Looking at a plot with PP but scary soil survey!

[Dreadnaught]

I am now looking at my second plot but the soil survey (from by the vendor) is rather intimidating and scary. I could do with some calming down and some advice.

• plot is near rectangular 13m by 19m
• soil: sand and gravel 0.15m to 1.35m depth
• underlying Gault clay
• groundwater below 2 - 2.5 metres depth
• the site has two major trees. "Live roots were recorded within the boreholes to depths between 0.90m and 2.80m 

"

Foundations


 recommendation

• Sleeved piles
• Driven piles not possible because of proximity of neighbouring buildings
• "Single 300mm diameter bored piles, installed to 10m and 16m depth at the locations [… ], would have estimated working loads in the order of 170kN and 310kN (FoS=2.5). Different pile lengths, or diameters, from those detailed above would give different available working loads, which could be tailored to suit the working loads required. A piling specialist should undertake final design of piles."

Floor slab

• "The floor slab for the proposed structure will need to be suspended on the piled foundations, in order to avoid differential settlement between them. As the floor slab will lie within the range of influence of mature trees, and desiccated clays are present, the ground floor should also be separated from the underlying soils by a sub-floor gap, as indicated in Table 7 of the above cited NHBC Standards."

This all sounds firghteningly expensive and as I read it I thought of the phrase "you're not out of the ground until you're out of the ground". 

 

What should I make of this? I have not made an offer on the plot as yet but I suspect that this should impact the price. I would be happy to ping the full report to anyone who'd be willing to give me some advice.

Edited March 22, 2018 by Dreadnaught

[MikeSharp01]

You don't necessarily need piles of money for piles @recoveringacademic can give you a feel as his place is built on piles. ( @Onoff will probably want to talk about other sorts of piles) The obvious trick is not to try and build a solid reinforced concrete edifice on the piles, something made of balsa wood will reduces the costings.  Give what the survey says if you have 10 x 170Kn piles you can pop a 170 Ton (approx) house on there and that is probably quite a big timber framed job. I recall @JSHarris telling us here how much his place weighed - ex foundation, was it 70T or some such Jeremy?

[ToughButterCup]

Well, I wouldn't worry too much. Here's why. 

Ian

[Ferdinand]

I am honestly not exactly sure. 

 

YOu need to allow for the potential costs in your offer should you choose to accept this mission (!). And that means you need to have some idea of what you think the costs of carrying this out should be. You could try asking or paying for an estimate from groundworks engineers, or looking for similar plots in your LPA (prob via reading lots of decision notices for Conditions) and asking the applicants. Walking up to self-builders on their sites may be fruitful if you have the front and some specific knowledge about the site to get past the protective barriers, but there are some accounts of resentment at 'nosey parkers' on BH.

 

The positive side may be that the report will scare everybody else off, though third party fools may rush in. You cannot account for them. It is what it is worth to YOU. It is also better to have the report than not. It is also in your vendor's favour that he has actually given you the report.

 

Do not be bound by any idea that you need to make an offer within a felt-fair range of the asking price; that may be your internal self-conditioning speaking. 

 

You also have the advantage that he has sunk money into the plot already and is on a ticking clock of 3 years if he has PP.

 

My approach would perhaps be to get an idea of the cost and all the background info I could in the time, then widen my contingency and make what I think is a reasonable offer. But costing piles before you dig holes is probably another black art.

 

Then you need to deal with the difference between your wrong estimate and what is actually needed once yo have bought it .

 

Or you can treat it as a lesson and do the dating 'date 10 then make a choice with the next one who is better than those 10" thing.

 

Where's that graphic? Ah yes:

 

 

@recoveringacademic has some good advice in his blog posts.

 

Ferdinand

 

Edited March 23, 2018 by Ferdinand

[Dreadnaught]

Thank you all for your advice. I am grateful. Two things:

 

1) Who to ask? 

 

Before I play @Ferdinand's spot-the-ball on costs, who to turn to for a ball-park indication of price for groundworks? Would it be: a piling contractor? A specialist piling consultant? A structural engineer such as @StructuralEngineer? (I appreciate that only the broadest guesses are possible at this stage dependent as they are on the ground and house weight).

 

2) How much does a house weigh? Might a non-piled insulated concrete raft be an alternative?

 

To @MikeSharp01's point, I foresee building a @JSHarris-style home of two bedrooms, 120 m² and timber-framed and so of a weight of perhaps 70 tonnes, or less. And perhaps closer to @TerryE estimated mass of his passive house, which I recall he calculated as in the order of 41 tonnes excluding roof covering.

 

Given this, and having read some comments in @recoveringacademic's blog of how the industry defaults to conservative solutions, might an alternative be an insulated-concrete raft, with its advantage of distributing the load and in combination with a light-weight house? Who to turn to for such innovative ideas?

 

Some extra background on the plot:

• access to the site is limited. It is down a tight 40m road, the width of a car-and-a-half and flanked by buildings
• of the three-year PP development window, less than a year now remains
• trees adjacent to the site necessitate a no-dig foundations solution. The trees are without TPOs but are on neighbouring land
• the reason I am persevering with the plot is that its in a truly fine location

[nod]

13 hours ago, Dreadnaught said:

I am now looking at my second plot but the soil survey (from by the vendor) is rather intimidating and scary. I could do with some calming down and some advice.

• plot is near rectangular 13m by 19m
• soil: sand and gravel 0.15m to 1.35m depth
• underlying Gault clay
• groundwater below 2 - 2.5 metres depth
• the site has two major trees. "Live roots were recorded within the boreholes to depths between 0.90m and 2.80m 

"

Foundations


 recommendation

• Sleeved piles
• Driven piles not possible because of proximity of neighbouring buildings
• "Single 300mm diameter bored piles, installed to 10m and 16m depth at the locations [… ], would have estimated working loads in the order of 170kN and 310kN (FoS=2.5). Different pile lengths, or diameters, from those detailed above would give different available working loads, which could be tailored to suit the working loads required. A piling specialist should undertake final design of piles."

Floor slab

• "The floor slab for the proposed structure will need to be suspended on the piled foundations, in order to avoid differential settlement between them. As the floor slab will lie within the range of influence of mature trees, and desiccated clays are present, the ground floor should also be separated from the underlying soils by a sub-floor gap, as indicated in Table 7 of the above cited NHBC Standards."

This all sounds firghteningly expensive and as I read it I thought of the phrase "you're not out of the ground until you're out of the ground". 

 

What should I make of this? I have not made an offer on the plot as yet but I suspect that this should impact the price. I would be happy to ping the full report to anyone who'd be willing to give me some advice.

Have I read the size correctly 13x19 That will be very tight to work on

[Dreadnaught]

3 minutes ago, nod said:

Have I read the size correctly 13x19 That will be very tight to work on

 

You did indeed read it correctly. Very tight indeed. Its the end of a garden.

Its one of the worries about this plot, especially when combined with a 40m narrow road leading up to it. I will need a Chinook  

[nod]

1 minute ago, Dreadnaught said:

 

You did indeed read it correctly. Very tight indeed. Its the end of a garden.

Its one of the worries about this plot, especially when combined with a 40m narrow road leading up to it. I will need a Chinook  

Yes it can be a problem

Ours is not a big as many on here

Around 25 mtr wide x 40 long 

We intended siting a container 

But had nowhere to put it that wouldn’t be in the way 

It’s surprising that once the scaffold goes up How tight things can get

 

[Dreadnaught]

6 minutes ago, nod said:

We intended siting a container 

But had nowhere to put it that wouldn’t be in the way 

 

Good point. I am tempted to ask the seller (with the rest of the garden) for some extra temporary space during the build.

[Ferdinand]

Just now, Dreadnaught said:

 

Good point. I am tempted to ask the seller (with the rest of the garden) for some extra temporary space during the build.

 

You should also ask the seller for an informal water supply, too - for obvious reasons, and perhaps electric.

 

A water sub meter is cheap. Not sure about electric.

 

Ferdinand

 

[billt]

Electricity meters are cheap too, e.g. https://www.shmmetershop.co.uk/shop/elster-a100c-100a-single-phase-direct-connection.html

(No connection, just the first price on a search.)

[nod]

53 minutes ago, Dreadnaught said:

 

Good point. I am tempted to ask the seller (with the rest of the garden) for some extra temporary space during the build.

Exactly what we did 

 

[StructuralEngineer]

Interesting site -- and almost precisely the same ground conditions as the question in my mock IStructE exam

 

Weight of a house:

Masonry clad timber frame, say 10-15kN per m2 according to the structural engineer's pocket book.

So for a 100m2 footprint, 1500kN.

 

Piles:

Forget about CFA piling or getting a large rig onto the site. Yours is a very small scale project. Noise and vibration could be a bit of a non-issue. If you use bottom driven steel mini-piles, you could perhaps get 50-100 kN per pile, thats 15-30 piles, and the team could quite happily complete the job in two days. All it needs is a compressor and the vibrating "mole" does about 85 decibels. Google "Grundomat" and you'll be amazed. I'm not sure off the top of my head if water table would be an issue for these piles though.

 

Raft

If it were me, I'd consider creating a reinforced concrete raft. It would be relatively tolerant to roots.

 

I second what others are saying -- it would be valuable to get the vendor to supply access to services and welfare during the build if it's a reduced access site.

 

[Dreadnaught]

20 minutes ago, StructuralEngineer said:

If it were me, I'd consider creating a reinforced concrete raft. It would be relatively tolerant to roots.

 

A concrete raft instead of piles? Or a concrete raft somehow connected to piles? Sorry for my beginners question.

Edited March 23, 2018 by Dreadnaught

[Mr Punter]

Also check what the fire service requirements are.  They sometimes insist on being able to turn around on the site, but are not always consulted at the planning stage.

[StructuralEngineer]

1 hour ago, Dreadnaught said:

 

A concrete raft instead of piles? Or a concrete raft somehow connected to piles? Sorry for my beginners question.

Yes, I was thinking, since you said there was reasonably competent granular strata overlying the clay, you could potentially go with scraping the topsoil away, import stone bed and compact, cast RC raft over the top. NHBC allows for that over desiccated clays (see CH4.2 p9) as long as your building is rectangular on plan and the raft is designed (or signed off) by a chartered engineer. I tend to prefer rafts for the simple reason that you have cost certainty, pretty much whatever you find on site and whatever the BCO says, and you can do a final design and get quotes even if the building hasn't been designed yet.

[Mr Punter]

12 minutes ago, StructuralEngineer said:

Yes, I was thinking, since you said there was reasonably competent granular strata overlying the clay, you could potentially go with scraping the topsoil away, import stone bed and compact, cast RC raft over the top. NHBC allows for that over desiccated clays (see CH4.2 p9) as long as your building is rectangular on plan and the raft is designed (or signed off) by a chartered engineer. I tend to prefer rafts for the simple reason that you have cost certainty, pretty much whatever you find on site and whatever the BCO says, and you can do a final design and get quotes even if the building hasn't been designed yet.

 

Are you familiar with the insulated raft type foundation that many on here have opted for?  Would it work for @Dreadnaught?

[Jeremy Harris]

We have a passive slab type reinforced raft over hard clay.  The make up is 200mm of crushed clean stone (18-35 no fines) laid over terram in a 200mm deep excavation below ground level, with perimeter perforated drains just outside the edge of the building envelope, grit blinding laid over the stone and very carefully levelled, 300mm of EPS insulation laid directly on the blinding in 100mm layers, with the DPM being between the top layer and the lower two layers.  The edge has a 200mm wide upstand that is 400mm above ground level, and creates a 200mm deep reinforced ring beam around the edge of the slab.  Most of the slab is 100mm thick RC35, laid on to A142 reinforcement fabric sat on 40mm chairs, the ring beam is 200mm thick.  The UFH pipes are tied directly to the 200mm pitch of the fabric.

 

In structural terms it's an absolutely massive overkill, as the bearing load on the ground, allowing for the additional load spreading from the 200mm thick layer of stone underneath, is less than 15% of the max allowable for the actual soil conditions.  The main advantage (apart from the main one of being very thermally efficient, with virtually no thermal bridging) is that the whole foundation laying process, from the start to the completed dead flat power floated concrete slab took four days, for three people plus the concrete truck.

[Luckylad]

22 hours ago, JSHarris said:

passive slab type reinforced raft

Hi

Do you have a cross section plan of this you could post here please?

[Jeremy Harris]

11 minutes ago, Luckylad said:

Hi

Do you have a cross section plan of this you could post here please?

 

 

Sure, it's here as a pdf on my blog:  http://www.mayfly.eu/wp-content/uploads/2013/09/Elevation-Sections-for-BC-A4.pdf

 

Here's the blog entry with some photos of the foundations going down: http://www.mayfly.eu/2013/10/part-sixteen-fun-and-games-in-the-mud/

 

 

[StructuralEngineer]

If you increase the depth of concrete and add additional reinforcement this should work well as a raft over less competent ground too.