Full height concrete abutment walls subjected to scour

[minorchord2000]

We have been tasked to design full height concrete abutments on spread footings subjected to scour. The bridge structure will be an ACROW steel bridge similar to a Bailey bridge. The single span is 110 feet from CL bearing to CL bearing with HL-93 loading. My question concerns scour from stream flow. During the scour condition the depth of flow down to the scour elevation is 15 feet. I plan on setting the tops of the spread footing 2 to 3 feet below the scour elevation. Of course the backfill behind the wall remains. Do I design the wall with the active backfill soil load and traffic surcharge load acting on the back side of the wall and the hydrostatic stream flow pressure acting on the other side of the wall? Do they act simultaneously and use the resulting loads to the design the concrete stem?

I never have encountered this condition before since I have always used stub abutments founded on drilled shafts.

Any help would and suggestions will be greatly appreciated

Thank You

 

[SlideRuleEra]

minorchord2000 - I believe the soil/water pressure diagram is more complex, with submerged earth on one side. I would consider both Figure 9 and Figure 12 (shown below), with Figure 9 (no water in the stream and submerged earth behind the wall being the upper bound. Figure 12 (water in the stream and submerged earth behind the wall) being the lower bound.

Those footings will be really expensive, appears a 30'+ deep cofferdam will be required to construct each one.

http://www.SlideRuleEra.net

 

[MIKE_311]

wow, you are in for some fun. You are going to have really large heels to resist that all that earth pressure. The last time I needed to design an abutment that tall we had huge footings.

Are deep foundation an option, I suspect with that potential much scour they should be. You could also do a shorter stub abutment on piles and wrap it with sheet piles. Considering you will need to drive sheet piles to construct this, it might be a more cost effective option.

 

[Xav1]

I have a question on this though:
When a particular scour depth is provided as in the sketch above, how do we determine the extent of wingwall, as scour does not only happen in front of the abutment (horizontal extent of scour back of the abutment). We need to retain earth and protect the approaches behind.

If you can do a plate model with wingwalls (a rigid abutment), the wingwalls would help reduce demands on the stem wall. Note that earth pressure considered for rigid abutments are higher compared to flexible cantilever retaining walls for example.

 

[BridgeSmith]

Depending on how quickly the stream level in front of the wall fluctuates, you may have to consider the differential in water pressure due to what's called rapid drawdown.

The last time I faced a situation like this (on a somewhat smaller scale), I put 30" drilled shafts directly under the 36" wide abutment wall and sweptback wingwalls. The DS under the abutment wall provided bearing capacity for the superstructure loads and downward load from the overturning moment, while the DS at the ends of the wingwalls provided the uplift resistance for overturning.

Rod Smith, P.E., The artist formerly known as HotRod10