The Brew Cabin

What is the spec. for this, please? Would 20mm washed gravel suffice?

I understand the importance of a geo-membrane to prevent the trench material getting clagged up with soil and roots etc. But what stops the top of the geo-membrane itself getting blocked with soil and thus reducing the draining capacity? I'm particularly thinking about a collector drain scenario.

There's no hard and fast spec for 'selected free-draining material' - basically, if the RE (Resident Engineer) says it's OK, then it's OK, and I'd hazard a guess that any 20mm clean gravel would be more than suitable. :)

SHW (Spec for Highway Works - the groundworkers' bible) states "Natural gravel, natural sand, crushed gravel, crushed rock other than argillaceous rock, cushed concrete, chalk, well-burnt colliery spoil or any combination thereof"...and then there's a load of technical stuff about grading and plasticity indices etc, but your 20mm gravel will be fine.

Just how geo-membranes work is a PhD dissertation in itself, but, the gist of it is that, because the sediments/silts/clays are not carried into the voids between the 'selected free draining material', there is less migration of non-soluble particulates and just the groundwater passes through the membrane, under hydrostatic pressure. We may find, in 20-odd years time, that the outside of the membrane does indeed become clogged, but the indications so far are that a geo-membrane extends the working life of a land drain indefinitely.