Sometimes, a bridge span has reached the end of its service life before its foundations have. In cases like this, it can be cost-effective to replace only the span, leaving the existing piers or abutments. This has been done successfully many times, but there are a few points to be wary of during planning.
Reducing cost is one of the main reasons existing bridge foundations are reused. The money saved from the reduced demolition and rebuilding of two abutments and a pier or two can allow another project to be completed with the same yearly budget. Not having to change a bridge’s length saves even more when you consider that site grading or embankment work will not be needed. Bridge length changes can result from constructing new abutments behind existing abutments or hydraulic permitting headaches can be introduced by reducing the waterway opening when constructing in front of existing abutments. Both of these options have construction staging and track outage constraints in addition to cost considerations.
There are key conditions on a reused substructure to look for during inspection. Any cracking or deterioration should be repaired. Superficial cracking or spalling is typical in old stone or concrete structures, but deep cracks can be a symptom of settling. Any rotation of the piers or abutments or noticeable elevation changes at the bridge seats are signs that there could be foundation problems that need to be fixed with the new construction. However, limited bridge seat or backwall deterioration do not immediately rule out reusing the substructure.
If the only foundation problems are in the bridge seats of the piers or abutment, or if the problems are found only in the abutment backwalls, these areas can be addressed when the superstructure is replaced. Once the spans are removed, the bridge seats or backwalls can be replaced with precast concrete pieces. Dowels are installed during the changeout window and held in place with fast-curing epoxy or grout. This ensures that the new components have a positive load path in the new condition.
A final possible concern is the extra dead load that will be placed on the existing foundations. Going from an open deck to a ballasted deck structure greatly increases the dead load. Precast, prestressed concrete spans are also much heavier than the steel spans they are likely replacing. This is a bigger concern with weak soils or timber piles. Spread foundations that rest directly on rock can typically support the additional load without concern. This is where inspecting for signs of settlement or tilting is important. It is easy to make a foundation problem worse with the extra load.
If you have questions about replacing bridge superstructures on existing foundations, please contact Travis Painter, an associate project manager, at tpainter@hanson-inc.com.