Ian
I don't have a 'minimum' depth applicable to all sites, every site is different and is considered separately. I have asked for 1m before (and upwards), but that would be very unusual. A more typical depth would be 0.5 - 0.8m. Using the BRE methodology almost always comes out with 0.6m, but you also really need to have specified the source cover material and the contaminant levels within it. If the developer/consultants are proposing the BRE system you need to decide whether the considerations it undertakes match the ones you have about the site - future use etc.
Typical formation would be topsoil - perhaps a subsoil - perhaps terram geotextile - marker layer/break layer of crushed concrete etc.
The topsoil layer should really be sufficient to allow domestic vegetable gardening in residential plots I always think, and that suggests 0.5m or so, as this is roughly two spade depths (the depth dug to using 'double digging'). The main purpose of the marker layer is to act as a visual marker for anyone digging down in their gardens etc, as an indication they might want to think about stopping (or at least contacting the Council/developer). It also serves a role in preventing mixing processes between deeper contaminated soil and the clean topsoil, it may also function as a capillary break layer, but that is rarely an issue for sites in Gloucester.
On most sites the issues to consider are more about confidence and future use of the site than about any objective engineering assessment of soil mixing, groundwater uplift of contaminants or similar.
Like all issues where there is a large amount of subjective professional judgement involved, there's plenty of scope for disagreement !
Hope this helps a little.
> Steve Moreby
>
> Environmental Health Department
> Gloucester City Council T 01452 396 312
> Herbert Warehouse F 01452 396 340
> The Docks [log in to unmask]
> Gloucester, GL1 2EQ www.gloucester.gov.uk
>
>
-----Original Message-----
From: Ian Grand [mailto:[log in to unmask]]
Sent: 24 January 2007 11:43
To: [log in to unmask]
Subject: Residential gardens - capping layer depths
To anyone from a Local Authority.
What is the minimum capping layer depth required by your local authority,
for residential gardens undergoing remediation through the planning
process. For instance in Nottinghamshire the cover depth is set at 1m.
Is it possible that you could forward details with the depth of
topsoil/subsoil and whether this includes a capillary break layer /
geotextile membrane.
If you do not set a requirements, do you undertake any form of assessment,
(other than the BRE Cover systems for land regeneration), to justify the
use of a lesser capping depth.
Regards
Ian
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