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Building on Montmorillonite

Building on Montmorillonite

Building on Montmorillonite

If you had an opprotunity to purchase 5 acres of prime view property for almost 33% of averge market comp knowing the whole site is underlain with andesite flow, flow breccia and  leharic breccia. A report done 8 years ago stated that the site is suitable for development as long as earthwork spec and basic guidelines are followed. Is it possible to compensate for the hazards inherent in building on a floor containing high concentrates of montmorillonite?
Thanks for any 2 cent thoughts you can contribute.

RE: Building on Montmorillonite

This group are designated expansive clays by engineers, because they can accommodate many water molecules into their structure and they all possess an overall negative charge.  They are subject to swelling and contraction.  

There are a few foundation systems that can be designed to allow for these soil conditions.  Stabilizing the high activity clays for some kind of foundations is very expensive. However, the consequence of not applying such procedures can be loss of life and destruction of property.
Contact the same Geotechnical Engineer that published the report ask about local building practices and cost associated with his recommendations.  

In some areas post-tensioning slabs are used on expansive soils with good results.

RE: Building on Montmorillonite

A common construction technique used on expansive clay sites is to support the structure on piles and leave a gap (crawlspace) between the soil and the superstructure.

RE: Building on Montmorillonite

On the one hand, if the real estate market is efficient the discount on the property reflects either the risk or the excess building costs.

On the other hand, andesite, lahars, and breccia are not local conditions. Your neighbors are most likely dealing with the same situation.

On the third hand, make sure that the property isn't on a detached slab sliding toward the valley floor.

Groundwater is the key to shrink/swell conditions.


RE: Building on Montmorillonite

I have seen montmorillonite layers/seams slide on a dip or slope as low as 6 degrees or even less once water is present (or is already present).  If the montmorillonite is in a distinct layer, you may have a big problem.  Excavating it out, or piling through it with piles designed for the shear loads may help.  Tendons from the downslope to upslope through the layer may help.  In rock work in mining and civil, this situation usually means $$$.

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