Hanson tackles unique challenges for Ameren transmission line river crossings

This image shows the west half of the Mississippi River transmission line crossing.

There are unique challenges when designing transmission line structures and foundations across rivers — especially large, navigable rivers. Hanson provided geotechnical, structural, civil and hydraulic engineering services for three such crossings as part of Ameren Transmission Co. of Illinois’ Illinois Rivers Project. This transmission line crosses the Illinois River near Meredosia and Beardstown and the Mississippi River near Quincy. Hanson provided foundation designs and access support for 14 lattice towers as high as 460 feet with spans as long as 3,700 feet. Foundation types included all-steel tripod foundations with driven H- and pipe piles at two crossings and concrete pedestals and footings with driven pipe piles at the third crossing.

Hanson’s services included geotechnical engineering, land and bathymetric surveying and engineering analysis for design load cases, geotechnical parameters and the hydraulic aspects of the river crossings. Design considerations included flood levels, scour, debris and ice.

Two foundation types were selected to adapt to the unique conditions at each of the crossings. All-steel tripods were chosen as one of the types to eliminate the need for delivering or producing concrete for the foundations on islands in the river. The tripod and pile combination also reduced wetland impacts in sensitive areas. Where adequate access was available, more conventional, pile-supported concrete foundations were used.

Civil engineering services involved the design of construction access to each river crossing tower location. At the Mississippi River crossing, Hanson designed and permitted a 200-foot temporary riprap causeway from a sand bar at the shore of an island out toward the main channel of the river. The causeway allowed construction equipment and materials to be unloaded from barges onto the island, where one of the transmission line towers is now located. The design necessitated obtaining a floodway construction permit, or no-rise certification, from the Illinois Department of Natural Resources’ Office of Water Resources. For the permit, Hanson had to demonstrate that the causeway construction would not cause an increase in the water surface elevation of greater than 0.01 feet.

All three crossings are complete. Each location offered significant and unique challenges in permitting, access, subsurface conditions and the varying and often unpredictable river levels.

For more information about Hanson’s services for transmission lines, contact Terry Barnett at tbarnett@hanson-inc.com.

Hanson provided geotechnical, structural, civil and hydraulic engineering services for the Mississippi River crossing as part of Ameren Transmission Co. of Illinois’ Illinois Rivers Project.