Footing for self-build extension 6x5

[Hmmm1987]

Hi there,

Looking for some advice regarding my footings.

My house sits on a road which slopes downward to the centre. So houses are stagged in height, every 4 houses.

We happen to be the last semi-detached so next doors house (semi also) is roughly a 1 meter drop lower than ours.

I started digging the foundations yesterday to try and expose the sewer pipe, after digging about 1m. I realised the terrace next door, has a manhole lifted the cover to discover the pipe is 1.7meters deep.

As the semi is lower next to us, I imagine it starts slopping lower in ours to go under the retaining wall, we'll it must do.

I dug to 1.5m today, cannot find any clay... it's likely to go ahead with a trench fill foundation I'm looking at going over 2m deep. Which just doesn't seem cost effective.

The architect has mentioned a raft foundation, concrete pads with steel beams and also spoke of piles.

Any advice would be helpful, I have a drainage survey booked for this week.

 

[Notch7]

it's likely to go ahead with a trench fill foundation I'm looking at going over 2m deep. Which just doesn't seem cost effective.

if you build over foul drainage, foundations need to be 150mm lower than invert level.
Obv you also need a build over agreement

If the ground is good, some digger drivers can go down to about 3 metres with a machine -but it needs a 3 ton machine Id guess.

2.4m deep is doable -clearly nobody can worked on a trench that deep, but if it can be dug with a bucket its ok.

personally I would choose to go for a trench fill if you can -the cost of concrete and muckaway is prob less than other options.

the major factor is whether the trench caves in -then you can end up with loads of extra concrete (might depend on whether you need new drainage runs)

given the sloping site, Id say you will need block and beam suspended floor.

 

[Hmmm1987]

if you build over foul drainage, foundations need to be 150mm lower than invert level.
Obv you also need a build over agreement

If the ground is good, some digger drivers can go down to about 3 metres with a machine -but it needs a 3 ton machine Id guess.

2.4m deep is doable -clearly nobody can worked on a trench that deep, but if it can be dug with a bucket its ok.

personally I would choose to go for a trench fill if you can -the cost of concrete and muckaway is prob less than other options.

the major factor is whether the trench caves in -then you can end up with loads of extra concrete (might depend on whether you need new drainage runs)

given the sloping site, Id say you will need block and beam suspended floor.

Hmmm, thanks for you're input, so you feel a trench even at possibly 2.4m deep may still be cheaper than a raft foundation.

I'll have to look at prices, guess the drainage survey will help tones on working out the price of this.

Block and beam sounds a good idea also, cheers.

 

[Hmmm1987]

So let's say I dig down to about 2.4m, put footings in, then lintel over sewer pipe, would I then need to bricklay all the way up? Is there any other alternative. Seems a long way and alot of materials?

Could it be concreted up after the lintel has been installed over the pipe and drainage sorted.

 

[AsleepInYorkshire]

When reviewing costs for foundations in cost order I have a rough guide I've always used

1. Deep fill concrete trench fill (up to 2m deep) with precast concrete beams
2. Raft
3. Piles

I work in volume housing (commercial) so my guide may not apply to a small build. However, I can't imagine on an extension of this size cost will really be the decision driver. I'd look at ease and speed of build. As you've been advised you will need to take your footing below the local drainage invert level. A raft may well be better suited to the conditions you've described as the depth of dig will be reduced. A raft is called raft because it spreads the load of the building over the entire slab area and not, as with a strip foundation, in a specific point. This will depend upon the depth and distance of the local drainage. if you do opt for beam and block see if you can obtain a warm floor design. Look at Rackham Housefloors website to see th difference between a Beam and Block floor and a warm floor design.

Hope that helps and do come back and interrogate if needed.

AiY

 

[Hmmm1987]

When reviewing costs for foundations in cost order I have a rough guide I've always used

1. Deep fill concrete trench fill (up to 2m deep) with precast concrete beams
2. Raft
3. Piles

I work in volume housing (commercial) so my guide may not apply to a small build. However, I can't imagine on an extension of this size cost will really be the decision driver. I'd look at ease and speed of build. As you've been advised you will need to take your footing below the local drainage invert level. A raft may well be better suited to the conditions you've described as the depth of dig will be reduced. A raft is called raft because it spreads the load of the building over the entire slab area and not, as with a strip foundation, in a specific point. This will depend upon the depth and distance of the local drainage. if you do opt for beam and block see if you can obtain a warm floor design. Look at Rackham Housefloors website to see th difference between a Beam and Block floor and a warm floor design.

Hope that helps and do come back and interrogate if needed.

AiY

Thank you for your reply, I think if I'm to go below the sewer pipe, the depth will need to be about 2.3m.

The architect has mentioned a 1.2m beam which sits on two concrete pads. This would be above the sewer pipe. He states once we know the exact location of the drains he can calculate angles, so the pipes bear no load.

This would require a build over agreement with severn trent.

This is what's coming out the ground at 1.5m, I guessing this is just soil and sand? Is this adequate for a concrete trench foundation, as even at 2.3m I might not find clay, as the area has been excavated.

Any other suggestions?