Foundations on Silt

[countervail]

Does anybody have experience of foundations on silt / siltstone. Or know any decent guidance.

We have a small domestic job where we will be building on silt / siltstone. We have a lot of trees on site and im wondering if silts are subject to heave and shrinkage due to trees in the same way clays are?

We will be using a raft but im interested to know if the ground is going to move.

Thanks

Countervail

 

[zakmuh]

Hi,

Based on code BS 8004:1986, silt soil has a NIL bearing capacity and yes, it triggers 'heave' warning!

 

[GeoPaveTraffic]

Silts can behave very differently than other soils. It all depends on the type of silt and the depositional environment. Some silts are plastic and tend to behave in a similar fashion as clays while others are non-plastic and behave more like sands.

You need to get a geotechnical engineer with local experience involved with the project.


Mike Lambert

 

[cvg]

most silts have low plasticity and low expansion, however depending on how they were deposited, may have significant collapse potential. they are often intermixed with sands and clays which makes it difficult to classify. siltstone or mudstone can degrade when it gets wet. what I have seen in the field was very soft and in my mind, questionable if it was actually "rock" or just very dense "soil".

 

[darthsoilsguy2]

thousands upon thousands of buildings in the North Carolina piedmont (the middle part of the state that isn't in the mountains or on the coast) are built on SILT, but there are also SILTS that get hauled away as well. you need a geotech. i don't use the term "domestic" job unless it's at my house...., but i have a hard time imagining a residential construction application where foundations bearing on siltstone would bother me... on-site septic might be a bad deal though.

 

[ACtrafficengr]

You didn't mention the climate at your site, but you've got one of the three ingredients for frost heave - susceptible soil. Sands and gravesl usually don't have enough capillarity to draw water up from the water table. Clay has huge capillarity, but is too impermiable and can't draw water up fast enought to cause problems.

If you also have a water source and cold winters, it could be an issue.

 

[countervail]

Cheers chaps

Most appreciated, by domestic job I mean a house extension where I can't persuade the home owner to get an geotechnical investigation done!

In this case I did some in house testing and decided it's silt with a little clay (probably low plasticity) and treated it as a high plasticity soil with an allowable bearing pressure of 50kN/m^2 (i.e close to soup, which it isnt) and went for a raft on 1m of compacted gravel.

I have a real problem with small jobs persuading clients to pay for a site investigation as they always ask "will it save me money" and when we say it might they won't pay "I'm not paying that if all its not gonna save me money and it's gonna take me weeks to get". I need to work on my people skills.

 

[nonplussed]

If it is a house extension, I think you already have far more useful information than a geotech analysis would give you. You should inspect the existing structure to determine if there's any damage and, if there is no damage, you know that the existing footings have performed well and you should be able to use the original design as a guideline (assuming you have access to the original design).