Pore Pressure Distribution with Wick Drains
Pore Pressure Distribution with Wick Drains
(OP)
I am looking at designing a preload and using wick drains to speed consolidation. The site is a 50 foot thick clay deposit and the site grade will be raised about 10 feet in addition to the preload I am looking at. The project is in the very preliminary stage so there are no borings or soil data.
In the past, I have used piezometers and settlement gauges to update and calibrate the field performance with respect to time and magnitude for the preload settlement, as well as strength gain if stability was an issue. But this was for sites that did not use wick drains.
I'm not sure if piezometers would be useful for wick drain applications since the drainage would be radial and the pore pressure distribution would be fairly complex. I'm guessing the wick drain spacing will be 5 to 7 feet, so during installation of the piezometers, you would not have an accurate estimate of the distance of the piezometer to a wick drain, which I think would be fairly important.
Does anyone have experience with this situation, or have any useful references?
Thanks
In the past, I have used piezometers and settlement gauges to update and calibrate the field performance with respect to time and magnitude for the preload settlement, as well as strength gain if stability was an issue. But this was for sites that did not use wick drains.
I'm not sure if piezometers would be useful for wick drain applications since the drainage would be radial and the pore pressure distribution would be fairly complex. I'm guessing the wick drain spacing will be 5 to 7 feet, so during installation of the piezometers, you would not have an accurate estimate of the distance of the piezometer to a wick drain, which I think would be fairly important.
Does anyone have experience with this situation, or have any useful references?
Thanks





RE: Pore Pressure Distribution with Wick Drains
f-d
¡papá gordo ain’t no madre flaca!
RE: Pore Pressure Distribution with Wick Drains
With a wick drain spacing of 5 to 7 feet, there would be no way to know how close the piezometer is to the wick drain since both the wick drain and the piezometer will wander from true vertical during installation. The piezometer may be right on the drain in which case there would be very little excess pore pressure, or it could be between two drains which would have a much higher excess pore pressure, but you wouldn't know which.
Even if you did know the position of the piezometer relative to the drain, I do not know if you could estimate the the overall excess pore pressure dissipation in the total clay layer. Does anyone know of a technique?
I have estimated the pore pressure distribution and corresponding percentage consolidation in preloads without wick drains by comparing them with standard pore pressure dissipation isochromes. This can be done because you know the approximate distance to a drainage boundary, and the isochrome is easier to discern if you are in the middle of the layer (for 2-way draiange).
However, I do not know if there is any benefit in spending the money on piezometers in a wick drain system if I cannot make an estimate on percentage of consolidation.
Thanks
RE: Pore Pressure Distribution with Wick Drains
What I did on my last wick job was to push the piezometer into the muck by hand. We'd an a surcharge and just like that the water level would bounce up! I also had settlemetn plates to gauge settlement with time (nothing fancy, just metal plates with standpipes). It was perfect watching the settlement v. time correspond to the dissipation of excess pore pressure.
I don't fault you at all for looking to increase the precision. I'm just saying that you can install a handfull of piezometers and get a good areal perspective on the effectiveness of the wick drains with out much added cost (for the job referenced above, I used metal pipe with a filter sock and just pushed the pipe into the ground by hand - as the hydraulic fill was right at the ground surface).
Good luck.
f-d
¡papá gordo ain’t no madre flaca!
RE: Pore Pressure Distribution with Wick Drains
Consider settlement monitoring with baseplates on the foundation surface. That may be a more direct indication of the progress of settlement, although they are a pain to work around with large equipment.
RE: Pore Pressure Distribution with Wick Drains
I'm still looking for some good techniques to evaluate pore pressure.
I am also going to use vibrating wire settlement gauges. I have used them in the past and they work great. They take a bit more time and money to install, but you don't have to worry about surveyor error and disturbance from equipment.
RE: Pore Pressure Distribution with Wick Drains
f-d
¡papá gordo ain’t no madre flaca!
RE: Pore Pressure Distribution with Wick Drains
RE: Pore Pressure Distribution with Wick Drains
Cao, et.al., "Back-calculation of consolidation parameters from field measurements at a reclamation site", Can. Geotech. J., 38, pp 755-769 (2001)
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The first goes into all the theory you would ever want to know.
RE: Pore Pressure Distribution with Wick Drains
RE: Pore Pressure Distribution with Wick Drains
f-d
¡papá gordo ain’t no madre flaca!
RE: Pore Pressure Distribution with Wick Drains
RE: Pore Pressure Distribution with Wick Drains
We installed six piezometers and twenty six settlement plates. Two piezometers was installed outside the influence of surcharge on groundwater (datum piezometer); other four in the surcharge area (service piezometer). Because the groundwater would fluctuate you will need the datum piezometer to calculate the induced pore pressure due to the surcharge. Remember to place some of the settlement plates near the service piezometers.
Piezometer is necessary to install for the following reasons:
(a) To get the representative value of ch/cv. This data will be required to adjust the preliminary settlement curve with time.
(b) Degree of consolidation with time and thus provide an excellent check with the settlement from the settlement plates.
(c) Dissipation of pore pressure with time.
For the design of wick drain, you will get the best help from the following book: Prefabricated Vertical Drains, Volume I (FHWA-RD-86-168) and can be downloaded from:
www.fh
You also need to generate a bunch of spreadsheets to monitor and to plot the graphs for settlement and pore pressure dissipation with time.
Good Luck!