Foundations - extension near trees - help!

[ninnage]

We're having an extension built on our garage. The outer wall is double skin (no cavity) - top of the existing found is 250mm below the top of the garage slab and the existing found is 280mm thick and aprox 500mm wide.

Problem is the large Sycamore approx. 3m from the wall (on neighbouring land). Now, here's the tricky bit - the Sycamore is raise up on a mound approx. 1m above ground level. The ground slopes down towards my wall. Here's a badly drawn sketch:-

Tree

• ninnage
• 30 Jul 2010
• 1

At the moment my architect and building regs can't agree on how deep the founds should be. Architect says no underpinning should be necessary, building regs want 4ft founds!

I'm quite happy to underpin if that's what's needed, but am very reluctant if it's not 100% necessary. The current wall and tree have been there for 40+ years and there's no sign of any movement at all.

Who wins - architect or building regs?

Andy

 

[freddiemercurystwin]

Who's going to be signing the Building Regulations Final Certificate?

If you know the answer you've answered your own question.

Ask your Architect if his Public Indemnity insurance covers him should any problems occur in the future regarding the foundations and the tree.

 

[ninnage]

I don't care who's signing it - I'll get a second opinion and take it to appeal if needs be. I don't want to underpin if 2 experienced professionals can't agree that it's needed.

Guess my question is, with the tree being 1m higher than the founds is this deducted from the required foundation depth?

Andy

 

[Static]

Assuming its in clay, the depth is determined by the lower point.. assuming a medium clay then the footing should be about 1.8m deep from the lower ground level.. 4ft to me sounds pretty shallow.. even for low demand clay which should be around 1.5m..

 

[theoldun]

Quote.
Who wins - architect or building regs?
Nine times out of ten Building Regs.
Quote
My question is, with the tree being 1m higher than the founds is this deducted from the required foundation depth?
Technically yes.

Most architects and BC follow NHBC guidelines Building near trees.
Sycamore. Deciduous. Moderate water demand. Mature height 20m.
On your sketch, draw a line from the left hand side of base of tree to bottom of brick wall, and that is your nominal ground level..
As I do not know the plasticity index of your soil, will assume it to be medium shrinkage, as you say that there has been no known ground movement to date.
Therefore, working to NHBC guidelines foundation depth should be 1800 below nominal ground level. As your nominal ground level is already 1000 below, depth of dig required is 800mm. As most BC, to be on the safe side base depths of foundations on high change potential, depth of dig required will be 1 metre below ground level.
Worst case. High shrinkage soil. Engineered designed foundation.
Low shrinkage.1450 below nominal ground level. (Will still be 1000 deep to comply).
Quote.
We're having an extension built on our garage.
This guidance is based on new foundations only, as you do not say what you are underpinning.
oldun

 

[Static]

With all due respect theoldun your incorrect.. the level is based on depth from the lower existing level, so should be 1.8m below the ground level around the existing garage..

View media item 24630
ps.. all this may be irrelevant if hes founding on chalk.. and not clay

 

[jeds]

You haven't said what the soil change volume is. If it's low the found would need to be about 1.4m deep. If it's high you will be closer to 2.1m

If the notional slope between the base of the tree across the site is less than 1 in 7 then the slope should be ignored and the depth taken from the ground level adjacent to the trench.

The foundation should be designed by somebody suitably qualified and insured. It's that person you will have a claim against if things go wrong. BC are not insured to give design advice and will take zero responsibility if things go wrong.

 

[ninnage]

May help to give you a badly drawn plan view since the tree is actually on a diagonal to the foundations...

Plan

• ninnage
• 30 Jul 2010
• 1

So the foundation in question is the bit at ground level. All very complicated...

 

[RonnyRaygun]

1.2m would be deep enough if the soil is LOW volume change potential.
Maybe BC are basing their depth on this...

Although...

If the notional slope between the base of the tree across the site is less than 1 in 7 then the slope should be ignored and the depth taken from the ground level adjacent to the trench.

Notional slope is 1 in 3, as shown on the original sketch...

So how would this affect things?

all this may be irrelevant if hes founding on chalk.. and not clay

Why would BC insist on underpinning if on chalk or gravel?

 

[Static]

1.2m would be deep enough if the soil is LOW volume change potential.
Maybe BC are basing their depth on this...

Although...

If the notional slope between the base of the tree across the site is less than 1 in 7 then the slope should be ignored and the depth taken from the ground level adjacent to the trench.

Notional slope is 1 in 3, as shown on the original sketch...

So how would this affect things?

NHBC quote that embankments greater than 1:7 are beyond the scope of their guidance and Engineers advice should be sought..

all this may be irrelevant if hes founding on chalk.. and not clay

Why would BC insist on underpinning if on chalk or gravel?

So far the OP has not answered any questions relating to the ground conditions.. in the end of the day BC dont design, the Architect might not know ass from elbow and potentially no-one has actual put a spade in the ground and looked at whats there.. this could be remedied by a quick dig and point..

 

[theoldun]

Quote:
If the notional slope between the base of the tree across the site is less than 1 in 7 then the slope should be ignored and the depth taken from the ground level adjacent to the trench.

Agree with this.

Quote:
NHBC quote that embankments greater than 1:7 are beyond the scope of their guidance and Engineers advice should be sought..

Never read this in NHBC.

Quote:
So far the OP has not answered any questions relating to the ground conditions.. in the end of the day BC dont design, the Architect might not know ass from elbow and potentially no-one has actual put a spade in the ground and looked at whats there.. this could be remedied by a quick dig and point..

Agree with this.
One other thing, my apologies for mistake in foundation depth for medium shrinkage soil and moderate water demand tree.
oldun

 

[stuart45]

Our local BCO is allowing me 700mm off the foundation depth of the extension as the ground level is that much higher next door where the trees are, as oldun stated. Right or wrong I'm not going to argue with him.