Project Description

CH 21 Bridges over Lake Fork Creek and Tributaries
Macon County, IL

Three bridges on County Highway 21 in Macon County were in need of replacement. Some of the bridges were posted for less than legal loads which limited the use of the road to transport grain and other freight between neighboring towns. The hydraulic conditions of the streams had also changed, causing high waters to encroach on the underside of the existing bridge structures. Clark Dietz prepared reports, studies and plans for Phase I and II services according to the State and Federal requirements. The County utilized Federal Funds for construction of the bridges.

The three bridges were designed and replaced over several years beginning in 2012 and ending with construction of the last bridge in 2018. All three bridges have similar features although each with different span lengths and geometry with respect to the stream being crossed. The bridges ranged in length between 73’-0″ to 96’-0″ with span lengths up to 34’-0″ and a width of 28’-0″ between railings. To minimize the superstructure depth and provide adequate freeboard without making significant changes to the roadway profile, three span continuous cast-in-place concrete bridges were used. The bridges were supported by integral abutments and solid wall encased pile piers. The integral abutments have no open joints between the roadway and bridge, and such joints require practically no maintenance. The solid wall encased piers provide a smooth surface to permit passage of debris through the stream. A shortened reinforced concrete approach pavement was also used instead of the longer approach pavements typically found on the State highway system.

County Highway 21 has a very flat profile with some of the bridges having nearly zero-degree vertical grade. Type SM metal bridge railings were used to allow water to quickly shed off the bridge sides. The metal railings also required less superstructure width and significantly reduced bridge costs. Use of the Type SM railing on a shallow cast-in-place slab bridge was coordinated with the IDOT Bridge Office and higher strength concrete used in the bridge decks to conform with testing that had been done on the Type SM railing.

Phase I planning consisted of preparation and submittal of all required documentation including the Bridge Condition Report, Hydraulic Report, Environmental Studies, Joint Application, and Project Development Report. In-house work included survey, hydraulic analysis, environmental investigations and reporting, roadway and bridge design. Geotechnical borings and the Structure Geotechnical Report were prepared by another consultant and the effort coordinated by Clark Dietz.