CPT data without any dissipation test, how to calculate GWL

[promken]

Hi All,
just received a bunch of cone data and they have not done any dissipations (So all U is Umax, no Ufinal).
I have done drilling so can infer soil/drainage characteristics.
I have some thicker strata of free draining sand to work off of.

is there any way to reliably calculate Ground Water Level, without dissipations?

thanks,
P

 

[Ron]

Don't calculate GWL...measure it!

 

[EireChch]

You dont need dissipation tests to "estimate" groundwater depth. CPT-u should give you a U graph.

Note: the above test is in Sand where it is easier to estimate groundwater depth, more difficult in fine grained of course..

 

[GeoEnvGuy]

Reliabley no. As Eirchch mentions you can look at the U plot, and you have to graph the data to a usable scale and find the hydro static triangle in sand and plot that up to find the estimated phreatic level.

 

[coneboy]

Promken,

Like EirChch mentioned: Look for some kind of hydrostatic Pp response line in the sand or sand layers. Draw a straight line (best fit est)
through all the Pp data in the sand and where it roughly crosses the "Zero" mark would equal the water table. If all the Pp data is very low to negative
then maybe the soils are only moist and true water table does not exist.

Another way: In some scenarios where you have a relatively shallow water table then you will see the friction sleeve and tip values drop at the water table.
It is more noticeable with the fs values. Compare your water table estimate across the site and if most of the CPT tests show the same estimated water table depth then you will have a higher level of confidence in your assessment.

A few extra points:
If you have silt/clay layer it may act a boundary layer or aquitard. The water could drain down from the ground surface and pool on top of silt/clay
layer.
Watch for silty clay layers that exhibit a large pore pressure response. If the Pp response is large the silt/clay layer is moist likely moist to sat.
You can use this a general idea as to where the water table might exist.

If you can, take all the ID off the coneplot so we can't tell where the test was conducted. Then post the coneplot and we could give you better advice.

Coneboy