PVD or Wick Drain guidelines

[Shakta]

Is there any guideline or design manual to design Wick Drain - how deep they need to go, spacing, loading and waiting time? Would Consolidation test result useful to determine that, how?

 

[jgailla]

You need a consolidation test to determine the parameters of the wick drain spacing if you want to know how long it will take to consolidate.
The wick drains should go to the bottom of the layer you are trying to drain.
The spacing will affect the waiting time. Farther spacing equals more waiting time.
You will also probably want to surcharge the area. The surcharge will "squeeze" the soil and quicken the consolidation.
Be sure to calculate the secondary consolidation as well.

 

[Shakta]

@ jgailla: Thank you very much for reply. Is there any relation between the wick drain spacing and waiting time considering same overloading/surcharge?

 

[oldestguy]

Knowing the stratification also is necessary, since you may have plenty of drainage already with laminated soil. That situation frequently results in much faster settlement than would be predicted by not knowing the drainage paths. For example in my many years of doing settlement estimates, only two sites (out of many) ever settled at the rate predicted. All others settled more rapidly.

 

[L0k]

hi Shakta
First of all you have to do the site investigation and followed by the soil laboratory test. The site investigation will determine the length of soft layer to be consolidated.
While the lab test will give you the vertical coefficient of consolidation using Terzaghi apparatus and the horizontal coefficient of consolidation using Rowe cell.
Using both coefficient of consolidations you may find the wickdrain spacings. Good luck.

 

[L0k]

Hi Shakta, first of all you have to do a site investigation to determine the length of soft clay to be consolidated.
Followed by the soil laboratory testings to determine the vertical coefficient of consolidation using the well-known Terzaghi apparatus and the horizontal coefficient of consolidation using Rowe cell apparatus.
Both coefficient of consolidation will determine the wick drain spacing's. Good-luck.

 

[BigH]

There is quite a bit more to it than what has been mentioned above. There is the difference in ch vs cv. There is also the consideration of smear on the drains as they are installed. Do you want to do rectangular or triangular patterns? You will also need a sand layer at the top of the drains (below the new fill you are adding) in order to move the expunged water out.

See:

http://www.ceteau.com/assets/pvd-design.pdf

as an example of some additional information.

We had wick drains installed on a job in West Bengal near Howrah - they worked well. We had about 6 m of very soft clay (Su<15 kPa, N of 0 and 1) overlying a stiffer clay N~8. We found that with the equipment we had that we could not push in the drains into the clay where the N value was in the order of 8 - say Su of about 50 kPa

 

[Shakta]

I have a 20' soft clay layer below 15' stiff layer where wick drain is going to be installed.
The fill (120 pcf soil) height at the top of the soil is 25'.
We got laboratory testing Cv = 2.5 ft2/day (for 1 ksf), and 1.25 ft2/day (for 2 ksf and higher load). The wick drain spacing is 6' in a triangular formation. I am using Ch = 2.0 ft2/day.

Can I use time Rate of Cosolidation formula like t = Tv (H^2)/Ch, where Tv = 0.85 for 90% consolidation and H = half of wick drain spacing ?

 

[bdbd]

Shakta, nope.

For wick drain consolidation analyses, you have to consider radial drainage. This is how they increase the rate of consolidation.
Also, if your stiff layer is not permeable, you should still use H for drainage path when you use wick drain. Wick drain does not reduce the vertical drainage path.
You can check out Jie Han (2016) Ground Improvement or Das-Principles of Foundation Eng. (8th Edition) for further references.

jgailla,

Why did you say wick drain should go to the bottom of layer? There is no need like that. Design becomes more complicated, true, but you can still use drains on half of the settling layer and calculate according to. Am I missing something here?

Oldestguy,

You are so right. Lommler in his book (Geotechnical Problem Solving) mentions the same issue. It is not easy to identify the sand bands located between clay layers. Thus, he recommends CPT with pore pressure measurement. Are you Lommler, he should be in your age
He tolds a story that an engineer put a settlement plate below an embankment, but he waited for one or two week to take readings due to something irrelevant. When he started to take the readings, he didn't observe any settlement. Lommler summoned as a consultant and he had noticed that settlement took place already due to thin sand layers.

 

[jgailla]

bdbd,
You are correct. The drains don't have to go all the way to the bottom.
In my limited experience with coastal plains soils, there is no reason not to go to the bottom. Wick drains are cheap once mobilization is accounted for, and time is very valuable to developers and contractors.

 

[bdbd]

I got it. You are right. As a preliminary tip, it may be useful.