Culvert Nesting Ledge Issues

Multiple box culverts with a clear height of 4 ft. – 0 in. and greater shall be provided with 1 in.

ledges, 4 ft. – 0 in. long on each side of all interior walls near the downstream end when these walls contain a single plane of reinforcement bars located at the wall center. These ledges provide suitable nesting sites for certain bird species (phoebes and barn swallows) that tend to nest in man-made shelters. The ledge detail is depicted in Figure 2.3.11.5-1.

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Figure 2.3.11.5-1

Jan. 2012 Page 2-149 2.3.12 Retaining Wall Selection Process

Many factors such as geometric limitations, geotechnical site conditions, aesthetics, structural feasibility, construction equipment access, and traffic staging affect retaining wall type selection.

The engineer responsible for the TSL plan should identify and evaluate these issues, and conduct feasibility analyses which leads to a retaining system selection that is cost effective.

To reduce both initial and long term maintenance expenses, efforts should be made to minimize overall wall length and exposed height. This can be accomplished by utilizing slopes either in front or behind the wall as well as evaluating various wall locations/alignments. During this process, there are a number of right-of-way, roadway cross section, drainage, utility, and construction limitations that should be considered.

The exposed retention height for a wall is established from finish grade elevations found within the roadway cross sections. The engineer responsible for TSL development should work closely with the roadway engineer when determining finish grade cross sections along the length of the wall. The bottom of wall elevations should be determined to accommodate any drainage/utility excavation proposed in front of the wall and satisfy the minimum embedment necessary for the wall type selected. The final top of wall elevations should be established to satisfy the cross section retention requirements while forming an aesthetic top of wall profile.

Where the wall face is visible to traffic or commonly viewed by the public, the use of form liners or other wall face texturing should be strongly considered. The need for coping, traffic barrier, noise wall/sight screen, railing or fencing mounted on the top of wall should also be determined and coordinated with the District.

In addition to establishing the required wall retention geometry and other site design constraints, geotechnical issues have a substantial impact on the wall type section process. The majority of loadings applied to the structure as well as the capacity of the foundation are controlled by the soil conditions present. The SGR generally contains all geotechnical analyses, foundation and wall type recommendations, and design parameters required to assist in wall type feasibility and cost analyses. Substantial coordination between the structural and geotechnical engineer during the SGR development process is necessary to ensure appropriate design parameters and recommendations are provided. Section 2.3.6.3 should also be referenced when evaluating foundation type options. Section 3.11 and the IDOT Geotechnical Manual should be referenced

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for detailed technical information concerning the design and subsurface investigation of retaining walls.

There are four retaining walls types commonly built in Illinois. These are mechanically stabilized earth (MSE), cast-in-place concrete T-type, soldier pile, and permanent sheet pile. Soil nailed, precast modular, segmental block, gabions, and other specialized wall systems have been utilized by the Department on a limited basis when their unique wall properties lend themselves to specific project conditions. MSE and cast-in-place walls are most economical and better suited for use in “fill” sections, while soldier piles and permanent sheet pile walls are generally most advantageous in “cut” section retention applications.

Typical cross section and details should be shown on the TSL plan. Sample retaining wall TSL plans are given in Section 2.3.14. Policy and procedures for the design of retaining walls are given in Section 3.11.

2.3.12.1 MSE Walls

MSE walls provide one of the most cost effective and durable wall structures available. The walls internal stability is designed by the wall vendor during construction to provide over 75 years of design life. The cost savings advantage is most prevalent on projects with large bid quantities or on structures where the maximum wall height is relatively tall. Locations with short wall heights or lengths often lend themselves to other wall types.

Precast panels avoid the typical cracking that occurs on CIP walls and provides superior aesthetics due to their articulated panel pattern. Panels can be cast with a smooth face or form liners can be specified to produce a variety of cast patterns. MSE walls can also be constructed along curved alignments. Both design and construction time is reduced when an MSE wall is selected.

The TSL shall provide the “top of exposed panel line”, the “finished grade line at front face of wall” and the “theoretical top of leveling pad” which is normally set 3 ft. – 6 in. below the finished grade unless there are other geotechnical or geometric limitations. Using these wall heights, the reinforced mass should be assumed to be 0.7 of the height for feasibility and economic analyses. The engineer responsible for the TSL, assisted by the recommendations contained in the SGR, is responsible verifying external stability and foundation soil adequacy. The reinforced