Soon, we will be starting phase 3 of our rebuild.

First we need to demolish and clear existing walls as they have no foundations, but that's easy :-)

Then we need to dig the trenches for this phase.

Straight forward in theory, but this bit of the land has clay so SE has specified 2.4m depth or deeper until we get 'good ground' where they is clay.

These may well be stepped as the existing part of the build is on sand and there is probably another 10m of sand before we hit this pesky bit of clay.

This is a corner to join up with the rest of the walls already rebuilt. Long wall is 18m (south), shorter wall is 9m (west)

There are also some internal walls that will need foundations, but these are very unlikely to be on the clay bit unless it's more extensive than our original trial digs showed.

This means that we will have to get someone in as our excavator is only little and won't go that deep.

Still all okay so far.

Now, what I need to know is how to shutter and brace. All our previous trenches have been 1m so didn't need anything.

Do said shutters need to be removed before, during or after pour and how manageable is this for 2 people.

So, please advise on shuttering and will BCO be happy to look at trench with shutters in place or does he need to see the sides.

In the worse case scenario that we get rain after we've dug the trenches then do we need to wait for them to dry before we pour.

SE has specified type3 concrete, will this 'settle' or do we need to vibrate as it's poured.

Is there anything else that I have totally forgotten that is obvious to consider.

Would piles not be easier ?

Worked well for us on a demolish rebuild.

Ours were 4-5 meters for two storey spec, 64 piles for a 200 sq m build, though a site next door went to 7.5 meters deep.

It all seemed a bit “suck it and see” when they were driving the piles in , just keep driving until they stop moving!

Edited 8 hours ago8 hr by FarmerN

35 minutes ago, FarmerN said:

It all seemed a bit “suck it and see” when they were driving the piles in , just keep driving until they stop moving!

That's how it works. The structural engineers want to see a number to carry their calculated load. The pile is hammered/augered in until it attains that number. Ours were 9m ....

I'd also be asking the SE to consider using pipes or a ground-bearing slab.

5 hours ago, LSB said:

probably another 10m of sand before we hit this pesky bit of clay

Slightly confused by this, the clay is at 10m depth but you are digging to only 2.4m? Why would you dig down to the clay?

Piles are expensive but at least the cost is more certain, there seems to be a lot of risk with this approach. 2.4m is very deep before you even consider the "or deeper" and someone getting into the trench seems risky.

How do they get the excavated material out of the trench? If you plan to lift it out with the digger then they have to hoist it up to wherever the bucket can reach, and you have a digger on the edge of your very deep excavation... Can you even get in a digger around the bracing you need? What if the bucket accidentally collapses the bracings with someone at the bottom? What does your principal designer say about the risks, as they have a duty to design out risks like this if possible? You're also doing it in Autumn where very dry clay (after this summer) starts getting potentially inundated with water which wont help the stability.

Are you sure the trench will be above the water table? What about perched water?

Once you start stepping a foundation that deep you are starting to need a lot of concrete.

Can you do it in short stretches so you dont open too much up at once and minimise instability?

Specifying 2.4m deep footings is a big CDM fail by your designer. Esp on a small residential project. It's just not practical to do safely on a typical self build budget. Back to the drawing board. Ground bearing slab / piles might be a better option.

Dig a trial hole where the new foundations are going. Clay trenches can hold up pretty well as long as it is not mixed with, say sand, gravel or alluvium. You will want to dig it out one day and pour it the next if possible so you need a big machine (17t) and an experienced operator. You will also need perhaps 6 trucks of concrete and they will need to be able to get close to the trenches. You will need lots of space for the muckaway pile. If the trenches are not self-supporting you will need to do piles.

Edited 3 hours ago3 hr by Mr Punter

We had trenches from about 2m at the deepest to about 1.3m at the shallowest. Very experienced digger driver made it look very easy. Dug 5th October last year filled (with 9 loads of concrete) 7th October.

I would say that it was only possible due to the skills of the driver.

I wouldn't want to be at the bottom of that trench or responsible for anyone else being at the bottom of it, and if you may have to go even deeper, the cost and risk will rise further. I'd ask your SE for alternatives.