EXTRA DEEP FOUNDATIONS

The problem is, that you won't really get anyone to quote because you haven't got any drawings. A lot of time and effort goes into doing a proper quote from a reputable builder. At this stage, you're sat on the fence a bit (through no fault of your own) and as there is plenty of work about, you may struggle to get the info you need.

What is the access like? Can you get a machine in to dig out?

 

Is 10m the perimeter or 10m in length of one side ?

It may sound strange but it is harder to dig a deep hole that is short in length than a loner one.

Since building control has specified a beam and block type floor you could consider piling our screw in foundations that will eliminate the need for conventional digging.

 

Surprised B.C. didn't mention a raft.

 

Supended floor could be concrete beams laid out & infilled with concrete blocks (block & beam), whereas as raft concrete floor is reinforced with steel mesh.
And with raft foundation, the two most frequently used is the flat raft and the wide toe raft.

 

The lengths are part of your problem. Digging 2m is at or near the maxim of most mini diggers around 1.5t. The problem is squaring off the bottom of the trench where it meets the house - due to the depth and angle of the bucket. Normally you would finish this by hand but at that depth (requires shoring) and at a 600mm width it going to be difficult. So either side of your footing trench you may have to get the machine to batter back the footing to get access and then shutter back up to the correct height.

It does seem accademic to put the foundations this deep on three sides without putting them against the house as well. Since the house is unlikely to have foundations this deep and the roots will work their way in through the open side of the extension.

The other thing to take into account is that you are going to need around 3 skips to take away all the waste material

Rather than try general builders, I would contact some groundwork specialists and then once the foundations are in place a lot of general builders will be more amenable to give quotes

 

The JCB 8014 CTS mini digger has a digging depth of 2.648m, which is the best in it's weight class of 1634kg(1.5 ton).

I agree with Sos, very,very few general builders would want to tackle this job.

 

Quite a few groundworrking firms use their civil engineering skills for difficult housebuilding projects,etc.

http://www.tensar.co.uk/site/

 

Some rough costs off the top of my head digging the foundations is probably going to be :

2 days hire of digger and driver £500
2 days hire of skip loader £120
3 skips £450
3 shuttering ply and misc timber £100
manual labour 2 people for 2 days £600

So probably set your expectations around £2,000 as we don't know soil conditions, access, local costs etc.

Foundations

2 full loads £1400
if it can go from the mixer straight into the footings 1 days labour £150

So again £2,000

Obviously very rough costs

 

We have a few of these around here, they have a reach of around 13 metre with the 360 degree rotatheam conveyor. .

 

Yep, one of the worst jobs is when the shutes don't reach and you have to put it on the deck because the mixer won't wait to put in barrows. Then the race to level it before it goes off - and of course it is never the right weather either too hot or rainy

 

That is a very well designed conveyor, it solves a lot of the issues with unstable ground too. Must cost a fair few quid
Rotatheam build some very good conveyors, there is one truck round here with the sprider, seen it in action a few times. Good way to loose the barrows.

 

When the were repair the gas mains down here they crew had a truck with a giant vacuum on an arm, as the digger worked the arm sucked up the spoil and dumped it in the back of the truck !

 

As said above many, many variables. I would guess you will add around £3k with the extra depth (there will be extra dig with the suspended floor as you generally need 225 clear gap under and some authorities want 300).

You say it's an oak that's causing the problem: how far away is it and how high is it now. Also what type of oak is it? NHBC list English, Red, Holm & Turkey in their tables and the max height varies between 16 & 24m: this makes a huge difference. If the tree is not full height then you should be able to argue that if the current height/distance/type indicates a depth <1500mm then the ground under the new floor will be isolated from the tree and the heave precautions are not required and a ground baring slab will suffice.

The founds will still need to go to a depth dictated by the max height of the tree but this doesn't need to be constant: it is entirely reasonable to step the founds as the distance to the tree increases.

 

I would never argue with a Building inspector over the potential for root damage. The tree will no doubt have a preservation order, If he decides to get awkward he can just say that the roots are with the RPA (root preservation area) and then you have to go and do battle on a whole new front and engage with arbologist and costs will spiral as you argue compliance and non compliance

Sometimes best to agree

 

There's a world of difference between arguing with BCO: which as said you'll generally lose and clarifying things to use the design codes properly.

I generally assume an oak to be english at 20m max but if your BCO is very cautious then he might say red at 24m max.

As an example take an oak at 12m in high PI soil NHBC chart 1:
If Red D/H = 12/24 = 0.5 so 2.5 depth
but if Holm D/H = 12/16 = 0.75 so 2 deep

Going further if the oak is off to one side then the distance to the far foundation is going to be 16m so for the Holm D/H is now 1.0 so 1.5 deep.

Get someone to do you a design PROPERLY looking at the foundation depth: a few minutes of calculating beats the bejazus out of digging oversize holes and filling them with concrete.