Small creek bridge abutment design?

[Slackdaddy]

I figured I would start a new thread for how to design/build an abutment,, as my other thread was about beam sizing.

So, my creek runs through a bottom that is about 100 yards wide and absolutely flat.

The soil is silty clay,, problem is when I put in a foot bridge 10 years ago, I concreted the post in about 1.5' back from the bank,, but over the years the bank has washed away, exposing the footer.

So, I will be putting my Tractor bridge in upstream about 100 yards,
The 2x4 in the picture is 12 feet long,, and has 1 and 2 foot marks on each end (black lines)
The water is somewhat low for this time of the year, but gets a few inches lower mid/end of summer.
One summer it did dryout,, and most spring/summer it over flows it's banks once or twice, putting a foot or so of water across the whole bottom.

Any Ideas on Abutments? I am very skilled in Masonry and structural concrete.
I was thinking trying to dig down even with the bottom of the stream (even with the top of the bank) and pouring a footer about as high as the water is now,, (this would be easier if the water goes down come late summer.)
Then forming and pouring a abutment wall on the footer.

Other idea was to get some used "sheet Piling" about 4' long and driving it down just below the lowest part of the stream bed, digging out behind it about 2' back, and back filling with concrete and stone.
No mater what, I want the abutment to abutment distance to be no tighter than the top of the stream bank,, I do not want it to be a constriction when the bottom floods.

Any ideas or words of wisdom??

 

[Streetcar]

Can you drill piers below creek level

 

[2manyrocks]

Is there bedrock below the water or just more dirt?

 

[Slackdaddy]

Is there bedrock below the water or just more dirt?

No bedrock, nothing but clay, silt and soil in this area.
You may find a stray rock 2-6" dia on a rare occasion.

 

[Slackdaddy]

Can you drill piers below creek level

Hmm, drilling them would not be a problem.
Depending how close to the creek, they may fill with water.
That "should" not be a problem come late summer.

Hoping to get my abutments (or piers ?) about where the top of the stream bank is,, that is ~8 foot span,, I could easily span that with 5" High I beams, and transporting them would be no problem.

 

[2manyrocks]

No bedrock, nothing but clay, silt and soil in this area.
You may find a stray rock 2-6" dia on a rare occasion.

Our bridge is sitting on the limestone bedrock in our creek. Not really knowing how to make this work on pure dirt, I'm going to shut up and hope someone who actually knows what they are doing will offer some suggestions.

 

[dodge man]

I’ve done construction staking on highway and a couple of railroad bridges. There is a couple of different ways it’s done. The most common way is piles driven down. This are usually steel I beams. Then a concrete abutment or pier is formed on top.

The other method is just a concrete spread footing. It usually ends up pretty large for highway bridge but the one I remember wasn’t that deep. I’m sure it was below frost depth.

Some kind of concrete is probably the only practical way for you unless you have access to some heavy equipment. It should probably be deeper than you are planning but unless you plan on building some kind of coffer dam. On bigger bridges they also have a lot of rebar in them, once again I’m not sure it’s needed for you.

I‘m not sure just a chunk of concrete behind the creek bank won’t work for you and be the simplest. If you have a backhoe it can be deeper than the creek, just dumping concrete in a wet hole won’t hurt anything.

 

[Streetcar]

Hmm, drilling them would not be a problem.
Depending how close to the creek, they may fill with water.
That "should" not be a problem come late summer.

Hoping to get my abutments (or piers ?) about where the top of the stream bank is,, that is ~8 foot span,, I could easily span that with 5" High I beams, and transporting them would be no problem.

Drill hole 12 in or more in diameter . Go below existing creek bottom. One hole at each end of abutment
put rebar in hole, rebar should extend into your abutment
fill hole with concrete, pour abutment on top of piers with rebar in abutment

 

[EddieWalker]

If it was me, I would dig out the footing in the shape of a T. I would want the long part of the T to come back from the creek at least as far as the T is wide, but probably twice that distance would be ideal. Depth would be two feet, but I don't have to deal with freezing temps. If freezing is an issue, maybe drill down with an auger to get extra depth at each end of the T.

For retaining walls, I believe this is called a Deadman.

 

[Streetcar]

You want to be at least a foot back from existing creek bed with your piers. The pedestrian bridge is failing from scour. Having new bridge footing below the creek bed helps protect against scour