Soil at rest pressure coefficient (Ko)

[SteelPE]

This is a simple question but for some reason my mind is cloudy today.

For reasons that I will not explain here I need to know what the Ko (at rest pressure coefficient) is for a soil with an angle of internal friction of 34 degrees? I believe it is Ko=1-sin(34 degrees) but that yields a value of .4408 and at a soil weight of 120 pcf will give an equivalent fluid pressure (efp) of 53 pcf. Now the reason why my mind is so cloudy is that I have always been told to use a efp of 70 for propped walls. So my typical design value is off by a bit (note the 34 degrees was given by the geotechnical report).

 

[GeoPaveTraffic]

Your equation is a generally accepted approximation of at-rest pressure, but not that the actual at-rest pressure can be much higher depending on the soil's stress history.

One reason your efp is much lower than you may generally use is the fairly high phi angle that you are using.

To be of any additional help, would need to know how you are going to use the at-rest pressure.

Mike Lambert

 

[SteelPE]

the EFP of 70 came from my old mentor based upon what he used throughout his long career. In this particular instance, the client is adding a large surcharge load against the wall. I am checking to make sure the existing wall can support the surcharge. Using an EFP of 70 causes some problems, but using an EFP of 55 and everything checks out fine.

 

[steellion]

There's nothing magic about 70 PCF for at-rest pressure; it is simply a conservative assumption (I typically use 60 PCF in the absence of more detailed geotechnical information). 53 PCF is the correct EFP for an internal friction of 34 degrees.

 

[DaveAtkins]

Sometimes engineers use a conservative value for at-rest pressure if the wall will be partially below the water table. The conservative value helps to account for the higher lateral pressure below the water table.

DaveAtkins