Civil Engineering Reference
In-Depth Information
the material has to be tipped. The same applies to used bentonite suspension, which should
be passed to appropriate plants as liquid waste.
Working in groundwater
Bentonite suspension and additives for soil conditioning have both been used for many
years in tunnels below the groundwater table. The same applies to the grout used for
grouting the annular gap and the biologically degradable grease used for the sealing of the
annular gap.
Tipping
The material removed from the tunnel should be processed and recycled if at all possible.
If this is not possible, the soil will have to be tipped. When conditioning agents are used,
attention should be paid to whether the excavated and conditioned soil complies with the
chemical and physical requirements for tipping.
The tipping of conditioned soil is regulated in Germany by the guidelines of the Länder-
arbeitsgemeinschaft Abfall (LAGA) (States working collaboration on waste) and particu-
larly by Guideline 20 “Requirements for the material recycling of mineral residues/wastes
- Technical rules”. This guideline governs the recycling of excavated soil and thus the tip-
ping of excavated material from tunnelling. Only when the analytically determined value
of chemical content rules out open tipping according to LAGA Guideline Nr. 20 (tipping
classes Z0 to Z2), does the material have to be tipped in a regulated landfill site or even a
tip for special waste (tipping classes Z3 to Z5). This is regulated in the Technical instruc-
tions for recycling, treatment and other disposal of municipal waste (TA Municipal waste).
For ecological reasons, unrestricted or restricted open tipping is preferable.
Material from the coarse and medium stages of separating plants can normally be recy-
cled. The fines content is mostly less than 5 %. However, these soils have special soil me-
chanics properties. The bentonite residue can swell again on contact with water and result
in material with similar properties to cohesive soil. The material should therefore only
be tipped in locations protected from water. For example, it can be used for backfilling a
road tunnel beneath the carriageway. Alternatively, a further stage can be provided in the
separating plant to wash the material. The simplest method is to spray the soil with water
on the oscillating dewaterer. This can significantly reduce the residual bentonite content in
the soil, which increases the quality of the soil and the possibilities for recycling.
Muck transported as sludge and the product of band filter presses and centrifuges cannot
generally be tipped without further processing, as it tends to plastic flow. Conceivable
ways of improving the suitability for tipping are consolidation through the addition of
lime or storing on an intermediate stockpile until the material has dried and thus gained
strength.
Concerning the suitability for tipping of soils, which have been treated with additives, the
information about biological degradability supplied by the manufacturer is not sufficient
on its own. The relevant regulations concerning pollutant content for each tipping class
should be complied with to check the permissibility of tipping, particularly with regard to
the residual content of hydrocarbons. Not least for economic reasons, the use of additives
with slurry shields and earth pressure balance shields should be reduced to a minimum.
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