Trench vs Raft: Garage Foundations

Postby nightg » Mon Jan 25, 2010 5:36 pm

Hi There
I am looking to build a detached garage 6mx6m. It's close to the boundary between us & the neighbours who very kindly have 2 eucalyptus trees planted right up to his side of the boudary. My technician who has sent me the draft drawing has specified trench foundations down to 2.5m along the side closest to the trees & 1.7m for the other 3 sides. This seems excessive & will consume masses of concrete & expense for me. Can anyone give me a guestimate of how this cost might stack up against a raft foundation. I think I'll speak with BC & get their opinion before I submit as once the plans are submitted I guess I'm locked in.
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Simply Build It

Postby stoneyboy » Tue Jan 26, 2010 10:36 pm

Unless you can get down to very hard bedrock subsoil I don't think BC will allow you to use raft foundations.
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Postby Baie21 » Wed Feb 02, 2011 4:51 am

Thank you for the replies.

Its not without minor frustration that I have posted this as I have been researching for over a week, and have found no similar situations or reference to this type of issue, in fact its how I found this forum and the previously mentioned topic.

When we originally built, the borough engineers were not really interested and had no suggestions, only when I proposed the split foundation did they agree with some conditions such as minimum 1 meter depth apron to the raft and reinforcement to both the raft slab and every course of bricks up to dpc level, rather serious for a single thermalite block building I thought, however I cant argue as it hasn't moved!
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Postby andy01 » Wed Feb 02, 2011 1:33 pm

NHBC 4.2 states guidance on foundation depths to be dug within certain distances of trees. this is due to dessication and heave problems. i'm not sure a raft is capable of dealing with these problems as the heave will still move the slab, hence the deep foundations.

the alternative is usually driven piles or vibro piles with RC ground beams and a RC slab or beam and block spanning between the ground beams.

NHBC states min depths for strip founds when not near trees, dependant on soil, off the top of my head its generally between 750mm and 1m in clay
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Postby elwood » Thu Feb 03, 2011 8:37 pm


Just finished a raft/ringbeam 6x6 garage oversite, structural designed by professionals total dig depth 1.6m (very soft clay)
Price £8.2k supply and fix

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Postby elwood » Thu Feb 03, 2011 8:42 pm


Also trees on the back about 1m away (leylandii)

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