Lateral pressure from CLSM fill material 5

[Awre]

Hello,

I'm analyzing a bulkhead that will retain CLSM back fill.
I understand that CLSM has two stages; flowable and hardened.

What is the angle of shear resistance, phi (?) and the cohesion that is best to represent this material in both of the stages above?

Thanks

 

[fattdad]

is there some reason that you don't explain the meaning of clsm?

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

"controlled low strength material" aka "flowable fill" aka "lean concrete". Around here, standard specs for CLSM range from 1/2 to 1-1/2 sack mix with 6 - 8 inch slump. While it is used as a soil replacement for backfill, it behaves more like concrete.

 

[Awre]

The CLSM is an engineered self-levelling and self compacting soil that is mix of sand, cement and fly ash.

http://www.cement.org/basics/concreteproducts_clsm.asp

Also:

http://www.osti.gov/bridge/purl.cov...4BA4652DB2EA?purl=/505263-DMiGgX/webviewable/

and click on the pdf icon

I need to estimate the lateral pressure in both stages (flowable & hardened). What are the C & phi that best represent this material in both stages. I don't think it's hard as concrete, and may still have a lateral pressure component. Thanks

 

[dgillette]

I wonder whether C, phi, and Ka or Ko are the right model to use. I have heretofore assumed that CLSM pressures (prior to curing) had to be treated as if it were a fluid (>110 pcf?), especially if gets a little vibration. Is that not correct?

Once it sets, it would seem to me that it's out of the range of typical soil mechanics, and would not need any lateral confinement to keep it in place (unless the fill is so high that its shear strength comes into play, or the CLSM wants to swell against rigid boundaries). Either way, the flowable stage would, I believe, govern the lateral support requirements.

How would you test that? Maybe start by assuming an unconfined compressive strength of what, 100 psi? This gives a shear strength of roughly 50 psi or 7200 psf (conservatively assuming a flat strength envelope). If you were to analyzed a vertical cut in material with that strength, you would probably find that it's stable to a pretty good height. If it is, then the lateral pressure would be pretty small, unless the CLSM tries to swell against rigid boundaries, or something like that.

Bon chance!
DRG

 

[Awre]

Thanks everybody!

I didn't see the hardened CLSM before, but I was told that it's something between the sand and weathered rock (in terms of the properties), and it may slum if you remove the side barrier (?). If this is the case, I think a corresponding Ka, and C may be a valid model for lateral pressure. (what do you think?)

I came across the following paper, titles "Strength properties of fly ash based controlled low strength materials ". Please read the abstract at:

http://www.sciencedirect.com/scienc...serid=10&md5=d010ccc3a9c709ec31d98169ad26cffd

It's not free though!.

If the above information are correct and by taking everything together, do you still think that phi, ka and c are not the correct model to use? thanks

 

[cvg]

CLSM is a very broad term and if your mix has a very low cement content and/or poor quality aggregate, perhaps you will end up with sand. However, most CLSM that I am familiar with is readymix variety using concrete sand, water, portland cement and fly ash. It comes in compressive strengths up to about 500 psi and definitely resembles concrete or rock more than sand when it cures.

Both the PCA (Portland Cement Assoc) and NRMA (National Ready Mix Assoc)have information online.

 

[dgillette]

From adfo's second link: "A major consideration in using highly flowable CLSM is the hydrostatic pressure it exerts. Where fluid pressure is a concern, CLSM may be placed in lifts, with each lift being allowed to harden before placement of the next lift."

This supports my expectation that the containment needs to be designed for something like full fluid pressure. Force = 1/2 * 110 pcf * H^2. I suppose you could do an active-pressure analysis with c and phi for the intact material, which would tell you that active pressure is zero unless the fill is quite high or the CLSM quite weak.

The first of adfo's links says 200 psi is a typical value with much higher ones being possible. If it's 200 psi once cured, it ain't goin' nowhere under its own weight.

 

[TDAA]

How wide is the fill going to be?

If it is thin, I would ignore for the hardened state, and us the information of the retained material.

Flowable will depend on the mix to some degree.

 

[Awre]

Thanks dgillette,

The lifts idea is what I did. I assumed that the flowable material may be viscous (not pure fluid, although it may worth to look at this case "more conservative")

This is what I analysed:
For flowable case:
? dry= 90 pcf
c=100 psf
?= 32 deg

For hardened case:
? dry= 90 pcf
c=200 psf
?= 34 deg

I think the above figures are on the conservative side, any comment?

Also TDAA, has an interesting point, which I occasionally come across and think about, regarding evaluating the thickness of the fill. Is there any guidelines or rule of thumb to determine when fill is considered thick enough to account for and when is thin to use of the retained materials behind it?

Thanks again

 

[dgillette]

No and yes. I think your flowable case is significantly unconservative - it's fluid and flowable, and you're proposing a strength like that for a lightly compacted granular fill. On the other hand, the strength you propose for the cured case is very conservative if the CLSM has any significant cement content. 34 degrees and 200 psf is reasonable for SM with a moderate amount of compaction (say 95%). 200 PSI, not PSF, was cited as typical compressive strength in one of those links.

Looked at another way, the 32-degree friction angle might be OK for an effective-stress envelope, ACCOUNTING FOR THE PORE PRESSURE in the CLSM during placement and flow. However, the flowable slurry would have a pore-pressure ratio near 100%, which puts you back to treating it as essentially fluid.

DRG

 

[TDAA]

Look at how a failure wedge would come back from the heel of the wall. How much of that failure plane passes through the CLSM? If the plane passes essentially only through the soil behind the wall, that is the material to analyze.

While I do not advocate being overly conservative all the time (we are paid to engineer) I think that there becomes a point when trying to include every detail to try and shave cost is counter productive for the average project. Get a big enough project, then it could add up.

BTW, for comparison to the numbers posted above, can someone say what are the typical values used for fresh concrete in form walls? Say normal weight, 4-6" slump.

 

[dgillette]

Here's one on self-consolidating concrete:

http://www.nrmca.org/research_engineering/Documents/CIF-July-Aug-09-Formwork-Pressure.pdf

If I understand their experiment correctly, they placed SCC in a tall form, a little bit at a time. For the first 5 meters / first half hour, the pressure was just short of hydrostatic, then it began to diverge, becoming significantly less than hydrostatic. (Figs 3-5) I believe that's a result of the slow placement, which allows some curing to take place before the last of the SCC was in the form. If you dumped in the full 12 m all at once, you would probably see something close to hydrostatic.

Fig. 5 shows a decrease in pressure with time after placement was complete.

USBR Concrete Manual recommends using the full 150 pcf for vibrated concrete.

 

[dgillette]

Correction: The USBR manual says "While concrete is being internally vibrated, the pressure against the forms IN THE IMMEDIATE AREA OF THE VIBRATOR approaches the full pressure of a liqued weighing about 150 pounds per cubic foot. [emphasis added] Lateral pressure of concrete is incluenced by many variables...rate of placement, termperature of concrete, and effect of consoiidation by vibration."

There is also a plot that shows pressure vs depth for two different cases of temperature and placement rate. For 6-foot depth per hour at 50 F, the slope is about 111 lb/ft^3. For slower and warmer, (2 ft/hr, 70 F) it's more like 83. Then, it drops off with time during cure.

 

[TDAA]

Thanks dgillette,

So using a similar approach, adfo could be using the wet density value of his CLSM, and to be conservative, analyze it as a liquid. If anything, that would give him a good point to compare to as a maximum value.

 

[Awre]

Thanks all for the great help! I got some good points from all your feedbacks. Thanks again.