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Lateral Load with Slurry Backfill

Lateral Load with Slurry Backfill

Lateral Load with Slurry Backfill

For the figure below, what is the end of construction horiz shear and moment at the base of the stiff cantilever wall?
The 90 pcf slurry (UCC = 50psi to 300psi) will be placed in 2 lifts at 2.25 ft tall per lift.
The top engr fill unit weight will be no more than 120 pcf, and will be lightly compacted.

RE: Lateral Load with Slurry Backfill

Assuming the second lift is placed after the first lift has hardened, the pressure on the wall is the lateral pressure from the slurry at the time of placement. Once the slurry is hardened, the lateral pressure depends on its deformability that may cause a permanent strain/stress on the wall. Upon completion, the silty sand fill will exert a pressure as usually assumed, which is in addition to the permanent stress caused by the slurry fill, if any. I could be wrong, let's see what the geotechnical/other experienced engineer engineer will say.

RE: Lateral Load with Slurry Backfill

It should be fairly straightforward, as the sum of loads exerted by the 3 layers individually, as if they were each placed at the bottom of the wall. The moment arm for each is different, applied for each layer at 1/3 of the height of that layer, measured from the bottom of the slurry.

I would suggest using the at-rest earth pressure (ko) for the engineered fill.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Lateral Load with Slurry Backfill

How does the water get out of the drain board and out from behind the wall?
Check that the "native weak silt" will remain stable when excavated to the indicated 45 degree angle, before the drain board and slurry are placed?
Check if the 45 degree slope is in accordance with any required excavation standards (OSHA?).
Are you using 90 pcf or 150 pcf for the cement slurry?


RE: Lateral Load with Slurry Backfill

Thanks for the helpful responses. I agree with all the comments above.
PEInc: good questions. My answers:
- What I did not mention in the post is that I expect the slope will remain stable primarily because I have seen the insitu soil, and because it has remained stable for many months, wet and dry, and because I expect a traditional coulomb wedge to be 45 deg + phi/2.
- The slurry fill is a high water, very high air entrainment, lightweight aggregate slurry mix.

One variant: If I place a temporary sheet of hardboard (say, 1/2" thick plywood placed vertically and in contract with the drain board), then remove it after after the slurry has hardened, then fill the void with fine sand and add the top engr fill, I would say that 99% of the total lateral is just the top 1.5' engr fill soil. Unlikely and unconventional, but a consideration if my motivation is to get the lateral loads as close to zero as possible. Agree?

RE: Lateral Load with Slurry Backfill

Sounds like a lot of work and cost for a 6' high, cantilevered wall.


RE: Lateral Load with Slurry Backfill

I doubt the plywood board will do any good, as it passes the lateral load the same as it does not exist. Crushable foam board might work though.

RE: Lateral Load with Slurry Backfill

Good luck pulling out a sheet of plywood after flowable fill has pushed against the plywood and has hardened and bonded to the plywood.


RE: Lateral Load with Slurry Backfill

Gents - thanks again.
And yes, the last variant shall be stricken from the record.

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