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Pressure from water in soil
6

Pressure from water in soil

Pressure from water in soil

(OP)
I came across this articles discussing pressures due to water in soil.
https://www.readingrock.com/sites/default/files/reconmistakes.pdf

There article makes a distinction between moist and saturated soils ... and a system which retains moving water.

My question is: How do I know if I've got a moist/saturated soil or the latter. Care is required in defining "saturated", the last time I used the term, I didn't place too much emphasis on the meaning but in this instance it does not infer that water is retained.

Where I'm coming from, is if I have a large rainfall event lasting a few hours or a leaking water pipe, which case is more representative. Are my loads increased by the order of 1/4 or are they doubled?
The retaining wall may be either gravity masonry wall without mortar, with mortar or concrete. Does that make a difference?
The soil may be sand or clayey sand.

RE: Pressure from water in soil

Quote (LR11)

How do I know if I've got a moist/saturated soil or the latter.

Soils below the phreatic surface are always saturated. If you have a water main break the phreatic surface typically reaches a mounded steady state until the break is shutoff and fixed. This results in an increase in the unit weight of the material and a increase in pore pressure.

RE: Pressure from water in soil

(OP)
Thanks for responding.
Can you please advise the reference.

RE: Pressure from water in soil

2

Quote (GeoEnvGuy)

Soils below the phreatic surface are always saturated.

Gotta disagree with you on this one. Soils below the phreatic surface are not always saturated. Many clays and some silts do not fully saturate even below the water table.

RE: Pressure from water in soil

Without knowing the type of soil, void ratio, water absorption characteristic, and conductivity, I don't think there is a rule-of-thumb kind of answer to your question. The linked article may help you to get start in the long journey. Link

RE: Pressure from water in soil

2
I agree with Ron, many clays and plastic silts below the water table are not saturated.

Mike Lambert

RE: Pressure from water in soil

(OP)
Thanks r13 I will look into that article.
Ron and GeoPaveTraffic, the point is noted.

I don't do a lot of retaining wall design but usually stability seems to be the issue for the ones I've dealt with.

I'm thinking that a moist soil, while imparting extra lateral pressure, in terms of stability and if there's a heel on the retained side, the heavier soil would partially offset this.

The situation which I can't deal with yet ... is when you have a lot of water and that exerts a hydrostatic pressure, and maybe (???) makes the soil buoyant, to a degree maybe (???) ... so that stability is a real issue.
But surely this would not be a real case would it. And not for a rainfall event.

RE: Pressure from water in soil

Your concern is real, but I think it is site and application specific, for which has many unknown variables. Please provide a graphical example for others to provide meaningful response and help.

RE: Pressure from water in soil

(OP)
Thanks for the response r13. The typical retaining wall is shown below. Limestone blocks are used, natural or reconstituted, with and without mortar. Footing is sometimes present more often not.
The other common option is either a reinforced brick wall with a footing or all concrete. Both of these rely on the heel, whereas the first option relies on gravity.

The soil is typically sand if brought in as fill, or clayey sand if natural. Some areas are closer to clay but this is not as common.




RE: Pressure from water in soil

For wall shown below, I would design for the maximum hydrostatic pressure, treating it as a retention pond, if the permeability of the original soil is sufficiently low (clay for instance); design for the reported/expected ground water, that is lower than the top of the wall, if the original soil is consisted of mainly gravelly soil, which can easily conducting and divert the storm water away from the wall. For both cases, weep holes are required if safety factor for sliding and rotation is lower than 1.5.

RE: Pressure from water in soil

(OP)
OK thank you. For the situation of designing it for groundwater, I assume this means like a damp/wet/saturated case, and not the hydrostatic pressure case.
The clayey sands will be a bit tricky as their permeability is not as high as the sands. But then again, they tend to repel water in my area, so a rainfall event may not last long enough to fully soak through.
Thanks for the comments, I have a way forward now I think.

RE: Pressure from water in soil

Quote:

The clayey sands will be a bit tricky as their permeability is not as high as the sands.

Note that clayey soil absorbs water much slower than sand, so for a rain storm, it may only wet the top surface. Once the sun is out, the surface water will evaporate. But for clay backfill, watch out tension crack that may occur, witch will collect water against the wall.

RE: Pressure from water in soil

(OP)
OK thank you.

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