WEC Energy Group’s Zoo Interchange Project is a prime example of how nontraditional modifications to electric infrastructure can create lasting benefits for both a utility and its customers. The Zoo Interchange is a freeway interchange on the west side of Milwaukee, Wisconsin. It forms the junction of Interstate 94, Interstate 894, Interstate 41, U.S. Highway 41 and U.S. Highway 45. It is one of the busiest and oldest freeway interchanges in the state of Wisconsin, accommodating over 300,000 vehicles per day. When it was originally built in 1963, three double-barrel tunnels were constructed under various portions of the interchange for electric transmission and distribution facilities.
Recently, the Wisconsin Department of Transportation (WisDOT) began a multiyear effort to increase capacity and improve the safety of the interchange. As a result, two of the electric transmission and distribution tunnels needed to be increased in length, the Zoo Tunnel by 300 feet and the Bluemound Tunnel by 500 feet. The south 350 feet of the third tunnel, known as the Adler Tunnel, were in direct conflict with multiple new highway ramps. In order to continue the critical operation of these assets, WEC Energy Group’s utility subsidiary We Energies needed to follow an aggressive construction schedule to extend and modify the tunnels and, in the case of the Adler Tunnel, lower a portion of the tunnel. “All of this work needed to be done while simultaneously maintaining circuit capacity and keeping electric assets in service for our customers,” explained Steve St. Amour, manager — substation engineering and transmission support for WEC Energy Group.
The already challenging project was complicated further by WisDOT’s plans to widen the freeway and change the grade as much as 40 feet in some areas, resulting in significantly increased bearing pressure, or load, on We Energies’ tunnels. Engineers performed a design integrity analysis to determine the best course of action. The analysis determined that the original design of the tunnels was conservative for the existing loads applied, but significant settlement was likely due to the proposed abrupt changes in grade. However, a structural analysis showed that the tunnel could support the additional loads resulting from the increase in overburden with modification.
Based on their findings, engineers opted to transform the existing tunnels into duct packages and install numerous control joints by saw-cutting into the existing tunnel concrete structure in strategic locations. This allowed the utility to control how much a tunnel would settle under the additional load at specific locations and therefore mitigate the risk of damage to the electric cables contained in the tunnel.
“This option resulted in reduced construction time and costs, and even decreased the cost of future maintenance,” said St. Amour. “However, it did create a potential problem. The duct packages’ increased mutual heating had a significant impact on the cables’ ampacity.” The existing paper insulated, lead covered (PILC) cables were replaced with compact copper, ethylene propylene rubber (EPR) insulated cables having flat strap neutral wires. Most cross link polyethylene (XLPE) and EPR insulated cables are limited to a maximum continuous conductor temperature of 90° C. Operating a cable above 90° C can result in a reduction in circuit capacity and cable life as well as an increased risk of joint or cable failure.
To mitigate this risk, engineers needed to develop a backfill material for the existing tunnels with a minimum thermal resistivity ρ (Rho) of less than C-cm/W but, ideally, less than 60° C-cm/W to minimize cable ampacity de-rating. Engineers also had to take into consideration the need for high flowability and pumpability characteristics, low cure strength, and longer than typical setup time for the backfill material. The final backfill design consisted of a mix of fly ash, sand and water with an 8-10 inch slump that was pumpable and flowable, had a minimum 72-hour setup time, and achieved a low 200-300 psi ultimate compressive strength. In addition, the tested thermal resistivity was 35° C-cm/W when wet and 91° C-cm/W when dry, which exceeded the strict, desired thermal resistivity of 40° C-cm/W when wet and 110° C-cm/W when dry. A distributed temperature sensing (DTS) system to monitor cable temperatures was also installed as part of the tunnel modifications.
“Thanks to the ingenuity of the project’s engineering team, we were able to address cable ampacity concerns despite a significant potential increase in mutual heating and address the significant settlement issues resulting from the abrupt changes in soil depth overlying the tunnels,” said Paul Gogan, director — electric distribution asset management for WEC Energy Group.
The unique project design also provided significant additional capacity to expand the electric assets into the foreseeable future, while substantially reducing future maintenance costs and maintaining typical expectations for cable life. Monitoring via the DTS system since the project was completed suggests that the thermal design is delivering expected results. Total installed costs were also well below other considered alternatives. Finally, the modifications allowed We Energies to keep assets in-service throughout construction, and finish nine months ahead of schedule. “From modernizing networks, to new innovations, we are dedicated to keeping the lights on and energy flowing,” Gogan said.