26
Geotechnical News • June 2016
GEOTECHNICAL INSTRUMENTATION NEWS
ing was thus mostly meant to reassure
residents, because humans feel vibra-
tion up to ten times less intense than
those that normally pose a threat to
buildings. Having this system in place
also ensured that if any blasting event
was higher than expected, it would be
quantified and any resulting damage
could be assessed subsequently.
With event-based and general monitor-
ing of blasting in mind, an automated
data collection system with cellular
modems was put in place to ensure
that data were transmitted rapidly to
the server. Specifications required that
the blasting foreman must be alerted
within 15 minutes by the construction
contractor if any vibration crossed the
threshold. To this end, the specifica-
tions written by the city engineers
required alarms to be sent out to the
construction contractor upon 2.5
mm/s peak vector sum (PVS) for any
frequency. This type of arrangement
is fairly common to ensure that work
cannot continue while generating
harmful levels of vibration.
Peak vector sum is defined by the fol-
lowing equation:
in which
tran
,
vert
and
long
are
respectively the transverse, vertical
and longitudinal PPV. However, the
datalogger could only relay alarms
on the PPV and not on the PVS. This
raises the issue that each axis could
be below 2.5 mm/s PPV while their
PVS is above 2.5 mm/s, and no alarm
e-mail would be sent out. As a com-
promise, alarms are relayed if any one
of the axes are above 1.8 mm/s, which
leads to a maximal possible peak
vector sum of 3.11 mm/s according
to equation (1). Lower values could
have led to too many false positives
and hampered progress of the tun-
nel construction. Figure 3 shows the
measured
tran
,
vert
,
long
and PVS
values over a one-month period. The
green line at 1.8 mm/s shows the
alarm threshold. It can be seen that the
measured vibration are typically much
lower than the 1.8 mm/s threshold,
blasting events have created PPV as
high as 11 mm/s. There are also clear
lulls during weekends where little to
no vibration is measured.
The automated system was required
and expected by the client to be
functioning twenty four hours per day.
Clients and construction contractors
expect this to be a cheap and straight-
forward affair that requires little to
no maintenance. However, the large
number of components (batteries,
casing, logger, sensors, and cellular
modems) make these goals difficult
to reach. The loggers and cellular
modems are finicky and sometimes
unreliable, occasionally requiring to
be reset on-site. Having staff available
to check on the systems weekly and to
replace batteries and recharge units,
made vibration monitoring much more
involved than originally planned.
This project proved to be fairly
straightforward once the technical
issues were settled. A lesson to be
drawn from this project is that, vibra-
tion criteria can be chosen for their
effects on residents rather than only to
protect buildings and infrastructure,
and systems were designed to provide
automated alarm e-mails.
Conclusions
In every vibration monitoring project,
technical requirements come first:
frequency range, sensitivity, measure-
ment range, etc. Choosing thresholds
according to the specific needs is pos-
sibly the most critical decision for this
type of monitoring. Other important
considerations include that humans are
much more sensitive to vibration than
structures and that there can be older,
more sensitive structures. However,
creating a good monitoring project
that fulfills its duty also requires
deliberate planning and communica-
tion with the client, from the planning
phase to its final execution. This is an
often overlooked point that proves to
be very important in vibration moni-
toring, perhaps even more so than in
“traditional” geotechnical monitoring
because it is chiefly implemented for
safety, legal and wellbeing reasons.
Vincent Le Borgne
GKM Consultants, 1430 Hocquart,
Saint-Bruno, J3V6E1, Canada
Tel. (450) 441-5444 (ext 207)
E:
PVS tran vert
long
=
+ +
2
2
2
1( )
The Vancouver Geotechnical Society
and the
Canadian Geotechnical Society
69th Canadian GeoteChniCal ConferenCe
Topics and specialty sessions of local and national relevance to geotechnical and geo-environmental engineering
October 2 to 5, 2016 • Westin Bayshore Hotel • Vancouver British Columbia
