Geotechnical

Hi,

Can anybody refer me to a technical paper, article, or a book that talks about static pore water pressure corrections due to surface/ground settlements. Thanks!

 

[dgillette]

Not clear what your are looking for. Provide more description of the question you want answered, the material involved, etc. Otherwise, we can't figure out what reference(s) to recommend. Better to provide too much description than too little.

 

[easygo]

Sorry for providing too little information. An MSE wall was constructed on soft clay soil conditions and the wall has settled considerably (60 inches)in 2 years. Piezometers were installed at various stations along the wall and readings were obtained over a period of 2 years. I am interested in determining the pore pressure differential with time.
Delta P = Pore pressure readings-static pore pressure.
For determining Static pore pressures, piezometer tip elevations are required. Unfortunately tip elevations were were measured at time of installation only. So my concern is that with settlement the tip elevations would have changed and corrections to the tip elevations or static pore pressures are required. Knowing the ground settlement are there any corrections taken into account to correct tip elevations or static pore pressures.

 

[BigH]

You in India? IF the soil is that soft, did you construct by stage loading? Just wondering as I was involved with some that settled up to 1100mm but we used wick drains and stage loading. For as "rough" an estimate that you would have even trying to estimate the porewater pressure response, I am not sure that your piezometer settlement issue is one to worry about. If your pore-pressure measurements (static) before construction had stabilized prior to construction, you would have, basically, the t=0 pore pressure - unless, of course, your soft clay was still consolidating - i.e., it hadn't yet consolidated fully under its own weight. Sometimes, there is a fill placed above soft clays - that put the clays in consolidation - and you may have underconsolidated clay to consider.

 

[easygo]

Nope not in India. Not the Engineer or the Contractor for this case. Just a reviewer. Definitely agree that stage loading is key to such degree of settlements.
I believe the clay was still consolidating during construction. I agree that for vey small change in settlement, change in elevations would'nt have much effect on static pore pressures, but with considerable settlement static pressures can have a bigger effect. That's my take on it, i may be wrong. Just looking for any papers or articles explaining effect of settlement on static pore pressures. Thanks

 

[dgillette]

Still not sure I understand the question. Are you trying to predict the static PWP based on the current PWP and the amount of settlement that has taken place so far, with the goal being to predict how much more settlement you might get?

"effect of settlement on static pore pressures" Why would the settlement affect the STATIC pore pressure at all? Isn't that governed by the local groundwater, nearby streams, rain, etc.? What am I missing here?

 

[BigH]

I intimated what dgillette says - the only caveat is if the layer was still consolidating and the porepressure would not be the "true" static.

 

[easygo]

Yes thats where i am getting at. Thanks BigH. Because the layer is still consolidating the pore pressure measured before construction is not the "true" static.

 

[fattdad]

Pore pressures measured prior to the imposition of new loads would be the "true static" unless the clay was underconsolidated from the onset.

However, if you have a normally consolidated clay, you have a piezometer in the middle of the layer and you then increase the loading by 1,000 psf at the mid-point of the layer, the piezometer level should go up by 16 ft. The attenuation of this excess pore pressure is then governed by permeability and the length of the flow path. The 1-D consolidation test gives you the information you need to solve this sort of problem.

f-d

¡papá gordo ain’t no madre flaca!

 

[aeoliantexan]

If your piezometers are open standpipes, they are self-correcting for settlement; the water level in the riser is the piezometric elevation. If they are pressure transducers, you are correct, you need to know the current elevation of the transducer and add the measured pressure, in feet or meters of water, to the tip elevation to get the piezometric elevation. You may be able to estimate the settlement at the tip elevation if you know the profile well enough. Otherwise, you have a problem, as the calculated piezometric elevations will be higher than actual.

As for the static condition, you may be able to go outside the influence of all construction and install one or more piezometers to get the natural static condition. If there is a sand layer below the clay, you could put a piezometer in the sand - it should be at static. The elevation of drainage ditches around the site might be another clue to the eventual static level.

Don't forget, as the load settles into the ground, it is bouyed up by the water table. When calculating the effective stress at the piezometer, correct for the change in density of the compressible soil and use the current elevation of the base of the load.

 

[easygo]

Thanks fattdad and aeolintexan for the valuable information.

 

[BigH]

You might wish to look at the following for reference:

http://sed.siiv.scelta.com/bari2005/076.pdf