided an efficient transfer of bending loads around the ring.
As part of this revised construction plan, exploratory holes were drilled into the host pipe to allow for assessment of supporting backfill which aided in determining when the timber box strutting could be removed. If voids were found, a “U” shaped cut allowed for the host wall to be opened up at the time of the liner plate assembly. These openings were located at the top of the voids and allowed the grout to fill the voids during the liner grouting process.
During the assembly of the Aluminum Tunnel Liner Plate, staging was conducted in a solid area of the host pipe where supports could be removed well ahead of the assembly at this location. Placement of blocking between the Tunnel Liner Plate and the host pipe allowed for immediate transfer of loads to keep the workers safe. The blocking provided uplift resistance during grouting as well. Grout was pumped through the new pipe wall through 2-inch diameter ports utilizing a three-lift grouting plan with a highly viscous, standard grout mix consisting of cement, water, sand and a plasticizer. It had a fluid unit weight of 125 pounds per cubic foot.
Even with the change to a different design, the County was extremely pleased with the outcome in what otherwise would have been a very costly and time-consuming replacement project. Brian E. Copley, P.E. of the Montgomery County DOT, stated, "We are very pleased with the outcome of this project. The unfortunate movement of the host pipe during construction caused a shift in our approach, but the final solution was highly effective."