Pore Pressure Distribution with Wick Drains 2

[moe333]

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

 

[fattdad]

You can install piezometers in the center of the wick spacing and use them to monitor the dissipation of excess pore pressure. You can also recover undisturbed samples as the pore pressure attenuates and do a UU to see if you have realized the intended strength gain.

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

fattdad,

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

 

[fattdad]

You do have a precision issue to deal with, but to do nothing out of fear of precision is equally errant. You need to go 25 ft into the compresible layer and have at least 2 or 3 ft from horizontal to stay "true". I'd go for it!

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!

 

[dgillette]

Sections 5 and 6 of Chapter 6 of NAVFAC DM-7 (1971 edition) provide some info on consolidation with radial drainage around sand drains. In the 1982 edition, see Chapter 5 of volume 1.

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.

 

[moe333]

I have used the vibrating wire piezometers with a push-in tip before and that is what I would use in this situation.
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.

 

[fattdad]

moe333: Won't the VWP evaluate the pore pressure? I mean if you pile on 5 ft of surcharge and the piezometer reads a corresponding head increase then the dissipation of that increase over time will show the extent of the pore pressure dissipation, eh?

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

Yes, the VWP's will measure pore pressure, but I am looking for a way to correlate the measured pore pressure with the average degree of consolidation for the entire layer so I can estimate the time to completion and the rate of strength gain at different depths. I cannot use the standard isochromes since they are for vertical 2-way draiange, not radial drainage.

 

[BigH]

See:
Cao, et.al., "Back-calculation of consolidation parameters from field measurements at a reclamation site", Can. Geotech. J., 38, pp 755-769 (2001)

http://adt.curtin.edu.au/theses/available/adt-WCU20050729.101614/unrestricted/

The first goes into all the theory you would ever want to know.

 

[BigHarvey]

When dealing with consolidation with drains, it is common sense to check the performance of the drainage system with both settlement gauges and pore pressure measurements. This dual approach allows the engineer to understand and check abnormal behaviours of some instruments. Just install tour pore pressure devices at centers of grids ( just after drain installation ) and settlement measurement devices ( settlement plates, deep settlement gauges etc ) and follow the values , if one of them is giving strange values or does not function , you will be able to detect it and explain it with the other mesaurements.

 

[fattdad]

moe333: Irrespective of correlation to isochromes if you are normally consolidated, have the wicks installed and install the piezometers, you'll have a pore pressure reading. If the piezometers are (about) at the furthest point from the wick that is controlling point for determining the relief of excess pore pressure. When the preload is installed, you'll see an increase in pore pressure. Gauging how much time it takes to realize 90 percent of recovery to the pre-surcharge pore pressure would tell all, wouldn't it?

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

Thank you all for your responses. I found what I was looking for in Charles Ladd's 1991 Journal paper. The reference from BigH is also very useful for this topic.

 

[Geoquery]

Four years age, I performed the geotechnical analysis for the similar project at Philadelphia Naval Business Center. The soft layer (CL, CH and MH) was 15 to 40 feet thick. The surcharge was 9 to 12 feet high. The area of surcharge was about 5700 x 100 to 140 ft.

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:

http://www.fhwa.dot.gov/engineering/geotech/library_arc.cfm?pub_number=16

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!