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Retaining Wall: Saturated Soil & Hydrostatic Pressure

Retaining Wall: Saturated Soil & Hydrostatic Pressure

(OP)
I understand that Hydrostatic pressure and Soil pressure exist cocurrently. I am curous if there are any additional variables at work when calcuating total pressure on a retaining wall.

Full disclaimer: I am not an engineer nor do I pretend to be one. I recognize that these are very basic questions, but I am at the early stages of my understaing. Please be assured I am not and will not attempt to engineer any actual structures.

  1. Does the pore size limit, in any way, the surface area available for hydrostatic loading on the back of a retaining wall? I assume it does not. It would stand to reason that water under pressure would suspend the soil particles and push its way to the point where most of the surface area on the back of the wall was under a hydrostatic load?
  2. Soil particules become suspended in water while a hydrostaic load is present. Does this (or any other water related factor) affect the overall soil load or does it remain the same?
  3. Is the total load on the wall simply (1/2 * H² * γ * Ka) + (1/2 * H² * 62.43) ? (Where γ = the soil density)?

RE: Retaining Wall: Saturated Soil & Hydrostatic Pressure

Consider the soil as marbles. No way would water "suspend" them unless they weigh less unit density than water, in which case they float to the top surface. Use their submerged unit density and internal friction and cohesion in finding the lateral pressure they add to that of water on the wall. Don' get into a rut where somehow the area of water held back has any influence on wall loads. One inch of water width against the wall exerts the same as 10 feet width.

RE: Retaining Wall: Saturated Soil & Hydrostatic Pressure

(OP)
Thanks for the reply.

The marbles analogy helps. I suppose I was thinking about liquefaction, which would only occur during an earthquake or similar event.

So, this goes back to water pressure; something I'm ashamed to admit I struggle to understand.

I keep imagining an L shaped PVC pipe, with the bottom of the L against the bottom of my wall (so it forms a seal). When it's fill to the top with water it would only be exerting a small force on the wall, equal the pressure times the surface area of the pipe opening.

For some reason I assumed the opening between our marbles would by analogous to many L shapes PVC pipes pressed against the wall. There would have gaps in our surface area where the marbles touch the wall.

This would exert a lot of pressure, but not the same amount as if the entire wall was in contact with a volume of water behind it.

I feel that this is wrong, and i'm struggling to figure out why. Any help correcting this picture would be much appreciated. I feel bad re-hashing such elementary subjects but I know if I don't have a firm grasp of the basics I wont be able to progress.

RE: Retaining Wall: Saturated Soil & Hydrostatic Pressure

The chunk of earth stuck against the wall may not have any water between it and the wall, but on the other surfaces of the earth chunk there is water pressure, transferring that to the stuck surface, as if there was no earth against the wall. Result the water pressure part of the loads covers the whole wetted surface. Then of course comes the submerged earth part in addition.

RE: Retaining Wall: Saturated Soil & Hydrostatic Pressure

(OP)
Okay, so my analogy breaks because the PVC as described, can't transfer pressure like soil can.

So if I had a 4 foot high retaining wall with a active soil pressure (Pa) of 830lbs per linear foot, and I also had a hydrostatic head 4ft high behind the wall this would give me an additional 500 lbs of added force:

4² ⋅ γw / 2 = 499.44 lbs per linear foot (where γw = cubic weight of water, 62.42 for this example )

This gives me a total of 830+500 = 1330lbs per linear foot of wall.

Is this correct? Is there anything I've misunderstood or didn't take into consideration?

(thanks again for your patience, I just want to make sure I understand this)

RE: Retaining Wall: Saturated Soil & Hydrostatic Pressure

The Pa value may be high but then your total is OK. Normally for a saturated condition it is unlikely that active earth pressure is involved, but likely an at rest condition. Let's say the soil is clean sand not loose or dense at 55 pcf submerged unit weight (I'm too lazy to go thru exact calcs.) then the lateral earth pressure unit weight at rest might run about half that. Then the force per foot of wall from this soil would run about half the water value, with the total quite less than your number total. Anyhow you are doing OK now? I hope so.

RE: Retaining Wall: Saturated Soil & Hydrostatic Pressure

(OP)
Yes, I'm good. Thanks for your patience. I really appreciate it.

RE: Retaining Wall: Saturated Soil & Hydrostatic Pressure

OG... good questions from a non-engineer...

Dik

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