Tricky plot - piling options

[JackOrion]

We’re building a 130sqm single storey timber framed house. Part sloping site with very tricky, limited access. It’s a beautiful spot but lots of hurdles to overcome… the latest of which is the piling cost.

 

SE seems to be taking the contractors’ lead in terms of piling method. As mentioned, access is limited (size and weight of plant) and also we need to install some of the piles on a slope. So to be fair to the SE, the pile specification is reliant on someone with good working knowledge of plant / installation etc..

 

First quote is approx £70k for 54 micropiles. These would be 115mm diameter drilled hollow bar up to 10m depth (based on ground investigation). Around £1300 per pile, excluding mobilisation etc..

 

Given the huge costs, we’re concerned that contractors aren’t necessarily exploring all possibilities, but just quoting based on their own experience and the plant they have available.

 

Wondering if some kind of independent specialist pile design consultant or similar might be an option here, to ensure we’re looking at as many methods as possible.

 

Does anyone have any recommendations as far as that might go? Or any other thoughts?

 

Noted the vibro stone columns mentioned elsewhere on BH which are something I’d never heard of before, although I’m guessing our access would rule this out. But this is exactly the kind of thing I’m wondering about… are other methods out there which might help us get these costs down?

 

Any thoughts appreciated.

 

[Mike]

45 minutes ago, JackOrion said:

timber framed house. Part sloping site with very tricky, limited access

If access is that limited, are you completely sure that it's good enough for the delivery & erection of a prefabricated timber frame?

 

55 minutes ago, JackOrion said:

we’re concerned that contractors aren’t necessarily exploring all possibilities, but just quoting based on their own experience and the plant they have available

That's certainly possible - micropiles are popular and an obvious go-to method. Helical steel piles may worth looking at - they're relatively easy to install & minimise the amount of excavation. Or, at the other end of the scale, if you can redistribute soil round the site, or remove it via the access road, it may be possibly to excavate a semi-basement and build off a raft, if the slope is the main reason for choosing piles; it may not be cheaper, but you may at least gain some extra usable space for the cash.

[Alan Ambrose]

10m sounds a long old way (although I should say that I know nothing about piles  ). Poss to post up a pic of the soil layers from the soil investigation?

[Great_scot_selfbuild]

1 hour ago, JackOrion said:

We’re building a 130sqm single storey timber framed house. Part sloping site with very tricky, limited access. It’s a beautiful spot but lots of hurdles to overcome… the latest of which is the piling cost.

 

SE seems to be taking the contractors’ lead in terms of piling method. As mentioned, access is limited (size and weight of plant) and also we need to install some of the piles on a slope. So to be fair to the SE, the pile specification is reliant on someone with good working knowledge of plant / installation etc..

 

First quote is approx £70k for 54 micropiles. These would be 115mm diameter drilled hollow bar up to 10m depth (based on ground investigation). Around £1300 per pile, excluding mobilisation etc..

 

Given the huge costs, we’re concerned that contractors aren’t necessarily exploring all possibilities, but just quoting based on their own experience and the plant they have available.

 

Wondering if some kind of independent specialist pile design consultant or similar might be an option here, to ensure we’re looking at as many methods as possible.

 

Does anyone have any recommendations as far as that might go? Or any other thoughts?

 

Noted the vibro stone columns mentioned elsewhere on BH which are something I’d never heard of before, although I’m guessing our access would rule this out. But this is exactly the kind of thing I’m wondering about… are other methods out there which might help us get these costs down?

 

Any thoughts appreciated.

 

@JackOrion sounds like a great plot, and very similar to our own one. We haven't decided just yet, but I'm looking at these helical screwpiles from Quadrabuild - https://www.quadrabuild.com/services/quadrapile/

 

The part that was particularly appealing (other than they meet the same loading capacity as the typical tubes with large helical discs), was that the installation equipment is hand-carried into place and they can run an 80m hose to the generator.

 

Although I haven't got personal experience of them yet, my sister-in-law used them on her extension and it's gone well, avoided huge foundations and allowed them to keep the stablished tree in their garden.

 

Good luck - would be interested to hear more about your build as you progress. 

Quadrapile-1.jpg

[JackOrion]

47 minutes ago, Mike said:

If access is that limited, are you completely sure that it's good enough for the delivery & erection of a prefabricated timber frame?

 

Unsure if we're going down the prefab route yet or not, but certainly can't get an artic anywhere near site. Spoke to a few suppliers (e.g. Eden Insulation), who suggested dropping their kit off at a delivery yard nearby, which would then be shuttled up to site on flat beds. The alternative would be to stick build.

 

Quote

That's certainly possible - micropiles are popular and an obvious go-to method. Helical steel piles may worth looking at - they're relatively easy to install & minimise the amount of excavation. Or, at the other end of the scale, if you can redistribute soil round the site, or remove it via the access road, it may be possibly to excavate a semi-basement and build off a raft, if the slope is the main reason for choosing piles; it may not be cheaper, but you may at least gain some extra usable space for the cash.

 

Pretty sure helical piles have been ruled out for us due to the ground conditions. Same goes for any major excavation of the slope – would likely require soil nails (more cost + large plant required!)

[Russell griffiths]

We used a steel driven pile, piling rig was about the same size as 5 tonne excavator 

the rest of the kit could be moved with a small 1.5 tonne machine. 
 

you might need to do a re design and stop thinking English house and think American or Australian, they build loads on far steeper ground than we have, minimal number of piles with a steel frame rising from these supporting the house. 
 

cut the plot into two flat areas and work from them instead of the slope. 

[ProDave]

A picture of the plot might help?

 

A neighbour here on a tricky plot built on piles, but not with a piling rig, but with a very skilled digger driver excavating a square hole at each pile position pouring a 1M square concrete pad, then forming a concrete pile on top of that, then put the soil back.

 

A bit like the post above, not many piles, widely spaced and the house was then built on a substantial timber base formed over the pile tops making it an above ground house with clear space under it.

 

Nothing more heavy on site than a 6t tracked digger.

 

Here is the resulting piles he built up from

 

[JackOrion]

58 minutes ago, Alan Ambrose said:

10m sounds a long old way (although I should say that I know nothing about piles  ). Poss to post up a pic of the soil layers from the soil investigation?

 

A photo attached here.. although I suspect you were after a borehole log?

The ground investigation took three boreholes... in summary they found up to a metre of 'cohesive made ground', below which is 'an extensive thickness of reworked clays with occasional gravel lenses revealed to variable depths' (4.5m to 10m), beyond which was the rockhead.

 

 

strata.pdf

Edited December 29, 2024 by JackOrion
Attachment failed

[Russell griffiths]

[JackOrion]

50 minutes ago, Great_scot_selfbuild said:

@JackOrion sounds like a great plot, and very similar to our own one. We haven't decided just yet, but I'm looking at these helical screwpiles from Quadrabuild - https://www.quadrabuild.com/services/quadrapile/

 

The part that was particularly appealing (other than they meet the same loading capacity as the typical tubes with large helical discs), was that the installation equipment is hand-carried into place and they can run an 80m hose to the generator.

 

Although I haven't got personal experience of them yet, my sister-in-law used them on her extension and it's gone well, avoided huge foundations and allowed them to keep the stablished tree in their garden.

 

Good luck - would be interested to hear more about your build as you progress. 

Quadrapile-1.jpg 2.38 MB · 1 download

 

@Great_scot_selfbuild Thanks for this. Will check these out. Given our position, anything man-handleable sounds really appealing but again I suspect they won't do the trick for our ground conditions, as previous screw / helical piles were deemed unsuitable. I'll keep you posted on how we get on though. Likewise keen to hear how you get on...

[Russell griffiths]

Do you have a pic of the site, and your proposed design. 

[JackOrion]

15 minutes ago, Russell griffiths said:

you might need to do a re design and stop thinking English house and think American or Australian, they build loads on far steeper ground than we have, minimal number of piles with a steel frame rising from these supporting the house.

 

Funnily enough that's pretty much what we've got... the slope is only beneath one part of the house, which is already split into two sections to make the most of the flattest areas of the plot. At this point the building is essentially on stilts with a steel frame above the pile caps.

[Great_scot_selfbuild]

1 minute ago, JackOrion said:

 

@Great_scot_selfbuild Thanks for this. Will check these out. Given our position, anything man-handleable sounds really appealing but again I suspect they won't do the trick for our ground conditions, as previous screw / helical piles were deemed unsuitable. I'll keep you posted on how we get on though. Likewise keen to hear how you get on...

@JackOrion I'm guessing your subsoil ground conditions are very different to ours if you're looking at possibly 10m depth. Ours are likely to be 2-3m before refusal (sand). Our ground investigation results were topping out with a N=50 around 2.5m on average.

[JackOrion]

1 minute ago, Great_scot_selfbuild said:

I'm guessing your subsoil ground conditions are very different to ours if you're looking at possibly 10m depth.

 

Exactly that! 2-3m refusal on each ground investigation borehole would have been a much better result for us.

[JackOrion]

@Russell griffiths @ProDave

3D model attached here, alongside elevation showing sloping 'stilt' detail. (completed design has quite a bit less glazing etc, but this should give the idea)

 

The new house is in two parts and wraps around the hillside, utilising the flattest parts of the plot (and avoiding protected trees).

 

Existing structures are greyed out – it's an old walled garden, which was pretty much a gardeners' tip when we bought it, but is in the grounds of a listed building (downhill, out of shot). So along with the geography we are fairly limited to what we can do.

 

Interestingly, the existing brick potting sheds (greyed out on the left of 3d model) are on minimal foundations (built in 1800s), and slope downhill with the natural terrain, and haven't moved an inch.

 

Image 1.pdf Image 2.pdf

[ProDave]

That design is so similar to what my neighbour did.

[JackOrion]

27 minutes ago, ProDave said:

That design is so similar to what my neighbour did.

 

Would be interested to see it!

[ProDave]

33 minutes ago, JackOrion said:

 

Would be interested to see it!

Sory i don't have a picture.

 

But those piles in the picture above support a rectangular building.  The wall in the foreground supports the end of the building at ground level and the rest of the building is supported on the widely spaced piles.

 

The point of the post is rather than try and get a piling rig on soft sloping ground they did it all with a digger. Removed a lot of top soil and then for each pile dug down to something solid then cast a 1 metre square of concrete then formed a poured in place concrete pier onto that concrete pad.  Then put most of the soil back then membrane and gravel to top it all off.

 

All the tops of the piers are at exactly the same height and gives you an idea of the slope.

[JackOrion]

3 minutes ago, ProDave said:

Sory i don't have a picture.

 

But those piles in the picture above support a rectangular building.  The wall in the foreground supports the end of the building at ground level and the rest of the building is supported on the widely spaced piles.

 

The point of the post is rather than try and get a piling rig on soft sloping ground they did it all with a digger. Removed a lot of top soil and then for each pile dug down to something solid then cast a 1 metre square of concrete then formed a poured in place concrete pier onto that concrete pad.  Then put most of the soil back then membrane and gravel to top it all off.

 

All the tops of the piers are at exactly the same height and gives you an idea of the slope.

 

Ah, I see what you mean there now, helpful to see it. I think the problem here comes down to how far beneath ground 'something solid' is...

[Russell griffiths]

I like the design, exactly what I was thinking. 
 

the problem I’ve seen with mini percussion pile rigs is there a bit pathetic, so you end up putting in more piles than is needed to achieve the same goal. 
 

I would do an assessment of your access and send it out to a few companies and see what sort of rig they have to fit your site. 
 

TBH I can’t see it being a cheap job. 

Edited December 29, 2024 by Russell griffiths

[JackOrion]

Thanks @Russell griffiths

 

I think that's where we're at. The quote we have so far is for a mini excavator with a drill mast (which will work to install the piles on the slope as well as on the flat part of the plot)

 

The problem with the small diameter piles is as you say, many more are needed (£££) than if we had a more regular rig installing 300mm piles for example.

 

Will follow your advice and reach out to more companies though.

 

Agreed, it's definitely not looking like a cheap job.

Edited December 29, 2024 by JackOrion

[saveasteading]

7 hours ago, Alan Ambrose said:

10m sounds a long old way (although I should say that I know nothing about piles

I've had a lot done so let me try to help.   I'm not disagreeing with anything above.

10m is a long way indeed. That doesn't seem right for a domestic loading, so a completely different piling method may be better.

These will be end bearing and the ground above must be very poor. Do not discount the big companies as they have knowledge and various options that are not end bearing.

 

You say 10m long but it is on a very steep slope. Is the underlying solid surface the same slope then?

 

If the ground is so poor and sloping has anyone checked that it won't all slide downhill? Piles don't like bending.

 

The photo from @ProDaveis like a building I did 25 years ago on decent ground. Just pads and columns up to a common level. I used plastic pipes for column shutters too.

It's still there.

The bits you see are not necessarily by the piling company, but a groundworker, so cheaper.

 

We really are guessing though without the ground report. It says 'download failed' .

 

 

 

[ToughButterCup]

17 hours ago, JackOrion said:

...

Any thoughts appreciated.

 

 

In haste ... similar though maybe not so severe an access challenge. I've written about our piling in detail: quick site search will do.

Bottom line - ground improvement columns - Town and Country Vibro. Very professional approach 

£6500, 52 piles. 

 

£70k is a try-on

 

 

[JackOrion]

9 hours ago, saveasteading said:

10m is a long way indeed. That doesn't seem right for a domestic loading, so a completely different piling method may be better.

These will be end bearing and the ground above must be very poor.

 

Thanks @saveasteading

I don't have the ground report to hand at the moment (also, it's a huge file), but in short, bedrock is indicated to be 8m below ground.

 

The 10m depth includes 2m drilled rock sockets. It's a relatively lightweight building (pile loads calculated at 75kN compression).

 

 

Quote

If the ground is so poor and sloping has anyone checked that it won't all slide downhill? Piles don't like bending.

 

 

Yes, the slope is stable. This has been established by the original ground investigation engineers plus at least two other geotech consulting engineers. That said, the rock socketed piles can withstand potential lateral force from this slope if required. The plan is for the slope to be more stable after construction.

 

Quote

Do not discount the big companies as they have knowledge and various options

 

This is a good point. Initial thoughts were they wouldn't be interested in a small project and would charge the earth... but no harm in trying. Assuming you're referring to the likes of Bullivant / Van Elle etc? Any others from the top of your head?

[JackOrion]

54 minutes ago, ToughButterCup said:

In haste ... similar though maybe not so severe an access challenge. I've written about our piling in detail: quick site search will do.

Bottom line - ground improvement columns - Town and Country Vibro. Very professional approach 

£6500, 52 piles. 

 

Thanks @ToughButterCup – yes, I'd read your previous posts with interest... I can't reach their website at the moment, but I see from their Facebook page that they sponsor a football team not too far from us. Will definitely be giving them a shout, though I fear access constraints will rule them out.