Civil Engineering Reference
In-Depth Information
agent used can be estimated from experience and the characteristic values of the ground.
The design of the cutting wheel, technical machine parameters and the required support
pressure also have to be considered. Products should be chosen so that they do not flow
uncontrolled into the surrounding ground but enable a homogeneous soil mixture.
For shield machines with slurry support to the face, conditioning can be in the form of
liquid additives. For shield machines with earth pressure-supported face, conditioning in
non-cohesive soft ground is normally provided by tenside foams with the possible addi-
tion of polymers, while in cohesive soft ground, polymer, bentonite or clay suspensions or
even water can be used.
The purpose of any conditioning agent is the alteration of the properties of the excavated
material to ensure the most trouble-free and economic tunnelling possible. For shields
with slurry-supported face, this can mean that sticking and separation in the excavation
chamber are reduced or prevented. For shield machines with earth pressure-supported
face, non-cohesive soil can be processed into a plastic material by conditioning, sandy
clays can be conditioned for less abrasive properties and in clay, conditioning is often used
to reduce sticking and adhesion problems.
The additives added to soft ground should comply with the following minimum criteria:
- Simple and controllable dosage (ensured by the use of liquid additives),
- Avoidance of blockages in the additive feed and in the pipeline pumping the condi-
tioned material out of the excavation chamber,
- Rapid development of effectiveness, in order to be able to react to geological alterations,
- Avoidance of environmental hazards.
11.6.3.4 Soil separation
On a tunnel project with slurry-supported face, the soil is separated from the transport
medium (typically bentonite suspension) in a separation plant. Boulders and gravely and
sandy soil contents are mechanically removed from the suspension on screens (coarse
stage), cyclones and oscillating dewaterers (medium stage). Grain sizes below the sand
fraction are separated from the suspension by chamber filter presses, centrifuges or high-
performance cyclones (fine stage). Separation in centrifuges is improved by the previous
addition of flocculants.
The configuration and dimensioning of the separation plant is mainly based on the grad-
ing distribution and the suspension circulation quantity. It should be borne in mind that
ground improvement measures and breaking and grinding processes during excavation
of the soil can increase the fines content and can worsen the properties of the suspension.
High suspension densities and abrasive minerals increase wear to the excavation tools and
the hydraulic mucking equipment.
11.6.3.5 Soil transport and tipping
In order to fully investigate cost-effectiveness for the selection of a tunnelling machine,
muck transport and tipping also have to be considered. The characteristic parameters of
the ground can be significantly altered by excavation, any soil conditioning and the indi-
vidual control of the tunnelling machinery.
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