Methods to remove driveway water

[unclebob]

Good Morning all,

I have just applied to harrow council (NW London) to have my driveway kerb cut. As part of the agreement, i must make allowances for the water on the surface of my drive to soak away into the ground within the boundaries of my property.

The layout of the drive is as follows (assuming your standing on the pavement looking at the house):

Length from house to pavement is 4.84m
Width from edge to edge is 7.49m

This gives me 36.25 square meters.

The existing surface slopes to the kerb towards the left side.

The surface at present is non-permeable - and this for the next 5-10 years is not likely to be changed.

I will be putting channel drain in across the front edge (7.49m) kerbside to stop water flowing out onto the pavement/dropped kerb as per the council requirement.

My question is, what should i do with this water?

Speaking to the council they recommend 25% (approx 9m) of the hard surface to be removed, to a depth of 0.4m and filled with washed stone...

Not sure how they came up with any of these numbers but i don't see them or any other detail in The Town & Country Planning Act (General Permitted Development) Order 1995 (as amended in 2008) as they are requesting me to comply with or any other document they supplied me with.

What are the alternatives? I have been looking at soakaways, but not sure on dimensions, and it wouldn't be more then 5m away from the house(though i seem to read that this is a recommendation not a requirement?) Havent done a test hole as yet - need to break the surface to test but want to get options as to what i can do before breaking the surface in the wrong place!

Alternatively, are there any actual documented requirements that the area for drainage must meet?

I know of at least two other properties in the harrow area that have recently had their drive cut, with non-permeable surfaces but no where near the 25% soil/gravel. So are they asking me to do 25% but the requirement is less?

I'm looking to do the work myself, and would prefer not to loose 25% of the front space to gravel if possible - fitting two cars with the area that's left will leave it a bit tight to get to and from the front door due to where the dropped kerb will be.

Any help and or guidance will be much appreciated! Want to try and get this done before winter really hits us!

[lutonlagerlout]

you need a soakaway
our local building control look for 4 crates for every 16 M2
this can be expensive
LLL

[Tony McC]

As LLL has said, the solution is a soakaway rather than the 25% rain garden/infiltration trench your council has suggested.

The real problem is the size of a suitable soakaway. I have wasted a couple of years trying to turn the jargon and over-engineering of CIRIA documents and BRE digests into a contractor-friendly guide to what is required for any particular job. It's complicated. Very, very complicated, but the one essential bit of information needed regardless of which angle is used to approach the problem is ground permeability.

If you live on Old Red Sandstone, then water is more-or-less sucked out of the ground and a soakaway of 1m² footprint will easily serve a 100m² pavement, but for those of you on that nasty London Clay, I can easily believe that a footprint of around 25% is required, but to be sure, a permeability test is needed.

When it comes to permeability tests, the problem has been to develop a test that will give an indication that would be adequate for homeowners undertaking patios and driveways. The 'approved' methodology is way beyond the ability of most DIYers or driveway contractors, and the cost of bringing in a geo-tech specialist to undertake the test is just a non-starter. I have a simple 'assessment' test, but I've been advised not to publish because it might not give reliable results on heavy clay and if someone was to use it and then find their drains surcharging, I could be liable. Sod that!

So, to return to the problem in hand, the real stumbling block is the fact that the driveway itself isn't scheduled to be re-constructed for at least 5 years. The drainage works, whichever option is chosen, involve significant work to the driveway/front garden and just on econmoic grounds, it makes sense to have the driveway 'upgraded' in some way while the requisite drainage works are completed.

It seems to me that the easiest solution is a sub-pavement soakaway using either permeable block paving (PCBP), storm crates or a sub-base replacement system. Itseems that the driveway will have to be considered as part of the overall project.

[unclebob]

Thanks for the reply's, looks like the renew of the surface may have to be done now and end up with permeable block in that case, or live with the 25% loss for now and alter when the renewal comes up...

Tony McC are you able to pm me the test? i promise i wont hold you responsible in any way! just so that i have an idea of what I'm dealing with so that i can try and make the best compromise for me and the environment!


Also Looking at the Sub-base replacement, what layers would be needed above the crates? (ie how deep would i have to go between the base of the crate to the finish level? Could i mix the two - use crates to make up the 25% with a non-permeable surface on top? Water would then get into the crates though channel drain?

[stephen gibson]

The Planning Act was changed.

See EA Website link on drives in flood zone 1

Tony McC is correct. The problem is laws / codes work on the basis that everyone needs to do the same thing....

The problem is permeability is measured on an exponential scale. I know of sites where the rate was a factor of a hundred different depending on where you dug. What worked on the last job could be completely wrong on the next. The solution at each site can be entirely different.

The cost of a specialist geotechnical contractor/consultant can be considerable.

There are however plenty of small civil engineering consultants who do the design / calculations alongside small local groundworkers who do the physical test.

Its not expensive. The test and design is typically less than 1/3 of the construction cost of a single typical manhole.