Geotechnical News • June 2017
27
THE GROUT LINE
erties of the rock/soil for specific
purposes: for example a treatment of
fissured rock with highly deformable
resin using sleeved pipes allowed for
excavation with blasting of a highway
tunnel close to a rock mass contain-
ing inflammable gas. The energy of
the blast was dissipated by the shell
of deformable treated rock that sur-
rounded the tunnel excavation profile,
avoiding the propagation of the vibra-
tion waves and the upsetting of the
dangerous zone in proximity to the site
(Wolf E., Collini R., Balossi Restelli
A. – Attraversamento di una tratta
della galleria Capo Calavà (Autostrada
Messina Palermo) in presenza di gas
tossici in pressione – Gallerie e grandi
opere sotterranee n.°8, March 1979)
Warsaw Metro compensation
grouting
The new Warsaw Metro Line 2 ends
east of Vistula river. The last station
C15 is near a shopping center. The
latter is connected to the Metro Station
by an underground pedestrian passage
that runs very close to the shallow
foundation of one of the building’s
façades. During the execution of the
provisional lateral sheet wall dia-
phragm, the vibration de-tensioned
the sandy layer under the foundation
of the structure, causing remarkable
settlements on some pillars, up to
about 50 mm (Fig. 4).
The investigation showed the pres-
ence of a superficial layer composed
of an almost monogranular sand in the
foundation level, from 3 m to 12 m of
depth under the campaign level (see
res curves in the chart of the following
fig. 5), followed by a stiff clay layer at
-12 m u.c.l. (blue curves).
CPT tests were carried out both in
the area of the disturbance and in the
sector of the building façade where the
diaphragm wall was not constructed,
putting in evidence the weakening
caused by the works.
It was then necessary to re-compact
the soil under the building founda-
tions in the zone of the damage, and
to uplift the involved plinths in order
to reduce the differential settlements
between the adjacent parts of the
structure. Moreover, preventive treat-
ment was necessary in order to rein-
force the soil foundation of the plinths
in the sectors where the sheet wall was
yet to be executed.
The treatment was carried out inject-
ing in the first stage a cement grout,
with ratio C/W=0,4 stabilized with
bentonite and the aid of a stabilizer
additive, and in the second stage
silica grout. The consumption of
cement mix has been prevalent in the
decompressed zone for achieving the
re-compaction effect. In the natural
sandy soil a lower quantity of cement
was integrated by the silica grout that
achieved the requested improvement
of the ground by permeating the latter
with a smooth effect on the structures,
as hereafter described.
The treatments were carried out by
grouting the sands for a thickness of
about 9 meters (down to the clayey
layer) under the foundations, com-
posed by concrete plinths at the base
of the pillars, connected by beams.
Two lines of PVC sleeved pipes were
installed along the external alignment
of the building (fig.6), while two other
rows were installed working from the
Figure 4. Pillars’ settlements before the grouting work.
Figure 5.The granulometric distribution of the soil.
