There are a number of issues here. The answer depends on the purpose of the
cover ?
If it is intended as a simple separation barrier (i.e. to prevent physical
disturbance of the underlying contaminated material) then the relative
depths of top-soil and sub-soil should be governed by horticultural
requirements. It is the total thickness that matters. There is no advantage
in a large depth of topsoil where this is not required to sustain whatever
planting is to take place. For permanent grass as little as 100 mm might
suffice. Depths can be increased if necessary where trees etc, are to be
planted by creating planting pits or mounds. High fertility is not
necessarily desirable for landscaping schemes.
For vegetable plots more will be needed, say 0.45 m.
BS3882, the top-soil standard includes, a maximum suggested depth. Top-soil
is a scarce resource and should not be wasted.
Consideration must also be given to the installation of services, fences,
foundations etc. when deciding on the overall depths, i.e can these be
installed without disturbing the underlying material or are additional
measures required.
If it is intended to have a more complex function, e.g. to act as a barrier
to upward migration due to capillary action, then the depth of covering
material required will depend on the physical properties of the material
(e.g. particle size distribution, hydraulic conductivity) that is to be
used. You can have a small thickness (say 0.2 m) of a very suitable material
(free draining gravel) or a greater thickness of a less suitable material
(e.g. 2 to 3 metres of fly ash). Once the thickness of the capillary break
layer is decided, the depth of the overlying material (sub-soil and top-soil
combined) is dictated by practical considerations such as the depth of
foundations and services. It should be possible to install these without
disturbing the capillary break layer.
The design of simple and complex cover systems is described in "CIRIA SP106
Remedial Treatment for Contaminated Land: Volume VI - Containment and
hydraulic measures" and in various texts by Tom Cairney who led the DOE
supported research on this topic.
SP106 includes discussions on monitoring, maintenance and failure modes of
covering systems.
regards,
Mike Smith
+++++++++++++++++++++++++++++++++++++++++++
M A Smith Environmental Consultancy
10A Moorland Road
Hemel Hempstead
Herts.
HP1 1NQ
tel. 01442 258164
fax. 01442 253777
mobile 07961 312790
e-mail. [log in to unmask]
web. www.michael.a.smith.btinternet.co.uk
----- Original Message -----
From: "Clare Horton" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, July 29, 2008 9:58 AM
Subject: 'Clean Cover'
Dear all,
I appreciate this is a very old chestnut, but I'm looking for a consensus:
What would you say the proportion of topsoil to subsoil should be for 1000mm
clean cover and 500mm clean cover:
Scenarios are 1) garden, 2) landscaped areas. AND the CoC's would be HC' S
and metals and PAH's.
The reason I ask is that the choice of material for subsoil is obviously
much less stringent.
We also ask for validation testing os 1 sample per 100m3 for all clean
cover. Is this appropriate?
Thanks,
Clare
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
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