Geotechnical News September 2011
23
GEOTECHNICAL INSTRUMENTATION NEWS
characteristics (colour, smoothness,
material) also affect the range and the
precision
.
All these elements shall be
taken into account when designing a
site setup.
Obstructions
Due to their location in roads and
pavements the RSPs are likely to be
randomly hidden by obstacles such as
pedestrians or cars, in which case the
total station will take the measurements
but the data will be filtered during the
acquisition chain.
Weather Conditions
Rain, snow and fog clearly downgrade
the emitted distance meter signal and
can prevent some of the measurements
from being made. Snow, leaves or
mud on the ground will also change
the height of the
apparent RSP.
Results of
Field Studies,
Standard and
Mesh Methods
In this section
we will present
both
methods,
their advantages
and
their
drawbacks and an
assessment of the
precision.
The Standard
Method
For the standard
method
the
RRTS is simply
p r o g r a m m e d
to
sight
the
road
surface
in
predefined
horizontal
and
vertical angles.
The
RRTS
measures
the
inclined distance,
and the software
calculates
the
variations
in
vertical position
(only) of the
point.
It is possible
to automatically estimate an adjust-
ment of the horizontal and vertical
angles depending on calculated move-
ments of the point and of the stations.
This is to try to reduce potential errors
linked to the sighted point moving
on the ground (there is no search of a
prism centre as with the usual use of
a RTS, so a movement of the ground
or of the RRTS would lead to a differ-
ent point being sighted for unchanged
horizontal and vertical movements).
In Amsterdam (Netherlands) over
82 total stations
(See Figure 2) are used
to
measure surface movements above
the tunnel boring machine during the
construction of the metro line, both
with conventional RTS and with RRTS.
Due to the quantity of points measured:
5320 RSP for RRTS and
5820 prisms
for RTS
,
and the delivery period of one
hour, the standard method is used to
comply with the client’s requirements.
In addition to RRTS and RTS a net-
work of manual levelling benchmarks
on buildings, quays and on the ground
was set up. The 3590 levelling bench-
marks
confirmed the consistency be-
tween the precise levelling and the RSP
movements. The precision
obtained
was better than ±1 mm on the RSPs.
The Mesh Method
The mesh method uses a number of
RSPs around the point of interest to
smooth and eliminate automatically
any surface irregularities, through a
geographical statistical treatment of
the measurements. This method is
therefore more complex, but is has been
well proven in practice since 2005.
In Toulon (France) during the con-
struction of the south road tunnel a
network of 1830 RSPs have been mea-
sured over roads and pavements along
the tunnel excavation from 36 total
stations fixed positions (see Figure
3). They allowed the measurement of
cross sections every 9 meters, larger
or smaller depending on the urban en-
vironment and to deliver data every 2
hours.
An external control using traditional
precise levelling on benchmarks was
performed to validate the results with a
precision about ±0.5mm.
Figure 2. Total stations in Amsterdam sighting prisms and
RSP. In this case two total stations are installed to allow a
larger number of points to be measured more often. Both total
stations can measure both RSPs and prisms.
Figure 3. Reflectorless Robotic Total
Station in Toulon.
