Digging out front garden for a driveway - without house collapse?

[DaveFix]

I've received full planning permission to dig out half of my front garden and create a driveway.

The distance from the public footpath to the house is approximately 10 meters. So at half way (5 meters from footpath, 5 meters from house) I need to construct a retaining wall, at this point I will need to excavate approximately 1 meter deep (plus another 0.5 to 1 meter deeper for the footings). I'll be hiring a structural engineer nearer the time to get these measurements right.

I've never done a project this big so I'll be going very slowly, and therefore intend to buy a used micro digger and resell later, rather than hire one.

What I'm unclear about is regarding the risk of seismic movement while I dig out for the retaining wall and footings, then lay the concrete and start building the retaining wall - because I can't imagine completing all that very quickly.

Can I dig out the whole driveway (from public footpath to retaining wall) and then dig the footings and build the retaining wall (this method will give better access for using the digger) or is it essential just to dig a deep narrow trench for the footings and retaining wall, then construct all this before digging out the rest of the driveway?
Can this project be done in any weather conditions or should winter be avoided?
How do I know how much time I have from digging out to completing the retaining wall?

 

[Murdochcat]

Do you know where the water, gas, electricity and phone lines run into your house? And the drains?

Beware older houses can have these services at virtually any depth

 

[JohnD]

A useful rule of thumb is that you can excavate at a 45 degree angle from the house wall. So at one metre away, you can dig one metre deep.

It depends on local conditions, as sand and wet clay have no natural angle of repose and can slide out. A structural engineer or skilled builder may know, but you might have to dug some trial pits.

If you want to dig near your house, excavate a one metre bay, consolidate it and build your retaining wall, with a toe, then leave at least a metre and dig out another bay. When you have finished half the house, go back and do the bays you left.

 

[DaveFix]

Do you know where the water, gas, electricity and phone lines run into your house? And the drains?

Beware older houses can have these services at virtually any depth

Yes, I bought a Cat, Genny and Mouse about a year ago and mapped these out. Services-in are at one side of the garden and run quite shallow under a concrete path and steps which lead to the house, then they continue along the side passage to the back of the house. The sewer runs along the back of the houses. I intend replacing the path and steps with a wheelchair access ramp, using pattern imprinted concrete. By the way, are there any regulations which prevent me laying reinforced concrete over utility services?

However, there's a manhole for rainwater drainage at about 1 metre from the house, after about another metre it drops down incredibly deep and I couldn't trace it beyond that. Even the Mouse couldn't be detected after a metre. I tried an inspection camera but it didn't help. It seems to be constructed with short terracotta sections. I guess this is something I'll have to proceed very carefully with, and be prepared for repairs.

 

[DaveFix]

A useful rule of thumb is that you can excavate at a 45 degree angle from the house wall. So at one metre away, you can dig one metre deep.

It depends on local conditions, as sand and wet clay have no natural angle of repose and can slide out. A structural engineer or skilled builder may know, but you might have to dug some trial pits.

If you want to dig near your house, excavate a one metre bay, consolidate it and build your retaining wall, with a toe, then leave at least a metre and dig out another bay. When you have finished half the house, go back and do the bays you left.

Won't it make the retaining wall and foundations less strong if they're not constructed contiguously? I was expecting to lay long reinforcing rods or mesh across the whole trench of wet concrete.

When you say to consolidate the bays, do you mean like effectively build boxes with timber shuttering?

 

[lostinthelight]

Draw a side elevation sketch with what your after

 

[martygturner]

The retaining wall is for you garden, not for the house, gravity only works in one direction straight down, the retaining wall will support the mass of earth behind it in a 45 degree angle ( soil type dependant).
eg. none of these have fallen down the slope.... number 20 used to be my house and I did the terracing and the extension - two story counter pitched roof...the height difference is 24m between the rear of the house and the end of the garden, soil 70% clay.

Swindon

Swindon

www.google.co.uk

 

[JohnAnderson1]

What a lot of people do in situations like yours is dig and build in sections. You could dig a metre-wide trench where the retaining wall will go, get the footings poured, build that section of wall, backfill behind it, then move along and repeat.

 

[DaveFix]

Thanks guys for all the tips and advice, I have a much clearer idea how to do this now.

I'd describe the soil as relatively dry clay.

Would it be okay to dig the first metre trench, support it with wooden shuttering, then dig the next metre right next to it, support it, and so on? It will be about 5 metres wide. My reason for preferring this is so that I can lay long reinforcing rods into the foundations, and I think I'd build a stronger wall if it's contiguous. Would this still be okay?

 

[martygturner]

You will not be retaining muck in the way of soil just a 45 degree line from the bottom of the wall. given our summer you will find that a clay soil will suffer no or little slip. I would just dig the whole thing out then build the wall, a man with a digger and a muck away truck will get that done in a day.

 

[ToolboxTalks]

It’s better to dig and build the retaining wall in stages, not all at once. If you open the whole area before the wall is built, you risk soil movement, especially if it rains. Keep the trench narrow and pour the footing soon after digging to avoid collapse or water filling it.

Winter work can be done, but it’s harder as cold and wet weather slow curing and make the ground unstable. If possible, do the main digging and concrete work in dry months. Once the footing’s in, you’ll have more time to build the wall, but don’t leave open trenches sitting for long.