Settlement with a Soil Vapor Extraction System

[becks177]

I am currently working on a building that is remediating a PERC spill using a Soil Vapor Extraction System. While remediating, severe building settlement started happening around the column closest to the remediation. We know the column is pulling everything down with it but aren't sure exactly what is happening beneath the slab. We plan to excavate the column footing as there are other site problems that are coming into play but I was wondering if anyone else has experienced settlement when dealing with a Soil Vapor Extraction System? I have been researching the systems and find no data involving foundation settlement but find it hard to believe the installation of the system and the column movement are not linked somehow.
Thanks

 

[AussieGeoEng]

If you are changing the relative humidity of the pore space, then using Kelvin's law (see Fredlund & Rahardjo,1993) you are altering the suction. So if the relative humidity is decreasing, then suction is increasing resulting in the compression you are observing.

 

[fattdad]

. . . or you are somehow influencing the position of the ground water table and changing effective stress below the footing.

(I really like ausiegeoeng's explanation though. . .)

f-d

¡papá gordo ain’t no madre flaca!

 

[becks177]

I agree with both of you regarding the change in ground water and/or moisture content of the soil. In my opinion the use of this system would definitely affect the soils under our foundations. Of course the company that installed the system denies that this could be the case so I'm looking if anyone else has come across a situtaion like this. We will be excavating the area to observe what is happening, I guess we will find out when we see it.

 

[BigInch]

They don't call it Oz for nothing.

Independent events are seldomly independent.

 

[oldestguy]

In my view there are two things happening, possibly three. Settlements of column footings can be: by compressing the subsoil, as would be done by lowering the ground water table; shrinkage of clay type soils by drying action; and actual movement of soil out from under, as with uniform sand, sometimes accompanied by vibration.

I understand most cleaning solvents are heavier than water, so, before that stuff comes up, chances are a bunch of water has to come along first. Did this system remove water? If so, look at possible action such as either increasing of soil unit density by taking away the buoyancy of that soil or by the drying of clays.

The solution might well be the coming in and injecting grout as with compaction grouting. Other treatments might be underpinning or mini piles with jacks, but the question is how deep are you affecting the supporting soil? Grout injection can go on until you have brought the column up and no further settling takes place.