extension of a 3m diameter stormwater sewer siphon required for
the widening of the Seaforth Passage, Liverpool
by Dr Graham Richardson BSc(Hons) PhD FGS
& Dr Stephen Thomas MBA MSc DPhil MCIWEM FICE CEng Eur Ing
Location of Seaforth Passage - Courtesy of OGI Groundwater
Ports required the widening of the Seaforth Passage (Figure 1)
within the Royal Seaforth Dock at the Port of Liverpool. The
increasing size of modern vessels led to a need to increase the
width of the Seaforth Passage, in order to improve the
turnaround times for the larger vessels within tidal windows.
Farrans Construction Ltd was appointed the £11m contract to
carry out the work to widen the passage from 40m to 60m. This
paper details the innovative groundwater controls that were
necessary to reduce groundwater pressure around the cofferdam.
The Passage widening
works included a number of significant challenges, including
re-routing of electrical and water services, making provision
for future electrical and fibre-optic services around the port,
demolishment of the existing quay wall and construction of a new
However, perhaps the
biggest challenge, was to extend a 3m diameter storm water sewer
connecting the Liverpool storm drainage system with an existing
outfall. The sewer “siphon” runs directly under the Seaforth
Passage (see Figure 2 below), with an up-shaft and down-shaft
either side of the Passage, creating a siphoning effect.
Throughout the works,
both the sewer and the Passage were to remain ‘live’, i.e. flow
of effluent would continue through the sewer and container ships
would continue to access the Seaforth Dock via the Seaforth
Passage. Extending the siphon pipe would require excavation to a
depth of 20m below reduced ground level (circa 23.5m below
original ground level), alongside the original sewer up shaft,
within a retaining wall cofferdam.
The original up-shaft had
to be intercepted at high level and the effluent flow diverted
around the cofferdam and back to the main sewer further
downstream. Excavation could then take place within the
cofferdam to break out the original sewer at low level, install
the horizontal siphon pipe extension and to construct the new
2. Required sewer siphon extension - Courtesy of OGI
Being so close to the
quay, groundwater pressures were very high. To prevent failure
of the combi-wall, the cofferdam designers required significant
lowering of the groundwater pressure around the cofferdam.
Construction of a combi-wall
cofferdam, designed by Royal Haskoning, was required to -14mCD
from a ground level of circa 13mCD, with an internal excavation
formation at -10.5mCD, and external excavation to 4mCD. The
structural design required the groundwater piezometric head
outside the combined structure to be reduced to -9mCD in the
lower aquifer, and to +3mCD in the upper perched aquifer,
including head within the clayey sand, which is sandwiched
between the two clay layers.
The piezometric head
profile prior to dewatering and that required by Royal Haskoning
are shown in Figure 3.
Figure 3. Original piezometric head profile (top) vs
that required by the Structural Designer
The groundwater control
system was designed and installed in two phases as described
Phase 1: Involved installing, pumping and
monitoring from 5 (No.) deep wells (the minimal number of test
wells required to reduce the groundwater pressure based on the
geological and hydrogeological data available). Test pumping and
monitoring of the impact on groundwater pressure was carried out
and the scenario was mathematically modelled by OGI using the
SEEP/W finite element model.
The following conclusions and solutions were drawn:
The initial 5 No. deep
wells successfully lowered the piezometric head in the lower
aquifer to between -5mCD and -6mCD, but fell short by 4m of the
required target level of -9mCD.
The primary reason for this was that the groundwater within the
Sand & Gravel aquifer was effectively perched above the lower
permeability sandstone aquifer.
Using a finite element
model, calibrated against the test pumping results, OGI
determined that additional deep wells would provide some further
lowering of the piezometric head, but that the benefits would
have diminishing returns (Figure 4 below), requiring an
excessive [circa 36 (No.)] additional deep wells.
In practice, this number
of wells would be impractical. Not only would it be very costly
in terms of installation and running costs but each pump
requires a power cable and discharge pipe, and so would occupy a
huge amount of surface space on what was already a very
congested site, getting in the way of other site activities.
OGI also recognised that
the risks associated with relying exclusively on active external
pumping were considerable.
Any loss in pumping would
lead to a rapid rise in piezometric head within minutes and this
could cause structural failure of the cofferdam or ground heave.
Figure 4. Graph of water level vs number of wells -
Courtesy of OGI Groundwater Specialists Ltd
Phase 2: To
further lower the existing piezometric head to reduce lateral
stress from pore water pressure, required a non-traditional
approach. The design essentially consisted of a passive
dewatering system to allow groundwater to drain under gravity
into the structure via a series of lateral drains which are
installed through cut holes within the sheet pile wall sections
of the combi-wall.
The design comprised the following items:
Continued pumping of the
5 (No.) external deep wells installed outside the combi-wall.
10 (No.) shallow
horizontal passive pressure relief wells through the combi-wall
into the upper aquifer.
3 (No.) deep vertical
active/passive pressure relief wells inside the combi-wall.
40 (No.) shallow inclined
passive pressure relief wells through the combi-wall into the
Groundwater pressure outside the cofferdam was monitored
throughout the installation stages to ensure the safety of the
cofferdam and those working within it.
The system was installed
in the following stages as follows:
STAGE 1: External
The 5 (No.) deep wells,
located outside the combi-wall, installed with borehole pumps
were used to partly lower the water level and to assist the
STAGE 2: Horizontal
pressure relief wells into upper aquifer
From excavation level at
-1.00mCD, 10 (No.) horizontal drains were installed through the
combi-wall into the upper aquifer.
3 (No.) vertical pressure
relief wells (PRWs) were installed inside the cofferdam from
Vertical PRWs were cut
down as excavation progressed.
When excavation reached
-5.50mCD, pumping commenced from vertical PRWs.
STAGE 4: Inclined
pressure relief wells
From an excavation level
at -5.50mCD, the first set of inclined PRWs was installed
through the combi-wall.
Second set of inclined
PRWs were installed at -8.00mCD.
Third set of inclined
PRWs were installed at -9.50mCD.
Fourth set of inclined
PRWs were installed at -10.50mCD.
Vertical PRWs continued
pumping and were cut down as excavation progressed. Figure 5
(below) shows installation of PRWs.
Figure 5. Inclined drilling within the cofferdam
Courtesy of OGI Groundwater Specialists Ltd
STAGE 5: Drainage layer
at final excavation level and hydraulic connections
A clean stone drainage
layer, including perforated drainage pipe, was installed to
create hydraulic continuity with the pressure relief system.
Sheet pile in-pan well
casings were installed hydraulically connected with the drainage
Groundwater bleeding from
the inclined PRWs was collected by the drainage layer to a boxed
sump installed with a sump pump.
STAGE 6: Reinforced
concrete base slab
with an impermeable geomembrane, concrete blinding and
reinforced concrete base slab. When the reinforced concrete slab
cured, pumps from vertical PRWs were removed. The vertical PRWs
were sealed over the slab layer, but remained hydraulically
connected with the underlying drainage layer (Figure 6 below).
Groundwater bleeding from
the vertical PRWs and the lower row of inclined PRWs was
collected by the drainage layer to the boxed sump.
Section detail showing active pumping wells, vertical
and inclined pressure relief wells & drainage layer -
Courtesy of OGI Groundwater Specialists Ltd -
Click to enlarge
Groundwater depressurisation in action - Courtesy of OGI
Groundwater Specialists Ltd
STAGE 7: Construction of
sewer connection and siphon shaft
Before mass concrete was
poured, the inclined PRWs installed at -8.00mCD and -9.50mCD
were connected to the vertical in-pan casings to channel the
water to the boxed sump. The sump was extended using man-hole
rings to enable sump pumping to continue. Once mass concrete was
cured the sump pump was removed and the man-hole backfilled.
Groundwater was allowed
to bleed through the vertical in-pan casings and inclined PRWs
to the surface of the mass concrete (see Figure 7 above).
Benefits of OGI’s
There are a number of
benefits for utilising OGI’s solution including:
OGI’s design successfully
reduced groundwater pressures on the outside of the cofferdam,
to the design profile required for a safe and stable cofferdam.
The system enabled
construction of the siphon extension and new up-shaft, free of
the risk of cofferdam collapse or ground heave due to the rapid
rise in external water pressure due to active pump failure.
OGI’s approach dispensed
with the requirement to install a large number of deep wells and
to run a large number of active pumps, saving the main
contractor considerable expense.
The installed system
occupied minimal space, so had minimal impact on excavation and
other site activities.
The upper aquifer was
further depressurised using inexpensive and easy to install
Only three additional
deep active wells were required to protect the base of the
excavation as it advanced, and these were converted to passive
depressurisation wells when final excavation level was reached.
Although a total of 50
(No.) inclined pressure relief wells were installed as
excavation advanced, these wells were to shallow depth only, so
were inexpensive to install and with no running costs.
The water ingress was
handled by channeling the water to a single sump.
The simple nature of the
solution required fewer specialist components and equipment.
The siphon extension was
successfully completed on programme.
It is important to
recognise that without the trust, support and clear lines of
communication established with Farrans Construction and Royal
Haskoning, the success of this project could not have been
achieved. OGI’s role in this project was awarded the following
2016 British Construction
Awards: ‘Highly commended’, in the category of Temporary Works.
2017 Ground Engineering
Awards: ‘Highly commended’, in the category of Technical
The judges’ comments
described OGI’s approach as:
“A counter-intuitive but
simple and elegant solution to a difficult groundwater problem
that would have otherwise rendered the project virtually
unachievable. This was impressive in its effectiveness and
demonstrated a high degree of trust and collaboration”.
and publishers would like to thank Dr Graham Richardson,
Senior Groundwater Control Engineer, and Dr Stephen Thomas,
Managing Director, both with OGI Groundwater Specialists
Ltd, for providing the above article for publication.