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Technical Committee 207 /
Comité technique 207
seawater clearly flooded behind the wall, the groundwater
became salty.
There was therefore a discernible hydraulic gradient from the
power station site in the south, northwards towards the quay
wall. It was also clear that the bulk of the groundwater was
fresh water, flowing from the power station site towards the
South Quay. In addition it was quite possible that concentrated
groundwater flow was occurring in the fault zone in the vicinity
of the harbour, although in the location of the South Quay the
zone was buried beneath a cover of Glacial Till.
Figure 3 Water levels and crest movements, 13-14 Oct 2000.
The analyses showed that the high groundwater level behind
the quay wall was the predominant driver of the instability and
the observed wall movements.
3.3 Wall Movements (2000)
Figure 4. Crest movements during 2000.
The displacements of the quay wall measured at the traverse
lines are illustrated on Figure 4. It can be seen that there was a
noticeable effect upon outward wall displacements while the
site investigation works were being undertaken in April 2000.
Figure 4 also shows that the rate of outward movement also
began to increase markedly in September of that year, and was
accelerating. By September 2000 the worst affected area of the
quay wall(Traverse Lines B and C on Figure 4) had recorded
between 270 and 350mm of outward movement, relative to their
values in January of that year. At this time the analysis, design
and procurement of the remedial works was still being
progressed, but, based upon the evidence of the increased wall
movements during the investigation work, contingency plans
had already been put in place to deal with such a situation.
Eleven dewatering wells were installed in a line behind the quay
wall at approximately 10m centres, to lower the ground water
level behind the wall. At that time the wall was moving forward
by more than 10mm per day at the worst affected part, and was
rocking measurably with the tide. The pumps were started as
soon as the wells were connected, over a three day period, the
first three being commissioned on 17 October and the balance
by 19 October 2000.
As soon as the first pumps were started, the wall movement
virtually ceased, as can be seen from Figure 4. Measurements
eight days later showed that the amplitude of the rocking motion
was less than half its magnitude prior to pumping. With the wall
stabilised, work on installing permanent drainage and ground
anchors could begin. The effects of the dewatering upon the
groundwater levels behind the wall are illustrated on Figure 5.
A feature of Figure 5 is that a distinct ‘plateau’ can be seen
in the levels recorded for the water immediately behind the quay
wall. The level of the retained water is drawn down to 0.0mOD
by the pumping wells as the tide falls. It does not fall below
this level, however, giving the step-like feature in the graph.
The 0.0mOD level coincides with the invert level of the lower
access-ways through the quay wall, which were open to the sea.
When the tide was above 0.0m, on a falling tide, the retained
water was clearly draining through these accessways, as well as
being removed by the pumping wells. Once the tide fell below
this level, then the pumping wells alone were removing the
retained water, and, on the evidence of these readings, could
only maintain the retained water at about the 0.0mOD level
against the fresh water flowing from the landward side of the
quay wall.
Figure 5. Water levels and crest movements 27-28 Oct 2000.
4
OUTLINE OF INTERIM REMEDIAL MEASURES
(2001 – 2002)
It was recognised from the outset that the efficiency of the
wells was likely to deteriorate in the medium term due to bio-
fouling. Hence, additional measures were designed to provide
physical restraint to the worst affected section of the wall.
These consisted of seventeen 1050kN permanent rock anchors
installed through the face of the wall at an inclination of 43
o
to
the vertical; twenty-four 525 or 626kN permanent anchors
through the crest of the wall inclined to landward at 10
o
to the
