Sub base levels

[pagmore]

Thanks for the fantastic and informative website. I've been lurking for a while now and salute Tony for his efforts.

I've been a bricklayer for over tweny years and have now left site-work and am doing a little block paving for a friend. It's surprising how little notice I've taken of the lads on site. After finally grasping that these blocks don't stack one on another, seperated by mortar, I'd like to know if anyone has any tips on accurate formation of the sub-base. I'm doing a drive that's 9 metres wide to varying falls. I understand the screeded sand using straight edges and trammel bars, but would like to hear the techniques for the base.

Cheers in advance

[lutonlagerlout]

have a butchers here mate
cheers LLL

[pagmore]

Thanks for the pointer. Now when we build a wall, it's a simple matter of a couple a' datums and up she goes.
With a drive, while I'm happy to throw in a few ton of type one and set about it with the rakes and wacker, I read on this site that it should be within +/- 10mm.
As this drive that I'm foolish enough to have taken on is over nine meters wide, sloping down the pavement, away from the street door and back to the garage. The thought going into the later setting up of my screed rails has been no mean feat, but I'm happy with where I am.
To achieve the acuracy in the screed will be straight forward, but how do you pros achieve the sub-base accuracy without rails?

Thanks again

[Tony McC]

"Rack o' th'eye", as they say in this part of the world. It's something that comes with practice and experience but, after about 15 years, you develop the ability to judge the accuracy of a sub-base level by sight alone given just two or three reference points.

To be certain, and the way we do it on commercial projects, is to "dip" the sub-base, or to use boning rods. Dipping involves pulling a string line taut between two reference points and then measuring down to the sub-base. Say, for instance, you had the string line pulled between garage floor and public footpath and there was to be a simple flat bone between the two. If we were using 60mm blocks on a 30mm bed, then we'd want the sub-base to be 90mm (±10mm) below that line.

Similarly with boning rods: the backsight and foresight are established at known points and the traveller used to check the level in various places.

Later this year, I'm hoping to have a pictorial guide to these techniques (and others) as I'm currently writing a training course all about setting-out and establishing levels, so some of the info used for the course (which will be a one day offering via the HLTG - book early to avoid disappointment!) could be used on this website.

[pagmore]

Thanks, Tony.

I'd better get my 'eye in' fast then.

Cheers,

Pag

[squire]

For those of us without the "eye". I used a rotating laser level (very handy tool). I got about 10 350mm lengths of 6mm twisted bar and drove them into the ground. I used the level to ensure that the top of the pins were at the required finised level throughout the area (including falls). I then put red insulating tape on the pin to show where the compacted 804 should come up to (100mm below the top of the pin in my case). The pins were left in place and only pulled out when the paving block were being put down. I was able to constantly check and recheck my level and falls during all stages.

This might seems a bit complicated or unnecessary to the pro but for a novice working alone I can say that it worked out very well.