This 40 year old storm sewer runs underneath a major apartment building and adjacent parking lot in downtown Rockville, MD. The storm sewer was corroding and starting to buckle in several locations. This project was bid with 2 options, the first being slip-lining and the second being shotcrete. The bids for rehabilitating the CMP with shotcrete came in half the price as the slip-lining bids. Coastal Gunite was awarded the project. The parking lot had to be remain fully functional during construction. The storm pipe deposited storm water into an environmentally sensitive stream due to the local fish population. Noise restrictions were in place, due to the apartment complex, which limited the amount of time per day that Coastal Gunite was allowed to work. With all of these restrictions, the working conditions were challenging.
The pipe had deformed into an elliptical shape in several places. Care had to be taken to reinforce these sections with rebar as well as shotcrete while also maintaining the flow area. The shotcrete had to have a smooth trowel finish in order to achieve the minimum hydraulics required. The invert was also severely corroded in several locations and had curled up on itself. These sections had to be braced with timber, cut out and a new invert installed. The City of Rockville was very pleased with the utilization of shotcrete on this project since the apartment tenants were undisturbed, the flow area was maintained, and a new 50-year life cycle concrete pipe was installed inside the original CMP.