NATM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 654

20.3.6 Excavation and Support
Figure 20.38 shows the sequence of excavation and the means of support specified for
the tunnel heading in the layers consisting predominantly of claystone. The excavation
was subdivided into vault excavation and bench and invert excavation.
The tunnel cross-section was excavated using tunnel excavators which were also
equipped with a heavy hydraulic chisel. Thicker banks of sandy limestone were loos-
ened by blasting. The pullout lengths ranged between 1 and 1.5 m at the vault and
between 2 and 3 m at bench and invert. The invert was closed at 4 - 6 m behind the
temporary face (Fig. 20.38).
The water pressure taken into account as well as the loads resulting from rock mass
pressure which are governed by the increased horizontal in-situ stresses existing in the
claystone layers led to a required thickness of the reinforced shotcrete membrane of
30 - 35 cm. A systematic anchoring with 3 m long SN anchors was carried out locally.
Steel sets were placed at a spacing of 1 m (Fig. 20.38).
Figure 20.38 Standard heading in the claystone, excavation and support (Wittke et al. 2003)