NATM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 640

Jet grouting
Jet grouting vaults are constructed for the advancing support of tunnels excavated in
soil or weak rock (Fig. 20.22, upper). They consist of jet grouting columns (Fig. 20.22,
lower left) that are constructed as follows: Cement-based suspension is injected into a
borehole under very high pressure (generally 20 - 50 MPa) via a rod with a nozzle at
its end. The nozzle rotates with the rod which is slowly pulled out of the borehole. In
this way, due to the mixing of the suspension with the surrounding ground, a column is
formed. After hardening of the cement this column possesses a high strength in com-
parison with the undisturbed ground. The surplus mixture of suspension and soil exits
as backflow through the annular gap between borehole and rod (Wittke et al. 2002).
Figure 20.22 Advancing support by jet grouting
Jet grouting columns are normally constructed in slightly ascending boreholes that are
carried out from the temporary face at the roof of a tunnel (Fig. 20.22, upper) with
about 15 m length and have diameters of 60 - 90 cm. If the tunnel is located in a ground
layer with high permeability, a bulkhead made of jet grouting columns may be con-
structed at the end of each vault (Fig. 20.22). Two successive jet grouting vaults overlap
by about 2 - 3 m.
A jet grouting vault transfers loads in transverse and longitudinal tunnel directions. Thus
the green shotcrete close to the temporary face and the area of the temporary face are less
strongly loaded. Furthermore, with jet grouting vaults subsidence can be limited, collap-
ses are avoided and the safety of the tunneling staff is ensured (Wittke et al. 2002).