Analysis of embedded cantilever wall in clay. Effective or total stress anlaysis?

[cyphos168]

Hi all, in the analysis of embedded wall, I see many many example where earth pressures are calculated using effective stress. This is because the backfill material used is generally cohesionless.

If the soil is of a clay nature (say stiff clay of Cu around 150kPa) where a contiguous bored pile is being installed, do I use effective stress analysis or total stress analysis? And if total, how can I calculate the horizontal earth pressure? Sorry I feel like I should know this but when it comes to clay I am kinda stumped.

I have read some articles online suggesting that even for temporary or short term design, it is best to use effective stress.

 

[Okiryu]

Even for clay, the buoyant density is used (earth pressures coefficients [ko, ka, kp] apply to buoyant densities), so my understanding is that this is "equivalent" to consider effective stresses analysis. I would also like to see more opinions from the engineers here in the forum.

 

[GeoPaveTraffic]

The short answer is you need to consider both effective and total stress. The longer answer is much more complicated.

For stiff clays the lateral pressure under total stress conditions is going to be small because the undrained strength of the clay is enough to stand vertically. Therefore there is little or no lateral pressure. However as the clay drains the lateral pressure is going to increase. This process continues until effective (drained) conditions are reached.

Think about a slope stability analysis of a clay slope. The greater the strength relative to the height of the slope, the greater the factor of safety. A high factor of safety would indicate that there is little force on a wall at the face of the slope.



Mike Lambert

 

[cyphos168]

@Mike. Thanks. That was the answer I was looking for.

 

[Okiryu]

Hi Mike, could you summarize also the longer answer? Or can you provide a reference so I can do some reading about it? Thanks!!