Soil survey

[Gus Potter]

2 hours ago, oldkettle said:

 

Thank you Declan

 

3.5t digger is too wide for our existing passage, only have about 1.20m. Thought I could dig 2m with a 1.5digger then use an auger. The question is whether it's good enough for an SE: I was asked for "factual and interpretative report for a potential insulated raft foundation, soil parameters, ground water and shrinkability of the soil as well as desiccation". I must admit this particular SE was talking about screw piles, no idea why - as I said, no existing house needed those. 

Hello old Kettle.

 

I'm not sure how to link properly but have copied a post I made below that may help.

 

Technically when you design a strip found you have a bulb of pressure on the ground below that is not that big.. width and depth, hence why you see folk often getting lower depths of dig for strip founds and a typical SE price less than £1000.00. But rafts.. ICF (insulated rafts) behave in a different way.. the bulb of pressure extends much deeper than for a strip found, roughly you need to dig deeper. But not always when you are say extending or adding something near an existing dwelling.

 

Oh, can anyone tell me how I reference a post I made a while ago?

 

 

 

 

Edited March 6, 2021 by Gus Potter

[oldkettle]

1 hour ago, Gus Potter said:

Hello old Kettle.

 

I'm not sure how to link properly but have copied a post I made below that may help.

 

Technically when you design a strip found you have a bulb of pressure on the ground below that is not that big.. width and depth, hence why you see folk often getting lower depths of dig for strip founds and a typical SE price less than £1000.00. But rafts.. ICF (insulated rafts) behave in a different way.. the bulb of pressure extends much deeper than for a strip found, roughly you need to dig deeper. But not always when you are say extending or adding something near an existing dwelling.

 

Oh, can anyone tell me how I reference a post I made a while ago?

 

 

 

 

Thank you, Gus. 

 

I must admit I am slightly confused here. I thought that the whole idea of a raft - apart from making sure it moves as a whole - is spreading the load. Hence the usual depth is no more than 650mm - MOT, insulation, concrete. Then the weight of the building is spread across the area. There must be a reason why they are happy to design a raft where strip foundations would have to be 3m deep. 

[Gus Potter]

Hello Old kettle.

 

I'll have a stab at this.

 

You are correct in that a raft spreads the load, we have a gut feel for this, I think you are on the right track Old Kettle. Also as a general spec I think you are on the right lines give or take a bit.

 

As a bit of background and starting from the most common. I  touch on couple of points which I hope helps discussion.

 

I see you mention a  value of 3.0m depth for a strip found. This can be (the 3.0m is a clue for me) more related to building near trees, It's two different issues? See NHBC link below.  Three metres, this is where you often get a big depth of a strip found when near trees as the roots play havoc with the soil moisture content in some types of ground (certain type of clay soils will shrink and swell for example by a few inches), the roots also grow larger, if you have a leaky drain, they will follow the nutriants you provide and grow accordingly! You can see the effect this has when you are out for a walk.. pavements near trees are often lifted up by the roots.. nature at work.

 

https://nhbc-standards.co.uk/4-foundations/4-2-building-near-trees/

 

You may have silty gound, a thin layer of peat, maybe filled ground and so on. Here the SE will judge that it is cheeper to dig down to something solid, trench fill and build on top. That too can give a deep strip found depth, but often not 3.0m unless you have wide span ground beams and poor gound.

 

Normally a strip found where there is no tree influence will be some 1.0  / 1.2 m to 0.45 m below ground level to the underside of formation. You can also find good info on this on the NHBC website, a legacy of their previous research.

 

Old Kettle. I can see / hope / helps  as it seems you are interested in this. Imagine you have strip found in a field on it's own. It loads the ground but as you go down in depth you have the weight of the soil above. This soil each side as you go down in depth acts to confine the pressure, hence when you look at the shear stress profile in the soil it's like a bulb shape, not the 45 - 60 deg principle you may see from a say lintel loading. But for a raft, or closely spaced piles the bulbs start to interact so the depth of the overall bulb goes further down, as each bulb interacts.. That is why rafts need a deeper depth of investigation which is not often discussed on BH.

 

As an aside.. There are plenty of keen gardeners here on BH, who know that tree roots often don't go that deep. I have seen many cases like Old Kettle mentions of 3.0m.. but many designs follow the standard warranty provider safe / conservative standard guidance that is also adopted by BC as a fall back / standard position.

 

Arboriculturists may not be the flavour from time to time on BH.. but a word with them on root depth could save you a fortune, and then a word in your SE's ear off the back of that?

 

Old Kettle. I hope that I have covered some your points but have the odd doubt.The technical aspect of foundation design and how that interfaces with the super structure is not yet been fully explored on BH.

[oldkettle]

Hi Gus,

 

thank you very much, very comprehensive reply.

 

Let me clarify first. 3m indeed came from a similar link some time last year. My point was that quite a few articles about rafts state that they can be used where strip foundation would be uneconomical. In my case I've just removed a row of Lawson's cypresses on the "top" boundary and some of them were 8m high. The remaining trees on the left boundary are much less dangerous hollies and on the right one - 4-5m cypresses again. Since I want to build 2m away from the border it's all very close. The ground has not been touched for at least 70 years. I didn't discuss a strip foundation with anybody as I am trying to use this outbuilding as a "rehearsal" for the main house  so really want to use insulated raft. Frankly, for a single storey building the load can't be that high.

 

Basically, if you think 4 holes of 3m is the right thing to do (and clearly I can't use a digger as this would lead to a disturbed ground and make things worse) I will just go for it. I thought it was improbable as existing houses tolerate all those trees just fine hence my layman's view was "it can't be a shrinkable clay otherwise our 1m founds would have given way a long time ago". Again, the BC said "low to medium shrinkability soil".

 

I feel a section on rafts ("Granular infill beneath raft foundations in shrinkable soils") describes what I expect to happen really well: https://nhbc-standards.co.uk/4-foundations/4-2-building-near-trees/4-2-9-foundation-depths-for-specific-conditions-in-shrinkable-soils/

 

Thanks again.