Sub Base

[Biffo17]

Hi my first post. Im working on my house 1890 . approx 30Metres Ive had to lower the ground level by approx 18 inches as there were 2 x2 slabs layed on 150mm sand on top of blue victorian pavers, all removed and approx 150mm below the blue pavers
The ground was high ,causing damp problems
can I use 20mm ballast as the sub base approx 100mm well wacked and then the screed ?? there will not be any heavy load on it ,its just a rear yard.
I hope this makes sense
Thanks Paul


(Edited by Biffo17 at 10:07 pm on June 9, 2004)

[alan ditchfield]

Not entirely sure what you meen but it sounds like the ground level was well above the dpc.? correct ?.this should be 150mm bellow finished level (2 brick)
Anyway you want to be laying the block on about 50mm of screed, course grit sand over a layer of hardcore whether its load bearing or not, but as i say im not entirely clear what you meant so i dont know whether this will help or not.

[Biffo17]

Yes ,the level was high ( the damp was bridging the damp course )
The ground im left with is very compacted ,do I still need to use MOT or can I get away with 20mm Ballast then screed
Im on skip number 5 !!!!!!!!!!!!!!!

[alan ditchfield]

If its for light use only then you should be ok if you can get an even, level screed of sand over the sub base, but only if the ground is well compacted if youve put all this work in now the last thing you want is a sunken patio next year, and have to do it over. Good luck.

[84-1093879891]

As the sub-grade is so well compacted, if you promise that there will be no vehicular traffic on this pavement when it's complete, then you can get away with using just 40-50mm of sub-base material, then a 35-50mm bedding layer.

I'm sorely tempted to suggest using a membrane between sub-grade and sub-base, just to be on the safe side, but I'd still actually prefer a thicker sub-base.

There's a 'rule of thumb' that we use for fill layers that they should always be at least twice the thickness of the largest particle size. This is done to ensure that no 'points' are created within a layer, that is, no particle or lump of stone can be in contact with both upper and lower surfaces at any time, but by definition must be 'cushioned' by finer material either above, below, or both above and below. This ensures that the layer acts to spread the loads it carries, but also ensures there are no 'rocking points' within the layer.

DTp1 has, in theory, a max particle size of 38mm, so using this 'rule of thumb', the min layer thickness would be 38 X 2 = 76mm.

You're proposing to use a 20mm ballast, which is less than ideal to start with, but, using the rule described above, this means a 40mm layer thickness would be required. I'd increase this to 50mm, just to be safe, and also because, as I hinted, ballast is a bit crappy when it comes to load distribution, in comparison to DTp1 and DTp2 materials.

So, if you use a 50mm thickness of sub-base, this will actually save you 50mm X 30m² = 1.5m³, which is roughly one-third of a skip. Is it worth it?

[Biffo17]

Thanks for the replies, youve talked me into doing it right.
Another snag!!!
Im working with 2 drains , as I have lowered the groung to lay the blocks at 150mm from damp the crock pipes and bends have been exposed.
Ive got all the perimiter done on 100mm - 150mm conc
This is leaving the pipes approx40 mm from the block screed level .
How can I get around this ?? I dont want to fracture the pipes with a wacker DO I !!!
Be vert gratefull for and suggestions
Thanks Again Paul

[84-1093879891]

If there is a 40mm sand cushion between the top of the pipes and the underside of the block paving, that will be sufficient to protect them, provided that the pipes are in good condition.

If there is any reason to suspect they may be a touch on the fragile side, then it's worth re-piping the exposed elements with new clayware or plastic haunched with concrete.

[Biffo17]

Hi again , Ive got the blocks ( marshallsDriveline Exel)
the website states that Joint filling and Keybond is reccomended , not sure what Keybond does or how to use it . Can anyone explain
Thanks Paul

[alan ditchfield]

Keybond is a joint stabiliser used to help the kiln dried sand from being washed from the joints im not sure of the price but there are many products on the market and they are all worth the small extra cost.

[84-1093879891]

Which website says Keybond is recommended? The Marshalls site? Well, there's a surprise, what with Keybond being a Marshalls' product! ;)

There are some jobs where Keybond, or other JSCs, might be necessary, but they really are the exception, rather than the rule.