Hi,

negative pore pressure is generated if the soil tends to dilate or if the boundary conditions and material behavior result in loads that tend to increase the volume of the soil-fluid mixture locally. Cohesion is usually not related to dilatancy, hence not directly related to negative pore pressure. However, a change in cohesion changes the material behavior and thus the behavior of the system as a whole. This may locally change pore pressure distribution.

In case of an elasto-plastic constitutive equation, the stress state may never leave the elastic domain if cohesion is sufficiently large. In such cases the effective stress can be positive, i.e. tension stress, with large absolute value, which may cause negative pore pressure at a given load.

Regards,

Daniel

Dear Daniel,

Thank you for your response. Actually, I should say why the volume of the soil medium tends to increase, so the pore pressure tends to decrease? Please note that soil constitutive model is Mohr-coulomb and the pore pressure values are calculated then based on the Finn formulation.

Dear Mr.Amin,

The kind of soil response i.e. dilatency (increase in volume by increase in strain) mentioned in your question is mainly relevant to granular material. All granular material is dilate and the amount of dilation depending upon the relative density and normal stress. Literature says for loosely packed soil medium dilatency is almost nil but it may dilate maximum up to 2 to 4 degree. However, volume expansion is significant in case of a densely packed soil medium. Since soil particle is densely packed and void ratio is less as compared to loose sand, the volume of soil get decrease at lower strain level but beyond certain strain level (at higher strain level) the volume of soil is going to expand. So, during the volume decrease there will be a +PWP and during the expansion of volume _PWP. Since particle is closely packed at higher strain level soil particles are to be reoriented due to which soil volume is increased. hope i have answered to your question. All the best in your research..

Dear Karthigeyan,

Thank you for your response. Is there any other reason except volume increase for decrease in PWP?

Dear Amin Iraji,

First, do you mean negative gauge (excess) pressure or negative absolute pressure? Gauge (excess) pressure is zero-referenced against ambient air pressure or hydrostatic water pressure, while absolute pressure is zero-referenced against vacuum.

To answer your last question: yes, there is. If you pump resp. withdraw the pore water at an outflow rate that is sufficiently large, you will get negative excess pressure or even negative absolute pressure through the generation of "tension stress" in the water. Note that negative absolute pressure will not occur in reality because the water vaporizes (cavitation). However, for those simple water models used in soil mechanics, negative absolute pressure is indeed possible.

Regards,

Daniel

Thank you for your consideration!

Hi Amin: to my understanding the model was proposed to accumulate positive excess pore pressures under cyclic loading, i.e., volume trend to decrease due to the cyclic loading, and no dilation should occur. What is the reason to assume high cohesion in a granular material? have you tried a more realistic condition? i.e., zero or negligible cohesion?

Hi Eimar

Yes I am modeling realistic condition, negligible cohesion.

Eimar! I assume e.g. 10 kPa cohesion such as many realistic granular material.

What relative density and confinement stresses you have?

I reached to these results in mass scale simulation. I see liquefaction in element test simulation, but in mass scale the liquefaction is not observed. N1 60 was assumed 7 and I simulated a saturated soil mass with height of 1 m. Dry density was 20 kN/m3 and there was no parameter to define relative density in FINN model.

Dear Amin:

I believe you should not have negative pore pressures for such N value; even for a denser material, I believe Finn's model for cyclic loading does not reproduce dilative behavior, which could occur under monotonic loading in dense materials. You should take a look of the boundary conditions in your problem to try to figure it out.

Here is a picture of triaxial tests, executed using the frictionless triaxial apparatus (for improved uniformity of volumetric strain).

As you can see, during liquefaction each loading cycle transitions between contractive and dilative response. Notice, these are not simulations, this is the real thing - measured empirically and plotted in Matlab.

Negative pore pressure also occurs in monotonic testing (visible in the same plot). Notice, there is a physical limit to how low water pressure can go, before water becomes gas (vapor), due to cavitation. The tests shown were executed using deaired (vacuum boiled) water, to remove all soluble gasses, thus pressure near -100 kPa (absolute vacuum) is reached.

You can find more examples of complex loading scenarios executed using the frictionless triaxial apparatus by Ibsen, Vardoulakis and recent work of mine too. It all experimental, real life test results - not a simulation. It is worth to notice, that models are only as good as their ability to capture patterns and regularities observed experimentally. Thus, by exposing oneself to empirical evidence - one become more capable of making good judgement.

Thank you Eimar and Tomas for your useful responses.

Mr.Amin,

Sorry for late reply..answer to your comment on my earlier reply..the reason for decrease in PWP is depending on the moisture content. When the water is in contact with soil particle the inter particle forces of adhesion takes place, which is tension in nature. Due to this there will be a decrease in PWP or negative pwp develops within soil void. all the best..

I come from rock-mechanics, geo-mechanics and fracture-mechanics background. I was curious what does the experimental capability (above plot from Tomas) of negative pore pressure ~ -100 kPa or -14.5 psi mean. Does this mean loose sand whose cohesion is 0-psi will pose 14.5 psi strength under tension?

Dear Umesh,

The absolute pore water pressure is always non-negative, and near zero cavitation in the pore water will occur. I am sure that Tomas' figure plots the excess pore water pressure (difference of absolute pore water pressure and ambient pressure).

In your example, Umesh, the sand would "gain" an additional strength through "prestressing" caused by 100kPa additional mean effective stress (correspondig to -100kPa excess pore water pressure). This is similar to prestressed concrete. In fact, you can apply tension load to the "prestressed" sand-water-mixture, but this tension load reduces the prestressing of the sand grain skeleton. The grain skeleton alone in cohesionless sand-water-mixture has no tension strength.

Regards,

Daniel

In the plot, the pore pressure starts around 150, which may be the consolidation stress in the triaxial test, thus, the plot may not correspond to excess pore pressures but to absolute pore pressures. I believe the negative value is due to problems with the input parameters using the loose-coupled Finn's model, which only update the pore pressures at each loading reversal.

Dear Eimar,

yes, the figure plots the consolidation pressure plus the pressure due to loading/deformation (excess pore water pressure). This is simply the gauge pressure of pore water, but not absolute pore water pressure which is gauge pressure plus atmospheric pressure, zero-referenced against a perfect vacuum. Moreover, as mentioned by Tomas, these are experiments, not simulations.

Regards,

Daniel

Dear Daniel, yes, the plot must not include the atmospheric pressure. What I meant was, it must not be excess pore pressures due to shear, which must start in zero. Thanks for the clarification, I had not realized it is an experiment and just thought about the initial Amin's question. Best regards.