Some basic questions on clay swelling and heave

[RichardHat]

Hi,
I had a quick search but couldn't spot anything that I could understand!

Briefly:
a: What mechanism causes clays to swell when damp?
b: What is the maximum pressure that swelling clay can exert?
c: Do cellar floors lift in the centre from heave because the walls are sufficiently loaded to defeat the uplift?

Thanks,

Richard

 

[oldestguy]

Swelling clays are made up of plates with surface electrical charges. The plates are held together by electrical forces. Water is a di-pole molecule and it is attracted to the charges on the surface of the clays. However the plates also are attracted to each other. The swelling occurs when water attraction overcomes the attraction of plates to each other. Pressure on the plats due to earth above can be such as to keep swelling from occurring. Likewise when there is lack of water due to drying, the attraction of water to dry zones overcomes the attraction to the platelets. There is a maximum pressures that can come from swelling, but that usually is from weights on it that are greater, and it could be as deep as 30 feet to be effective. As to walls having and effect, yes sometimes that pressure negates heaving. The subject is quite complicated and this is only a brief explanation, much simplified. Different clay minerals have different make-up, not all being plates. Montmorillonite is one that is made up with platlets. Here is a link,

https://en.wikipedia.org/wiki/Montmorillonite

 

[GeoPaveTraffic]

Additional information to address your questions specifically:

a. Changes in water content of the clay causes volume changes. If the clay is on the dryer side and water is added, then it swells. If the clay is on the wetter side and it dries, then it shrinks.

b. The max pressure varies based on the clay in question. Laboratory tests are required to determine the max pressure.

c. Swelling clay being restrained by the footing is one explanation for what you describe.

A local geotechnical engineer should be able to look at the material and give you an idea of its swell potential. However, laboratory testing is needed to quantify the possible amount of swell/shrinkage and the swell pressure.

Mike Lambert

 

[RichardHat]

Thanks both.

In fact I have a detailed borehole survey being made on-site this week.

We may have a situation where a concrete barn floor is sitting on a meter or so of clay which is sitting on a rock layer.

We are planning to place a single storey timber frame house on the concrete floor.

It looks like piles will be needed to satisfy the engineers - they are worried about clay heave.

I just can't get my head around the fact that I'll have to spend £15k to compensate for possible heave in such a thin clay layer.

 

[fattdad]

I'd think if the Liquid limit is below 50, there'd be nothing to worry about. Swelling clays are typically related to the smectites, clay minerals with a very active diffuse double layer (refer to OG's post).

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

I'd first do the lab tests. If you think you have a swelling clay, it may well be less expensive to remove it and replace with granular soil, compacted. If you use piles, you have to, do something to avoid the lift of that clay on your slab, lifting it off the piles perhaps. Now that's an engineering problem that may be costly.

 

[Mad Mike]

Richard,

Your post has resurfaced since being discussed under Foundation Engineering some time ago...I'd await the results of your soil report before speculating further over the conclusions and costs.

As you previously relayed, the existing concrete floor is not showing signs of heave distress, therefore heave may not be a real problem at all. Besides, if the rock is only 1m down, alternative foundations to screwpiles could probably be used successfully. In fact, replacing 1m of swelling clay with a soilcrete at your foundation positions and simply founding on top of the soilcrete would almost certainly be more economic.

I would be interested to hear more once the detailed soil profiles are known- until then, you're not going to get the reassurance you're looking for.

All the best,
Mike