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Geotechnical News • December 2014
www.geotechnicalnews.com
GEOTECHNICAL INSTRUMENTATION NEWS
tion link. Again the actual option will
depend on the location, and a wired
‘hop’ to a GSM modem could be pos-
sible, a DSL link via a phone line, or
an Ethernet connection, but this will
very much depend on local circum-
stances.
The potential of wireless
The common drivers for the use of
wireless for geotechnical monitor-
ing have been cost, low maintenance
and the ease of installation. Wireless
sensors should typically always be
cheaper to install than wired systems,
as they don’t need wires and should
be much quicker to deploy. That has
collateral benefits in terms of hazard-
ous locations where access is time
restricted, and may incur access
and additional personnel costs. The
elimination of wires itself may incur
savings through reduction in support
and maintenance during the life of the
deployment.
However there are further potential
benefits to using wireless. Using wire-
less for backhaul gives remote access
to data. But the use of wireless mesh
sensors allows for much more flexibil-
ity in terms of system deployment. It
should be possible to add sensors to a
system, irrespective of sensor type, to
extend the specific application as well
as reconfiguring the system as needs
dictate, with a minimum of effort and
without the need for specialised skills.
Wireless also offers the possibility
of monitoring where wired or other
systems, such as optical based (robotic
total station) systems are not feasible,
because of space and other constraints.
Wireless also lends itself to tactical
deployment where sensing is required
in a dynamic environment, as engi-
neering and construction works move
over an asset. Again this should be
possible without specialised skills.
Finally, the evolution of electronics is
going to continue to drive evolution of
wireless sensing, with units becoming
ever more energy efficient, smaller so
they are simpler and less obtrusive to
deploy, and falling in cost so that it
will be ever more economically viable
to deploy sensors comprehensively on
assets where it has not be considered
possible in the past.
Simon Maddison
Chief Operating Officer,
Senceive Ltd.
Hurlingham Studios, Ranelagh
Gardens, London SW6 3PA,
England
Tel: +44 7679 5720
email: smaddison@senceive.com
Widespread misconceptions involving liquid or vapor flow in
geotechnical monitoring applications
Glenn Tofani
This article presents examples of two
geotechnical monitoring scenarios
where liquids
or gases are transmitted
across what are commonly perceived
to be relatively impermeable barriers.
The first case involves the transmis-
sion of groundwater hydrostatic
pressure to a pressure transducer
embedded within a column of cement/
bentonite grout. The second case
involves the transmission of vapors
(Volatile Organic Compounds or
VOCs) across an engineered bar-
rier placed below a floor slab that is
intended to block their transmission.
In both cases, there have been wide-
spread misconceptions within the
engineering and regulatory commu-
nities regarding the degree to which
transmission occurs.
Fully-grouted piezometers
The first case involves the develop-
ment of fully-grouted installation
procedures for pneumatic or vibrating
wire piezometers during the 1980s
and 1990s. During the early 1980s,
piezometer installations in southern
California typically consisted of open
standpipes or, less frequently, pneu-
matic or vibrating wire transducers
embedded in sand backfill. It was
generally recognized that open stand-
pipes could give misleading results at
stratified sites where multiple zones of
groundwater occur, or where signifi-
cant vertical flow gradients are pres-
ent. The use of transducers to measure
piezometric levels at discrete points
or within relatively isolated zones
was found to produce more reliable
and useful data. However, the con-
struction of multi-stage installations
with transducers embedded within
sand intervals isolated by bentonite
seals was difficult, time consuming,
and often resulted in damage to the
transducers, bridging of the borehole
during backfilling, or other installation
problems.
Fully-grouted installations were
considered as a means of eliminating
these installation problems. However,
many clients, consultants, and regula-
tory agencies were reluctant to utilize