26
Geotechnical News • June 2017
THE GROUT LINE
hydraulic with expansion joint). The
injection mixture opens the valve and
spreads the grout into the surrounding
soil. After grouting, the valve closes,
preventing the injected material from
returning into the pipe. Once the pipe
has been washed inside, the valves are
ready to be used eventually for other
subsequent grouting operations.
There are many advantages to this
method. One of them is that the injec-
tion can be carried out in more than
one stage and, if necessary, using dif-
ferent cement based mixtures.
One of the typically employed grout-
ing processes is
Permeation grouting
,
which consists of filling the natural
voids in the soil with a mixture, substi-
tuting the in-situ fluid (normally air or
water). The result is a reduction of the
hydraulic conductivity and an increase
in the mechanical characteristic of
the soil. Cements, chemical mixtures
and resins reinforce the soil structure,
improving the strength and the elastic
modulus of the treated ground. The
granular composition of the soil also
plays a fundamental role in the success
of the permeation grouting: the finer
the dimension of the soil grains and
the higher the dis-uniformity degree
(U= D
60
/D
10
), the lower the volume of
the voids. So basically it is important
that in the first phase of the grouting
design, a detailed investigation cam-
paign including soil coring is planned,
in order to obtain the granulometric
sieve from samples of the soil layers
where the treatment shall be carried
out.
On the other hand, the capability of
a grout mix to permeate the soil is a
function of its composition. The rheol-
ogy study of the mixtures allows for
definition of this parameter.
In order to achieve a good permeation
grouting homogeneity, cement mixes
are stabilized by adding a filler (for
example bentonite) that helps to main-
tain in suspension the cement grains.
The use of fillers normally increases
the viscosity of the mixture; the use
of polymeric-based additives allows
viscosity and stability to reach optimal
values.
As a matter of fact, an accurate set of
preliminary tests has to be foreseen
before starting permeation grouting
work, in order to define the correct
mixture to be used for the grout and
to provide the benchmark values of
its rheological parameters. The latter
will be considered during the on-site
work in progress controls. The use of
sleeved pipes brings a series of advan-
tages to permeation grouting:
• it allows for the injection of the
proper grout quantities, in a con-
trolled way, in the different zones
of the soil mass to be treated;
• for each sleeve it is possible to
record the operative parameters
(pressure and flow rate) during
the injection process, obtaining
the “story” of each grouting and
allowing the evaluation of the pa-
rameters for the following steps;
• it is possible, if necessary, to inject
additional quantities of grout in
further stages;
• it allows for the injection of dif-
ferent kinds of mixtures from the
same sleeve, for example follow-
ing an order of growing penetrabil-
ity.
The use of sleeved pipes, often
coupled with a polypropylene bag
(Fig.3), allows for the reduction of the
number of drillings required. This way
the injection process is faster, without
any influence on the type of mixture to
be injected and/or on the parameters to
be used.
Grouting is not only required to
improve the mechanical character-
istics. It has also been used in the
reduction of the mechanical prop-
Figure 2. Grouting injection through
a sleeved grouting pipe.
Figure 3. High tenacity polypropilene bags.
