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Housing - New Build: Mini-piles vs padstones

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  1.  
    Hi all,

    I have posted about my foundation woes before on ebuild, but sadly it is no more so I'm updating here instead :)

    Our site is overlaid by 1-1.7m of organic topsoil, which not only is uneconomical to get rid of (no space to lose it on the urban site) but we cannot drop the level of the house that far down, as it is surrounded by other buildings and we would lose a lot of the light into the ground floor.

    So, we have the choice of mini-piles or padstones. Both would tie into a network of ground beams, which will be formed as part of our insulated slab (MBC), which will sit on top.

    We have received a quote of approx 5.5k for 35 driven 4m piles, with a meterage rate below that. I'm waiting on another piling quote to see whether this one is way out.

    So, what's going to be most cost effective? Piles or padstones with rising columns? The ground floor layout is 205sqm, access for a large digger if required.

    The piling guy has estimated a week to complete the piles, but we have a very good digger guy.

    Any advice or experience most appreciated.

    dj
    •  
      CommentAuthordjh
    • CommentTimeMay 28th 2016
     
    Posted By: divorcingjackSo, we have the choice of mini-piles or padstones. Both would tie into a network of ground beams, which will be formed as part of our insulated slab (MBC), which will sit on top.

    I'm confused about how padstones would work here. My understanding is that padstones are small isolated foundations that sit on soil that has good bearing and that serve as a base for lightweight post and beam construction. So I don't see how you could use them on poor soil to support a concrete raft. I'd expect the bearing pressure of a concrete raft to be lower.

    What is the bearing capacity of your soil, by the way?

    The piling quote doesn't sound expensive to me - it's about half the quote I got - but how deep will the piles actually need to go? What would be the effect on cost if they needed to go down say another metre?
    • CommentAuthorEd Davies
    • CommentTimeMay 28th 2016
     
    For comparison, my 36 concrete bases (0.5m cubes, basically) cost about 3.5k (IIRC) for labour, digger, concrete, threaded rod and resin (but not the steel brackets).

    The geology was pretty simple: topsoil, clay, broken rock, solid rock with the solid rock only about 500 mm down. The topsoil had already been removed and hardcore laid down (separate contract with somebody else as part of the site access preparation - actually done the previous year after I had planning permission but before the building warrant - yes, I checked that was OK with the building control people).

    https://edavies.me.uk/2015/03/bolted-to-scotland/

    Day 1: one chap with digger dug most of the holes using some of the extracted hardcore to extend the area a bit (I'd not made any allowance for using more hardcore beyond the end concrete pads as I'd assumed they'd use some shuttering).

    Day 2: said chap finished off the holes and BCO came mid-morning, peered in the holes and pronouncing himself happy if we got the SE's approval for a few of them which had got to solid rock before 500 mm below the intended tops of the concrete or we chipped them out to full depth. SE uncontactable so I chipped one out by hand (well, chisel) while the builder went for a breaker and did the other two.

    Day 3: Builder plus two labourers filled the holes with about 6 m³ of concrete delivered by a lorry using barrows for the rows away from the access track then compacted them down and smoothed the tops.

    Day 4: Builder drilled the bolt holes and pushed in threaded rod for most of the brackets.

    Day 5: Builder did the last brackets in the morning.

    The other quote I got was a lot more but that builder was busy with bigger projects and not exactly desperate for the work. He'd have put shuttering around the pads, at least at the top, which would have used a bit less concrete but a lot more labour.

    So 5.5k for 35 piles seems to me to be quite sensible. Two people (I'd assume) for a week, a more specialized bit of kit to knock them in and quite a lot of steel.
    • CommentAuthordickster
    • CommentTimeMay 28th 2016
     
    Both builder and architect said trench foundations would be OK, we had a ground survey done first, mini rig with big weight and 1 metre sectional core. It disappeared on first hit, the bloke said it didn't look too promising.. Having found clay, gravel and incredibly loose sand within 5 metres of each other and a stream of water underneath a 1 inch layer of black clay, I insisted on piles.

    Only 10 of them, the pile driver man suggested we stop when at 11 metres, we were still going strong. Given that the house weighs virtually nothing, we were happy with that and now live above a ten legged concrete topped table.

    Cost around 11K and lots of cups of tea.
    • CommentAuthorEd Davies
    • CommentTimeMay 28th 2016
     
    Posted By: dicksterOnly 10 of them, the pile driver man suggested we stop when at 11 metres, we were still going strong.
    Do they have a rule of thumb for that? Something like when it takes more than so much impulse (newton seconds or however they'd put it) to knock it in a millimetre it's good enough?

    If the house is on piles then what's the point of concrete above? I'd have thought a pure timber structure (as I think Dickster's is above ground level) would make a lot more sense as if 11 m isn't quite enough, so there's some small movement in the future, timber would absorb the flex much better.
    • CommentAuthorborpin
    • CommentTimeMay 28th 2016 edited
     
    I looked at a system called 'system first' from Roger Bullivant which was minipiles and a galvanised steel framework that held the insulation. Looked really good although I think they may not do it any more. http://www.roger-bullivant.co.uk/products/mini.cfm There are some vids on YouTube.
    •  
      CommentAuthordjh
    • CommentTimeMay 28th 2016
     
    Posted By: Ed DaviesDo they have a rule of thumb for that? Something like when it takes more than so much impulse (newton seconds or however they'd put it) to knock it in a millimetre it's good enough?

    That's why I prefer the concept of screw piles - you know how much load they'll take as you put them in.

    If the house is on piles then what's the point of concrete above? I'd have thought a pure timber structure (as I think Dickster's is above ground level) would make a lot more sense

    When I priced timber on top of piles it was going to be a lot more expensive, not to mention difficult to even find a quote.

    Which was why I ended up with a big hole full of hardcore and a passive slab.
    • CommentAuthordickster
    • CommentTimeMay 29th 2016
     
    I would have loved to have had an all timber construction, but it was a step too far. Finding an architect and builder willing to build a weird house out of timber I beams was hard enough, couldn't find any good references for timber ground beams 7 years ago and we are 600mm below ground level in places to avoid ridge height restrictions.

    Have just seen some brand new eco lodges with timber ground beams, lovely, but well above ground on stilts.

    The pile driver said if piles only travelled 10 mm per thump, then that was good, we nearly managed it, but we would go through what I assume were layers of gravel and then keep going! I think it's the friction of the surface of the pile (steel tubes) that builds up which is how it works.

    The 11K was for the complete foundation.

    Hope this helps.
    • CommentAuthorEd Davies
    • CommentTimeMay 29th 2016
     
    Posted By: dicksterand we are 600mm below ground level in places to avoid ridge height restrictions.
    Yes, that makes timber more tricky. Apart from that, Segal on piles ought to make sense; it was something I had in mind as a possibility when looking for a site before moving to an area which is rock a few hundred mm down (if that) pretty much everywhere. Screw piles looked attractive for the reasons DJH mentioned.

    …if piles only travelled 10 mm per thump, then that was good…
    Was puzzling about what the units would be, joules, kg·m·s⁻¹ or what. Thumps will do. :tongue:
    • CommentAuthorbarney
    • CommentTimeMay 29th 2016
     
    You don't necessarily need to thumping in the piles

    we often use continuous flight auger piling - ie basically drill the hole, drop in a re bar cage and pump the concrete.

    You can get pile cap formers, so you can level for vertical members, ground beams etc to suit the ground conditions

    Even a 300mm dia augered pile has a hell of a bearing capacity

    You do need a good idea of the ground conditions though

    There is a small development of what appear to be totally timber frame houses (although I think the sub frame at GF level is steel) sitting on "short" 600mm dia augered piles (circa 1.8m) - the whole site for about 14 houses was done in less than a week - that's in Cumbria, so rock head my be close with just clays and gravel (basically junk) over it

    Regards

    Barney
  11.  
    <blockquote\>I'm confused about how padstones would work here. My understanding is that padstones are small isolated foundations that sit on soil that has good bearing and that serve as a base for lightweight post and beam construction. So I don't see how you could use them on poor soil to support a concrete raft. I'd expect the bearing pressure of a concrete raft to be lower.

    What is the bearing capacity of your soil, by the way?

    The piling quote doesn't sound expensive to me - it's about half the quote I got - but how deep will the piles actually need to go? What would be the effect on cost if they needed to go down say another metre?</blockquote>

    Thanks so much for all the responses, I will try to answer the questions as they come :)

    We are going for an insulated MBC slab and the SE has suggested that he can move a layer of the insulation to create formwork for ground beams - essentially thickening the slab instead of having separate beams as part of the pile quote. He has advised that this may add a little to the steel cost, but not too much extra concrete and would be the most cost efficient way of doing it. The soil is not poor quality, it's good bearing sand, just covered with too much black earth/topsoil. The piles/padstones are solely to avoid dropping the house level and losing available light.

    The theory behind the padstones/piles is that they would penetrate the insulation to meet the ground beams within the slab, so losing a little thermal efficiency, but hopefully not too bad.

    We've not had a full soil investigation (2K!) but dug a big hole to 4m, and were hitting cobbles/stone at that level. It would be about £1000 per extra metre below 4m for the whole site.
  12.  
    eddavies, thank you so much for that detailed breakdown, it really helped me understand what's involved in the padstones. As our site is on a slope, I believe that they would have to use shuttering, so my 5.5K for piles doesn't look too bad.

    The guy said it was a 2 man crew for a week, so you're right.
    •  
      CommentAuthordjh
    • CommentTimeMay 31st 2016 edited
     
    The simplest thing to do is dig a big hole and fill it with either hardcore or a bulk of EPS - that will have the lowest ground pressure. If that would be so deep that it proves too expensive then piles are a good alternative. The concern with piles is how to engineer the connection to the slab so that there is no thermal bridge.

    Who is doing your thermal calculations? (and how are they modelling the foundations?)

    Has your piling contractor had a look at your hole and given you some comfort about the depth of piles required?

    Here's a brief description of a passivhaus that used GRP piles to avoid thermal bridging. http://www.building4change.com/article.jsp?id=2311
  14.  
    <blockquote><cite>Posted By: Ed Davies</cite><blockquote><cite>Posted By: dickster</cite>Only 10 of them, the pile driver man suggested we stop when at 11 metres, we were still going strong.</blockquote>Do they have a rule of thumb for that? Something like when it takes more than so much impulse (newton seconds or however they'd put it) to knock it in a millimetre it's good enough?

    If the house is on piles then what's the point of concrete above? I'd have thought a pure timber structure (as I think Dickster's is above ground level) would make a lot more sense as if 11 m isn't quite enough, so there's some small movement in the future, timber would absorb the flex much better.</blockquote>

    We would certainly consider a timber floor structure, but we have to have a build warranty for a mortgage, and MBC are offering a 10 year warranty if we go for their insulated slab - also the peace of mind of having one contractor for the whole thing.

    The quotes we have are using 150mm permanently steel cased, cast-in situ piles , driven using a grundomat. The other one I believe is speccing 220mm piles, but is 30% more expensive.

    Barney - this continuous flight augering sounds good, as do screw piles, but I can't find a company to bloody quote for them! It seems like a very bad idea to pick one off the internet at random, so if anyone has recommendations for people that work in Scotland, they would be much appreciated.

    Hopefully getting a digger guy to give us an estimate this week.
  15.  
    djh - thanks for your comments, answers below.

    It wouldn't be a square hole, it's basically a wedge, varying from 1.7m at one end to approx 1m at the other. Full footprint is 205sqm, we reckon about 5-600 tonnes to move out.

    Couple of questions:

    - Is is possible to form hardcore to that depth eg. 1.7m deep?
    - What do you mean by bulk EPS? Just filling it with blocks of EPS eg extending the foundation slab downwards?

    That sounds like genius!

    Thermal calcs are by Hilliard Tanner from MBC, he admits that there will be a small thermal bridge where the piles penetrate the EPS layer, but the cross section should be small and he says the thermal impact should be minimal.

    dj
    •  
      CommentAuthordjh
    • CommentTimeMay 31st 2016
     
    Posted By: divorcingjack
    It wouldn't be a square hole, it's basically a wedge, varying from 1.7m at one end to approx 1m at the other. Full footprint is 205sqm, we reckon about 5-600 tonnes to move out.

    You mean 1.7 m deep to 1 m deep? That's almost exactly the same as the hole under my passive slab and about the same area as well (we have trees).

    Is is possible to form hardcore to that depth eg. 1.7m deep?

    Yes you can make it as deep as you like - the only trick is to roll it in layers of 6" or so as you go. My contractors had a full size digger and a middling size roller on site and they have a whole bunch of lorries for muck away and to bring the hardcore.

    What do you mean by bulk EPS? Just filling it with blocks of EPS eg extending the foundation slab downwards?

    Yes, it's quite a common way to build railway and motorway embankments apparently. Presumably cheaper on the transport costs than hardcore. I just went with ordinary Type 1 because it was simpler.

    Thermal calcs are by Hilliard Tanner from MBC, he admits that there will be a small thermal bridge where the piles penetrate the EPS layer, but the cross section should be small and he says the thermal impact should be minimal.

    Hilliard did the structural calcs for our slab and he was very helpful, but my architect had to do the thermal calcs. I'm not sure exactly why Hilliard didn't want to do them. Both my architect and myself were a little surprised because we thought he would have some formulae already worked out for his passive slab designs. We are going for PH certification though, so we did need them done properly.

    ABC Anchors were who I talked to about screw piles. There's also another newish system that uses several poles put in at angles and tied together at the top that looks quite interesting.
    • CommentAuthorEd Davies
    • CommentTimeMay 31st 2016
     
    Posted By: djh: “Presumably cheaper on the transport costs than hardcore.”

    That can be a big part of the deal. The hardcore itself is a small part of the price, delivery can be a bigger bit.

    We (the contractor doing the access and site prep, actually) got a quote for hardcore from a quarry about 25 km away by road. In the end we got it from my neighbour's borrow pit just up the road which involved a bit more labour for the contractor (they needed a digger in the borrow pit and a couple of dumper trucks to shuttle back and forward) but still knocked about £1000 off the price. Neighbour said “split the difference” on the cost so I guess getting it from the quarry would have been about £2000. Contractor paid him directly so I don't know the exact amount.

    That was for entry service bay, 70 metre track, parking area, and 200 m² house site.

    https://edavies.me.uk/2014/05/access/

    Getting rid of more than 200 m³ of soil won't be free, either, if you can't just scatter it on the rest of the site.
    •  
      CommentAuthordjh
    • CommentTimeMay 31st 2016
     
    Posted By: Ed DaviesGetting rid of more than 200 m³ of soil won't be free, either, if you can't just scatter it on the rest of the site.

    Indeed. Local farmers are the favourite place to look for somebody willing to accept it. My neighbour was happy to help unloading and positioning my bales (which he also couldn't supply) but didn't want soil so the contractor took it who knows where.
  19.  
    Posted By: divorcingjackbut we have to have a build warranty for a mortgage, and MBC are offering a 10 year warranty if we go for their insulated slab - also the peace of mind of having one contractor for the whole thing.


    Have you checked with your finance supplier that what MBC offers is what they require? It sounds like they require something like a NHBC warranty which would be very different from a company guaranteeing their own work for 10 years. Unless MBC are doing the whole lot (ie handing over a fully finished house) then I would be surprised if the finance company would find their offering sufficient even if it is insurance backed (which I don't know if it is).
  20.  
    Hi again all,

    Some interesting updates for you over the last couple of days.

    djh, that is really interesting to see that you did a similar thing and very reassuring. Did you build the whole lot up to the lowest level, or take it above surrounding ground level? We would ideally like to build it up to the high point. If you don't mind me asking, how many tonnes of hardcore did it take? Did you hire a roller to compact it or have a contractor?

    All of you have a fair point about removing the soil, but this week a vague acquaintance has come into the need for approx 400 tonnes of soil, which by a lucky coincidence is roughly what we have to remove from site. The acquaintance has offered to pay for the removal and transportation of the material from the site.

    So, after that development, it looks like the digging out and hardcore option is going to be the cheapest. However, we have a couple of things to bear in mind.

    We are an urban site and digging close to some other buildings. Firstly, a row of 1950's prefab garages, which are sitting on thin concrete slabs with no foundations as far as we can see. Next, a wobbly stone boundary wall with very shallow foundations and missing stones.

    Obviously, we don't want to cause any damage - I have uploaded the site survey with distances from the wall and garages - do you think we would be safe digging so close. Maximum depth is about 1.7m at the garage end, going down to 1m.

    Thanks,
    dj
  21.  
    willie.macleod - you're right, the mortgage company will require an official self build warranty, but it's more having the peace of mind of one contractor doing the foundations and connecting superstructure. We have heard some horror stories about companies blaming each other, projects delayed, spiralling costs etc. We can just about afford to have them do it all, and it seems a small price to pay to avoid the potential hassle. The guarantee is just an additional bonus.
  22.  
    Files here - site survey and footprint plans, hope they make sense.
  23.  
  24.  
    Posted By: divorcingjackSo, after that development, it looks like the digging out and hardcore option is going to be the cheapest. However, we have a couple of things to bear in mind.

    We are an urban site and digging close to some other buildings. Firstly, a row of 1950's prefab garages, which are sitting on thin concrete slabs with no foundations as far as we can see. Next, a wobbly stone boundary wall with very shallow foundations and missing stones.

    Obviously, we don't want to cause any damage - I have uploaded the site survey with distances from the wall and garages - do you think we would be safe digging so close. Maximum depth is about 1.7m at the garage end, going down to 1m.

    As a general rule of thumb if you take a line of 45 deg downwards and outwards from the bottom of the foundations of the adjacent structure then your excavations should not go below this line. This however may be modified by actual site conditions and soil type and a surveyor should be consulted if you are close to this line, but it should be a good indication as to whether to expect problems.
    • CommentAuthorringi
    • CommentTimeJun 3rd 2016
     
    Posted By: divorcingjack
    All of you have a fair point about removing the soil, but this week a vague acquaintance has come into the need for approx 400 tonnes of soil, which by a lucky coincidence is roughly what we have to remove from site. The acquaintance has offered to pay for the removal and transportation of the material from the site.


    Would they take more soil? If so what about a basement....
    •  
      CommentAuthordjh
    • CommentTimeJun 3rd 2016
     
    Posted By: divorcingjack
    djh, that is really interesting to see that you did a similar thing and very reassuring. Did you build the whole lot up to the lowest level, or take it above surrounding ground level?

    We dug the hole then filled it with hardcore such that when the EPS plus slab was added, the top of the slab was just about level with the ground at the back. There's a slight slope down to the front, so the rest of the slab is a bit above ground. There's a 600 mm wide corridor around the house where the French drain sits in the top of the hardcore and the space above is filled with gravel to 150 mm below the top of the slab. The patio at the back extends across the gravel and gives us our level access into a sliding door.

    We would ideally like to build it up to the high point. If you don't mind me asking, how many tonnes of hardcore did it take? Did you hire a roller to compact it or have a contractor?

    I don't remember quantities I'm afraid. I had a firm of contractors that did all the groundworks and laid the insulation and slab.

    All of you have a fair point about removing the soil, but this week a vague acquaintance has come into the need for approx 400 tonnes of soil, which by a lucky coincidence is roughly what we have to remove from site. The acquaintance has offered to pay for the removal and transportation of the material from the site.

    That's brilliant!

    So, after that development, it looks like the digging out and hardcore option is going to be the cheapest. However, we have a couple of things to bear in mind.

    We are an urban site and digging close to some other buildings. Firstly, a row of 1950's prefab garages, which are sitting on thin concrete slabs with no foundations as far as we can see. Next, a wobbly stone boundary wall with very shallow foundations and missing stones.

    My garage sits on a raft that does NOT have a big hole of hardcore underneath it. If the ground in front of the garage ever heaves, I want the garage to heave with it. But it is a raft so if it ever moves it will do so as a unit and not break.

    You'll need to check with the engineer and surveyor about what precautions and support are needed for your hole. Don't forget that the hole will be a bit bigger than the slab, to allow for load spreading and to provide space for the French drain, so check the drawings carefully.

    Good luck. Cheers, Dave
  27.  
    Hi all,

    Thank you for your additional comments - we're now awaiting the contract to sign so that we can get an approximate start date and start putting together the building warrant application. The edge of the hole is very tight on one edge to the wobbly wall (about 800mm), so we are meeting an engineer on site next Wednesday to get his input.

    The general feeling that we've had so far is to dig a trench in sections, then either fill it with concrete or packed hardcore to stabilise the ground close to wall and garages, then dig out the middle bit.

    This is all fine, but our friend who wants the soil really needs it ASAFP, so my question is - do you think we could proceed with this section under the banner of "site preparation" without a building warrant? After all, it's not the foundation, it's just an area of hard-standing that happens to be underneath where the house will go!

    Apparently, areas of paving or hardstanding under 200sqm are exempt from building warrants.

    Thoughts?

    dj
  28.  
    ringi - noooooooo! We have been round the houses with the basement vs no basement idea, and eventually came to the conclusion that it had the potential to just be too hideously expensive and complicated.

    We decided that we'd rather have a smaller finished house that we could enjoy, than an unfinished basement and a load more debt. The footprint is very large, so the costs were going to be insane.
  29.  
    Posted By: divorcingjack
    we're now awaiting the contract to sign so that we can get an approximate start date and start putting together the building warrant application.


    Stop! If you are in Scotland you are not going about this the right way and signing any contract just now is madness. You need the building warrant sorted out first, otherwise you can find you have contracted with someone to do something you aren't going to be allowed to do. A building warrant needs to be approved before any work can begin and depending on what council you are with it can take months to process, especially for something big and different. Don't start doing anything till you have the paperwork in order. You will get fewer BC visits than those south of the border but they will want to see the groundworks as one of the visits. BC is a very different matter north of the border. There are far higher standards to be met and you can't contract it out to any 3rd party companies.
    •  
      CommentAuthordjh
    • CommentTimeJun 8th 2016
     
    Posted By: willie.macleodBC is a very different matter north of the border. There are far higher standards to be met and you can't contract it out to any 3rd party companies.

    It's interesting how different the systems are - I for one hadn't realized quite how much.

    There does seem to be a possibility to use third-party Approved Certifiers for Design and Approved Certifiers of Construction and certified contractors for services according to
    http://www.gov.scot/Topics/Built-Environment/Housing/BuyingSelling/self-build/guide/buildingwarrants

    How does that all work?

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