Insulated raft when piling is required

[MoDo]

Was nearing a decision on timber frame supplier and drawn to MBC's package of founds and frame. However soil survey shows we needs piles and MBC have sent a general sketch of how they might implement piles with their insulated raft. I'm no engineer but to me it seems criminal to bridge the EPS with the piles as the drawing seems to illustrate. I'm told the area is small and thus the cold bridging isn't significant enough to negate the overall benefit of the insulated raft. Apparently they have done a few like this - is there anyone here with their foundation and piles? I recall @TerryE calculated the impact from rebar to the external masonry footing was significant but I guess that involved significantly more metal than the limited number of piles typically needed for a raft.

[MoDo]

Would also be grateful for any opinions as to the significance of the piles affecting the thermal performance of the slab - is it a reason to avoid this foundation type entirely or is the impact minor and nothing to be concerned about?

[ToughButterCup]

Our piling design included Foam Glass blocks at the head of most of our piles to get over the problem you raise.  (Those supporting a structure outside the 'lived-in' envelope didn't need the foam glass blocks )

 

Edited November 6, 2022 by ToughButterCup

[MoDo]

Thanks looks like a creative solution. So the piles don't tie in directly with the concrete - just support it from below? 

 

Did you use a local contractor for the job? Or was it an MBC job? (Sorry haven't had a chance to dig through your blog etc yet). 

[ToughButterCup]

Our design was provided by Tanners (Eire) . One sentence in an email from him about the foundation design saved his fee many times over. The design 'type' is called an Insulated Raft. - One of the consequences is - for example - that we have had no heating at all in the house so far this year - and the ground  floor is tiled throughout. 

 

We used ground improvement columns - not piles - because they cost about one third (£6,500 ) that other piling contractors were threatening to charge  (10m by 10m house, 64 'piles' )

 

Heres the build-up from piles upward 

 

Edited November 6, 2022 by ToughButterCup

[George]

Vibro-stone columns? Often a good choice.

 

CMCs are another option but fewer companies here have the rigs.

[MoDo]

13 hours ago, George said:

Vibro-stone columns? Often a good choice.

 

CMCs are another option but fewer companies here have the rigs.

Thanks, but do either of these avoid the cold bridging issue with MBC's insulated raft? 

[George]

11 hours ago, MoDo said:

Thanks, but do either of these avoid the cold bridging issue with MBC's insulated raft? 

 

Oh I was commenting on the above post. Vibro-stone columns are what he probably had.

Both VSCs and CMCs are displacement piles so they improve the ground - so yes this would avoid the thermal bridging as once the ground is improved, you could build up treating the soil as a competent strata. 

 

Definitely worth getting that looked at as an option and may have come about in value engineering anyway. The thermal bridge itself was probably pretty minor. Or you could have switched to a structural raft with insulation laid on top (this would be my preference in poor ground - even with VSCs, put down a structural slab then insulate on top). A decent thick screed would make up a lot of the thermal mass otherwise lost. 

 

Edited November 7, 2022 by George

[MoDo]

Thanks, would you think the potential for cold-bridging of the piles is significant enough to consider a structural raft - which would mean finding a separate contractor for the works. Whereas MBC offer the foundation (insulated raft only it seems) along with their timber frame - thus reducing the possibility of timber frame contractor blaming the foundation contractor for any issues. Having one supplier for the lot is one of the main attractions with MBC. But I appreciate your point that a structural raft would be a better way to do it in this case.

[MoDo]

@ToughButterCup Thanks for your detailed previous posts. Can I just ask which contractor you used for the raft and what your proximity to other buildings is?

 

(T&C Vibro - although their website states VSC's "Installation produces very little noise and vibration, reducing disruption to the surrounding areas compared to traditional piling techniques." - They felt our site might be unsuitable due to proximity to other properties; that'd be a real shame as it does look like a good solution.)

[ToughButterCup]

Shame: T+C Vibro were - for us - excellent.

I would not inflict our contractor on anyone.

 

Proximity - 13 meters.

[MoDo]

Thanks very much, we have 14m to the nearest house - so I may try them again. Appreciate your help.

[ToughButterCup]

On 17/11/2022 at 23:25, MoDo said:

..... we have 14m to the nearest house.....

 

Remind them that they did our house (Salamander Cottage) which is 12 meters away from our other cottage, and that they put a vibration meter in; and that reports to their HQ in real time.  Vibration was no problem at all.

 

And this (below) is as bad as it got .... 3 meters from the piling rig while working

(I used the well-known bowl, water and food colour meter)

 

 

[MoDo]

Thanks for that, will give them another try! Like your ingenious test method!