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I'm casting a 4m long triangular-profile reinforced gravity retaining wall against soil movement due to a 1 metre jump in ground level.

The lateral forces and moments on the formwork have been easy to handle, but I'm trying to find a good approach to the vertical uplift hydrostatic force on small-scale/DIY formwork while concrete is poured, which is a general problem for which I can't seem to find a good solution.

Details -

I'm using a very fluid self-consolidating (SCC) mix since access for compaction would be hard once poured (slump-flow ~ 550mm, full spec available if needed). The form has one vertical face and one face with a 2:1 slope; it's this second face that creates uplift forces. The actual height of the visible wall above ground is about 92 - 100 cm but I've dug down to about 1.2 - 1.4m to allow a "heel" and more mass under the "soil" side, and also because then it can be braced against slipping by the other concrete under the pathway itself. The wall has about 0.5 m2 cross-section area. I've gone for an "over engineered" approach - it's also being laterally braced at both ends by perpendicular concrete works, and will have 10mm (A393) welded mesh and 12+16mm rebar and "L" bends added along the length and at the corners. The form is lined with polythene for easy removal of the formwork. I'm indifferent to surface finish.

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I'm confident in the actual formwork - plywood backed by C16 2x4's (50x100 mm), which are in turn backed by 3x6's (70x150 mm) held together with heavy duty structural screws, 12mm s/steel threaded bar between the formwork timbers of the two faces, all the entire form ultimately being braced laterally against solid ground on both sides nearby.

My concern is countering the vertical uplift forces from the hydrostatic pressure of the fluid concrete when it's poured, which will be about 4 - 5 tons equivalent (40 - 50kN), ie the weight of the "missing" concrete above the slanted face of the form. Basically, I want it not to be lifted off the ground by the fluid concrete. I've thought of several possible approaches to offset this force, but the reality is that I'm just not sure which is best or most foolproof, or if there are standard solutions used in civil engineering that I'm unaware of.

Possible solutions + sketches I've come up with -

Possible solutions

  1. Reduce the footprint and hence uplift, by making the wall slimmer. Perhaps with strong lateral restraint, rebar, and concrete under an adjacent path, the usual 50% rule of thumb is excessive and I can reduce the width of the wall to ~ 40cm (and therefore the upward force by ~ 30-50%)?
  2. Allow the concrete to fill the full vertical space of the wall's footprint, removing the excess pour when set. Precisely zero hydrostatic pressure guaranteed as all concrete is at the same top level; the extra volume can be cast with plywood dividers so it comes away easily in sensible sized slices.
  3. Cast in 2 parts, using a smaller 1st cast steel-linked to the form, as a deadweight for a larger 2nd cast. Cast the bottom 1/4 only, but with steel rod embedded to link it to the form, and vertical rebar for the join. Then cast the top 3/4 about 48 hrs later. The bottom 1/4, attached to the form itself, acts as a deadweight to hold down the form against upward pressure when casting the narrower but taller top 3/4. Extra weight still needed but only 1/4 as much; hopefully the vertical rebar linking the pours will ensure long term integrity.
  4. Add a 'false floor' in and fixed to the sides of the formwork so the weight rests on the formwork not the ground; remove and underfill this space beneath the wall once the 1st pour is set. 5 tons of concrete along a 4m length can easily be supported on 80 side-by-side 2x4 (50x100mm) timber (weight per 50mm 'slice' about 62kg which is small for a 0.8m clear span). Because the 'floor' is itself supported by the formwork, there is no net uplift force - it's replaced by vertical tension in the formwork which is much easier to handle. After the form has set, remove alternate 50cm sections of these 2x4 timbers, and pour concrete into the void below, to properly support the wall. When that's set, remove the other half of the timbers and pour under the rest of the wall. Needs 3 pours but again, guaranteed to work since there is zero net uplift force and the weight spread across many 50mm slices is small for each slice. Also simple to set up.
  5. Brute force. The simple solution. Add about 5 tons of weight. For stability it's added to the heavy members at the base of the form, not on top. problem is that 5 tons is a heck of a lot to move.

How can I easiest ensure the formwork doesn't rise at the base and the pour escape, when it's poured?

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Solutions

Yes, this is a very real problem. In "professional" construction I have seen similar situations handled by either:

  1. Pouring in layers
  2. Placing the bucket of construction equipment on the form
  3. Tying (wire) the form down to something embedded in the ground (posts, columns, other concrete).

As an example, when we specify pouring concrete (or flowable fill) around a pipe culvert, we typically specify pouring layers of 6"-12" at a time along with holding the pipe down.

Personal Use

I would say for smaller scales, a combination of methods would be easiest. I would try to pour in thin layers if your concrete supplier can handle it. Also place whatever you can on the form to help weigh it down (concrete blocks, etc).

To pour in layers, you shouldn't have to wait 48hrs. The concrete should set enough sooner than that, but you still may not be able to do multiple pours in one day.

A Different Mix?

This might be easier if you use a mix of concrete that isn't so fluid (not SCC). You will have to vibrate the concrete and you will likely have more honey combing on the sloped side, but that side will be buried anyway.

Final Words

You are thinking about the right concerns, but there isn't really a simple solution. You may need to combine as many of your methods as you can reasonably do to be safe.

No matter what you do, be ready to stop pouring the instant that you see any movement in the form! You don't want to have to shovel a bunch of concrete by hand!

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  1. Pour your wall slowly, .5 meters an hour?, that way you wouldn't have the pour pushing full force. A lot of times folks will specify pour rate for this exact reason. It reduces the pressure and allows you to dial back your formwork needs a bit. put some bar in at the bottom of your pour - and attach ties to it holding down your formwork. Let the bottom set up a little bit (pour slowly to attain consolidation) and your ties will hold down your formwork.
  2. Fasten your formwork to the ground if you can.. Pour a mat of concrete or a footer that you can anchor your wall to? Again, pour in phases or stages.
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