DGA Backfill as 'brace'

[Cosimo]

When constructing cofferdams for 'small' bridge footings we often backfill the void between the concrete footing and temporary steel sheeting with dense graded aggregate (DGA). This allows us to remove the lower tier waler and cross braces. In essence, it provides a continuous brace. When the space between the footing and steel sheeting is relatively narrow with respect to footing thickness, this works well. In some cases we would just extend the footing width and pour tight against the sheets and use a bond breaker if approved by the engineer. How would you evealuate a situation, i.e., determine the adequacy of the DGA to resist loads where the space is wider, say maybe 6 to 8' with a 3' thick footing? Any input would be appreciated.

 

[civilperson]

Use the Ko of the dense graded aggregate on the area, compare to forces to be resisted, (minimum SF = 1.5).

 

[DRC1]

Not sure exactly what Dense Graded Aggergate is, put crushed stone is often used for backfillging in a coffer dam. If you are planning on bracing across the excavation, I would pour the entire bottom(unless it is really large compared to the footing. Teh concrete will provide a reliable brac and the savings of forming ,stripping and backfilling will offset the cost of the additional concrete. A sheet of poly is often used as a bondbreaker.