Footbridge foundations over railway cutting

[Llessur]

Hi,

I am trying to gather information regarding the construction of a footbridge over a 10m wide railway cutting.

The cutting runs through strong, fresh to slightly weathered granite with an overburden of up to 1m of boulder clay/till.

I am looking into the preliminary design of the foundations for this bridge but am having trouble finding much relevent literature about foundations for bridges across cuttings. I understand that the lack of confining pressure due to the presence of the cutting will influence the type of foundation required.

The bridge will be relatively small-scale (i.e. approx 10m across) but will be required to carry large groups (up to 50 people at a time) as well as the occasional tractor and other grounds maintenance machinery.

If anyone can give me any tips as to the various types of foundations which I might consider as well as any benefits/problems which might be encountered then I would be most grateful.

Many thanks for your help.

Llessur

 

[Focht3]

The easy answer: remove the overburden and bear directly on the granite. This is the best - and simplest - approach in the long run.

YES, you CAN bear on the mixed materials. But why bother?

 

[Llessur]

Thanks for your prompt reply!

Very true, but removal of the overburden was always my intention - my concern is due to the presence of the cutting itself.

This is the first time I have approached the study of bridge foundations and I can understand the construction of footings directly onto granite but what I need to know is how much the presence of the cutting will influence the design of the footings.

My biggest concern is the need to avoid initiation of failures of the cutting side-slopes in the vicinity of the bridge caused by the downward force excerted on the granite due to the weight of the footbridge.

Presumably, if the downward force excerted by the bridge becomes too great (due to the lack of a 'confining pressure' caused by the presence of the cutting), failure of the side wall will occur.

I am interested to know how much this will affect the strength of the foundations and if any alterations are required to the depth & type of foundations which will need to be used to take this into account. Is there a special type of footing which can be used in this situation?

Any info anyone can provide will be greatly appreciated.

 

[jheidt2543]

I should think that if you place your footbridge footings back far enough from the face of the cut, such that the influence line from from the footing does not intersect the cut line until the footing pressure is below the allowable on the rock face, you should be OK. If the cut were through soil, you would do the same thing, but with a lower allowable pressure.

 

[Focht3]

jheidt2543 is correct - move the bridge foundation back from the cut.

How far back? Well, that depends on a lot of things you haven't told us. Is the granite massive, or is it heavily jointed? Dip and strike of the joints? Any observed instability in older, similar cuts nearby? Dead loads? Live loads?

If the granite is massive with no significant secondary structure, then a ten meter setback may be more than sufficient. Then again, a twenty meter setback would probably be sufficient under most circumstances - unless you have a bad problem with adverse fractures/joints. These are fairly conservative suggestions - a more detailed study could well justify a setback of only a few meters. However, we are talking about a footbridge over a railway cut - I suspect this is in an urban area. Some conservatism seems advisable.

I'd suggest getting a qualified engineering geologist to look at the site and tell you what he sees before you make a final decision. (Hopefully you have one on staff.) This is a fast, relatively cheap way to help you get to an answer that you can be comfortable in choosing.