Piles & Trench Fill Combo

[benben5555]

Hi folks. We had a lot of trees on our site, which have now been removed. Based on the trees that were there the main house requires piled foundations (there was a large hawthorn tree & a lot of conifers). To the other side of the house there is a link corridor and an attached garage with room above. Due to its position away from the trees this would only need a strip footing of 1.3m deep. I've been told that it is not 'acceptable' to have part of the property piled and part on a trench fill foundation. Does anyone know if this is accurate? I don't think there would be an issue with differential movement and I would think a joint between the two could be introduced between the two if necessary. Any thoughts and opinions welcome.

[PeterW]

What are the ground conditions ..? Clay ..? 

[benben5555]

16 minutes ago, PeterW said:

What are the ground conditions ..? Clay ..? 

Yes, moderate clay

[Russell griffiths]

Are you not messing about over a couple of grand. 
Get the piles found in and sleep soundly. 

[benben5555]

11 minutes ago, Russell griffiths said:

Are you not messing about over a couple of grand. 
Get the piles found in and sleep soundly. 

Probably about £8-10k. That's a lot of money to me. And not really messing around, if its a feasible solution I will sleep soundly on a mattress stuffed with the £10k I've saved.

[markc]

£8-10k extra for that part or total?

If you don’t want piles you could look at digging out all the roots and clay and replace with compacted stone but piling would be a lot less disruptive and probably much cheaper

[saveasteading]

Hawthorns and conifers are not a big problem, and piling seems excessive.

 

It depends enormously on the size and type of trees, and their distance.

 

Mostly on trees remaining but also of those removed. Any more info?

 

[Bonner]

We have trench footing for the main house, single storey offshoot has a raft found due to made up ground. Movement joint in between, all specified by structural engineer which is what you need.

[Makeitstop]

I think the long and short of it is that an engineer might not like the different methods of foundation support. They're likely to suggest this could result in differential settlement.

 

If structure "A" is on piles and you're seeking to add an adjoining structure "B", an engineer is probably going to advise piled foundation for the new build.

 

Of course, if you have a perfectly sound structure on conventional strip foundation, and are looking to add to that, then ground conditions may dictate that piling could prove the optimal choice, but perhaps not.

 

Mixing foundation techniques (piled and strip) on any new structure however is likely to be less than ideal and an engineer may advise against it. Your best bet is to discuss it with maybe two different engineers, to get some idea of how its viewed.

Edited April 15, 2022 by Makeitstop

[Russell griffiths]

10 hours ago, benben5555 said:

Probably about £8-10k. That's a lot of money to me. And not really messing around, if its a feasible solution I will sleep soundly on a mattress stuffed with the £10k I've saved.

You won’t though, you will spend it on a more expensive kitchen 🤣🤣

[saveasteading]

In approximate terms , and in principle, I would go for all trench fill up to 2.4m , or mini-piles if deeper.

 

The trenches all go to different depths according to the tree effect, 

ie could be 2 to 3m if near an oak tree, getting less deep and eventually to no extra depth if distant from an oak or near a conifer.

I have designed for such circumstances many times, without any problems.

 

Logically then, it wouldn't be  a problem shifting from piles to trench footing.

 

This is what Engineers understand and do,  although the cost side of things isn't necessarily a universal skill.

But if you tell them the cost effect then they will try to help.

 

 

[George]

Because of the different way in how piles and trench foundations work there would almost certainly be some level of differential settlement. In large commercial buildings the extra expense of a movement joint is worthwhile, but will be a headache in a domestic house. 

 

Once foundations get atypical, what should be driving the design is a ground investigation. If the existing design is based on estimates and typical details, it may be worth spending a grand in getting a ground investigation done.