undrained vs. drained soil condition 1

[MPorada]

I have some confusion over how to apply these load conditions appropriately when designing a sheet pile wall. As I understand it, a drained soil condition assumes there is no pore pressure behind the wall and total unit weight values are used. The location of the water table is not needed. In this scenario, one typically assumes a cohesionless soil (phi>>0, c=0) since this condition also emulates the long term condition of cohesive soils. Is an undrained soil condition only considered when there is a differential height in the water table in front of the wall vs. behind the wall (assuming the water table falls within the height of the wall)? If the water table is the same on both sides of the wall, does this condition need to be considered since it will only reduce earth pressures since the buoyant unit weight will need to be used instead of the total unit weight? Also, if the soil is cohesive, in the undrained condition I would use shear strength values and assume phi=0, correct?

Any clarity/suggestions to shed some light on how to approach/differentiate these load conditions would be greatly appreciated.

 

[DaveAtkins]

You always need to include the effect of the water table, even when considering the drained condition. I am a structural engineer, not a geotechnical engineer, so I won't get into soil properties--but from the standpoint of sheet pile wall design, you consider cohesion (for cohesive soils) for the undrained condition, but not for the drained condition. That is the only difference.

You are correct, when the water table is at the same height on both sides of the wall, there is no net effect (the water pressures cancel each other out). But it is good practice to include the effect of the water table, even for this scenario.

You should always check both undrained and drained conditions, because it is difficult to know which condition will control.

DaveAtkins

 

[r13]

Note, for water on both side of the wall, one condition always has to be kept in mind, and checked - quick drawn down as unbalanced load will result.

 

[PEinc]

If ground water is above the grade in front of the SSP, then it needs to be dewatered if you need to build in front of the wall. In that case, there may be an unbalanced water head. If the SSP makes an enclosed retaining structure, around the construction area, then dewatering in front of the SSP will cause an unbalanced hydrostatic head unless the dewatering is capable of lowering the water table in front of and behind the SSP. If the SSP is just a straight wall on one side of the construction area, water can probably drain around the ends and under the bottom of the SSP when you dewater in front of the wall. You can't have a water retaining SSP cofferdam unless you have closure (a closed triangular cofferdam, a circular cofferdam, or closed polygon cofferdam) of the SSP around the area to be dewatered. A drained condition exists when the pore pressure is allowed to dissipate even if the soil is still saturated. How long that dissipation or drainage takes is the hard question to answer.

http://www.PeirceEngineering.com

 

[geotechengineer1]

Drained : Excess pore pressure has dissipated
Undrained : Excess pore pressure has not dissipated

 

[HENRYZAU]

Drained or undrained conditions is normally related to saturated clays (drained condition is always assumed for sands due to high permeability although fine sand under earthquake can also behave as undrained). For soils above ground water table, unsaturated soil mechanics should be applied.

To quantify whether clay is in drained or undrained condition for a retaining wall design, time factor Tv = Cv x t / D^2 may be used: if Tv is within 0.01 then assume undrained consition (use Cu or Su and phi_u =0, total unit weight); if Tv > 0.4 then assume drained condition (use c' and phi', effective unit weight, pore pressure estimated from a flow-net or steady-state seepage analysis).

 

[Retrograde]

@HENRYZAU - do you have any more info or references on that time factor method?

Also, if using something like WALLAP, how should one apply unsaturated soil mechanics?