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Assumptions for water table height for a retaining wall design.?

Assumptions for water table height for a retaining wall design.?

(OP)
I have a project given to me to design a retaining wall. It will be back filled with gravel with weep holes to act as drainage. However the problem is how high should I assume the water table to be. If I design it for a full water table, this gives me a very over engineered design. But it Is safer. If assume that the weep holes will take care of the water and put 0m, This could disregard possibility's in the future.


What is a safe assumption used in the industry, half?

Soil conditions are thick clay.

Im seeing people do different things, some say 0m, some say full, some say 60%. please help

RE: Assumptions for water table height for a retaining wall design.?

the water to the springline of the weeps, at a minimum.

Now inboard of the weeps, there will be some phreatic surface. At some distance removed from the actual wall, do you have some sense of the water table? It may just require some guess work (or a confocal parabola) to forecast the water table inboard of the wall.

So, what data do you have to generate the boundary conditions?

f-d

ípapß gordo ainÆt no madre flaca!

RE: Assumptions for water table height for a retaining wall design.?

(OP)
Im afraid i do not have any data of the height of the water table in the area. The wall is 1.8m, so quite small. What would be a safe assumption.

RE: Assumptions for water table height for a retaining wall design.?

even if you connsider the water level at top of wall, there shouldnt be any significant raise on thickness of wall. Anyway, there is no any specific code for this situation.. You already consider that there will not be any water pressure due to weep holes and etc. Anthow you can consider the water level at one third of the wall height to be on the safe side.

RE: Assumptions for water table height for a retaining wall design.?

For a wall that height, with weeps, I would think the lateral hydraulic loading would be minimal and would normally neglect it... using multiple weeps with a good drainage backfill the consideration of the weeps being 'clogged' would also be neglected.

Dik

RE: Assumptions for water table height for a retaining wall design.?

How's come no one comments on the correct backfill? What is meant by "gravel"? If it is single sized stone, it is not a filter and erosion out the weeps can happen, or more likely everything plugs up in time. Knowing the gradation of the earth that is to be retained, a proper filter can be figured out for the drainage layer. However, the short-cut way is to use ASTM C-33 concrete sand as the main drainage zone and use a filter of that between the sand and the weeps, probably a well graded gravel.

RE: Assumptions for water table height for a retaining wall design.?

Quote (abuh11)

I do not have any data of the height of the water table in the area.
The wall is 1.8m, so quite small. What would be a safe assumption.

IMHO, 1.8 meters.

Assume, for a moment, that you assume 1.0 meters. The difference in that (uninformed) assumption and full wall height is 0.8 meters... an extra 0.8 meters head is not much additional hydrostatic pressure. Don't try to "fine tune" a retaining wall design, there are too many other factors... say, maybe an unexpected surcharge in the future.


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RE: Assumptions for water table height for a retaining wall design.?

If your worried about the watertable, add an additional drain line at the top of the footing on the backside with pervious backfill, all drained via gravity away from the wall.

Mike McCann, PE, SE (WA)


RE: Assumptions for water table height for a retaining wall design.?

unless the "water table" is artesian, weep holes rarely flow ground water. instead, you will be handling surface and shallow subsurface water which flows down a slope and gets behind your wall. since this is clay, I would expect it to happen right behind the wall where the gravel or sand might come to the surface or there might be a crack in the soil which would allow surface water to migrate downward behind the wall. you need to prevent that. proper surface drainage behind the top of the wall is crucial. otherwise, OG hit on the main issue which is plugging of the drainage layer. as far as stability goes, check your factor of safety with plugged weepholes and make sure it is 1.0 or greater. your normal design FS for overturning should be 1.5 or 2.0

RE: Assumptions for water table height for a retaining wall design.?

Perhaps, I'm repeating myself. . .

I an involved in a project where the retaining wall is moving. In reviewing the initial design calculations, it's apparent that the geotechnical design did not consider water table at the spring line of the weeps. They just figured, there are weeps, so there is no water pressure.

When I develop the free-body diagram, consider Ka, Ka and tan(delta) and show no water pressure, there is no problem. When I consider water pressure acting below the weeps, there is a design problem.

The wall has slid over 14 inches.

The pavement above the wall has cracked.

(Yes, this is a small wall, but in answer to the question, the design, "Should" consider water pressure acting below the weeps, which are rarely at the exact foundation bearing level.)

f-d

ípapß gordo ainÆt no madre flaca!

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