Groundworkers Trial Hole

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We are in the process of getting quotes for ground workers to excavate our foundations, Our architect has stated on our plans 600mm X 1m for the footings but subject to LBC. We have had groundworkers come in and they have quoted for the whole job in the architects requests but have stated they know the area is on Heavy Clay and have done 1 or 2 jobs close by where theyve had to dig closer to 2.1m which changes the costs massively.

Theyve asked if we can have 2 trial holes dug in the garden close to where the foundations will be and dig them down to around 1.3m so they can get a look at the ground before giving a more accurate quote they also stated that we should dig the trail holes and get LBC to come out and take a look at them and they will confirm if they will be happy for the footings at the required depth. Does this sound correct to or can anyone give some advice on what should be done?

Thanks in Advance
 
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We are in the process of getting quotes for ground workers to excavate our foundations, Our architect has stated on our plans 600mm X 1m for the footings but subject to LBC. We have had groundworkers come in and they have quoted for the whole job in the architects requests but have stated they know the area is on Heavy Clay and have done 1 or 2 jobs close by where theyve had to dig closer to 2.1m which changes the costs massively.

Theyve asked if we can have 2 trial holes dug in the garden close to where the foundations will be and dig them down to around 1.3m so they can get a look at the ground before giving a more accurate quote they also stated that we should dig the trail holes and get LBC to come out and take a look at them and they will confirm if they will be happy for the footings at the required depth. Does this sound correct to or can anyone give some advice on what should be done?

Thanks in Advance
Clay substrate only usually makes a difference (foundation depth wise) if there are other mitigating factors - namely trees and shrubs in proximity to the proposed build. You would not arbitrarily suggest deeper foundations in clay if there were zero factors influencing.
 
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Have a chat with bco first, the council ones are usually clued up about ground conditions in local areas. After that, yes a test pit isn't uncommon.
 
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I'm on heavy clay and went down 1.2m (same as house foundations)
As noseall mentions ... are there any trees that could cause problems? A neighbours footing had to go down to 2m due to an oak tree 15m away!
 
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When I dug out the footings for my front extension, we came across a clay pipe so I called the bco in. He said it was an old land drain and told me what to do with it. However, he told me to stop digging as I didn't have permission! I showed him the planning permission and he said something about building regs that I didn't have! I had to go back to the bloke that did the plans as he said there were no trees nearby! Anyway, he pointed out a tree in my neighbours front garden and gave me a chart that showed different distances and different soils with different trees which meant I needed to dig down to 2.1 metres at that end of the footings....or remove the tree. Luckily my neighbour let me remove his tree and I replaced it with an identical one after it was passed off.
 
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Thanks for all the replies we have a lot of trees mainly at the bottom of the garden over 35+m away which the groundworkers have all said were fine same with the architect and structural engineers. The closest tree is an apple tree around 18m away. We used the NHBC depth calculator app to identify them all and the only thing looking at and identifying all the trees the depth calculator doesn't change. I have to pay my fees to the LBC for all my inspections so may give them a call and go from there.
 
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I needed to dig down to 2.1 metres at that end of the footings....or remove the tree. Luckily my neighbour let me remove his tree and I replaced it with an identical one after it was passed off.
Interesting. But you still should have dug to 2.1m. Removing the tree would only give the potential for heave (which could take years).

Ah well, fingers crossed eh
 
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Interesting. But you still should have dug to 2.1m. Removing the tree would only give the potential for heave (which could take years).

Ah well, fingers crossed eh
That was 21 years ago now. The BCO told me to, rather than go to 2.1 metres. I removed an 8ft tree on the day of sign-off and the one I replaced it with is now well over 20ft! No problems so far.
 
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have done 1 or 2 jobs close by where theyve had to dig closer to 2.1m which changes the costs massively.
Personally I don’t see an increase from 1m to 2.1m having a massive increase in cost.

I’d guess it depends on what access you have. From memory a 2.1m depth needs something like a 3 tonne digger and say a 1400mm wide access.

The annoying thing is hiring a mini digger, going down 1 metre, the inspector says deeper and that means paying for a new bigger digger, new hire charge and more delivery cost.

If access is good and spoil is being collected by grab lorry, that cost won’t go up much, the Labour cost won’t be that much more providing all work can be down from above.
 
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That was 21 years ago now. The BCO told me to, rather than go to 2.1 metres. I removed an 8ft tree on the day of sign-off and the one I replaced it with is now well over 20ft! No problems so far.
Glad that’s the case. But it’s not what the BCO should have advised. Foundation depth should be based on the mature height of the tree, whether it’s removed or not (or actual height if less than 50% of mature height when removed)
It’s not an exact science though. Maybe the guidelines were different 21 years ago.
 
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Glad that’s the case. But it’s not what the BCO should have advised. Foundation depth should be based on the mature height of the tree, whether it’s removed or not (or actual height if less than 50% of mature height when removed)
It’s not an exact science though. Maybe the guidelines were different 21 years ago.

He provided me with a chart with types of trees down one side, distance from footings along the bottom and where they intercepted, it gave the foundation depth. I would imagine it would have been based on the maximum root spread for any particular tree. It was 2.1 metres at the nearest point of the footings going to 1.1 metres at the furthest point.
 
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A few thousand, minimum. £5k for the average 30sq/m fella.
Yes, thats the figure I had in mind - In the context of an extension I wouldn’t call it a massive extra over cost.

It’s when the inspector insists on changing from concrete floor slab to block n beam when it starts getting expensive when combined with deeper founds.

The biggest nuisance is having to reschedule everything
 
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He provided me with a chart with types of trees down one side, distance from footings along the bottom and where they intercepted, it gave the foundation depth. I would imagine it would have been based on the maximum root spread for any particular tree. It was 2.1 metres at the nearest point of the footings going to 1.1 metres at the furthest point.
It would, but if you need to go 2.1m for a tree that’s in the ground, you still need to go 2.1m when you remove it (unless it’s less than 50% of mature height in which case you can take height at removal).

There are also guidelines regarding how close you can plant a new tree.

The reason is that a tree can suck moisture from the clay causing it to shrink, and causing the foundations to subside.
If the trees have already caused shrinkage of the clay removing it will eventually cause the clay to rehydrate to its original size, causing heave of any new foundations.

Maybe the reason you have been lucky is that another tree was planted so the heave never occurred.

Whatever, bad advice by the BCO and totally pointless to remove a tree only to replant with another one the same a short time later ‍
 
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Yes, thats the figure I had in mind - In the context of an extension I wouldn’t call it a massive extra over cost.

It’s when the inspector insists on changing from concrete floor slab to block n beam when it starts getting expensive when combined with deeper founds.

The biggest nuisance is having to reschedule everything
...as long as the trenches behave that is.
 

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