Geotechnical News • June 2016
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GEOTECHNICAL INSTRUMENTATION NEWS
it preferable that an engineer would
monitor the data in real-time to make
any work stop in under a minute.
Lessons learned
The unique work conditions of a large
wastewater sewer pose significant
difficulties. Work in sanitary sewers
is accompanied by a slew of worker
safety rules. Installation of instruments
was conducted by workers accustomed
to confined spaces who had never
installed geotechnical instrumenta-
tion. The first step was to prepare a
course to teach them how to install the
instruments in the tunnel. This was
achieved with hands-on demos that
had the workers install instruments
on a concrete jersey (a modular road
barrier) and with preparation of drill-
ing templates with every tool needed
properly identified. Despite thorough
preparation, we rapidly came to the
conclusion that it was necessary to be
available during installation should
any issue arise. It would be very dif-
ficult and costly to fix an improperly
mounted or damaged instrument and
we made sure to provide whatever
help we could through an unreli-
able radio link. In addition to these
considerations, working in a sewer
raised logistical issues. Workers wear
a special combination with respirators,
heavy boots, a rubber dry suit, a radio,
and three pairs of gloves that hinder
their.
Due to the high water level and flow,
protective equipment for the instru-
ments had to be designed. After instal-
lation of each geophone, a metal cover
was bolted on top to protect it from
impacts from smaller debris and to
deflect heavy debris carried by water.
Geophone casings were also filled
with epoxy resin to make them fully
waterproof and their cables were fed
into a flexible metal conduit that was
bolted to the wall. This is illustrated in
Figure 2, where a geophone installed
inside the tunnel, with the protective
cover, the conduit for the cable, and
one of the large cracks running along-
side are displayed. Similar protection
was provided to the crackmeters.
This was all done because mainte-
nance would have proven challeng-
ing. Access is difficult and restricted,
cables ae bolted to the wall and vision
and dexterity are severely limited
in the tunnel. Flowing water during
rainstorms did not significantly affect
vibration measurements. Water flow
barely registered on the geophones
and was not anywhere near the 2 mm/s
threshold. Finally, crackmeters showed
that the cracks expand and contract as
the tunnel heats up and cools down.
The main goal of this project was to
ensure that the tunnel would remain
stable during construction work. It did
remain stable and no crack opening
or contraction were observed beyond
thermal effects.
Project 2 brought up a plethora of
challenges that needed very careful
planning. In this project, as a follow
up to project 1, we have seen the value
of putting a deliberate effort into com-
munications with the client from the
very beginning of the planning stages.
Doing so ensured rapid and correct
installation of the instruments. To sum
up, conducting a successful vibra-
tion monitoring project goes beyond
simple technical considerations.
Project 3
Technical requirements
The last project is a new 5 km long
sewer tunnel being constructed under-
neath a densely populated area. Simi-
lar to project 1, vibration had to be
monitored around the shafts and along
the tunnel route. In addition to vibra-
tion monitoring, “traditional” geo-
technical instruments were installed
(inclinometers and multipoint bore-
hole extensometers) to measure the
effects of tunneling and to ensure that
no convergence or settlement would
threaten the surrounding structures.
Lastly, noise monitoring was also
undertaken to ensure compliance with
bylaws concerning noise emissions.
Lessons learned
It was estimated that the blasting
schedule would pose almost no risk of
damaging buildings. Indeed, 25 mm/s
is the accepted threshold for modern
buildings and the blasting schedule
was designed to keep vibration much
lower for any single event. Monitor-
Figure 3. One-month sample of vibration measurements near a shaft.
