Lifting force needed for a bucket of saturated soil underwater?

[Martin.H]

I'm not a geotechnical engineer so just wanted to check my thinking:

If a soil has a saturated density of 22kN/m³, then what force would be needed to lift a 1m³ bucket full of this soil off the sea bed (assuming the bucket is weightless and has no 2volume)?

Assuming a sea water density of 10kN/m³, my calculation is that the force needed is 22 - 10 = 12kN (aka the buoyant density of the soil?). And once the bucket is out of the water, it will weight 22kN/m³.

Is that right?!


Martin.

 

[LittleInch]

Ignoring drag forces or suction forces from mud etc, in a lift sense that looks correct.

As the bucket emerges then it gradually goes from 12kn/m3 to 22kN/m3.

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[Martin.H]

Thanks for confirming LittleInch.

Is quantifying the effects of suction simple, by any chance?

Martin.

 

[LittleInch]

Not my area of expertise but seems to depend a lot on the "stickiness" of the mud.

Mud anchors rely on it to work so look up that design.

Something like this?

https://iopscience.iop.org/article/10.1088/1757-899X/1094/1/012048/pdf

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[Jack Walter]

Hey Little Inch,

I am guessing this is for a dredging project? The dynamic forces acting on a bucket from the operator banging it against the bottom will most likely be more than the static lifting forces calculated. I would be intreated to know the context of this one.

 

[LittleInch]

I'm not the OP....

Ask Martin.

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[Martin.H]

The actual application is mooring piles for a houseboat that, rather than being driven in to the ground are to be fixed to a ballast box, dug into the river bed and filled with the river bed arisings. The area of the river bed is tidal but usually dry and only flooding a few times a month, so excavation is feasible, and experience of the ground suggests that for small excavation depths (1m) the cut faces will hold up.

My original question was a bit abstract as I was just trying to get a handle on what the effective weight of the ballast would be when submerged with a head of water.

As I type this, I now wonder if the hydrostatic pressure on the underside of the ballast box will reduce it's effective weight further, or as the box it open topped and the ballast permeable (I'm assuming), the hydrostatic pressure on the inside face of the ballast box base will offset it (I this so). And what if the fill has low permeability and has dried out a bit, is there a temporary buoyancy as water rises and the time it takes to fill to fully re-saturate? I have realised I am out of my depth and have guided the client to seek advice from someone suitably experienced!

Thanks for input, and I'd still be interested to hear more from anyone with useful input or ideas - I've shared the link to this thread with the client so he would appreciate it too I'm sure.

Martin.

 

[LittleInch]

So basically a gravity anchor?

Not sure how this will work when buried or located in the river bed.

Screw anchors might be better?
Or just a big lump of concrete?

There's a reason why piles are, well, piled into the ground. They work, they are very strong and only need a small amount of river bed.

The forces you will get could be substantial and hence the size of the box also very large to prevent movement / dragging etc when you add in a FoS of 2 or more.

I think you need local professional help here and not scratching around on the internet for ideas.

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[Martin.H]

I agree with everything you say LittleInch.

This is essentially a self-build project so the owner/builder is keen to explore ideas, even if a bit novel, to find a 'cost effective' solution that works with the access difficulties and geology (relatively thin layer of riverbed silt over chalk). For what it's worth my first and persistent advice has been piles.

Martin.

 

[LittleInch]

It won't be "cost effective" when his home suddenly starts moving or drifting downstream in a storm....

I would stick to offering friendly conversation rather than encouraging any wild and wonderful 'cost effective' ideas....

Stick with the persistent advice IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.