The Fun Irish (House)

  On 13/04/2023 at 09:11, Iceverge said:

don't know what the engineering term for this is.

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Trench fill.

It depends how wide the footing needs to be. If very wide it can be a lot of concrete, so you might split the difference and start blockwork at about 1m down.

  On 13/04/2023 at 09:11, Iceverge said:

These guys are in Cork. 

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1. Their home page kind of confirms what I say above. 

Infrastructure, temporary buildings. 

 

2. Basements are silly expensive. Have done one successfully, only because the original architect had designed it in ( as a car park that cars couldn't exit) and it became something else. 

So expensive, and such a lot of hassle.

Built to keep water out, but it also kept it in, during construction. Does it rain in Ireland?

  On 13/04/2023 at 09:28, saveasteading said:

Their home page kind of confirms what I say above. 

Infrastructure, temporary buildings

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https://www.screwpile.ie/housing

 

This page has some residential links. From memory when I looked at their Facebook (before I quit it) the screw piles were to bolster a concrete ring beam. 

 

I toyed with the idea of a screwpile only foundation. I liked the lack of concrete and speed and novelty. I quickly ran back to concrete when I saw the cost and our ground didn't need any special treatment. 

 

  On 13/04/2023 at 09:28, saveasteading said:

Basements are silly expensive. Have done one successfully, only because the original architect had designed it in ( as a car park that cars couldn't exit) and it became something else. 

So expensive, and such a lot of hassle.

Built to keep water out, but it also kept it in, during construction. Does it rain in Ireland?

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Rain . LOL. 

 

Very occasionally it doesn't rain is probably a better way to put it. 

 

It depends on site value I guess but if you had to put 2.6m concrete "walls" into the ground it would seem a missed opportunity not to put a floor in too. A sump and pump and you're done, even just as storage area/plant area. 

 

A contractor built me a 4.6m X 17.7m X 2.7m deep slurry pit in 2020 for €17.5k. 

 

 

 

 

Nice work and price. But add waterproofing the walls and base, muck away, the stair, fire escape provision,  and it is expensive for a wine and potato store.

So TSD came back with a proposal using 2.5m Foundation Screws with a trial screw and pull-out test to prove its capacity. Based on another post on this forum that used this method I think this is the answer I was hoping for. Less weather dependant and while it drives up the cost it avoids 2.2m trenches less than 2.5 meters from the existing house and having to check the depth of those foundations (you draw a 45 degree line from the bottom of those existing foundations and if you cut into those you're in trouble). They will design a ring beam to carry the outer block leaf and design the beams within the foundations to span from screw to screw. The cost for the design is slightly more but is affordable. Will need to see what the Screw system costs and then the raft foundation itself when it's repriced.

Now just need legal issues sorted out to proceed with my sale (easements rejected by buyers solicitor) and find a plumber! There's a trade show next month so will harass the vendors there! 

Thanks for all the discussions! 

 

  On 14/04/2023 at 09:10, mike2016 said:

less than 2.5 meters from the existing house

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Interesting. Please keep us informed how this proceeds.

In summary, the screw system will cost a fair bit more but reduce hassle?

 

Please remember that, from your SE information, the foundations will not be 2.5m deep everywhere, some being distant from the trees. It reduces to 1.3m, and you will still be digging at least 0.5 out to concrete around the screws.

 

It's up to you of course. Make sure you get a fixed price for the screws and the SE knows your plans....I am guessing that SE will add reinforcement to the concrete, and the groundworker will have to hand dig around the screws....but I look forward to hearing more.

 

One more concern. How long have the trees that were in the way, been gone? It takes a year or so for the ground to adapt to the reduced water demand.

From Wikipedia "Screw piles were first described by the Irish civil engineer Alexander Mitchell in a paper in Civil Engineer and Architect's Journal in 1848".

 

I wonder if there are any lifetime analyses anywhere? Seems to me with steel, we like to go with the working assumption that 'it lasts for ever' when it clearly doesn't.

 

I'm not trying to generate any extra anxiety here - but I would like to know.

  On 14/04/2023 at 10:34, Alan Ambrose said:

From Wikipedia "Screw piles were first described by the Irish civil engineer Alexander Mitchell in a paper in Civil Engineer and Architect's Journal in 1848".

 

I wonder if there are any lifetime analyses anywhere? Seems to me with steel, we like to go with the working assumption that 'it lasts for ever' when it clearly doesn't.

 

I'm not trying to generate any extra anxiety here - but I would like to know.

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The West Pier in Brighton was founded on screw piles.  Opened in 1866, closed 1975 so over 100 years in a marine environment.

  On 14/04/2023 at 10:34, Alan Ambrose said:

with steel, we like to go with the working assumption that 'it lasts for ever' when it clearly doesn't.

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These piles would  be encased in concrete at the top, then very tight within the clay beneath. Rust needs oxygen, so these should last a very long time.

There was one tree created by a seed from bird droppings - no idea what type it was, the others were Pyrocanthia or Firethorn. I plan to dig up those roots as much as possible over the next few weeks. The taller Upright Hornbeam are to the front of the site and will be untouched but could have large roots reaching over to the foundations, will see then! 

Have stood down the first SE and even though my groundsworks guy wanted to just excavate down to a meter and backfill he wouldn't be able to stand over it as the ground report clearly stated the need to dig to 2.2m. A case of experience vs insurance/prudence. But not to worry, will see what the prices turn out like and thanks for the tip about fixed price. Should be an interesting foundation! They will also detail the thresholds to minimize thermal bridging using Bosig Phonotherm or Almatherm under the door/window frame to spread the load onto the EPS upstand without compromising the thermal envelope. 

Bringing us back on topic? Like herding cats no 

I'm now considering building a Pier....!

  On 14/04/2023 at 15:12, mike2016 said:

the ground report clearly stated the need to dig to 2.2m.

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Can you show us that statement please? I'm hoping for your sake that this is a misunderstanding. It is unfortunately common for a ground report to summarise the worst part. They aren't designers. Then the SE works to that.

I made my living partly from finding such overdesigns.

 

These shrubs are zero risk.

How far away is the hornbeam?

It says on a drawing "clear trees", so that seems to be wrong.

Digging down to 2.2 meters for the strip foundation was the SE's recommendation. My mistake, the ground investigation just shows the table at the start and other findings but the SE was left to determine how to adapt to the ground conditions. The closest Hornbeam is 6 to 7 meters away. The site is cleared but the Hornbeam are on council property or the front of the existing house 8 meters away. Thanks! 

Just a thought.. have not really examined all the detail but.. what about this line of thought..

 

You have a confined site, trees, it is in a bit of a hole, probable ground water and neighbours close by that could be "touchy". Good news is you have refusal at 2.6 m so lets say that is where we have something we can work off. But we need to know more about the ground water.. and how fast it will flow will you do trench fill. Could we do a test and use a sump pump?

 

I would explore doing a trench fill  1.0 - 1.4 m in from the boundary. You look at the stand up time of the excavation, you are working in a safer zone (further away from the boundary and loads beyond the boundary  / unexpected things that are more difficult to control) as not to close to the boundary so if things start to move you have more time to recover as opposed to getting complex by working right on the boundary.

 

Also for example it takes you further away from the trees, primary roots etc and you have a "soft zone" to run all services around the outside.

 

What you then do is cantilever your structural slab out to the boundary / where it needs to go and put the superstructure on the edge of the cantilever. We do this a lot when we are fitting a commercial building into a gap site.

 

I have no objection to using screw piles myself but they are not so good at carrying sideways.. wind loads.. a good circular pile has more lateral bearing area against the soil. But I would alway look at the simple stupid first even if just to rule out.. like trench fill and a cantilever slab.

 

Is this worth exploring for you?

 

 

 

 

 

 

 

 

 

 

 

 

I bear good news. Hornbeam are low water demand trees. At 7m you only need 1.1m depth of foundations.

 

Your SE should have known this. Or perhaps they based the design on simplicity rather than materials required.

 

Foundations can be stepped.

 

Now is the time to brief your new SE  with whether you prefer the screw method or conventional. Guessing £2k to £3k difference but I have not analysed it. More difference if you can use foundations at 1.1 ish for a lot of it.

 

  On 14/04/2023 at 21:51, saveasteading said:

I bear good news.

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Post overlapping here.

 

Good point about the Hornbeam.

 

Have had a scant back at the ground report, seems to be a confined site and access. There could be a lot of ground water flow or negligable.. depends on the lenses of gravel / sand and the topography, water regime. But the elegance of trench fill is that even if 1/3 is rubbish you still have plenty gas in the tank.. where as piles.. especially screw piles.. taking lateral load.. does not float my boat.

 

Also don't yet know how high the structure is that is going on top. But in the round I'm thinking trench fill, cantilever slab say 300 - 350 thick with 20m rebar in the top and associated anticrack rebar.. that will carry a good load from a house? My thoughts are to make all the stuff about the trees become a mute point, make any party wall agreement easier to negotiate.. mitigate risk as the site seems to be lower than the surrounding buildings.

 

In Scotland the rules are a bit different but same principles apply.

 

For me it's looking at the job in the round and not getting hung up on piles.

 

 

 

 

 

 

 

 

 

 

Thanks all. Will discuss with new SE. Have an immediate problem with sale of old house though, easements rejected by buyer to water & broadband 

. We'll get there in the end but may have to re-advertize all over again. At least that gives time to submit fresh planning application! 

  On 15/04/2023 at 12:00, mike2016 said:

Will discuss with new SE

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Want to summarise your brief to us first?

  On 14/04/2023 at 23:08, Gus Potter said:

 But in the round I'm thinking trench fill, cantilever slab say 300 - 350 thick with 20m rebar in the top and associated anticrack rebar.. that will carry a good load from a house

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Again for the curious, any links or a quick sketch for this method?

 

 

Have now looked up Pear Tree.  Again this is not onerous. I think your design depths are wrong.

And therefore I am still with traditional footings 1.1 m deep, perhaps slightly wider footings than average,  up to 1.5m deep looking for the optimum ground and narrower footings . 

 

Then either ground bearing slab or beam and block. 

 

Then any builder can do it which is another big advantage in price and progress.

 

my way  x

Gus: add £3,000

Your original way add another £3,000

screw  piles ditto.

 

to which add for extra SE time.

 

The issue is clay plasticity and its tendency to expand as it re-wets (heave). This causes subsidence. Trees make heave worse because they dry the clay over the summer and it re-wets more over the winter.