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AASHTO Drop Wall Requirements
What is the justification for drop walls on box culverts?
Drop walls are an AASHTO requirement. See specifications below from AASHTO-LRFD Bridge Design Specifications, Second Edition, 1998, Section 12 - Buried Structures and Tunnel Liners:
- 12.6.5 - Scour: Buried structures shall be designed so that no movement of any part of the structure will occur as a result of scour. In areas where scour is a concern, the wingwalls shall be extended far enough from the structure to protect the structural portion of the soil envelope surrounding the structure. For structures placed over erodible deposits, a cutoff wall or scour curtain, extending below the maximum anticipated depth of scour or a paved invert, shall be used. The footings of structures shall be placed not less than 2.0 ft below the maximum anticipated depth of scour.
- 12.8.6.4 - Hydraulic Protection
- 12.8.6.4.3 - Cut-off (Toe) Walls: All hydraulic structures with full inverts shall be designed and detailed with upstream and downstream cut-off walls. Invert plates shall be bolted to cut-off walls at a maximum 20.0 in center-to-center spacing using 0.75 in bolts. The cut-off wall shall extend to an adequate depth to limit hydraulic percolation to control uplift forces as specified in Article 12.8.6.4.4 and scour as specified in Article 12.8.6.4.5.
- *12.8.6.4.4 - Hydraulic Uplift (specification): Hydraulic uplift shall be considered for hydraulic structures with full inverts where the design flow level in the pipe can drop quickly. The design shall provide means to limit the resulting hydraulic gradients, with the water level higher in the backfill than in the pipe, so that the invert will not buckle and the structure will not float. Buckling may be evaluated as specified in Article 12.7.2.4, with the span of the structure taken as twice the invert radius.
- *12.8.6.4.4 (commentary): Structural plate structures are not watertight and allow for both infiltration and exfiltration through the structure's seams, bolt holes and other discontinuities. Where uplift can be a concern, designs typically employ adequate cut-off walls and other means to seal off water flow into the structural backfill.
- *Generally a problem in structural steel pipes.
- 12.8.6.4.5 - Scour (specification): Scour design shall satisfy the requirements of 12.6.5. Where erodible soils are encountered, conventional means of scour protection may be employed to satisfy these requirements. Deep foundations such as piles or caissons should not be used unless a special design is proved to consider differential settlement and the inability of intermittent supports to retain the structural backfill if scour proceeds below the pile cap.
- 12.8.6.4.5 (commentary): Structures with full inverts eliminate footing scour considerations when adequate cut-off walls are used. For arches, reinforced concrete invert pavements, riprap, grouted riprap, etc, can be employed to provide scour protection.