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Geotechnical News • December 2014
29
THE GROUT LINE
Requirements of the project
The Stans tunnel is 750 m long,
and has a polycentric section 12 m
high, and 13 m wide. The minimum
thickness of the jet grouting shell
was designed 2 m thick, with the jet
grouted soil designed for a minimum
strength of 5 MPa.
The entire tunnel was divided into 38
compartments, each 20 m long. The
division into compartments allowed
testing of the residual water inflow,
with a maximum flow of 5 liters/sec-
ond.
The design specified also allowed
displacements for each of the existing
works. For the motorway bridge and
the railway a vertical and horizontal
displacement of ± 2 mm, was set.
One of the main problems was the
presence of a high velocity ground-
water table, created mainly by the
Inn River and by two perpendicu-
lar creeks. The presence of these
underground flow conditions created
turbulence and continuous changes
in direction of the flow with possible
wash-out of the grout mix used for
the jet grouting. For this reason Trevi
designed and patented an innovative
technique, in which the cells created
by the columns are tightly interlocked.
This technology has been considered
reliable in responding to the technical
problems.
The works
A preliminary full scale test using a
double fluid jet grouting system (air
and cement slurry) has been performed
to confirm the jetting parameters
necessary for the design assumptions
(Figure 3).
The test involved the installation of
some columns using the spacing,
parameters, and sequence devised for
the entire work. After the hardening,
the set of columns was cored to assess
the strength of the jet grouted soil.
Based on the results of the test, the
final design was prepared for the
improvement of a total volume of
120,000 cubic metres of soil.
Trevi ‘s method created a structure
composed of three different series of
vertical and/or inclined columns, over-
lapping each other by at least 100 mm.
A special sequence of installation was
studied and patented to grant the fea-
tures of water tightness and strength of
the jet grouting ring requested by the
design.
Initially, the most permeable layers
of soil were saturated with cement
slurry before commencement of the
jetting process. Then, for each area, the
primary columns were installed. The
secondary columns were then installed
midway between primaries, resulting
in a hexagonal cell lattice. After the
columns hardened, the tertiary columns
were installed in the centre of the cell,
closing the structure (Figure 4). The
Figure 1. The Trans-European Trans-
port Network, with Berlin-Rome line
highlighted.
Figure 2. Aerial view of the tunnel site, where motorway and railway are
crossing each other close to the Inn River.
Figure 3. A full scale test (section and plan view) was performed to assess the
constructability.