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Geotechnical News • June 2013
29
GEOTECHNICAL INSTRUMENTATION NEWS
In addition, at two locations 25 metre
length multi-point borehole exten-
someters (MPBXs) were installed that
were modified to measure conver-
gence in the backfilled area and reflec-
tive pile targets were also placed at the
top of sheet piles adjacent to the exist-
ing road for monitoring of horizontal
and vertical movement of the wall.
A typical section of the monitoring
plan is shown in Figure 3.
The ground conditions proved to be
more challenging than initially fore-
seen with the initial assumptions with
greater than predicted movements. It
was anticipated that the bottoms of
the inclinometer casings would be
anchored in stable ground and used as
a fixed reference point for calculation
of movements as is usual practice. The
lengths of the casing were determined
by the engineer with the available data
at the time. Review of the borehole
inclinometer plots, in conjunction with
the other monitoring data, particularly
unexpected divergent movements in
the MPBX data, suggested that the
bottom of the casings were not in a
fixed position but in ground that was
experiencing significant movements.
In order to continue to provide valu-
able subsurface information at these
inclinometer locations, the top of the
casings were surveyed using a total
station and a survey prism pole placed
at a specific point on the casing. The
survey determined the geodetic posi-
tion of the top of the casing that was
then used as the reference point from
which movements were calculated.
The resulting data showed a shift of
the inclinometer profile adjusted for
each reading with the changes in the
x and y co-ordinates according to the
survey at the top. An example of the
adjusted inclinometer plot is shown
on Figure 4 showing this shift in the
profile.
Lessons Learned
Lesson learned 1: Benefits of a
complete monitoring program.
This case history highlights the value
Figure 2. Section of proposed widened road with piles.
Figure 3. Typical section of monitoring plan.
Figure 4. Sample of inclinometer
plot.