June 2010 - NHBC Home

Standards
Extra
Contaminated land –
Cover system validation
Introduction
Clean cover systems (also known as
‘simple capping systems’, ‘capping’
or ‘covering’ amongst others)
are frequently used methods
of remediating contamination
encountered on brownfield sites.
Their simplicity means that they are
often highly effective in ensuring that
the future occupants of residential
developments won’t be affected by any
contamination that remains on the site.
Background
Clean cover systems predominantly
involve placing an appropriate
thickness of suitable subsoil and/or
topsoil over areas of a site affected
by contamination. They work best
when the contaminants present are
low to moderate concentrations of
heavy metals or similar substances (i.e.
arsenic, lead). They are not appropriate
when high levels of contamination that
pose a significant risk to human health
are present, nor when petrol, other
hydrocarbons, asbestos, a high water
table or potential flooding are present.
In order to identify whether a cover
system is appropriate for a site, it is
essential that the nature and extent
of the contamination present has
been fully investigated by a suitably
experienced consultant or specialist.
Capping and topsoil validation
Where a clean cover system is adopted
to remediate a contaminated site,
the successful completion of the
remediation works will need to be
proven (‘validated’) by a suitably
experienced consultant or specialist.
Two of the major aspects requiring
consideration during validation are:
Photographic record of trial hole
1. Confirmation that the designed
thickness of the material has been
placed.
2. Confirmation that the materials
comprising the cover system are not
in themselves contaminated.
The requirements for validation are
outlined in NHBC Standards Chapter
4.1 ‘Land Quality – Managing Ground
Conditions’. NHBC requires that
the works are managed under the
supervision of suitable consultant or
specialist and we are provided with a
validation report before the homes
are finalled.
Further guidance on validation can also
be found in BRE 465 ‘Cover systems for
land regeneration – thickness of cover
systems for contaminated land’.
Validation of thickness
The thickness of the cover system is
typically validated by digging trial holes
in treated areas once the cover system
has been placed. Alternatively, on some
sites it may be convenient to use data
from level surveys made before and
after cover system placement from
which the thickness of cover materials
can be calculated.
Where trial holes are used to confirm
the cover thickness, the validation
report should clearly state the
locations and numbers of the trial
holes. Confirmation of the thickness
of the cover can be evidenced by a log
describing the materials encountered in
the trial holes and/or by photographs of
the trial holes with a tape or staff clearly
showing the hole depth. Placing a piece
of wood or similar flat item (clipboard
etc) at the ground surface immediately
adjacent to the tape/staff can make it
easier to indicate the depth.
Chemical testing
Chemical analysis of capping material
used on contaminated sites is essential
to demonstrate that the materials
are not themselves contaminated
and are suitable for their intended
use. The analysis should consist of
an appropriate set of tests for the
presence of possible contaminants.
This may be undertaken in several
ways by:
1. Using soils (including manufactured
soils) from a commercial provider
who can provide the results of
quality testing conducted ‘at source’
and who can confirm that the
materials delivered to the site are
representative of those tested. Any
test certification from suppliers
should be current and representative
of the material actually being used
on site.
2. Identifying a suitable source
of material on site or at other
construction sites (i.e. perhaps
generated from excavation
activities) and testing this prior
to incorporation into the works
or importation to the sitebeing
remediated.
10