Civil Engineering Reference
In-Depth Information
11.9.2.3 SM-V1 (full-face excavation, face without support)
This type of machine can only be used in stable, predominantly water-impermeable, co-
hesive soft ground with high fines content. The stability of the face should be verified by
calculation. It should also be verified that the sides of the excavation are temporarily sta-
ble until the final tunnel lining has been installed. Loosening of the ground, which could
reduce the bedding, should be ruled out. If there is building on the surface susceptible to
settlement, deformation of the subsoil and loosening should be verified using the usual
damage classes (e.g. gradient of the settlement trough).
In rock, this type of machine can be used in rock classed as liable to rockfall to brittle, also
with water in strata or joints. The strength of the rock mass can be greatly reduced even
if the rock strength is good. This corresponds to a joint spacing of ≈ 0.6 to 0.06 m and a
RQD value between approx. 10 and 50 %. In general, however, this type can be used in
rock with compression strengths less than 5 MN/m
2
, for example strongly weathered rock.
The stability of the face and the sides of the excavated cavity should be verified with cal-
culations. In case of high water ingress, appropriate measures should be planned.
11.9.2.4 SM-V2 (full-face excavation, face with mechanical support)
Due to its failure on numerous projects, this type of machine is no longer recommended.
11.9.2.5 SM-V3 (Full-face excavation, face with compressed air application)
The application of compressed air enables machine type SM-V1 to be used in stable
ground even under the groundwater table. The air permeability of the ground or the air
consumption, the verification of the formation of an air flow and safety against blowouts
are the essential criteria for the use of this type of machine. The groundwater table should
be above the tunnel crown with an adequate margin of safety.
11.9.2.6 SM-V4 (full-face excavation, face with slurry support)
The main area of application of slurry shields is in coarse- and mixed-graded soil types.
The groundwater table should also be above the tunnel crown with an adequate safety mar-
gin. As the ground is excavated, a fluid under pressure, e.g. bentonite suspension, supports
the face. Highly permeable soils impede the formation of a membrane. At a permeability
of over 5 · 10
-3
m/s, there is a danger that the bentonite flows uncontrolled into the ground.
The scope of application can be extended by adding fine-grained material and filler or ad-
ditives for the improvement of the rheological properties. Alternatively, additional meas-
ures to reduce the permeability of the soil (for example filling the pores) can be necessary.
Boulders and blocks too large to be pumped can be broken by a crusher in front of the
inlet. A high fines content can lead to difficulties with the separation. It should be borne
in mind that the rheological properties of the support fluid are worsened by fine-grained
material, as the separation of clay fractions and bentonite is not possible.
11.9.2.7 SM-V5 (full-face excavation, face with earth pressure
balance support)
Machine types with earth pressure balance support are particularly suitable in soils with
fines content (< 0.06 mm) of over 30 %. In coarse- and mixed-grained soils and rock, the
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