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Four output materials are produced from the soil Ground conditions within one construction zone in the
washing plants as sands (>2mm, generally 40%),gravels north of the park encountered an historical landfill,
(<50mm, generally between 45% to 50%), fine silts originating from the Victorian period and active up
and clays as filter cake (waste, generally between 15% until the 1960s.
and 18%), and fine/coarse organic matter and
Excavated materials required sorting on a complex
ashes/coke materials (waste, generally between 2%
level, prior to any main treatment of the soils, to render
and 5%). The design of the soil washing plants have
them suitable for reuse as an engineered fill. The
been continuously under review, as the site-derived
complex sorting machinery was procured with careful
materials have varied. The plants have required high
consultation and use of the specialist contractor, who
levels of technical support and backup laboratory trials
designed the combination of equipment uniquely for
from the specialist supporting teams. These provide
the site conditions. The equipment uses vibrating
either recommendations for site modifications within
screens to separate the soil from the general landfill
the plant equipment, or changes in water flush rates/
materials, before using belt electromagnets to remove
volumes and modifications in the density separation
materials. The soil then goes through a manual picking
additives used in the process.
zone to undertake a final visual separation. The process
Both external electromagnetic belts and magnetic has subsequently been refined and used for the
drums are used to fine-tune the output materials. manual abstraction of low-grade exempt radiological
This renders them suitable for the variety of materials wastes and asbestos materials.
used as general fill, specific engineering Class fill
One bio-remediation system has been established in
(CLASS 1a, 6F, 6N) and cover or break layer materials
the south treatment centre to allow for the processing
protective of the final intended land use. Fine-grained
of soft alluvium or cohesive materials with principally
derived materials from the enabling works have been
hydrocarbon (organic) exceedence values in contami-
processed through one of the two ex-situ batch
nation. So far over 12,000m
3
of site-derived soils have
pugmills, primarily for soil stabilisation of chemical
been bioremediated, with about 2,000m
3
still to be
contaminants. It uses proprietary and specialised
processed. The works have also encountered invasive
re-agents to produce chemically stable constituents/
species in the form of Japanese knotweed, Giant
materials.
Hogweed, Himalayan Balsam and Floating Pennywort.
The soil stabilisation equipment is also available to
The works have undertaken a carefully designed
enhance the material strength of soil using re-agents
sequence of treatments over the park, including
and/or cement. Chemical soil stabilization has to date
spraying and controlled removal, together with
processed 50,000m
3
of site-derived soils, with about
excavation and burial within a root barrier containment
10,000m
3
still to be processed from enabling works
system. A specialist horticultural management
and 50,000m
3
from the follow-on construction works.
contractor was procured to recycle about 28,000m
3
of
Soil stabilisation of river silts and soft alluvium for topsoil and sub-soils, and oversee the design, burial
material strength enhancement is also being under- and containment of 40,000m
3
of chemically contami-
taken using in-situ techniques, with up to three nated soil and Knotweed.
specialist Wirgen machines. So far over 150,000m
3
of
Less than 3% of soils infected by invasive species have
site-derived soils have been geotechnically stabilised.
been sent to appropriate licensed landfill.
ENVIRONMENT INDUSTRY MAGAZINE
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