Total and effective stress based on CPT

[kauri]

Hi,

In the case that I have CPT data, when calculating total and effective stress, should I be taking u=9.81*z or u=u2(pore water pressure from CPT)?

Cheers,
Kauri

 

[GeoEnvGuy]

You should be looking at the pore pressure dissipation tests. Then calculating u=9.81 x z. If there are downward or upward gradients the dissipation tests completed at multiple depths and in all strata should estimate the phreatic surface throughout the profile.

The pore pressure measurements during penetration is an indication of whether the material will generate positive or negative pore pressures during compression shearing.

 

[EireChch]

You can estimate a groundwater level from CPT data based on review of pore pressure data as GeoEnvGuy says, however when dealing with Clays, just because there is an increase in pore pressure in the layer it doesn't guarantee that that layer is below the water table.

In Sands any increase in porewater pressure is most likely due to groundwater level (or a purged water table).

You ideally should have a monitoring standpipe to determine the groundwater level or at least some groundwater level data from boreholes. Even though groundwater level from a borehole is likely affected by drilling fluid, if used.

 

[kauri]

Thankyou EireChch, if I'm dealing with clays and don't have borehole data, how would you suggest estimating the GWL?

 

[EireChch]

its a judgement call really. Post the CPT results and lets have a look