Surcharge on CLSM/Flowable Fill Backfill

[pokeng]

What is standard practice for determining lateral surcharge effects through a CLSM/flowable fill backfill? Many references indicate the surcharge loads can be reduced or even eliminated, but no one seems stick their neck out and indicate how much reduction is permitted.

 

[DBNodurf]

WHAT??!! Obviously, when the fill is first placed, it will induce the same pressures laterally and vertically as a fluid, since it is one... when it sets, these pressures are locked in place.

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[gandersen]

Pokeng and DBNodurf:

This is an interesting question. I would assume that what you are referring to is having used CLSM as a backfill material behind a retaining wall and then needing the backfill to support a surcharge load. I agree with DBNodurf that when you place the CLSM there will be lateral forces exerted on the wall. These forces would be due to the fluid pressure of the CLSM with a unit weight similar to concrete. I would expect that as the CLSM hardens, there would be a decrease in the magnitude of the lateral force if there is any shrinkage. In fact, it may go to near zero depending upon the geometry of the backfill plug of CLSM. If it is very wide top to bottom and there is enough sliding frictional resistance and overturning capacity of the CLSM plug to retain the soils behind it, then it would most likely function as a gravity retaining wall by itself.

The actual magnitude of the lateral force between the hardened CLSM plug and the retaining wall may be difficult to predict based on some of the issues above.

In terms of the effect that a surcharge may have, so long as the compressive strength of the CLSM plug is not exceeded, I would assume that after it sets up, the CLSM would act as a load transfer element and carry all of the surcharge load to its base. If the base of the plug is at or below the bottom of the retaining wall, I would assume that the surcharge would have very little impact on the lateral forces acting on the wall.

 

[Focht3]

Be careful here - the CLSM (Controlled Low Strength Material) "mass" may have vertical cracks, horizontal laminations, weak pockets, etc. - it ain't reinforced concrete, ASTM test procedures notwithstanding. This is a very broad category of materials.

If you need for the CLSM mass to act as a single unit and transfer anything other than trivial loads, it must be reinforced. This requires actual engineering design, not an owner's/contractor's prayer. My preference is to use geogrids -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[pokeng]

Thank you all for your replies.

DBNodurf,

Yes, I am aware that CLSM will exert the same pressures horizontally and vertically as a fluid during placement. However, I never intended for the _surcharge_ load to be applied while the CLSM is in a plastic state.

I am looking for the lateral loads applied to a wall when the CLSM has set up and a vertical surcharge load is applied to directly to the CLSM mass.

gandersen,

I was thinking along the same lines. A reference of some type confirming this would make me feel better about the design.

Focht3,

I don't need or necessarily want the CLSM unit to act as one. I think any criteria for lateral loads due to surcharge of CLSM would account for the likelihood of cracks, laminations and pockets. But then, this criteria would then likely need to be complicated to account for variances in different types of CLSM and make our lives even more difficult (sigh).

 

[gandersen]

pokeng:

I am not sure if I have a reference to the approach that I outlined. It appears to me that the problem will be driven by the magnitude of the surcharge that you are planning to apply and how fractured you expect the CLSM to become under the applied load.

You might be able to model your problem like a fractured rock mass and estimate what magnitude of lateral force you would need for a certain factor of safety. Two references that I use for rock mechanics are Introduction to Rock Mechanics by Richard E. Goodman (you may want to look at Chapter 8), and Rock Slope Engineering by Hoek and Bray (you may want to look at Chapters 7-9).

Clearly, the more fractured you make the CLSM, the more lateral force you will have to support by the wall. You also want to include the effect of water behind the wall.

If you decide on reasonable strength parameters, you could use off-the-shelf slope stability programs and put lateral forces (in place of the retaining wall) to increase the factor of safety to what you consider to be a reasonable value.

I hope this helps.

 

[ishvaaag]

Certainly it is expected of cemented foundations to be at least able to pass the loads vertically. This has always been the traditional understanding -and limit- of, say, well foundations made of poor concrete made with big chunks of stone: for sure the compressive loads should be taken with no problem. If the intent is the same with the flowable backfill, once hardened, it should show such behaviour (the ability to pass the vertical loads of the surcharge to one competent bottom) for in other case it wouldn't have the named property that serves to initiate this thread. If such is the case, and the backfill is so poor that this mechanical behaviour can't be reliably expected to take place, proper structural care of the surcharges needs be made as Focht3 points.