This document is the case study for N-12 Delivery is Right on Time Case Study. The I-85/I-385 stormwater drainage system flipped from concrete to plastic.

For the stormwater drainage system of the I-85/I-385 rebuilt interchange, high-density polyethylene (HDPE) pipe was going to be used sparingly. Originally, reinforced concrete pipe (RCP) was going to be used. When the contractor realized HDPE could be delivered on time, the project flipped to plastic pipe.

41,000 feet (12,497 m) of N-12 watertight pipe, which meets AASHTO M294, was chosen for the project, which is the third busiest South Carolina interchange. N-12 was the perfect choice for the project because it has a long projected life and could be delivered on a just-in-time basis to the design-build project. The staging area was within the footprint of the old interchange so pipe storage was at a minimum on the project’s site.

The light weight of N-12 pipe also allowed it to be installed about twice as fast as RCP could have been installed, which lowered the total installed cost of the stormwater system.

The installation was done in 2020 using 41,000’ (12,497 m) of 12”-48” (300-1200 mm) N-12.

Photographs of the project are included.