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CLSM Lateral Pressure

CLSM Lateral Pressure

We are planning to backfill behind abutment and wing walls of a new proposed bridge with at least 320 psi (preferably 600psi) foamed CLSM (unit weight of 65 pcf). After it is set, what kind of active pressure it can exert to the walls in dry state. We have considered 4 Ft lift height during construction and have already considered horizontal pressure from the fluid CLSM. Your help will be greatly appreciated. Thanks.

RE: CLSM Lateral Pressure

Zero! It is a cementitious material and would be capable of sustaining a vertical cut; therefore, no active pressure exerted on the wall. Its only pressure on the wall was prior to set and hydration.

RE: CLSM Lateral Pressure


Thanks a lot for your input. I agree with your view. However, there are people from an extremely reputed firm thinks otherwise. It will be great if I can present some evidence. Will you be able to get me some documents (research paper, guides, or past design performed on a live project, etc.). All I am getting over the internet is few old half done research paper.

Thanks once again.

RE: CLSM Lateral Pressure

well, I was going to say zero but I thought it was a trick question. then ron answered.

if it was a chunk of 3,000 psi concrete or solid bedrock behind the wall, would there be any pressure? What if you left an 1/8 inch gap between the back of the wall and the CLSM, would there be any pressure then? So what if you precast the concrete, then placed it behind the abutment wall, would that cause any pressure?

Does it transmit some force from earth behind it? Will it allow water to build behind the wall to generate full hydrostatic pressure?

RE: CLSM Lateral Pressure


FYI. There will be drainage board behind the wall and weep holes to avoid any hydrostatic build up. There should not be any gap between the flowable fill and the wall backface as the fluid CLSM should fill up any voids.

Thanks for your input.

RE: CLSM Lateral Pressure

i wasnt suggesting that you should do any of those things, just that it helps to visualize the fact that there is very little or no lateral pressure exerted by the solid chunk of concrete sitting behind the abutment wall.

RE: CLSM Lateral Pressure

I would love to have alternate thoughts if any. I truly appreciate your input. But I fail to see, like you have mentioned, how an equivalent fluid pressure in the order of magnitude of 30 PSF or more can develop when the CLSM is set.

RE: CLSM Lateral Pressure

it cant

RE: CLSM Lateral Pressure

I believe there will be no pressures as long as the CLSM fills the active zone behind the wall.

If someone believes there will be pressure from the CLSM, ask them to draw a load diagram explaining why there is load and the source of the load.

Mike Lambert

RE: CLSM Lateral Pressure

Thank you guys for your inputs. It will be a big help if you can provide any document(s) supporting your argument.

RE: CLSM Lateral Pressure


You are asking us to prove a negative, can't be done. You need to ask those who are stating that there will be pressure to provide a free body diagram showing the load. Then others can look at that diagram and determine where the errors/misunderstanding are.

Mike Lambert

RE: CLSM Lateral Pressure

Let me disagree. The CLSM won't put pressure on the wall if the soil behind the CLSM is sloped to a stable angle and the CLSM follows that slope. If the back end of the CLSM is vertical or placed against a steeply sloped ground surface, the ground will push laterally on the CLSM which will push on the wall.

See Page 6 of 8 for an example of what I have described.

RE: CLSM Lateral Pressure

the rebuttable presumption in designing a retaining wall is that risk exists on an vertical cut and some stabilizing influence is needed. So, if I was the owner, that's my reply to Mike's comment. I'm not suggesting that Mike is wrong, but when using innovation to omit a design concern, the obligation is on the designer.

I also agree that there is some chance that there is unbalanced earth pressure working on the back side of the CLSM. So is there sufficient shear strength to hold back the CLSM?


ípapß gordo ainÆt no madre flaca!

RE: CLSM Lateral Pressure

Lightweight fill such as EPS or CLSM can be used to reduce settlement at a structure and/or to reduce or eliminated lateral pressures that would act on a structure. If only for settlement control, the rear face of the material against the retained ground surface could be at any slope angle. If for reduction of lateral pressure, the ground surface between the fill and the original ground should be sloped. Whichever fill you install, it will be rigid. If retained soils push laterally on the rigid fill, the soil will push on the fill and the fill, therefore, will push on the structure. If the retained original ground is sloped, the lateral force will be reduced or be eliminated.

RE: CLSM Lateral Pressure

Hi F-D;

This is the fill which will be covered by an approach slab (a structural slab designed to span between the abutment and another approach structure on one side and between abut and a sleeper pad on the other side). The flowable fill below should not see any load from above the roadway. The fill will be contained in a 4 sided enclosure (45 ft by 30 ft approx.) behind one abutment. On the other side geotechnical straps will be provided to retain the soil beyond the limit of wingwalls. So, there should not be any material that will induce active pressure on the fill mass. Also, the grade on both approaches slopes down away from the abutments/bridge.

Say the minimum compressive strength of the 65 pcf flowable fill is 320 psi. The shear strength should be 2*fc^0.5 (assuming concrete type property), which will be around 36 psi. So, self weight of the fill (max height 15 ft approx.) is the only weight that will have to induce a shear stress (along the sliding plane from the wedge above) greater than 36 psi to initiate lateral movement. Is it going to happen? It seems like the factor of safety is too high for this lightweight material. Even if we assume that there will be some residual lateral pressure, what will be a reasonable value? 35 psf equivalent fluid pressure seems too high.

Thanks for all of your active participation in this discussion.


RE: CLSM Lateral Pressure

I agree that the material being "retained" by the CLSM will exert a load on the CLSM and that load "could" transmit to the wall. However, it all depends on the geometry and I expect that any load on the CLSM would be taken by shear along the base of the CLSM.

That said, I overstated in my previous post.

Really need a sketch to see what is really going on.

msrrr77 - you really need to have your geotech address this issue. I suggest that the first step would be a good sketch followed by a free body diagram to look at load paths.

Mike Lambert

RE: CLSM Lateral Pressure

I have used various foamed concrete and lightweight fills behind all sorts of retaining walls. They do exactly what one would expect and as described above. As noted above, the light weight reduces sliding and overturning resistance so external loadings need to be considered. On the good side, applied bearing pressures go down which is a typical reason for using such fills. Most do not drain so external drainage details have to be tended to. In large volumes, cracks can develop so some consideration should be given to adverse cracking and potential issues.

Keep in mind that a 320 psi compressive strength is a lot if you think of it in lbs/sq. ft. like soil cohesion (1000's of PSF) which does not take much to reduce earth pressures to nothing.

BTW, You might be able to make a case that an approach slab is not needed if the the entire zone behind an abutment will be filled with the material. Not sure how it would settle differentially without cracking. More chance of the settlement to be behind the lightweight concrete fill mass.

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