Hanson is providing geotechnical, structural, civil and hydraulic engineering services for Ameren Transmission Co. of Illinois’ Illinois Rivers transmission line, which crosses three major rivers. The transmission line crosses the Illinois River near Meredosia and Beardstown and the Mississippi River near Quincy. The project involves providing foundation designs and access support for 14 lattice towers that reach as high as 460 feet and feature spans as wide as 3,700 feet. Foundation types included all-steel tripod foundations with driven H- and pipe piles as well as concrete pedestals with driven pipe piles.
Hanson provided technical oversight and borehole logging for geotechnical investigations at each of the structure locations, in some cases, advancing soil test borings to rock 120 feet below the surface. Hanson performed soil laboratory testing, land and bathymetric surveying and engineering analysis of design load cases, geotechnical parameters and hydraulic aspects of the river crossings to develop foundation design parameters and performance criteria (strength and displacement) for the evaluated conditions. These conditions included flood levels, scour and debris, hydraulic and ice forces.
Two foundation types were chosen to efficiently adapt to the unique conditions at each of the three crossings. The all-steel tripods were chosen as one of the types to eliminate the need for delivering or producing concrete for the foundations located on islands in the river. The tripod and pile combination also reduced wetland impacts in sensitive areas.
Civil engineering services involved the design of construction access to each of the river crossing tower locations. At the Mississippi River crossing — the Herleman to Maywood river crossing — Hanson designed and permitted a 200-foot temporary riprap causeway from the main channel of the river to an existing sand bar. The causeway provided for the unloading of construction equipment and materials from barges onto the island that 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 foot.
The Mississippi River crossing and one of the Illinois River crossings have been completed successfully. The final Illinois River crossing — the Meredosia to Ipava river crossing — is underway. Each location offered significant and unique challenges in permitting, access, subsurface conditions and the varying and often unpredictable river levels.
An article on the Mississippi River crossing is included in the November 2016 Transmission & Distribution World Magazine's 2016 American Society of Civil Engineers (ASCE) Lines & Structures Supplement, http://tdworld.com/supplements/mississippi-crossing.
The article is an abbreviated and updated version of a technical paper co-authored by Ameren and Hanson personnel that was included in the 2015 ASCE Electrical Transmission & Substations Structures Conference Proceedings, http://ascelibrary.org/doi/book/10.1061/9780784479414.