Geotechnical News • March 2015
41
THE GROUT LINE
excess volume of grout necessary to
create the energy required to build
the geometrical elements designed.
Reflow is a “must” in the jet grouting
process, with exceptions in some spe-
cial cases and applications. Without
a continuous return of the reflow to
the ground surface, significant grout
pressure can build up in the ground
with consequent hydro-fracturing or
hydro-jacking of the soil.” Ref 1.
So another question: what does high
flow mean? More than thirty years
ago, on my first jet grouting job, I was
using a grout mix flow of 70 liters
per minute. The limitation, obviously,
was due to the pump and accessories
available at that time. Contractors and
Manufacturers evolved, and today
there are pumps on the market that can
achieve 500 to 600 liters/minute, at
400 to 500 bars, even though some jet
grouting projects are still carried out
using much smaller flows.
It is clear, and I am referring to the
conclusions of the accident at the
spillway of the first paper mentioned
above, that the annulus space between
the drilled hole and the monitor/jet
grouting rods, can play a fundamental
role,
depending on the flows used
.
Using 70 liters/minute can provide
different results in the spoil compared
to using 500 liters/minute!
We cannot use a Ferrari engine with
a FIAT 500 (FCA Group) brake and
chassis! Or, we can, but with adequate
precaution.
• Apart from the annular space, what
keeps me up at night, and this may
be the most important point, is that
in both cases the Contractor/s-En-
gineers didn’t have a trigger point
defined to stop the jet grouting
once the spoil/reflow was lost. For
how much time should the jetting
continue without having the spoil/
reflow returning to surface? 0
seconds? 30 seconds? 1 minute? 5
minutes?
Here again the
fluid flows
, decided by
the Contractors, can play an important
role.
I believe it is a good practice, or better
it should be mandatory, that in each
jet grouting project (and here I am
speaking to the Engineer/Designer)
there should be, in the Specification, a
clause asking the Contractor to define
a time limit in which the jetting can
continue without spoil/reflow return-
ing to surface before stopping jetting
and starting to ream the hole. This
time can be dependant on the risk that
can be managed in a specific project
(jet grouting done in the desert vs.
below a building) and from the flow
used for the jetting (70 or 200 or 400
or 600 liters/minute). In any case it is
something that should be discussed,
for each specific jet grouting project,
between the Engineer and the Contrac-
tor to mitigate the risk of uplifting,
before the start of the works, and after
the field test.
• Other factors that can influence
the spoil/return are, of course, the
grout mix composition (thinner or
thicker depending on the type of
soil) and the type of jet grouting
whether it will be single, double
or triple. And definitely the soil
conditions.
• Last but not least, and probably the
most important aspect is the atten-
tion and care of the operator at the
drill rig. The operator is the “key”
person in avoiding heave and other
damages.
Related to the operator, I am reminded
of a short episode that happened a few
months ago. On a jet grouting project,
the operator was jetting, sitting inside
the cabin of an excavator located
behind the jet grouting mast without
a direct view of the hole and conse-
quently of the reflow.
A third person was obliged to control
the spoil/reflow return and inform the
operator (by shouting?) about potential
spoil/reflow problems.
Definitely not an ideal situation in jet
grouting considering that the opera-
tor should “feel” the spoil/reflow and
ream the hole if needed.
I leave it to you to decide who needs
to take care of these important details
and I welcome discussion about these
observations!
Ref 1. Siu, Gazzarrini, and al. “John
Hart Dam - Back-up Jet Grouted
seepage cut off wall construction”.
ICOLD 2013 International Sympo-
sium - Seattle USA
And, as usual, the same request,
asking you to send me your grouting
comments or grouting stories or case
histories. My coordinates are:
Paolo Gazzarrini,
or
Ciao! Cheers!
