The Brew Cabin

Posted: **Tue Nov 13, 2018 4:43 am**

Hi,

About to finish a build but i've been having a bit of a dilemma with my finish surface. It's a pale gravel with white cast concrete slabs dotted through. A little inexperienced with gravel hard standings, originally i had planned on a limestone chipping and had thought a crossfall would be unnecessary as it would be free draining and a lot easier as i wouldnt have to put a fall on all my edging etc.

As i progressed with the build i have become more aware of using self binding gravels which would give a much softer, earthy look to the garden, a lot better than using boring old chippings. HOWEVER with all my corten steel edging in and slabs about the be set there is no adequate crossfall built into the surface.

By no means am i expecting you gents to recommend doing something the wrong way but if was to proceed how much of a disaster is it going to be? everything else is by the book, 100 compacted sub base etc. it's not a massive area. I suppose the question is am i overthinking it in this instance and for the sake of a much better look is it worth going against the recommended method or would it be a real mistake and essentially ruin the job.

Thanks for any thoughts or advice
Cheers, Steve.

Posted: **Tue Nov 13, 2018 9:23 pm**

Hi Steve
Is it to late to put a camber in or have you already laid slabs. For gravel drives I always use a type 3 Base so itâ€™s free draining. A lot will depend on the Sub grade Soil , if it drains
Well then I would think your be fine, but clay could give you problems
Regards
Craig

Posted: **Wed Feb 13, 2019 5:42 pm**

Cheers Craig. Only just seen this reply! i was a bit crafty with the way i laid the gravel it wasn't a straight self binder as i was trying to get a more natural look by mixing larger aggregates with the self binding stuff.

So essentially i had pockets of loose gravel so in my mind plenty of areas to drain away if the self binder becomes quite impervious. I think i was being overly cautious and freaking out which i have a tendency to do. all looks good currently and after a hell of a lot of rain this winter has performed fine. time will tell obviously but confident it'll be good.

Posted: **Wed Feb 20, 2019 9:43 am**

I've noticed a trend developing over the last couple of years, where SBG is claimed by its suppliers to be permeable or porous (there are technical differences between the two terms), and this disturbs me somewhat.

If we were to be really, really picky, then we have to accept that all of the popular SBGs, and even aggregates such as DTp1, are, just about, permeable. Given enough time, surface water will drain through and disappear, and, of course, we wouldn't use any such materials to, say, line a swimming pool or cover a roof structure.

However, it's the degree of permeability that matters - just *how* permeable are they? This is measured by the co-efficient of permeability (known as k or hydraulic conductivity), which is a reasonably simple measure of how fast a given quantity of water will travel through the material in a given period of time, which gives values in metres per second - the water will travel X metres in Y seconds, so to speak.

I'm not going to bore people even more with all the physics, but, as you will appreciate, the higher the coefficient of permeability, the more permeable is a material, that is, the faster water will drain through it.

Some of these SBGs, in their compacted state, have relatively low K values, so while they *may* be described as permeable, the degree of permeability is pretty poor and there is a damned good chance you will see standing water every now and again. Most of us will have seen areas of compacted DTp1 with puddles on it after heavy rain - they dissipate over time, but this is a classic example of an aggregate that is, to be strictly accurate, permeable, but only just so!

So, there's a need to be careful with drainage when creating gravelled or SBG areas. They *will* drain, eventually, but if the requirement is a puddle-free surface, then either consider drainage or choose a different material.

The Brew Cabin

Self binding gravel cross fall