Pore Water Pressure and Embankments

[KennyRogers]

Had a few questions on interpreting pore water pressures from VW piezometers installed in clay used for embankment construction - we are planning a staged embankment. Thought I ask the crowd.

1. If there is no activity in the area of an installed piezometer, will the pore pressure vary with fluctuations in the ground water table (GWT)? If it were sand, I would expect this would happen quickly and quite slowly for a clay or silt. Is there any way to estimate how fast the water table would influence the pore pressures if the permeability and increase GWT elevation is known?

2. I have seen data indicating that pore pressures were "stacking up" during embankment construction. For this particular scenario, there was a 2 meter fill was placed, a spike in the pore water was recorded, and then they began to dissipate. However, they didn't return to their initial value. I would have expected pore pressured to return to the existing equilibrium state which is relatively the GWT. Or does pore pressure tend to increase permanently with increases in the overburden pressures? If so, why does the internal pressure not return to an equilibrium state of pressure?

3. Would near by impulse loads (i.e. pile driving, vibratory equipment, etc.) affect the instruments? If so, does anyone know reasonable limits? Is there a way to tell if there has been permanent damage to the instrument?

4. If wick drains are used, and the GWT is used, I would assume this would reduce the effectiveness of the wick drains since the speed/flow rate of water is dependent on the head difference. Is my thinking correct? Within this context, is it possible that a wick drain that is too far from the drainage layer will be essentially ineffective since there isn't enough pressure/capillary draw to remove water?

5. Any decent references on how to interpret piezometer/settlement/displacement data in terms of embankment construction? Are there any guides available to help put together threshold values or warning values?

Cheers!

 

[fattdad]

piling up an embankment will change infiltration. The change can result in greater infiltration or more unsaturated soil to receive infiltration. Such change in boundary conditions can require the ground water table to adjust. So, yes, it makes sense to me that the piezometer may not recover to the pre-fill elevation even after the excess pore pressures have dissipated.

Irrespetive of the water table, wick drains will work to relieve the excess pore pressures. Any restriction of "head" differences will dim in comparison to the flow that's required to dissipate the excess pore pressure.

Check to make sure your VWP is not influenced by changes in barometric pressure.

f-d

¡papá gordo ain’t no madre flaca!

 

[aeoliantexan]

If the clay is saturated and the water table is in a sand layer above the clay, the pore pressure will vary with the water table. As to how fast, I can only say faster than I expected. I presume that a high undissolved gas content would slow it down.

100% consolidation takes a long time even with wick drains. Some residual pore pressure may remain as your "spike" dies down.

Also, your piezometer probably settles with the surrounding soil, and if the settlement is substantial, it is now a greater distance below the water table, so its final stabilized reading will be that much higher. You can correct for this effect.

Interpretation takes longer to discuss than I have right now, but Asaoka published a simple and useful way to interpret settlement data to see how much more primary settlement to expect. Evaluating slope stability using piezometric data is risky; I would see what Charles Ladd has said about it. Inclinometers can be very useful in warning of impending instability. Plot lateral displacement or strain vs embankment height and watch out for developing non-linearity.

Call your instrument supplier about dynamic loadings.

I don't understand your Question #4.

 

[KennyRogers]

Thanks for the responses!

aeoliantexan - My question #4 is hypothetical and might not be possible under normal conditions. For a wick drain to function, one would assume the pore pressure would be great enough to push the draining water through the drain (assuming the drainage path was upwards).

For example, assume a wick drain with the drainage path upwards towards a permeable layer. Now, lets say, a solid standpipe is attached to the upper end of the wick drain before reaching the permeable layer above. If this solid pipe length was long enough, the water would rise in the pipe until it reach equilibrium (effectively acting like a piezometer). Since the pipe is long enough, there is no flow and with no flow there is no consolidation.

Hopefully that is a little easier to visualize. Like I said, it's purely hypothetical, but I didn't know if there could be a case where the increased pore pressures were insufficient in magnitude to cause flow. I was thinking in my head an unexpected flooded condition...however I realized that the increase in resisting pressure from flood waters would be counter acted by the increase in total stress caused by the weight of the water itself.