Civil Engineering Reference
In-Depth Information
Uncontrolled ingress of formation water in shields without active face support can lead to
liquefaction of the muck, which should be taken into account in the planning of transport
and tipping.
Further information about tipping and conditioning is contained in Section 11.7.
11.7
Environmental aspects
Outside factors, which do not derive from the system behaviour (ground/tunnelling pro-
cess, see Section 11.6), can sometimes also influence the selection of a tunnelling ma-
chine. Particularly when two different processes of equal technical value are possible, the
factor “environmental impact” can be decisive. Particularly the suitability of the excavated
material for recycling or landfill can be of great significance. The soil conditioning used
with EPB shields, such as the addition of foams or polymers, can rule out the filling of the
material in certain landfill sites.
Conditioning
The purpose of conditioning agents is described in Section 11.6.3.3. They only penetrate
slightly into the subsoil, or not at all, but are transported out of the tunnel with the muck
and thus have a significant effect on the suitability of the material for recycling or tipping.
Conditioning agents can be classified into various categories. These include water pollu-
tion classes (WGK 0, WGK 1, WGK 2, WGK 3), degradability (min. 60 % primary bio-
logical degradability and min. 80 % biological degradability) and the toxological thresh-
old values for mammals (LD50) and water organisms (EC50).
Because of the wide range of conditioning agents, the composition of soft ground and its
properties, no general classification of conditioned soil is possible. It is necessary to inves-
tigate on a case-by-case basis which threshold values are complied with and how the con-
ditioned material should be processed. Information is given in the regulations in Section
11.2. According to the threshold, applicability of a conditioning agent can be so severely
limited that the result can affect the tunnelling process (see also suitability for tipping).
Separation
In the separation plant, the excavated soil is separated from the transport medium, as de-
scribed in Section 11.6.3.4.
When bentonite suspension is used as a transport medium, some residual bentonite content
will always remain in the separated soil. This bentonite does not, however, alter the LAGA
class (see below) of the soil. Recycling of separated soil is therefore possible, or not pos-
sible, according to the LAGA class of the excavated soil.
The separation of the fines in centrifuges is assisted by the previous addition of floccu-
lants. Because of the number of flocculants available on the market, no general statements
about the environmental acceptability of these products are possible. Information on this
point can be found in the safety data sheet supplied by the manufacturer.
The material produced by centrifuges, filter presses and high-performance cyclones is
very fine-grained and mostly of a pasty consistency. Recycling is therefore impractical, so
Search WWH ::
Custom Search