CLSM Passive Pressure for Integral Abutments

[OSUCivlEng]

Oklahoma has been using CLSM (flowable fill) as a backfill behind integral abutments for a while now. I just ran some numbers and am getting a passive pressure of 5 times that of granular backfill. I'm using a cohesion value of half of the compressive strength. I assumed a compressive strength of 150 psi.

Has anyone else ever looked into this?

 

[IDS]

I haven't looked into it, but a pressure increase of 5x seems reasonable for a cement stabilised fill, although obviously it would depend on the mix. A compressive strength of 150 psi seems very low, and what Young's Modulus did you use?

By the way, if you enter the full name, rather than an abbreviation, it saves people looking it up.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[BridgeEI]

90pcf for wet/fluid condition and 75psf for a cured condition. One of the local professors came up with those values for research they did but I don't know if it was ever published.

 

[bridgebuster]

He probably meant to say "unconfined compressive strength"

5x is interesting. I have a very unusual retaining wall project under construction where I'm using CLSM for backfill because I need to minimize active pressure. I'm not relying on any passive pressure but it's good to know it's very high.

 

[OSUCivlEng]

Yes I mean unconfined compressive strength. 5 times was based on one research paper I found where they assumed a friction angle of zero and a cohesion value of one half the unconfined compressive strength. Their predicted value for passive pressure was within 10% of what they measured.

Bridge EI, 90 pcf for a wet condition sounds about right. Do you mean 75 pcf for the in place unit weight/density?

FHWA says the friction angle is around 20 to 30 degrees with a cohesion value of 30% of the unconfined compressive strength.

https://www.fhwa.dot.gov/publications/research/infrastructure/structures/97148/app6.cfm

"Once set, the fill has very high passive pressure resistance since it is essentially a concrete product. For this reason, it should not be used behind integral abutments where abutment movement could generate large passive pressures against the fill." Go figure.

https://www.fhwa.dot.gov/bridge/prefab/if09010/03a.cfm

NCHRP Report 597 reported measured friction angles of 30 to 40 degrees and cohesion values of 835 psf to 7200 psf depending on the mix design.

Either way it seems like a poor choice to put behind an integral abutment. Granular backfill makes more sense to me.

 

[BridgeEI]

75psf applied as a uniform distributed load in the dry condition. Fluid has the pcf units since it's dependent upon the height. This was determined the from the research paper and the logic is that once it cured, the CLSM actually shrunk and wasn't in contact with the backwall until your embankment pushed it or the bridge moved.

The CLSM around here isn't very well cemented. You can run your finger along it and leave marks well after it's cured. I have a hard time imagining it creates high pressures since it crumbles so easily so to me it would resemble a more compacted granular backfill. Other parts of the country may have something that's more dense/solid and warrants higher pressures.

 

[OSUCivlEng]

BridgeEI is that what you use for design? I remember you saying you work in Oklahoma, we probably have met. I wish there was a way to send a private message on here.

I only have an expansion length of 114.5', so it's borderline if there's enough movement to develop the passive pressure.

 

[BridgeEI]

That's what I use as design values and the latest to my knowledge as directed by ODOT. These values have been around for quite some time (several decades).

 

[OSUCivlEng]

It would be nice if ODOT had a document where these design assumptions/criteria were stated. I have found they are willing to share with you if you ask.

I'm fairly certain that 75 psf is the active pressure against the wall. If the integral abutment starts moving into the CLSM, the passive pressures generated would be much higher than 75 psf or 0.5 psi.