Preconsolidation Pressure

A lazy day for me at the office. Started to read topics I kept on hold. Stopped at the theory manual of Settle3. It's a software for Geotechnical Engineers. Not really my forte.

Got into the topic of Preconsolidation Pressure. 

Now what is it? One could define it as a memory pressure for the soil. It represents the maximum effective stress experienced by the soil in the past. It's the Newton's third law of motion. 


Now you don't have to understand the graph completely. I don't get it too. But I am here to explain my part. 

First understand what a Consolidation is. It is the gradual release of water in the soil when weight is applied to it, causing the soil particles to reorganize and settle. 
It helps in creating a solid foundation stability for the structures, minimization of settlements and also improves the soil strength. 


Consolidation causes the application of force (weight). Preconsolidation pressure is the reaction to it. It is the maximum effective pressure the soil has ever experienced. Anything beyond that value has to be kept in consideration and acted accordingly. 

Effect of preconsolidation pressure.

Imagine a soil surface acted upon by a stress of 150 kN/m^2, the surface will deflect by a magnitude. Now remove the load and apply a gradually increasing load. What preconsolidation pressure does is it will not allow any deflection larger than the initial deflection as long as the stress is less than 150 kN/m^2. The soil surface has already reacted to the earlier load, hence the memory. From the image above, you can see the re-compression line, it is the path the second load would follow. Notice how it merges with the virgin line, that would be the case of 150 kN/m^2.

Now comes the part that made me write this blog. Imagine an excavation of 10 m. Taking a unit weight of 18 kN/m^3, the total stress exerted by the excavated soil is 180 kN/m^2. Like I said memory pressure, the soil surface at the excavated layer has had a stress of 180 kN/m^2 and it should be dealt accordingly. What if the stress by the structure is less than 180? The deflection by the structure is not much of an issue as we know it will be less than the unexcavated profile. The calculations should be done when the stress is more than 180. 

Preconsolidation pressure can be affected due to other mechanisms like pore water pressure, chemical weathering, environmental changes etc. But that's for the Geotech guys to decipher.

Cool! Isn't it? How much is considered in a real-life project. 

I got the above information while discussing with Karan Pandey. He is a Geotechnical Engineer at DAR. 





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