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# Soil unit weight for retaining wall design

## Soil unit weight for retaining wall design

(OP)
Hello,

I have a question regarding retaining wall design, specifically what unit weight to use for a soil above the water table? I’ve seen many textbooks that say use the saturated unit weight for a soil above the water table, why is this the case? It it just a conservative estimate?

### RE: Soil unit weight for retaining wall design

You need to ask yourself how high the water could be in the future after the wall is built. The current water level is likely not to be the water table following construction of the wall.

Mike Lambert

### RE: Soil unit weight for retaining wall design

to derive the horizontal earth pressure that acts on a retaining wall, you'd need to determine if the wall is free to rotate at the top (i.e., cantilever) or whether it's braced at the top (i.e., like a basement wall). For the former, you'd take the friction angle of the retained soil and use tan^2(45-phi/2), where phi is the friction angle. That will return some number typically in the 0.3 to 0.4 range. Knowing the coefficient of active earth pressure, you'd then multiply it by the soil's unit weight. If the wall cannot move, it's likely appropriate to use 0.5. Many folks use 1-sin(phi) for this value.

If there is water, use the buoyant unit weight against your coefficient, then add the full amount of hydrostatic pressure.

f-d

ípapß gordo ainÆt no madre flaca!

### RE: Soil unit weight for retaining wall design

It all depends on "will the water table be up in the backfill?" In other words, "is the backfill always drained"? If you do not have some positive method of draining, I suppose the answer might be "yes". Then the unit weight, NOT THE LATERAL EARTH PRESSURE FOR EQUIVALENT FLUID PRESSURE, will be water at 62.4 plus submerged unit weight of soil, maybe 60 pcf, depending on how loose or dense it is. So for wall design then you use the 62.4 for water and the at rest submerged soil unit weight, maybe 30 for the soil, in p/sf/ft of depth, as the equivalent fluid lateral pressure. That is still DARN HIGH. Better design is keep it drained.

I missed your point I see. The design lateral earth pressure for design would be proportionately greater according to the difference in density from bone dry to damp.

### RE: Soil unit weight for retaining wall design

As I have always seen it - the "bug" is in the "base" unit weight which is always a guess. Are you using 125 pcf or 120 pcf or 130 pcf as your "base" unit weight? The error imposed in this choice probably far outweighs whether you use Saturated unit weight or partially saturated unit weight - as noted by others, below the water table, you will have to use the bouyant unit weight (total less 62.4 pcf) and then add on the hydrostatic pressures. Keep the backfill drained.

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