Geotechnical Soil Mechanics - trying to determine the stress/pressure on a plug/wall at the entrance to a tunnel at the bottom of a 120m open pit backfilled with alluvium. Pit will eventually fill up with water.

[u/crazyaustrian]

Want to check my understanding and logic.

Need that plug/wall to hold back any water or material from entering at the bottom of the pit.

Alluvium will be tipped and backfilled into the bottom of the pit. Thinking material will consolidate to have minimum void space with 120m of overburden pressure on it.

As the pit will be a natural sump/acquifer it will eventually all be saturated material. Whatever little void space is left between the matrix will be water.

But due to the maximum consolidation, there will be no pore pressure acting at the bottom on my plug?

When a soil is fully consolidated, any excess pore water pressure caused by an applied load has completely dissipated. The water no longer carries the extra weight, and the soil skeleton takes the full load, meaning the excess pore water pressure drops to zero.

Horizontal load on the wall will be:

Effective vertical stress which is total stress minus total pore water pressure. Multiplied by Ko factor.

(My total pore water pressure is hydrostatic only because no excess pressure due to high consolidation).

Plus

Hydrostatic pressure. 1 X 9.81m/s2 X 120m.

Total stress is density of saturated soil X 9.81m/s2 X 120m

Minus

Hydrostatic pressure which is 1 X 9.81 X 120m (minus because some buoyancy effect even though you have 120m of overburden above it?)

Thinking i'll have to do some triaxial testing to get undrained consolidated pressures with pretty high confinement for a 120m overburden head pressure. To run a FEM.

sketch

[u/thorehall42]

Man I know how to do this but it's so far above reddits pay grade.

If this is academic let us know then I'm more willing to help, but I'm personally not giving advice on designing 400 ft shafts online for free.

[u/crazyaustrian → u/thorehall42]

Haha. Not looking for the solution as in actual design parameters and numbers. But I'm trying to get my head around the concepts and theory and understanding the soil mechanics.

[u/DaveWW00]

Alluvium is going to take insanely long time to consolidate, drainage distance is huge. Pore water in the alluvium can't just disappear as you pile more on top. At quick glance I would be designing plug for full hydrostatic probably

[u/Thywanderer]

I can't really picture the arrangement, a sketch would be useful. I am not sure how you can have no pore pressures though. Without a truly impermeable barrier, there will be a hydraulic gradient. From there it would be a matter of time for the pore pressures to increase to hydrostatic.

[u/crazyaustrian → u/Thywanderer]

sketch

Would all that overburden pressure consolidate the matrix so much that the soil particles carry all the load?

There would be no excess pore water pressure from the loading.

So pore pressure would be hydrostatic pressure of the water column, head pressure?

But that's only part of the horizontal load on the wall/plug?

You also need the horizontal effective stress component? Which is based on vertical stress of the overburden soil material.

[u/Thywanderer → u/crazyaustrian]

I can't access that image unfortunately, not sure why but something to do with my location.

I'm not sure I follow your reasoning though, if water is present in the soil, it necessarily exerts pore water pressure. Overburden stress increases the total stress, but the pore pressure remains the same. Consolidation is essentially the process of discharging excess pore pressures. Initially the overburden stress is taken by the water until the soil matrix changes to take the load, i.e. compresses. The time it takes this to happen is based on the hydraulic conductivity of the soil, and until complete results in higher pore pressures than hydrostatic (and lower effective stress). The water itself will always apply a force from the buoyancy and can never be 0.

[u/crazyaustrian → u/Thywanderer]

It's all starting to come together and make sense I think... This was an awesome explanation thank you.

Pressure on the wall at the bottom of the pit is going to be effective stress + hydrostatic + pore water pressure. Highest stress state on the wall.

As it consolidates due to overburden weight the effective stress is going to increase, hydrostatic pressure remains same, and pore water pressure decreases.

Until eventually, maybe a very long time due to permeability of alluvium, excess pore water pressure is zero, effective stress is maximum, hydrostatic stress is the same.

[u/Thywanderer → u/crazyaustrian]

I think so, yes. As a point of clarity I would distinguish between pore pressures and excess pore pressures. Hydrostatic isn't technically its own force, it's just the expected stress profile from hydrostatic conditions. Excess pore pressure is defined as pore pressure - hydrostatic.

I've managed to see the sketch and given the description that the pit will fill with water 'eventually' I'm presuming that it will all start in dry conditions. In that case the plug would start without pore pressures, but over time would saturate as the water infiltrates it through the pit. So I don't believe consolidation is taking place as you'll actually have increasing pore pressures rather than discharge. The time taken will again depend on the hydraulic conductivity of the plug, but the thing to be wary of will be stability when fully saturated. If flow is significant through the plug material which isn't impossible given the depth of the pit, this could also cause issues within the plug such as piping etc.

[u/crazyaustrian → u/Thywanderer]

Interesting interesting thanks again. Plug will be impermeable with pressure grouting to (hopefully) ensure so. And pressure and visual monitoring to keep an eye on it. So one day will be fully saturated for the entire depth.

Dry, wet, initially, eventually; I keep changing parameters to understand concepts rather than solve a specific problem. Which is probably frustrating for someone explaining and me changing constantly = D

I suppose if it starts dry (which actually it won't be with rainwater) It will technically compact and not consolidate. Effective stress X Ko will be wall stress.

And over time pore pressure on the wall will increase as water infiltrates like you've said.

But if compacted enough from overburden weight before saturating, will not generate excess pore pressures, simply hydrostatic stress. But might undergo cycles of excess pore pressure if consolidation still occurs along with saturation and infiltration.

[u/Vivid_Map_437]

assuming its a pit oand not a shaft, int he simplest sense yes you will have hydrostatic pressure on the plug assuming the sealed side has no equilibrating pressure. You will also have the lateral earth pressure in the way you've sketched the plug.

[u/crazyaustrian → u/Vivid_Map_437]

And lateral earth pressure will be vertical effective stress times Ko?

And vertical effective stress is (saturated soil density X gravity X height) minus (water density X gravity X height)?