NATM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 636

Driven spiles consist of 3 - 6 m long steel pipes which are driven into pre-drilled holes.
The diameter of the steel pipes is slightly larger than the diameter of the drillings.
Injection drill spiles are carried out with lengths of 4 - 12 m and consist of steel pipes
with a lost drilling bit. The holes for the spiles are drilled with diameters of 42 - 76 mm.
After drilling the pipes to the desired depth, the boreholes, together with the surround-
ing rock mass, are grouted. Injection drill spiles are used in rock mass conditions where
stable holes cannot be drilled and/or the rock mass needs to be improved by grouting
(Wittke et al. 2002).
Pipe umbrellas (forepoling)
If spile umbrellas do not offer sufficient support, pipe umbrellas, also named “fore-
poling”, are installed. Pipe umbrellas are often conducted in weak rock if streets or
structures are undercut with little cover. As with spiles the pipes are arranged over a
particular section of the circumference of the excavation profile (Fig. 20.19) preceding
the excavation. Pipe umbrellas extend further in advance of the excavation than spile
umbrellas.
If a pipe umbrella is constructed from niches, the cross-section of the tunnel must be
widened so that the drill points for the pipes are located outside the inner contour of the
standard profile of the shotcrete membrane. Thus, the geometry of the steel sets must
be adapted for each round and the niches must at least partially be filled by shotcrete
before the sealing and the internal lining are installed (Fig. 20.20).
If a pipe umbrella is constructed without niches, the drill points for the pipes are located
within the standard excavation profile (Fig. 20.21). In the course of further excavation
the pipe sections lying within the cross-section of the shotcrete membrane must be cut
off after each round.
Pipe umbrellas are constructed with lengths of 15 - 30 m with a heavy drilling rig. The
pipes remain in the rock mass as structural elements. After installation of the pipes, the
annular gap between the pipes and the rock mass and, as far as possible, also the sur-
rounding rock mass, are grouted with cement-based suspension.
The boreholes for a pipe umbrella have a spacing of 30 - 50 cm. They are drilled as-
cending at an angle of around 5° with respect to the tunnel axis. The pipes of two pipe
umbrellas should overlap by at least 3 m in the longitudinal tunnel direction (Figs. 20.20
and 20.21).