Salamander Cottage

I hate to say this, but you do know that you can get a torque driver head to allow your digger to place screw piles, don't you?  Useful gadget, as it uses torque measurement to determine soil bearing strength.

It's also another useful addition if you already own a digger.................................

Thanks for the nudge. Hmm, yes I had read that.  But have I got the confidence to use one? No, and here's why.

Ian

Hi Ian,

You might want to try http://www.roger-bullivant.co.uk/index.cfm. They are a customer of mine, I supply their reinforcing, plastic spacers and lifting systems (precast concrete is so glamorous). I'm not sure if they do one off residential projects but it might we worth a shot. They are a very professional outfit and they have an office in Chorley.

Rich

Thanks. I'll give them a try.

Ian

Ian

Another thing to thing about is if you actually need piles or not.  I was told by the SE that did my ground survey and a couple of others as well that I needed them.  But remembered reading Hillards report on passive slabs here that mentions On pages 3-4 how a slab can be used instead of piles by excavating out a soil under the slab and replacing it with stone, or even a greater about of EPS which is sometimes used in Scandinavia.

Under our slab is 750mm of stone.  You have a few ponds to fill to get rid of the stuff you dig out ?, and also have access to cheap stone, and even have a digger to do the digging!

Its something I think you should look at it you haven't already

Edited July 5, 2016 by Calvinmiddle

No, I haven't thought about it. Yet. That important little word. Yet.

More reading. 

Thanks for the nudge. Ian

If you can avoid using piles then I would, as not only is piling expensive, but is also adds cost in the form of the reinforced ring beam that joins the pile caps together and may create an access problem if you need a big rig.

Tradition is at work here, as the traditional solutions for building on clay were either to use piles or to use a deep trench foundation with anti-heave measures (often foam to take out the effects of heave). 

The alternative solutions, of using another material to spread the load out and down, as detailed in that report (which is also available from Kore, now, I believe) are relatively new here in the UK for houses, but have been used for decades in other civil engineering works.  For example, significant lengths of railway line across boggy ground has been laid on EPS, with ballast on top, and works very well. 

Hilliard Tanner has some good solutions for difficult ground, but it very much depends on the exact nature of the clay you're on.  For example, if it's like London Clay, then  piles or deep trenches are often the only solution, because of the relatively high heave potential.  If you're on other forms of clay, then this may not be an issue at all; we're on hard gault clay, and that doesn't heave at all to any measurable degree, so an ordinary slab works OK.  On the other hand, the gault clay we're on does shear easily when wet, so our retaining wall needed a large shear key at the base of the foundations, to prevent the base from sliding out under the side load.

Yes, thanks, J. 

The key issue for me is that I had my head in piles and wasn't looking around  to see the bigger picture. Calvinmiddle's post (above) and now yours have made sure that I won't  (in the words of the song) '...get fooled again...'

I will read the HT report (again) last thing tonight. So tomorrow's meeting at 10:00 will be interesting. 

More soon.

 

I think there is a natural tendency to tunnel vision within the building business, generally.  Solutions that have been used for hundreds of years (like piles) tend to looked at as the only solution.  It's really quite rare to find anyone who stands back and looks at any problem from first principles, yet often doing that opens up a range of options that may not have been considered, some good, some not so good.

For a foundation there are two key requirements, bearing stress on the ground underneath them and movement due to heave (and heave isn't just restricted to clay and moisture content - some places have to design for frost heave, too).

The bearing stress limit for the ground is pretty easy to measure.  The posh way is to use a penetrometer, the old school way is to use a bit of 2 x 2 and a lump hammer.  For most purposes the old 2x2 test (still used by some building inspectors) is perfectly OK, as it tends to err on the conservative side and there aren't many foundations that need a max allowable ground bearing stress of greater than 100kN/m², which is the rough result from that old school test.

Bearing stress will change with moisture content for many types of ground, usually decreasing with increasing moisture content in the case of clay, but the fact that clay is a very effective aquatard has to be taken into account, so clay more than a foot of so down won't change much in moisture content in practice, unless it's just a thin clay band with an aquifer beneath and run-off above.

Heave is more often than not the main problem with some types of clay, resulting from the relatively significant volume change with moisture content.  My view is that it causes more imaginary than real concern, and most of the concern comes from the failure of shallow foundations within the London Clay basin.  Many thousands of houses in and around London were built with shallow (often just corbelled brick) foundations, and these are particularly prone to heave damage.  The result has been thousands of houses that have had to be underpinned and the horror stories that abound about building on clay, that aren't always really justified.

All that's needed to remove the movement risk is to reduce the bearing stress down to a level that even wet clay will comfortably withstand, whilst providing an even load over a wide area and good surface drainage so that the clay moisture content stays fairly constant.  This is why the deep ballast bed system works so well.  The load spreads out into the ground at around 45 deg from the point of vertical load application, and the ballast provides a means of both spreading this load well and of draining the surface (land drains are put in around the periphery) so that the clay remains stable and well able to withstand the much reduced bearing stress.  I estimated the bearing stress on the ground under our house foundation and it was around 20 kN/m², which is very low, much lower than the typical bearing stress at the base of a traditional trench foundation

The clay we were on had good bearing capacity but the soil report was that it was dessicated and hence at risk of heave as the soil reverted back to its natural state (no one could explain why it wasn't at its natural state after being part of a garden for 70+ years)

SE that did the soil report said it would need piles ( with the added benefit to him that I would ask him to design them). Even Hilliard first mentioned piles when I first contacted him, it was only when I said that with piling quotes coming in at £25k-£40k the project wouldn't happen and we needed to think of something else that he looked at digging deeper to get under the heave risk.

We went down 750mm below the EPS and filled it up with 30-50mm stone whacked down in layers of 150mm.  He then added more steel to the slab to stiffen it up.

Simple solution if you have somewhere to get rid of the soil, we dug out 500-600 tonnes, not sure on the exact amount but farmer friend took away 33 grain trailer loads, trailers rated at 15t but some overloaded so much that rams couldn't lift trailer to tip soil out, they had to get a digger to help it up.

Paid him about £5k, and stone to fill it up was about £2k, but a hell of a lot cheaper than piles....

 

Hmmmm. Serious food for thought here. I can feel a few phone calls coming on and a chat with a couple of local farmers.

Benefit with farmers is that they generally have access to some pretty heavy equipment, my friend was able to borrow his neighbours 12t excavator, it "just" fit down the side f the existing house - but it sure didn't take long to keep the fill the grain trailers  - 5-6 scopes did the job.  He was doing it around the harvest when he had extra drivers, if there was a dew they would come to me and just keep four 15t trailers on the goes with one coming in just as one left.  Once the dew had lifted they left to harvest.

 

Started a new thread with some timelapse clips of the excavator at work...

 

Edited July 7, 2016 by Calvinmiddle

Ian

 

Curious if you read the report and chatted to Hilliard yet, anyway to avoid the piles and the hurt to your wallet?

HT's opinion was, that with 4.1 meters of topsoil to get through (we are on the top spoil of an old quarry) it probably won't be economical. 

Having said that, the final decision will be taken next week. Not looking forward to it.

£20K for 40 piles and an appropriate ring-beam.

Just bumping this to see how it is going?

I'm in a similar position, the SE has recommended piling based on the soil samples etc, but more tellingly because of the proximity of protected trees this leaves me with little or no alternative. (these are costing me a fortune)

 

I have 39 piles on the drawings.  After getting a number of quotes I'm also staring down the barrel of 20K for the piles and similar again for the ring beam....I think, I must admit it is often difficult to actually compare the quotes, what is and isn't included, any hidden extras.  Any tips from experienced pilers welcome.

On 9/18/2016 at 15:34, swisscheese said:

Just bumping this to see how it is going?

I'm in a similar position, the SE has recommended piling based on the soil samples etc, but more tellingly because of the proximity of protected trees this leaves me with little or no alternative. (these are costing me a fortune)

 

I have 39 piles on the drawings.  After getting a number of quotes I'm also staring down the barrel of 20K for the piles and similar again for the ring beam....I think, I must admit it is often difficult to actually compare the quotes, what is and isn't included, any hidden extras.  Any tips from experienced pilers welcome.

I'm surrounded by trees. With likely heave from the roots, piles were suggested.

 

But the engineer that I use prefers a deep trench with a low density polystyrene internal perimeter and a block and beam floor.

 

This should cut the cost by about 50%. But I'm on firm ground. Just the trees that have taken us down this path

Do you want the trees to survive? Deep trenching close to trees can kill them. Not always immediately but slowly over several years. Looks like we might loose one or two.

 

I was told that piling can work out cheaper if your trenches would need to be more than 3m deep.

On 9/18/2016 at 15:34, swisscheese said:

Just bumping this to see how it is going?

 

Lots of progress. Details to follow when  I am rained off....