The Ingenieur Vol 57 January-March 2014 The Ingenieur Vol 57 January-March 2014 | Page 18

INGENIEUR
that in most of the area measured the Lugeon
value (Lu) was less than 20 Lu, while in some
areas locally under the tunnel invert level there
were relatively high values of 50 to 135 Lu. The
result of the Lugeon Test is illustrated in Figure 16.
Furthermore, for the purpose of assessing
the permeability from the tunnel crown up to the
surface, JFT (Johnson Formation Test) was also
conducted by using five vertical boreholes. As a
result, the obtained coefficient of permeability
was within 10-6 to 10-7 m/sec. The rock core and
the series of permeability tests indicated the rock
condition around the tunnel at the river-crossing
area consisted of a low-permeable layer of silt,
fine sand and clay material. Consequently the
possibility of occurrence of high water pressure to
cause a large amount of ingress water would be
quite small with an earth covering as shallow as
approximately10-20m.
A series of high-density electrical exploration
surveys was implemented at the river-crossing.
The result of the high-density electrical exploration
survey is shown in Figure 17. The difference in
specific resistance values in the survey indicated
that relatively-hard rock would be located around
the tunnel crown right under the river, and above
this rock layer was a clay layer with low permeability.
From this result, it was speculated that the stability
of the tunnel crown could be more problematic in
terms of proceeding the tunnel excavation than
the permeability of the ground at the river-crossing
area.

Figure 17 – High-Density Electrical Exploration
Results
For the purpose of stabilizing the tunnel
crown during tunnel excavation, the installation of
25m long Steel pipe fore poling with silica resin
injection was selected as a countermeasure,
combined with the normal 6m long fore poling

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VOL 57 JANUARY-MARCH 2014
VOL 55 JUNE 2013

Figure 18 Tunnelling Under the Kerau River
to complement 25m long steel pipe fore poling.
These methods are designed so that the fore
poling pipes or rods are installed immediately
outside the to-be-excavated perimeter, and silica
resin material is injected from the pipes or rods
by fracturing or penetrating to “bond” the jointed
rock or decomposed rock, forming an “umbrella”
ahead of the tunnel face to pre-support the ground
in advance of tunnel excavation.
For this work, silica resin was selected as
the material for injection, taking into consideration
the economical aspects and the estimation of
low-permeability of the rock excavated right under
the river. Additionally urethane material with quite
short set time was also prepared in case of an
occurrence of a large amount of ingress water
(Figure 18).
Safety Aspects in the Tunnelling
Safety in tunnelling work is of utmost importance.
Special care has to given priority to prevent fire
accidents occurring. Emergency Response Plan
(ERP) should be in place and needs to