Milwaukee installed a combined sewer system in 1925 to collect rainwater runoff, domestic sewage and industrial wastewater. But the old sewer system is inadequate for a modern city with nearly 600,000 residents and 39,000 business establishments. So in 2013, Milwaukee embarked on a sewer upgrade, which included the largest fiberglass sliplining project in North America.

The upgrade was imperative because the sewer system overloaded during heavy rainfall. “The sewer was showing structural distress, developing cracks throughout,” explains Timothy Thur, Milwaukee’s chief sewer design manager. “There were multiple leaks through the cracks and significant ground water infiltration. Large mineral deposits were also present at various locations in the sewer.” As part of the upgrade, the city rehabilitated a 4,500-foot section of the existing combined sewer.

The pipes are buried 63 to 103 feet below ground. “Many of the older sewer pipes are now located under other utilities, streets or even businesses, so rehab [rather than pipe replacement] allows the utilities above to be minimally affected,” says Kim Paggioli, principal engineer, vice president of marketing and quality control at Hobas Pipe USA. To control the hydraulic capacity – the maximum amount of water that passes through the sewer system – the existing 144-inch diameter monolithic concrete sewer needed to be repaired.

Thur and his team looked at both cured-in-place pipe (CIPP) and sliplining for the repair. Because of the depth and amount of flow in the sewer, the city opted for sliplining using fiberglass mortar pipe. The project contractor, Michels Corporation, selected Houston-based Hobas Pipe USA to supply the pipe.

With sliplining, a smaller carrier pipe is installed – or slipped – into the existing host pipe. The space between the two pipes is grouted and each manhole is sealed. Sliplining is a very cost effective and easily installed rehabilitation method, which is especially important in populated areas since it minimizes the disruption of businesses and highways.

Hobas Pipe manufactured a 126-inch diameter centrifugally cast, fiberglass reinforced polymer mortar (CCFRPM) pipe for this project. “There are other fiberglass pipes and other centrifugally cast pipes, but the combination we have is unique,” says Paggioli. The pipe incorporates FRP and mortar (or sand). This was the largest diameter custom manufactured pipe ever made by Hobas Pipe and the biggest fiberglass sliplining job done in North America at the time, according to Paggioli.

Centrifugal casting is an automated production process: A computer program controls a feeder arm that continuously inserts three basic components – glass fiber, unsaturated polyester resin and aggregates – into a 6-meter-long rotating mold.