Joined: July 2004
||Posted: 20 Feb. 2019,10:43
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.
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