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Geotechnical News • December 2016
GEOTECHNICAL INSTRUMENTATION NEWS
One-point liquid-based settlement sys-
tems can be back-pressured to push
out the bubbles but it is not feasible in
this system given that there are several
measurement locations on the same
line and that bubbles can be trapped in
localised “kinks.”
Installation
There are several limitations that were
to be overcome during installation. The
line and sensors had to be installed in
cramped spaces, around beams, inside
doorframes and so on. The complex
arrangement in the building made it
impossible to avoid curves that could
trap air bubbles in the liquid, so the line
had to be filled before being attached to
the wall. However, filling the line be-
fore running it makes installation even
more demanding because of the added
weight.
To minimize inaccuracy due to temper-
ature changes in the liquid and to have
access to the full measurement range,
these settlement measurement systems
also require that the sensors be at the
same elevation, within 10 mm of each
other. While this is reasonably easy
to achieve on a single long wall or a
tunnel, it is much more difficult where
there is little to no line of sight for use
of laser levels, obstruction rendered the
use of water levels arduous, and where
floors and ceilings are either absent or
uneven. We modeled the effect of low-
ering or raising each liquid container
with respect to its sensor before we
were able to position each of them at
the right height. Moving any one of the
reservoirs up or down would have an
effect on the readings of the other mea-
surement points.
Results
We were asked to place some of the
sensors outdoors, where the sun would
heat up the sensor housing, yielding
unreliable data during daytime (i.e.
sunlight would heat one part of the
line). This can be seen in Figure 2,
where measurements (blue curve) shift
rapidly from daytime to nighttime as it
follows air temperature (orange curve).
It can also be seen that perceived set-
tlement changes over months in such
a way that is difficult to specifically
attribute to real differential shifting or
to temperature effects. There is a corre-
lation between the two curves, but the
exact relation between the two is un-
known. In addition to these concerns,
workers would occasionally operate
space heaters in the vicinity of the in-
struments without telling anyone, in-
ducing false readings of shifts.
One of the liquid lines was accidently
damaged and this has been a recur-
ring theme throughout this project, an
expected outcome of instrumenting a
demolition project. Though a cut can
be fixed, it makes comparison of data
before and after the break difficult to
perform.
Recommendations for future use
If there is critical safety and data rest-
ing on the settlement system, it is cru-
cial to protect the lines and they should
be put entirely out of reach or be pro-
tected by a conduit.
Ideally, settlement systems such as
these need to be installed in tempera-
ture-controlled environment to provide
best accuracy.
If the system cannot be back-pressured,
it is a better practice to fill it with liquid
before installation and make sure no air
bubbles remain in the system.
Using laser levels is the best approach
to install sensors at the right height
when the conditions permit it.
Multi-point borehole
extensometers (MPBX)
Purpose and description of
instruments
Over the course of phases (2) and (3),
instruments such as vibrating wire
MPBX (Geokon model 1280) and in-
place inclinometers were routinely
used to follow the effects of excavation
both inside and outside the building.
They provided independent data and
complemented measurements from
settlement systems.
Sources of inaccuracy
MPBX are fairly robust instruments
that do not have many sources of inac-
curacy once they are properly installed.
The main source of inaccuracy for this
type of instrument would be caused by
a mismatch between the soil and the
grout’s hardness.
Figure 2. Differential settlement measurement of an outdoor wall (blue)
and measured temperatures (orange)
