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Posted: Tue Aug 19, 2008 9:55 am
by BlueDragon
Further to my previous post 'block paving on a clay soil' the contractor has started to put the base down today.
Basically he has dug to a depth of max 7" and in places only 4". He has had the scalpings delivered that are what I can only describe as builders rubbish, eg bits of broken concrete, brickwork and a fair bit of fine stuff like soil!
He assures me that he is not using type 1 scalpings as this would simply sink into the clay so this larger stuff, mixed in with bags of cement will form a more keyed base.
It does not sound right to me and I am concerned.
Is this practice correct when working on a clay soil?
I equally dont think he has enough depth but again he says that if he keeps digging down he will only uncover more clay making it harder to work with!
Each time I question him I get these types of answers and I think he is starting to get frustrated with my questions. As I am no expert I can only go by what he tells me and assume this to be correct but from the general explanations on this forum and other sites I have not come accross this as good practice for a block paved driveway!
Can anyone advise, am I over concerned? I have basically left him to get on with it now as I fear an argument is just round the corner.
Posted: Tue Aug 19, 2008 10:56 am
by ambient
should be digging at least 12 inch out, stone going back in doesnt sound good if its got soil in your asking for trouble,we use recycled mot type 1 but that has no soil in it,it is though crushed brick ,flags and concrete all crushed 25mm to dust
Posted: Tue Aug 19, 2008 11:44 am
by BlueDragon
This stuff has dust in it (which is fine bits of stone, concrete, and possible earth but hard to tell) but also has lumps of concrete and bricks in it mostly around the size of 50-60mm up to the size of half a brick. I have also noticed bits of wood, plastic sheeting/bags and pipework mixed in (although not in great quantities).
Basically I am concerned about the depth they have dug down to 5-7" at best, the quality of the sub-base materials used, the depth of the sub-base which I am guessing is only 3", and the fact they are mixing in about 15 bags of cement with this to harden the base.
Reading other posts there has been mention that doing it this way can create a water trap and eventually the driveway will fail?
I admit I could be doing the guy a dis-service as he is a well established contractor that only deals in block paving and has done so for 20+ years. He does come well recommended, I have seen the quality of his finished work on a number of drives which was very good and as I know nothing more than what I have read up on this forum and other sites dont know if the practices he is employing are right or wrong.
But alarm bells are ringing and I dont want to end up regretting the job in the future.
What would peoples advice be?
Posted: Tue Aug 19, 2008 1:28 pm
by SelectPaving
Have you seen any of this guys drives that he has laid that have been down a few years?
There shouldnt be anything that resembles soil in the mot type 1.
In our 15 years of laying drives we have found that soil is to be dug out until coming across a firm soil which has compressed to its compacted final level over the years, take too much out and the crusher will still settle if too much of a depth has been added.
Posted: Tue Aug 19, 2008 2:20 pm
by Rich H
Crushed concrete has fines which can resemble dust when dry or mud when wet. It's a mix of concrete and brick dust which gives this effect. Doesn't mean it's not Type 1. However, the large chunks do mean that it's not. Added to that there should be little or no debris (rags, plastic, wood, etc.)
It can still form a good sub-base (if cleaned up) but not at the depths described. If the sub-base is firm you could just about get away with an 8" dig (110mm sub-base + 40mm bedding course + 50mm block) but with clay I'd advocate a minimum of 250mm, that's 150mm+ of sub-base, particularly if it is a bit coarse.
5"-7" is certainly not enough to withstand long-term use on a driveway.
As for mixing cement with a crushed concrete sub-base, this is pointless at best and counter-productive at worst. It won't form a solid layer but will instead hold up moisture in random areas. This could cause patches of fluidised bedding course to form which will 'flow' away from points of pressure, leading to ruts forming, etc.
As with all block paving, a cowboy can make a job look professional but only for a limited period. In this country though, people tend to move before the problems appear so firms like this one just keep getting away with it.
By failing to dig out that extra couple of inches the contractor will be saving himself £5/sq.m in waste disposal and MOT. £500 on a 100sq.m drive is too tempting for some.
Posted: Tue Aug 19, 2008 3:28 pm
by Tony McC
7" is nowhere near deep enough.
Ask to see delivery notes for these "scalpings": the material should be DTp1 or DTp2.
If he's claiming DTp1 would 'sink into the clay', what is to stop that happening with the stuff he's using? And if the clay is soft, why isn't he using a suitable geo-textile?
I can't help feeling this guy is a cowboy.