Stumbling at first hurdle?

[Adamantium]

Dear All,

 

Although we have a structural engineer on this, i would really appreciate any suggestions that i can put to him.

 

We are putting together detailed design drawings for the tender process.

 

There is a main trunk sewer running front to back under my property.

 

Thames water will not allow building within one metre of the sewer, so all my house plans involve the flank wall of the house running parallel to but a metre from the sewer.

 

We are constrained on the other side of the plot and so can't move the house further from the sewer, all we could do is make it smaller which creates significant delays, adds additional costs and might require planning permission - which I'm sure we'd get but I'd prefer to not have to start again and lose a year, nor do I want to make the house smaller.

 

The issue is that the main lank wall does not sit at the edge of the foundations. The proposed trench foundation is 2.4 metres wide, with the wall centred on the middle of the pad. (it's a heavy house so I think the structural engineer requires a pad of this size).

 

The wider foundation defines the closest point we can be to the sewer and so the wall if central to the pad has now moved 1.2 metres further away (minus the thickness of the wall).

 

We could potentially pile just down that side but if piling the minimum distance increases to 1.5m.

 

I'm wondering if there is any alternative type of foundation for a very heavy house with a 225sqm foot print that will see the outer walls sit right at the edges of the designed foundations.

 

Any suggestions would be greatly appreciated.

 

Thanks in advance, 

 

Adam

 

ps. I'm trying to keep cost down, but at the moment I'm just hoping that there is a solution rather than worrying too much about how much it will cost.

 

 

[le-cerveau]

A passive slab, many here have them, will allow you to build up to the edge of the slab (just about).  Our house is 250m² on the ground floor and the slab is the same size with a little extension at the sides.

 

 

I have attached the foundation layout to show this:

16-063-01B Foundation Layout.pdf

 

Others  here have used other manufacturers raft foundations but they all have the same principal.  I chose mine as it is the same supplier for foundation and wall and they interlock.

[Mr Punter]

This is one for your engineer.  We have just done a project which cantilevers over a retaining wall and a river.  The engineer designed a piled foundation with cantilevered ground beams so that the piling rig could still operate within a safe distance from the retaining wall.

[Jeremy Harris]

The obvious solution would seem to be to use a slab foundation, as then the outer walls can sit on the edge of the slab.  It's ground-condition dependent as to how close you could get the outer wall to the 1m line, and frankly I don't think any foundation system will get you closer than a few hundred mm to that line, even a slab, due to the need for stone underneath that spreads the load down and out at a 45 deg angle.

 

Helical screw piles are another option, and might get you closer.  You can put either a steel or reinforced concrete ring beam on the pile caps to stick the house on.

 

What sort of soil type do you have? 

 

Do you have a soil susceptible to movement with water content, like some types of clay?

 

Do you know the maximum allowable soil bearing stress? 

Edited January 16, 2018 by JSHarris

[Adamantium]

Soil sample showed london clay, but I'm told it isn't susceptible to movement but without looking at the soil investigation report I don't know the bearing stress.

 

The SE certainly does.

 

I was hoping a slab might be an option.

 

Are the terms slab and raft interchangable and are they significantly more expensive than conventional trench foundations? 2m deep and 2.4 m wide sounded like a lot of concrete/block work to me - can't imagine his preferred option was so cheap int he first place.

[le-cerveau]

What are you going to put on top of the, potential, slab, TF/ICF/SIPS/block? this potentially changes the whole house structure.

 

Do you have any diagrams/plans to share?

 

What are your thermal targets?

 

so many questions and options.

[Jeremy Harris]

London clay is very definitely a soil type that moves a lot with variation in moisture content, so needs care with the foundation design to reduce the risk of movement.  You can use a raft or slab foundation on clay, but you won't be able to get it that close to the 1m limit, as it will have to have a fairly deep layer of well-drained stone underneath it to spread the load out and down to reduce the soil bearing stress and ultimately limit any movement the clay could induce in the slab.

 

I think that a piled solution is far and away your best bet, given your space constraints.  Piles are pretty much the classic way of getting around London clay heave and would allow you to get the ring beam that supports that outer wall right up to your 1m limit.

 

You can put an insulated slab on top of a piled ring beam, plus there are several other options that could be explored.  I don't think that the suggestion you have at the moment from your SE is at all sensible, given the site limitations.  The SE has used what seems to be a fairly standard solution for the soil type, but hasn't taken account of your site restrictions, from the sound of it.

 

Much as I'm a great fan of insulated slab foundations, there are places where they cannot be easily used, and in your case, with the need to have a vertical load bearing foundation along a 1m line, with nothing going over that line, you are going to find that you're pretty much forced into using piles, I think.

 

There are lots of piling systems around, ranging from driven steel piles, conventional bored and reinforced concrete poured piles to helical screw piles.  The fastest to put in and most probably the ones that would get you closest to your 1m limit, may well be helical screw piles.  I looked at using them on a previous plot we were looking to buy and was impressed.  It's well-proven technology (some Victorian piers are still standing on ancient cast iron screw piles) and it's a clean and quick solution, with minimal excavation or soil disturbance.  It's also a system that is immediately load bearing, so the house can literally be put on the piles as soon as they are driven in.

Edited January 16, 2018 by JSHarris

[ToughButterCup]

17 minutes ago, JSHarris said:

[...]

I think that a piled solution is far and away your best bet, given your space constraints.  Piles are pretty much the classic way of getting around London clay heave and would allow you to get the ring beam that supports that outer wall right up to your 1m limit.

[...]

 

@JSHarris  is right. And it's exactly the problem we had. My heart sank when I realised we needed piles.

 

1 hour ago, Adamantium said:

[...]

ps. I'm trying to keep cost down, but at the moment I'm just hoping that there is a solution rather than worrying too much about how much it will cost.

[...]

.

Read this . It might go some way to reducing the rictus grin and sucking of teeth about piling: well the cost of them: intial quotes - silly money. Evental cost £6.500 for 64 piles.

 

This is written in haste: concreting today, so I'll come  back to this later.

With luck we'll soon have caught your stumble.....  

[Adamantium]

I have huge amounts of drawings since the detailed design of the house from the ground up at least is complete.

 

With regard to screw piles, I'm not sure if thames water would allow them. When they refer to piling being 1.5 m from the sewer, they insist on continuous flight augured piles. They make no mentioned of helical screw piles.

 

I would image they have the same impact on the ground for all intents and purposes.

[Adamantium]

Just want to say a big thanks to everyone, regardless of the outcome.

[Jeremy Harris]

14 minutes ago, Adamantium said:

I have huge amounts of drawings since the detailed design of the house from the ground up at least is complete.

 

With regard to screw piles, I'm not sure if thames water would allow them. When they refer to piling being 1.5 m from the sewer, they insist on continuous flight augured piles. They make no mentioned of helical screw piles.

 

I would image they have the same impact on the ground for all intents and purposes.

 

 

Screw piles are just augers that are the piles, so have less ground impact overall.  I very much doubt that Thames Water would have the slightest problem with them, as they present a lower risk to their pipe.  I suggested helical screw piles specifically because they are low risk.  Take a look at a helical screw pile and at an auger pile rig and you'll see what I mean.

[Adamantium]

The minute I saw the word helical it did make sense.

[Adamantium]

The SE has come back with this.

 

Seems to be CFA piles.

 

Should I push for helical screw piles?

 

Also, is there any reason why the entire foundations now have to be piled. Could the rest without the same limitation be a standard trench or is it a case of the majority of the cost being bring the equipment rather than the actual piling process, hence might as well get the whole lot done.

 

Finally, is there a huge variation in CFA/Helical piling costs? Should I be looking for a piling contractor myself now or leave it to the main contractor?

piled foundation suggestion.pdf

[Jeremy Harris]

Because you're on London clay, I think it would be very wise to stick to the same foundation support all the way around, to reduce the risk of differential movement.

 

When it comes to piling systems, then there are several different methods, each with their pros and cons.  I would suggest that you don't leave this to the main contractor, but explore the relative cost of different piled solutions yourself first. 

 

Often you are trading things like additional material cost, with less labour or mobilisation cost, making a direct comparison challenging.  For example, you will incur a one-off mobilisation cost for a CFA piling rig, plus spoil removal cost, plus steel cost, plus concrete cost, plus labour cost.  You would incur a probably smaller mobilisation cost for helical screw piles, a higher per-pile cost for materials, a lower labour cost (they are a lot quicker to put in) and virtually no spoil removal cost.  The same goes for other systems, there is always a trade off between higher costs in one area and lower in another, that make direct comparison a bit of a pain at times.

 

 

Edited January 16, 2018 by JSHarris

[ToughButterCup]

@Adamantium, is the SE insisting on CFA piles? If so why?

 

I ask because SEs recommend what they know about and or specialise in. And so do all professionals. Our SE (Tanners) were thoughtful enough to include one sentence in their recommendation. Paraphrasing: despite the Soil Analysis Recommendation , other piling methods are available.

 

The important thing is not the method it's the outcome

 

The matter of the distance from the Water Company's assets: we had an exactly analogous problem. So we read their policies carefully, and after a bit of research found out that if we did this (image below)

 

 

all would be well. And indeed it was.

As to piling mat. Are you on a slope? If so, make use of it in helping make the piling mat. 

Ian

 

[Mr Punter]

The section you posted shows a 215mm concrete inner leaf to the wall.  Why has this been specified?

[Adamantium]

The architect likes to build houses that are substantial in look and feel. This means making visible wall depths thicker. It also makes for a much heavier house hence the chunky foundations initially specced.

 

FYI sewer I am avoiding is 860mm across, not sure it's analagous to the example above.

 

I am hoping the proposed section will be suitable, but the architect is currently silent on screw piles versus augured piles. I would hope both are an option, but don't know why he doesn't seem to have an opinion yet.

 

Final question, can anyone recommend a reasonable and decent company to do this piling. I am based near watford.

 

I wondering with the extent of investigation done if someone has come across a more reasonable, yet competent company. Thankfully, this is not bleeding edge technology piling.

 

 

[RichS]

If the only reason for a 215mm inner leaf is because " The architect likes to build houses that are substantial in look and feel" then I would get rid of him sharpish

[Adamantium]

I specifically requested that I wanted to achieve the same finish as the houses of his that I'd seen. They lay the blocks on their side in the inner leaf on purpose. Yes it's heavier and more expensive but it gives rise to a more solid feel as well as creating nice features like deep window reveals etc.

 

Every door way is mounted within a thick wall which really make a different to the feel of the whole house. 

 

Why would i dismiss that?

 

 

[joe90]

Well I have got deep window reveals ( angled to let more light in, and it works really well) deep door reveals also angled, but 100 mm block inner walls, this enabled me to have 200mm cavity which is full of insulation. My build is very solid and i don’t understand why you would want 200 mm inner walls as the 100mm I have is rock solid.

[Triassic]

I'd suggest less wall and more insulation,. having less wall will also reduce to load on the raft and possibly make it simpler to construct. With more insulation you retain the thicker walls and have a lot warmer feel inside the house and smaller heating bills.

[ProDave]

57 minutes ago, Adamantium said:

I specifically requested that I wanted to achieve the same finish as the houses of his that I'd seen. They lay the blocks on their side in the inner leaf on purpose. Yes it's heavier and more expensive but it gives rise to a more solid feel as well as creating nice features like deep window reveals etc.

 

Every door way is mounted within a thick wall which really make a different to the feel of the whole house. 

 

Why would i dismiss that?

 

 

You would be paying a lot for extra blockwork and labour and a thicker than needed block wall adds very little extra to the insulation.

 

What is the wall make up in this proposed build? i.e. what goes on the inside of the blocks or is it "plaster on the hard" and what goes on the outside?  Where and what type of insulation?

[Brickie]

+1 to the above questioning 215mm blockwork. 

As said,you’d be far better off with 100mm & either a wider cavity (with more insulation) or standard 100mm cavity with iwi. Either option would give you the deep reveals you’re keen on at a far smaller cost & a warmer house. Personally,I’d go for Iwi in your position. 

Trusting your average brickwork gang to insulate properly is wishful thinking,and not easily rectified if discovered later on. 

Ps if I were pricing drawings that showed 215mm blockwork internally I’d be booking a villa in Spain for a month!

[ToughButterCup]

19 hours ago, Adamantium said:

[...]

FYI sewer I am avoiding is 860mm across, not sure it's analagous to the example above.

[...]

 

I should perhaps  have explained more clearly that it is possible to pile close to underground assets provided that the asset is insulated from the vibration by  an air gap.

I don't think that the diameter of (in this case) the pipe makes a difference to that principle.

 

If the costs of CFA piling is an issue, then it might be possible to persuade your authority to allow a different method to be used if the sewer were to be suitably insulated from vibration. 

Just a thought......

[Adamantium]

Completely understand. thanks for clarification.

 

As for the internal wall thickness, I'm open to suggestions, no question, but given that the eventual wall thickness is about 500mm that's sounding like a lot of insulation and a very large gap.

 

I recall bringing this up with the architect who mentioned a concern about the wall ties being too long for the larger gap. Apparently we'd then need stainless ties which removes a large chunk of what is saved.