Deep footings versus mini-piling

[Gasbag]

Hi.
I have just received approval for a detached double garage of 42m2 and the plans were for mini piled footings due to a neighbours hawthorn tree and the fact we are on heavy clay. Quotes for the piling seem expensive and would prefer concrete footings but the building regs say that the footings must be 2.4m deep and .500m wide then block and beam etc.
With quotes for the total build coming in at near £30,000, this is making it a very expensive garage.
Any suggestions how I can reduce the cost or get round the deep footings whilst still being within the regs?
PS - Already reduced the size so can't go any smaller.

 

[mikric]

When you say building regs, who do you mean ? Is this advice from your local authority ? How close is the hawthorn ? It could be that a good structural engineer may come up with a more cost effective design but you will have to pay for this advice, but then this is probably how you have come up with the current design ?? A bit more info required here really .

 

[^woody^]

You can't look at this purely on cost.

You have a situation where a tree could potentially affect a structure, so you need to have designed, and pay for, a suitable solution.

You could save £5k now and then pay £10k in remedial work after a dry summer.

What about a raft?

Has an engineer looked at the site and come up with something?

 

[Gasbag]

Hawthorn is about a metre away from the boundary and a metre and a half from the garage.
The chap who drew the plans actually does the inspections for the LA so he obviously knows his stuff. Just thought I would have got the garage for about £20,000 but perhaps I under estimated how much things cost.

 

[Shytalkz]

The chap who drew the plans actually does the inspections for the LA so he obviously knows his stuff.

Never, ever fall into the trap of thinking that that is necessarily the case

 

[^woody^]

Yeah, far from 'knowing his stuff' what he does know is how to draw plans with just enough detail to get them passed.

I'd bet a year of Shytalkz's fees that he has not actually 'designed' the foundations

More like he's just done some standard foundations to get his cash for the drawings, and is not really bothered whether the garage costs you £20k or £30k to build

 

[mikric]

You will find that unless he is a qualified structural engineer with a hawthorn 1.5 meters away from the foundation in clay according to the nhbc tables he is not qualified to design your foundation. I dealt with a hawthorn hedge 1.8 m away last week and the SE was happy to go at 1.5m in clay with mesh installed and trench filled .

 

[Shytalkz]

That must have been a low shrinkage soil then and even then doesn't comply with NHBC 4.2 tables, which would require 2.25m (ish) depth less any climate zone reduction, which maxes at 0.5m waaaay oop north; thus the abso min at 1.8m distance for the mature height of a hawthorn (10m)would be 1.75m, north of Hadrian's Wall.

The OP doesn't say where he is, but if he's on London/Weald clays, they go up and down like a whore's drawers with moisture content changes; whereas over here in carrot land, it's generally a boulder clay, with much less susceptibility to volume changes.

 

[mikric]

Yes I agree Shy we usually work on medium shrinkage in my area. There was some gravel present , and he lived 3 doors from the site , so should know ground conditions.
If a qualified SE wants to design and put it on paper we dont argue . Normally Its his insurance . Just wanted to point out a normal BCO should not be designing this foundation.

 

[isettaman]

I have just completed the foundation for my double garage, 39sm.

It was drawn as a strip foundation but BCO looked at the oak tree 2m from one side and told me I'd need to go down 2.8m.

He suggested I lay a raft foundation instead, 150mm hardcore with blinding on top, 300mm concrete at the edge with 150mm in middle, with rebar across the slab.

Hard work but now waiting for the brickies.

Dave

 

[Shytalkz]

I would assume that in your case the hedge is not very high then and, in the SE's view, unlikely to grow to its mature height (quite how that would be achieved is open to question); or is the hedgerow being removed and his foundation depth taken on the height at the time of removal (which is not an unreasonable approach)? If the hedge is staying then, on medium shrinkage soils, with a high water demand tree/hedgerow in close proximity, departing significantly from the NHBC guidance is just asking for a PI claim, if it subsequently moves....

And, yes, I would agree entirely with your point re BCOs designing foundations.

ETA to add this refers to Mikric's post two up!

 

[Shytalkz]

I have just completed the foundation for my double garage, 39sm.

It was drawn as a strip foundation but BCO looked at the oak tree 2m from one side and told me I'd need to go down 2.8m.

He suggested I lay a raft foundation instead, 150mm hardcore with blinding on top, 300mm concrete at the edge with 150mm in middle, with rebar across the slab.

Hard work but now waiting for the brickies.

Dave

Oh dear, lots of possibilities with this one. A raft as an alternative to deep founds is not just a case of a shallow dig, some hoggin and a slab with some adhoc thought-up rebar requirements. Even for a garage.

Clearly you're on clay, otherwise the BCO wouldn't have been saying about increased footing depths - although where 2.8m for a 20m mature height, high water demand tree comes from I don't know, as only low shrinkage soils give a depth (of 2.4m); medium and high say >2.5m depth, designed by engineer. The following spring to mind: was the clay analysed for heave/shrinkage potential; the depth of fill should not be less than 50% of the required strip/trenchfill footing depth up to a max of 1.25m and extend for the building footprint, plus angle of repose of fill material, plus 500mm; rafts should only be used for situations where the required standard footing depth is 2.5m or less; and should be designed by an engineer *not* a BCO. He doesn't seem to have suggested much in the way of compliance with this guidance.

If the raft is inadequately reinforced, then any differential soil shrinkage is likely to cause it to split. If it is stiff enough (unlikely for the probable dims to get 39m2), then it will stay bodily together, but still drop as a whole - including the superstructure - towards the tree, where the moisture extraction will be greatest.

And, if the tree is mature and is ever removed, then there should be some spectacular bobbling around of the structure going on after that, as you can't guard against heave with a raft, it's an impossibility.

In short, the BCO was a complete knob to advise you to build what you have. In my professional opinion .

The above comments might seem slightly at odds with my earlier post in another thread, but there we were talking about a shed construction, not a double/triple garage.

 

[Gasbag]

Thanks everyone for your thoughts and advice.

 

[mikric]

Well said Shy BCO s are not normally qualified SE s and should not pretend to be in my opinion. As I always say, if its off my guidance scales consult a SE £££££

 

[Shytalkz]

I've never known of one BCO who is also an SE. As Woody said in another post fairly recently, their role is to ensure compliance with ADs, not to design structural elements.

Always use an SE, you know it makes sense