Bailey Bridge on Timber Crib Abutments Filled with Large Stone

[canstruct12]

Hello,

I am a civil/structural engineer working on a site in Canada. We currently have a consultant designing a new bridge for us to cross an existing train track.

The consultant has provided a preliminary design which includes an ~23m bailey bridge passing over 2 railway tracks. This is a single lane bridge that will typically be used for emergencies or light vehicles. The bailey bridge sits on timber abutments (12m parallel to bridge, 7m perpendicular to the bridge and 8m high), constructed of 200mm x 200mm alternating direction timbers (leaving openings of 200mm between layers). The material inside the timber abutments is a rip-rap material consisting of stone 250mm to 400mm diameter as to not migrate through the openings. and common material to create approaches.

I have reservations about using timber for the abutments, as they are not as robust a material as piles, steel bins or MSE walls. I have not been able to find much information about it (between the Canadian Highway Bridge Design Code (CHBDC) and the FHWA literature), but have been told by our consultant and a 3rd party estimator this would be the cheapest option.
As well the use of rip-rap for inside the abutments is different than anything I have seen previously as fill material. Typically I would expect a common material and have any large stones adjacent to the structure removed, as not to create pressure points against the structure, as material settles shifts. Will the 250mm to 400mm diameter rip-rap material cause a similar pressure (active earth pressure) against the timber structure, same as material placed behind a retaining wall would?

Any information is welcome.

 

[miningman]

Is the site in permafrost ??? This would influence the suitability or otherwise of timber cribs , piles or MSE walls etc

 

[canstruct12]

@miningman

No, not in permafrost. Frost is out of the ground during the summer months

 

[miningman]

Frost being out of the ground in summer does not negate the presence of permafrost. Tell me your geographical area and I will tell you if PF is an issue

 

[PEinc]

How will the backwall be constructed, It may need to be about 39.5 inches higher than the elevation of the bearing seats. Upon what will the bearing plates sit? Wood? Rocks? Concrete crib cap? I would think a concrete abutment and backwall would be fast, cheap, and easy to construct.

http://www.PeirceEngineering.com

 

[canstruct12]

@miningman
Fermont, Quebec

@PEinc
The abutments are timber cribs. So all 4 sides are timber. The approaches will be common material to the back of the timber crib, with a layer of geo textile to stop fines migration from the common material into the rip-rap inside the abutment.
Bridge bearings will site on a solid step of wood on the abutment. Supplier recommends a steel plate under the bearings on top of the wood.

 

[miningman]

I would expect at a minimum, localized areas of permafrost , especially on north facing slopes. Be very carefull with engineering assumptions in such areas.

 

[dik]

That's hardly northern Canada... Crib structures like that are not uncommon... Baily bridges have a fair amount of ductility and can accommodate some movement... I'd be a little more curious about the bearing conditions other than the crib type of construction. To clear a train, the crib structure would have to be fairly high, and stability may be an issue.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[canstruct12]

@dik
Changed it to just Canada. You're right, it is not that Northern.

~8m high. Geotechnical report gives a 300kPa SLS bearing capacity, with 1 inch settlement and 1/2 inch differential settlement.

 

[PEinc]

Abutments still need backwalls to resist surcharges from traffic immediately behind the abutments.

http://www.PeirceEngineering.com

 

[cvg]

one layer of geotextile between the common fill and the rock leave a significant potential failure mode if you get a rip in the fabric. and given that it is sitting on top of the rock, I would expect it to rip sooner rather than later.

 

[STrctPono]

canstruct12, from the sounds of the design (Bailey Bridge Superstructure and Timber Abutments) this must be a remote location. A Bailey Bridge doesn't require a crane to erect since you can just launch it from one side so perhaps getting concrete trucks and heavy machinery back there is a major consideration. I'm guessing that the nearest concrete plant is far away? If so, then you may be stuck with what you got. Sure, they can always set up a mobile concrete batch plant but that is expensive. They could also precast the Concrete abutments but at 8m high that is probably not possible to lift. Concrete Abutments would be the most durable option but perhaps that doesn't fit within the construction budget and the consultant knows this. I have never designed timber abutments since the agency we consult for would never allow it but I can see a time and a place for them (if designed properly). Are you more worried about whether or not the consultant is going to design them properly or questioning their long term durability?

One solution is a GRS or MSE abutment with a stone gabion facing or Keystone block facing. That would be cheap and would not require specialized equipment but would require your consultant to know how to design a GRS abutment. It's still (relatively) new technology that not everyone is willing to design.

 

[dik]

I was joking...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik