Bridge - Structural Fill Approach

[sgs114]

Hello,

I am working on a steel access bridge project that is under the jurisdiction of the US Forestry Service. The bridge is crossing a stream and will be composed of a steel grating deck with (2) steel WF girders. The requirement of the forestry service is to have 2' of free-board above the 100 year flood level. The bridge is 12' wide and our loading criteria is H-10 vehicle. The bridge is basically serving as an access bridge for some of our personnel (we will own this bridge, but it is on Forestry Service land). Since the slope down to the stream is very gradual, we were thinking of building up the approach with some structural fill to limit the size of the girders. We would basically fill up the existing slope about 8' from the start of the bridge to a depth of approximately 2' (8H:2V triangle of fill). I cannot find any design guidance on this in LRFD-8. Are there any specs or anything I need to look at when specifying this fill? I would assume just compacted gravel would be acceptable, but have not done something like this before and could not find anything in LRFD-8. Are there any concerns about the fill sloughing out during repetitive traffic loading (note this bridge will only be used about 4 times a year by a vehicle).

Thanks

SGS

 

[BridgeSmith]

We would typically use a crushed base or crusher run base material for that purpose. We usually reinforce it with a geosynthetic fabric to reduce the soil pressure behind the abutment and thereby reduce the moments on the abutment backwall.

The forest service will likely require something of a wingwall to make the transition from the roadway elevation behind the abutment to the creek bank elevation in front. The wingwalls will just need to somewhat bigger for the larger change in elevation.

 

[oldestguy]

I'd refer also to the state highway design manuals or even call them for advice. Local contractors would be familiar with state standards, etc. A local geotechnical engineer also might be quite helpful. I'd want the embankment resistant to flooding erosive effects. You may even want to rip-rap it to prevent erosion. No point in losing a bridge for having not been prepared for flood flows. Not just any old rocks for riprap. It has to account for flood flow velocity. Is your proposed embankment(s) going to restrict flood flows and cause some problems there (raised flood elevation? fast flow?) An engineer experienced in these hydrology matters probably can be found by asking the DOT for some references they use.