Phreatic Surface with PCPT without Dissipation Tests.

[Nikel559]

Hey guys, as you know dissipation tests can be time-consuming especially for deep tests and when you have them in thousands. I was suggested by a collegue that if you perform the PCPT, and then make a Hydrostatic profile, where the u2 intersect the u of the Hydrostatic profile can be assumed as your groundwater table, is there any truth to this? Thanks

 

[EireChch]

How do you know the hydrostatic profile?

u2 is your plot of porepressure. Groundwater is more easily interpreted in granular soils given porepressure is likely coming form the head of groundwater above the cone tip. However in clay, it is a different story as increased porepressure is generated by the penetration of the tip.

 

[GeoEnvGuy]

Pore pressure dissipation tests take 5 min in sand and hours in clay. If you are suggesting to skip the test in sands to save the 5 min. The pore pressure profile can give you a reasonable estimate on phreatic level but you will not know if it is generating negative or positive pore pressure during penetration. If you are suggesting to skip the ppd tests in clay you will not attain any reasonable estimate of the phreatic surface. To save time clay ppd tests only go to T50 instead of completing the dissipation.

 

[Nikel559]

@EireChch: The Hydrostatic profile would simply be 9.81*thickness, regardless of the porepressure value at the level (It could be negative indicating suction or positive indicating porewater pressure), the Hydrostatic profile states that the Total Head at this level is 0 i.e. at this level energy loss would not occur i.e. flow of pore liquid from a high potential to a lower potential. But at the groundwater table the pore-water pressure is assumed as zero (unsat. soil mechanics), that is why I was confused when my collegue was showing me piezometric results and geomil PCPT-Hydrostatic profile comparative results (the Geomil software will make the charts automatically for you)and where the Hydrostatic profile was intersecting the u2, at a similar thickness GWT was indicated, I just don't understand how it works that way.

@GeoEnvGuy: Thanks, i'll take the T50 fact to knowledge. Yes clay dissipation tests are tooo long.