Approach Slab Omission from Bridge 2

[sponcyv]

We have a bridge located inside a county park and will have very low daily traffic. The G.C. just got bids for the bridge and they are higher than expected. We are looking at ways to cut costs. One idea is to remove the approach slabs, which are a big cost item for this small single span bridge. Does anyone have a good resource showing what type of subgrade should be placed under the asphalt at the bridge interface to help prevent settlement between the asphalt surface and the bridge?

Thanks!

 

[JAE]

Solid concrete 20 feet deep?

I don't think you can ever avoid settlement completely. This would become an on-going maintenance cost vs. the up-front one time cost of the approach slab.

The amount of traffic wouldn't change anything - the settlement occurs due to the build-up of the grade at the bridge end...i.e. the soil self-weight creates long term settlements.

 

[JennyNakamura]

I'm sure you have, but just a friendly reminder to check with the Owner. Most counties follow state DOT guidelines for bridge design, and many states require approach slabs now days regardless of projected ADT.

 

[Qshake]

Some states have taken to elminate expensive approach slabs on low volume roads. They'll use asphalt and when settlement occurs it can be repaved easily.

Other options are to preload the fills to accelerate any settlement that may be expected thereby minimizing.

Another option that is becoming frequent is to use a slab on grade as the approach slab rather than a one slab supprted on the pavement notch (abutment) and sleeper slab. This reduces the amount of reinforcing greatly.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[sponcyv]

Qshake,

Do you have any links with any white paper or other write-ups on this? I want to be able to provide the owner with examples that other DOT's are doing this.

Thanks!

 

[Qshake]

sponcyv - Not really just do a search for a variety of DOTs and view their policies or standards.

Here's an example result I found...

http://epg.modot.org/index.php?title=Category:503_Bridge_Approach_Slabs

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[sponcyv]

Thanks Qshake - I have found a few things, but figured I'd ask.

 

[ACtrafficengr]

I recently saw an article on using controlled low strength material (AKA flowable fill, K-crete) for utility trenches to control settlement.

Would that be a possible replacement for structural fill in some applications? I'm thinking it might be more suited for stub abutments than high abutments due to the need to contain it, higher cost than structural fill, etc.

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[huntley101888]

My firm has encountered some joint issues with County bridges not having approach slabs. One particular bridge had a concrete deck with no approach slab. They had a 1'-6" wide backwall that they brought up to the road grade in order to embed armoring angles for the joint. As the road settled, the backwall was repeatedly impacted by snow plows, and eventually grabbed the armoring angle, which caused a lot of damage to the plow. Our recommendation was to create "mini" approach slabs that were 4'-6" long (see attached detail). This didn't help with the settling, but it, at the very least, moved the impacts away from the joint. I haven't been back to the bridge to see if the slab has cracked above the back of the backwall.