Groundwater level rising resulting in Heave

[EireChch]

Hi all,

A bit of a trivial question..when the groundwater level is lowered (for whatever reason), a foundation resting on sand should suffer some additional settlement due to the increase in effective stress.

When the groundwater level rises (again for whatever reason), there is a reduction in effective stress due to the increase in porewater pressure. Theory would suggest that there should be some negative settlement (i.e. heave). This question arose as we are modelling the effect of a large soakaway (5m deep) beside a road. In our Plaxis model, when we locally raise the groundwater level beneath the soakaway we are getting 35mm of heave.

However I dont think this is the case in real life. For example, when there is periods of heavy rainfall which would result in a seasonal rise in groundwater level, foundations dont just start rising, at least in my experience...

Maybe I am missing something...

 

[emmgjld]

Heave with rising groundwater is slight & only obvious after any & all settlement/collapse under the existing load conditions has occurred. I have experienced this under a driveway slab which had been placed on well compacted sands over sandstone.The slight rise & fall of the slab was apparent for at least 5 years. The subgrade beneath the curb & gutter had not been as well prepared and some settlement was obvious.

 

[GeoEnvGuy]

It could be that increasing the water table in the soakaway pit next to the road increases the stress in the excavation. Your heave maybe due to compression of the soakaway pit pushing horizontally below your road.

 

[maxim22]

I have a dewatering insurance case at the moment where drawdown 10 m of water has been modelled to give a change in total stress below a footing and elastic theory indicates 10-20 mm of settlement which might be why the buildings have cracked a bit and then there has been a small amount of recovery when the pump was turned off. Ground is not reliably elastic but the model in the software might be.

 

[EireChch]

Thanks for the replies.

Maxim - i though that was the case and modelled it with hardening soil (i.e. not perfectly plastic), this gave 22mm of heave. Again, I just dont believe that its realistic. We have decided to ignore it by resetting displacements to zero at that stage of the calculation.

I doubt its 100% correct, but the settlement is minimal so were not concerned.