Trench Blocks & size



Hi, just a quickie'.
I'm about to lay a couple of courses of trench blocks for the start of the kitchen/bathroom extension. There's about 20 linear metres.
Spec' is for 100mm outer skin, 100mm inner skin & 50 mm cavity, half-filled with K8 cavity board.

I was hoping to use 255mm wide trench blocks but 275mm are much easier to obtain. Is there any problem with using 275mm blocks, just for the first two courses which will finish about 100mm below ground level?
Will the BI be OK with this?
Outer skin will then be semi-engineering bricks outer and 100mm concrete blocks inner to DPC.
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I can't see a problem

Bco may want you to point a mortar fillet as a belt & braces touch to allow any water run off should any find its way down there!
Cheers for that. It'll save me a few days of waiting for 255mm blocks.
BCO is easy-going and he doesn't want to see it again before I'm ready for floor slab. He's happy for photos of footings concrete and oversite to be e-mailed to him.
But it's no problem for a mortar fillet along brickwork/blockwork to trench blocks. In fact I tend to do that on blockwork to footings as well.
Do trench blocks have the necessary frost resistance for only 100mm below ground? Check first

I normally use dense concert or semi-engineers for 225mm below ground.

Don't bother with a mortar fillet, scrape the back of your trowel along to skim the corner off. Yee-harrr :cool:

But we never filleted the old proper stepped footings
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Thermalite Trenchblock is a tried and tested, economical alternative to the construction of cavity walls with concrete infill for foundations.
• For use in soils of up to Design Sulfate Class DS 4
• Available with tongue and groove joints and handholds
• Improves thermal performance

Lower down the page a 'product.pdf' available indicating very clearly the Thermalite T&G Trench Blocks being used about and above ground level.
"All Celcon Foundation Blocks may
also be used above ground for solid
wall construction."

"With exceptional resistance to freeze/thaw
conditions likely to occur below DPC level
BBA certificate 01/3816 confirms that all
Celcon Foundation Blocks are suitable for
use in soil conditions from DS1 to DS4."

Reply to an oldish post, but it might save someone else searching for the answer to the questions I had about trench blocks

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