What is best practice for piling where thin layers of clay-rock-clay-rock are present?

[RichardHat]

Hi,

This is my first post here, so please don't kill me if I break any etiquette rules!

I am converting a UK 13 metre x 9 metre barn into a small single storey timber frame house.

The frame is a Wiki House CNC 'jigsaw kit' design.

My structural engineer says that there will be about 20 - 30 tons load spread over 4 x 11 metre ground rails.
(One rail for each of the outside long walls, one rail to support an internal run of pillars, and one rail to support a wide section of floor)

There is an existing good condition concrete floor 10 cm thick then DPC sheet then 30 - 40 cm of hardcore.
This floor looks like it could support the house - I have parked my tractor there with no problems.

However as the site is firm clay, and trees are very close it looks like I will need screw piles through the floor to act as the main supports, to avoid clay heave etc.

Now, this seems fine as clay in my area normally goes down many, many metres ... but I have found that the barn is sitting on 1.6 metres of clay then 23 cm of limestone then another layer of clay then another thin layer of rock etc. The clay is fairly firm - its very very difficult drive a steel rod into it with a sledge hammer. The rock layer broke my hand auger. In other words, the ground is not especially weak.

The key question is: what the heck do we do about the rock layers? In such a case would the 23 cm (9 inch) limestone layer be regarded as strong enough to rest a pile on? Or would holes need to be augered through the rock layers to allow piles to be placed?

In my mind I imagine the concrete floor+hardcore+thin layer of firm clay+ 9" rock layer as being a single composite assembly. I have read somewhere that clay when squished between two hard layers is stronger than usual. Also, how does clay heave work? Would any heave from the thin 1.6 metre layer be significant?

Any advice on what any industry standard approach to settings foundations on, or driving piles through/onto a multi-layered sandwich of clay and rock would be most appreciated.

Many thanks.

 

[Mad Mike]

Richard,

This 23cm layer of limestone, have you only identified it in one boring/excavation? I assume so given the exact measurement.

Reason being, in limestone geology, or any geology for that matter, do not assume the layer to be consistent across your structure, that is, it's very possible one side of your barn has no rock layers at all while perhaps on the other side the rock layer is very thick etc.

One big question your description raises is that, if clay heave is the reason for piling, why is the existing concrete slab not badly damaged from the heave? I would get a professional second opinion on why exactly you need to pile...if piles were required to support your structural loads, taking them down onto the rock layer would be fine. If however piles are required to resist heave, not necessarily fine. Worst case is you install all the piles and still suffer from heave- get a professional opinion on alternative options first.

All the best,
Mike

 

[RichardHat]

Hi Mike,

The current slab is perfectly fine - level and no sign of damage.
It has been there maybe 15 years.

I assume that if there is heave, it lifts the rock layer below evenly.

Or the sandwich nature modifies the heave properties in some way.

If I use piling, it has to be decoupled from the concrete floor.

The UK regulations require 15 cm of clearance between concrete floor & building floor to avoid heave hitting the underside of the building, so heave damage post piling is unlikely.

My problem is that my SE says that he will act as if the rock is not there in his calculations ... which suggests that holes need to be drilled through it to run long piles into the clay. Or I'll need say 2 metre deep strip foundations which will require the rock layer to be dug out ????

It all seems overkill to support what is essentially a wooden hut!

I have a dark suspicion that various Rules Of Thumb, Official Standards and professional caution often lead to over-engineered and possibly inappropriate solutions in the building world.

 

[oldestguy]

With questions like this you may or may not get any usable help due to the site geology. I'd opt for using an experienced geotech who works in your area. He (she) may be able to come up with a different solution at less cost.

 

[RichardHat]

Hello oldestguy,

What sort of UK job title or firm should I be looking for in order to get the required expertise?

Thanks,

 

[EireChch]

Richard,

I practice in the UK for a geotechnical engineering company that does this work day in day out. We also have a structural engineering team as well.

 

[oldestguy]

In this country (USA) there are firms specializing in geotechnical engineering and those with that as a department. While the Yellow pages of the phone book may show some with big ads, I'd prefer to contact your Professional Engineering Society and ask for a list of those specializing in geotchnical engineering. Then get a brochure from them and look for the years of experience of the principals. I'd suspect FireChch may show up there. You likely need at least one test boring by them of a firm they recommend.

My advice to FireChch is "Let 'em find you". No point in getting in trouble here.

 

[RichardHat]

Clearly I can't post an email address.

However my name is real, and Google is your friend.

Richard Hathway RHAD MBCS CITP

 

[BigHarvey]

Richard
Any Site Investigation company in the UK will do the job. Hiring them might prove useful in the way that it's very likely that you don't need deep foundations. Your Structural Engineer is not a soil specialist. If anyway you need deep foundations see Roger Bullivant in the UK, they provide economical packages "piles + beams" for light housing.

http://www.roger-bullivant.co.uk/