2112
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2
BRIEF HISTORY OF THE SITE
The harbour walls were constructed in the dry within a large
embayment formed by advancing two curved embankments
from the shore to meet at a central point, which would form the
entrance to the harbour, as illustrated on Figure 1. This entrance
point was closed by a temporary dam for the construction works
and the site was drained by pumping. The harbour was then
excavated in the dry.
A typical section through the quay wall is shown on Figure 2.
The walls were constructed of mass concrete and, in the larger
sections, large sandstone “plums” were incorporated into the
concrete. The quay was intended for both passenger and
livestock traffic. In view of the large tidal range provision was
made for loading and off-loading at any state of the tide. This
was achieved by constructing access-ways through the wall at
two levels. These were connected to subways and stairways on
the landward side of the wall. On the seaward side the quay wall
was fronted by a heavy timber staging with continuous landings
at the necessary levels to give access from the steamers into the
subways.
Figure 2. Typical cross-section through quay wall.
As illustrated on Figure 1, the affected quay, known as the
‘South Quay’ was constructed on the harbour side of a central
spine embankment within the construction site. To the south of
this, the remaining area was planned as a future extension to the
harbour, but this extension was never constructed. In the 1970s
this area was infilled and a large power station constructed on
the site
In the early 1980s surface settlement was noted behind a
40m long section of the quay wall. This was repaired by
infilling and relevelling to grade. Further repairs were
undertaken in 1988, when a maximum settlement of 400mm
was reported on the quay surface prior to reconstruction.
Following these repair works the quay surface settled a further
30mm of settlement within a year, and the worst-affected area
was reported as extending over an 85 metre length of the wall,
although the effect was also discernible beyond this. Crane rails
which had been relaid along the quayside in 1972 were found to
have deviated by up to 100mm towards the harbour over a
100m length of the quay wall.
Detailed surveys taken in 1991 indicated that the South Quay
was showing a distinct bulge of as much as 430mm towards the
harbour, and the zone of movement extended for some 200m
along the wall. Shortly after these measurements were taken, a
break occurred in a water main some 13m behind the crest of
the wall, due to the continuing outward displacement of the
quay wall. At this time a remedial works solution using high
capacity ground anchors was proposed, but not proceeded with.
In late 1999 a further major leak occurred from a fractured
water main near the centre of the previous movements, causing
accelerated wall movements and severe ground settlements
beneath the quay itself. The next sections describe subsequent
evaluation and remedial works following this particular
incident.
3
GROUND INVESTIGATIONS AND OBSERVATIONS
(2000-2001)
In early 2000 Applied Geotechnical Engineering was
requested by the port owner to undertake investigations as to
the cause of the movements and to advise on the measures
required to stabilise the South Quay wall.
A targeted ground investigation was commissioned, to
supplement information available from previous investigations
undertaken in 1990/1992. In addition, inclinometers were
installed at five locations along the crest of the affected length
of wall and were continued through the base of the wall and any
superficial soils into bedrock at depth. A further three
inclinometers were installed behind the wall to monitor the
behaviour of the retained soils. Water observation boreholes
were also installed behind the quay wall to monitor groundwater
conditions.
In addition a series of “traverse lines” was established
perpendicular to the wall and extending back to stable ground
well beyond any zone of influence of the wall movements.
These were monitored by conventional surveying techniques to
determine horizontal and vertical displacements.
3.1 Geology and Ground Conditions at the Site
The ground was raised behind the masonry quay walls using
the material excavated to form the harbour. Beneath these
surface construction materials, the South Quay is underlain by
fine to medium grained sands of Quaternary age. In some
places these are underlain by soft sandy silty clays. These
materials rest upon either Glacial Till (typified as reddish
brown, stony, clayey silts), or, more commonly, directly onto
bedrock. At the western end of the quay, bedrock consists of
sandstones with subordinate marls of Permo-Triassic age. Over
the eastern end older sandstones and siltstones of Upper
Carboniferous Namurian (Millstone Grit) age are present. The
boundary between the two rock formations is formed by a large
fault, trending in a NNW – SSE direction, which cuts at right
angles across the line of the South Quay, slightly west of its
central point, and close to the centre point of the historic
movement of the wall. The fault zone was also identified during
the construction of the power station to the south.
This fault zone is associated with heavy flows of
groundwater under artesian or sub-artesian pressures. Before
the power station was constructed a freshwater lake formed in
the area of the old proposed harbour extension.
The investigations led to the conclusion that groundwater
flows from this source to the south of the South Quay were the
root cause of the wall instability, as described below.
3.2 Groundwater observations
It was soon apparent from the monitoring data that the
movement of the wall was driven by the groundwater held
behind it. The rate of drainage of the water below the wall was
relatively slow, so that the difference in water levels between
the front and the back of the wall was highest at low tide, as
shown in Figure 3. At these points the wall would ratchet
forward, and not quite recover its original location as the tide
level rose. By inference, the effect was greatest at times of high
tidal difference (Spring tides).
The water observation standpipes confirmed that close to the
quay wall, water levels varied tidally between about +4m OD
and +1.4mOD. Some fifty metres south of the face of the quay
wall, water levels varied between +2.9 and +2.7m OD with the
tide. At a distance of 100 metres groundwater levels were
almost static, with very little discernible tidal variation, and
were around +3.7m OD.
It was also noted that the salinity of the groundwater
changed from south to north. In the south the groundwater was
fresh or brackish. Close to the quay wall the salinity altered with
the tide. At low tide it was brackish or fresh. At high tide, as
