Pwp dissipation using CPT 4

[pelelo]

Engineers,

I have a question about the cpt.

I have worked in central florida, USA, for few years and noticed that projects involving CPT it is not required to perform PWP dissipation.

On the other hand, i have worked in few other projects like in Mexico and noticed that the engineer in charged does require pwp dissipation.

My question is: is it mandatory to run pwp dissipation or not?. At least the projects in mexico are dry stack embankments (silt), so i guess the pwp dissipation is useful for predicting consolidation ratio. But in FL, everything is pretty much sand, so i guess pwp dissipation is not needed as sands do not hold water pressures.

Please correct me if i am wrong.

Thanks.

 

[LRJ]

Whether or not this information is required very much depends on the design. However, you are right that there is no point in performing a dissipation test in sand as you would not get any useful information since the excess pore-water pressure would dissipate very quickly. In contrast, other soils (e.g. clays) take longer to dissipate excess pore-water pressure, so the information can be useful for design.

 

[L0k]

Dear Pelelo, the pwp measurement is required if you have to analyse the liquefaction in sand layer.
For the clay layer is required if you need the time consolidation, cv but you have to read till the pwp disappear.

 

[LRJ]

CPT dissipation tests in sand are generally very hard/impossible to interpret due to the short drainage time. This may differ depending on fines content, however.

Also, the parameters from the CPT dissipation test are horizontal rather than vertical, i.e. you get c[sub]h[/sub], not c[sub]v[/sub].

 

[JHeisenberg]

Like LRJ said, the dissipation test can be used the get horizontal parameters. An example of when you may need these values is the design of PVD drains, although these values are often estimated from Cv.

 

[pelelo]

Engineers, thanks a lot for your responses.

L0k, specifically what value do i get from CPT dissipation for liquefaction analyses?, I have never seen that before.

Please let me know,

Thanks

 

[fattdad]

If you jam a cone in the ground and it returns negative pore pressure, that'd mean the soil is below the critical void ratio and you can't get liquefaction.

I've spent today looking over CPTu data on a PVD job (the embankment is constructed). We still have excess pore pressures. I guess the consolidation and strength gain are not complete.

f-d

ípapß gordo ainÆt no madre flaca!

 

[L0k]

Hi Pelelo
This is the pore pressure data I use for liquefaction analysis.

 

[LRJ]

There aren't dissipation tests in the plot presented by L0k. L0k is using the PWP response to determine whether liquefaction is even possible in a layer, as per fattdad's suggestion regarding negative excess PWP. This isn't necessarily correct, however, since even the densest sand will accumulate positive excess PWP when subject to (fully undrained) cyclic loading; strong dilation will be followed by even stronger contraction in the load reversal.