soil-water characteristic curve for transient stability analyses

In an earlier comment on transient stability analyses, Ong said:

We (engineer) face a problem when we want to define the soil-water characteristic curve.  We have no idea what is the maximum suction can develop during the time of analysis, so we normally give up.

I asked Lori, one of our support engineers, if she could address this problem.  Lori says:

It’s important to understand that you only need to define the soil-water characteristic curve (also known as the volumetric water content or VWC function) if you are doing a transient analysis.   The VWC  function describes how water is stored or released due to changes in matric suction or negative pore-water pressure during the transient process.    

 

However, once you have a VWC  function defined, you can also use it to estimate the hydraulic conductivity function using established estimation techniques that are built into the GeoStudio software.  The hydraulic conductivity function is required for both a steady-state and transient analysis, so often a VWC function is defined regardless of the type of analysis that is being solved.

 

It’s true that if you expect the negative pore-water pressures to reach a particular value, then the function needs to at least be defined up to this value.  However there is a rule of thumb that you can use to determine what this value might be.   Unless you are considering evaporation and using a negative flux boundary condition at the ground surface, the maximum negative pore-water pressure that will develop above the phreatic surface will be hydrostatically negative.   

 

To estimate hydrostatic pressures above the phreatic surface, take the maximum distance between the ground surface and the anticipated phreatic surface or P = 0 contour, and multiply it by the unit weight of water (9.807 kN/m^3 or 62.4 pcf). 

 

So if the maximum distance between the phreatic surface and the ground is 10 m, then the maximum negative pressure that would develop  would be approximately 100 kPa. 

 

Please note that if you are going to use the volumetric water content function to estimate the hydraulic conductivity function, you will need to ensure that it has been fully defined, from saturation through to residual water content. 

 

There is no problem with defining the water content function over a range that is larger than the anticipated computed negative pore-water pressures.  Just make sure you have defined a smooth function that has a nicely defined air-entry value and a smooth transition to a residual water content if necessary.  

 

Within the GeoStudio software, you can estimate the volumetric water content  function using grain-size data, so you may be interested in watching the movie on function estimation that is available on our website at http://www.geo-slope.com/support/geostudio2007/tutorials.aspx?id=SepFnEstMov.

Does this help?  Please respond with your own comments or questions!