Piling vs. an alternative…????

[Wuey]

Hi everyone 

We’ve been told we need to have piling when we build a partial (3/4) subterranean 4-car garage (i.e. it’s a reasonable size space to dig out/work in etc) . This solution came about because those contractors who may end up with the contract have said that the safest way to carry out the work given our site.
This solution is obviously at a much greater cost than we were anticipating in terms of how we support the remaining ground whilst the work is being done. 

There is a difference in height from the land at the front to the rear of the proposed garage of roughly 7m. We obviously wouldn’t want to endanger any contractors during the build but we are questioning if this really is our only option, given the horrendous price involved for piling this area (approx 60k  )

Is there any way that steel piling or some other non-piled solution could be feasible?

We’re first time self builders so don’t have building/engineering knowledge other than what we’re picking up on this project so any insights from those who know far more than us would be gratefully received.

TIA

[Bitpipe]

Do you mean sheet piling around the perimeter to prevent landslip during excavation?

 

On a level site with sufficient space the cheapest method would be having a sloped mouth to the excavation  - known as battering back. The internal construction provides the long term means of holding back the surrounding ground and the gap is backfilled with something incompressible (rock) or layers of compacted spoil.

 

On a tight site, or if the ground is unstable (made ground etc) then sheet piling may be an option. They are removed and sold for scrap post construction.

 

Another approach is creating a wall of poured in situ piles which then form the perimeter wall.

 

OR do you mean piling as a foundation system for the excavation itself (i.e. under the slab) due to poor ground conditions?

 

Ultimately you should have a structural engineer making these decisions based on a comprehensive ground investigation report - probes, cores, water & gas levels plus contamination etc.

[Wuey]

Hi Bitpipe,

 

We'd originally hoped we could batter back then use the garage structure as the land support,  as you say in your reply.  The site is relatively tight (in that we have neighbours and the neighbouring gardens also have the same height drop as us, so contractors are concerned that with all the vibrations, the neighbouring gardens may slip too.  Sheet piling was never mentioned as an option, we seem to have jumped from standard battering to extreme contiguous piling in one go, but we're wondering if sheet piling is the compromise solution, in that yes it incrreases our costs but not as much as contiguous piles would.

 

The contractors asked our SE to do a site visit with them and after that meeting, contiguous piles was the solution - so the ground (ours and neighbours) doesn't slip whilst the work is being done.  I guess what we're wondering, is  - is this being proposed because it easily removes the risk from the SE and contractors but isn't necessarily our only option, its just the contractors prefer to work in this way?

[George]

Having seen a few cases where this has gone wrong (relying on vertical cuts to self-support), if you haven't got room to batter back then contiguous piling is probably your best bet. If vibrations are a concern then sheet piling is not going to be any better than a auger pile rig. 

 

Things like soil nailing and more elaborate temporary works is possible, but you'd need expressed permission (else trespass) and it'll be slower and overall may not save you much.

 

Although it's money to just get out of the ground, I wouldn't try and skimp. 

[Bitpipe]

7 hours ago, Wuey said:

Hi Bitpipe,

 

We'd originally hoped we could batter back then use the garage structure as the land support,  as you say in your reply.  The site is relatively tight (in that we have neighbours and the neighbouring gardens also have the same height drop as us, so contractors are concerned that with all the vibrations, the neighbouring gardens may slip too.  Sheet piling was never mentioned as an option, we seem to have jumped from standard battering to extreme contiguous piling in one go, but we're wondering if sheet piling is the compromise solution, in that yes it incrreases our costs but not as much as contiguous piles would.

 

The contractors asked our SE to do a site visit with them and after that meeting, contiguous piles was the solution - so the ground (ours and neighbours) doesn't slip whilst the work is being done.  I guess what we're wondering, is  - is this being proposed because it easily removes the risk from the SE and contractors but isn't necessarily our only option, its just the contractors prefer to work in this way?

 

Do the contiguous piles form the garage wall or are they in addition to it? Just wondering if you're being asked to double up on structure.

 

Ultimately your site needs to be safe to work on and the groundworker / SE will rightly be be risk adverse in that respect, however you may be able to get a second opinion - do you have a detailed ground investigation report, site topography plan etc that you could take to another professional for their opinion?

 

[Mr Punter]

We have done king post walling in the past as temporary support, but steel was cheap then.  These involved augering holes at 2.5m centres down 6m, lowering in a 6m I section steel and concreting in the bottom 2m.  Next day dig out between the steels in sections and fill the gaps with railway sleepers, trimmed to size with chainsaw.

[Wuey]

Thanks everyone for your replies.  We're working through all this now with our architect/SE to see if we can make savings anywhere without compromising safety, nor doubling up on required support etc.