2.3m Deep Foundations alternatives

[John Williamson]

I have just had a inspection from building control who are asking for very deep foundations for a single storey extension.

To quote the report:
"Lombardy Poplar tree to rear paced out at circa 18m from proposed rear elevation.
Eucalyptus at circa 10m away from the proposed flank elevation at worst case location.

I have assumed high PI clay index in absence of a soil investigation report and advised depths as per

NHBC guidelines.

Rear elevation trench to be 2.1m deep.
Front flank trench to be 2.3m deep
Rear flank trench to be 2.2m deep.

All above trenches require 75mm clay master heave precaution board pinned to internal faces of excavations to within 500mm from trench bottom."

Would it be cheaper and quicker for me to pay for an engineer and get some pile foundations? The builder has quoted an additional £7500 for 7 skips, labour etc to give an idea of the costs I am facing.

 

[John Williamson]

Sorry just to add, they are currently digging out by hand as the site has complicated tight access.

 

[stuart45]

Might be worth getting your soil tested.

 

[RonnyRaygun]

Would it be possible to find out the local soil plasticity? You might save a bit of digging if the soil is medium or low plasticity.

Are the trees mature? If less than 50% of mature height, could they be cut down and the current height used to determine depths?

What about pad and beam? If loads are low (which they will be for single storey) you would get away with some reasonably small pads. You would of course probably need to get an RC beam or two designed, or alternatively you could use steel beams encased in concrete. If spans were short enough, below ground spec prestressed concrete lintels are also an option. Do you know the bearing capacity of the soil?

There are mini diggers available which will fit through a standard door. Is access that tight?

 

[John Williamson]

Would it be possible to find out the local soil plasticity? You might save a bit of digging if the soil is medium or low plasticity.

Are the trees mature? If less than 50% of mature height, could they be cut down and the current height used to determine depths?

What about pad and beam? If loads are low (which they will be for single storey) you would get away with some reasonably small pads. You would of course probably need to get an RC beam or two designed. Do you know the bearing capacity of the soil?

There are mini diggers available which will fit through a standard door. Is access that tight?

Thanks so much for the response. I am a complete novice so how would I get the soil tested?

The tree causing the most problems is not mature and my neighbour who owns it also wants it chopped down but the surveyor says this will not affect the depth as it takes years to settle.

Pad and beams sounds like a possible option. I am guessing they would need an engineers drawings?

For the digger unfortunately yes it's that tight. Not even a micro digger unfortunately.

 

[RonnyRaygun]

Thanks so much for the response. I am a complete novice so how would I get the soil tested?

If Building control can't provide info then you would have to go to a geotechnics company - no idea of the cost of that sorry.

The tree causing the most problems is not mature and my neighbour who owns it also wants it chopped down but the surveyor says this will not affect the depth as it takes years to settle.

If you check NHBC 4.2 it does state that if the tree is less than 50% of its mature height when removed then the current height can be taken when determining foundation depths. This is because the roots will not have reached as deep as a more mature tree.

Pad and beams sounds like a possible option. I am guessing they would need an engineers drawings?

Yes.

 

[noseall]

2.3m deep? HAND DIG!

YOU HAVE GOT TO BE KIDDING ME!!!

For the digger unfortunately yes it's that tight. Not even a micro digger unfortunately.

A micro is no good for those depths in any case. We (just) managed a 2.4m dig with a 3 tonner and that was having to perch the machine right on the edge of the trench in order to get max dig.

It would be more cost effective to crane a digger in. How will you expect the piling boys to get their machinery in if you can't even access a micro?

 

[noseall]

The builder has quoted an additional £7500 for 7 skips, labour etc to give an idea of the costs I am facing.

Is that all? You have got a bargain there mate if we are talking hand dig and barrow. How on earth are they gonna get stuff out of the trench bottom and into the barrows?

 

[John Williamson]

The tree causing the most problems is not mature and my neighbour who owns it also wants it chopped down but the surveyor says this will not affect the depth as it takes years to settle.

If you check NHBC 4.2 it does state that if the tree is less than 50% of its mature height when removed then the current height can be taken when determining foundation depths. This is because the roots will not have reached as deep as a more mature tree.

Brilliant. Thanks a million. Checked with the Building control who said he will revise his estimates. If it's more than 20cm saving I owe you a pint..

 

[John Williamson]

The builder has quoted an additional £7500 for 7 skips, labour etc to give an idea of the costs I am facing.

Is that all? You have got a bargain there mate if we are talking hand dig and barrow. How on earth are they gonna get stuff out of the trench bottom and into the barrows?

Yes I know it's a bargain but it's painfully slow and definitely painful for the labourers. It's also costing me money so I really need to know how to reduce all these problems bargain or not.

 

[John Williamson]

2.3m deep? HAND DIG!

YOU HAVE GOT TO BE KIDDING ME!!!

For the digger unfortunately yes it's that tight. Not even a micro digger unfortunately.

A micro is no good for those depths in any case. We (just) managed a 2.4m dig with a 3 tonner and that was having to perch the machine right on the edge of the trench in order to get max dig.

It would be more cost effective to crane a digger in. How will you expect the piling boys to get their machinery in if you can't even access a micro?

Very good point. Having googled images of pilling machines there is no way.

 

[stuart45]

I am a complete novice so how would I get the soil tested?[/QUOTE]
Cost me £40 last time I had it done in Dorchester, but look around on the net. Could save you a fair bit of cash.

 

[John Williamson]

I am a complete novice so how would I get the soil tested?

Cost me £40 last time I had it done in Dorchester, but look around on the net. Could save you a fair bit of cash.[/QUOTE]

Certainly better than the quote I got earlier.

Product Qty Unit Price Price
Standard Classification Suite 1 £ 115.00 £ 115.00
Mobilisation 1 £ 300.00 £ 300.00
Total: £ 415.00

 

[stuart45]

Shop around, you don't want to spend a lot to find out it's High anyway. There is a simple test the man in the lab showed me by rolling the clay. Low breaks up easier.

 

[RonnyRaygun]

It would be more cost effective to crane a digger in. How will you expect the piling boys to get their machinery in if you can't even access a micro?

Very good point. Having googled images of pilling machines there is no way.

I'm sure there are more cost effective solutions, but look up the Grundomat piling system - you park the rig on the road and just bring a flexible pipe through and vibrate the piles in. Have spec 'd the system once or twice.