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FILLING PAULL SHAFT WITH WATER FROM THE BOREHOLES MARY BEFORE HER DELIVERY TO HUMBERSIDE MARY REACHING THE SHAFT AT PAULL

he River Humber pipeline is glacial till, Flamborough chalk, The ground surrounding the Paull
an important one, connecting Burnham chalk, peat and even sand. shaft had a very small particle size and
an import location for gas at The material condition of this geological was fully waterlogged. This meant that
Easington, on the East Yorkshire layer varied greatly in compaction, during TBM breakthrough, water and
T coast, to the national network bearing capacity and porosity. particles surrounding the shaft could
and delivering gas supplies to millions Mary, our slurry tunnel boring wash into the shaft, leading to a cavity
of customers throughout the UK. machine, helped us tackle the forming under the tunnel or shaft, and
The existing pipeline crosses the geological issues by using an potentially damaging it catastrophically.
River Humber in a trench on the river engineered fluid to excavate the To resolve this challenge, we dewatered
bed, between Paull on the north bank tunnel. The fluid compensated for the ground surrounding the shaft
and Goxhill on the south bank. Over weak ground and acted as a medium and filled the shaft with water. This
time, tidal patterns have eroded the to carry the heavier dense ground. balanced the pressure on the outside
river bed covering the pipeline. The tunnel segments, which form the and inside of the shaft, reducing the
On 10 September 2019, at 12.08pm, skin of the tunnel, were designed to likelihood of water and fines ingress
‘Mary’, our 160-metre long tunnel accommodate the expected conditions. into the shaft during breakthrough.
boring machine (TBM) digging the The aforementioned ground treatment
tunnel to house a replacement gas The geology of the tunnel drive also significantly reduced the chance of
pipeline 30m beneath the River was especially challenging material washing into the shaft.
Humber, completed her epic journey.
The breakthrough, into the specially- due to the large variation Pipeline problems
constructed shaft at Paull on the north in the mass and material The water of the River Humber is rich
bank of the river, marks the end of her properties of the soil and rock. with sulphur bacteria. When these
18-month journey from Goxhill on the bacteria are placed in an oxygen
south side and the start of the next The variation in the type of deficient environment, the anaerobic
phase of this world record-breaking feat material we encountered meant reaction results in a highly corrosive
of engineering. we dug through glacial till, environment in the tunnel. This leads
Recently, Mary was carefully Flamborough chalk, Burnham to the rapid degradation of the steel
dismantled and lifted to the surface pipeline. Our solution was to carefully
ready to be shipped back to Germany to chalk, peat and even sand select the water for filling the gap
be fully refurbished ready for her next between the pipeline and tunnel from
tunnelling challenge. The final section of the tunnel was nearby sources that would not easily
Mary has excavated 160,000 tonnes in weak soil (very variable, low load degrade the pipeline or tunnel lining.
of material – mostly chalk – which has bearing, water-logged and organically The pipeline has a double protective
been carefully transported back to the rich). Such material would be expected layer, one of standard epoxy coating
surface, graded and reprocessed at our to settle when excavated through and a second made of a bespoke
Goxhill slurry treatment plant. The and settle further after it has been impinged concrete with iron filling
excavated material is being used to help excavated. The magnitude of risk coating. These coatings act as barriers,
restore a former quarry nearby. expected was high, with predicted but in case any weaknesses develop
At peak we’ve had 250 people on site outcomes ranging from a small over time a third protection system
at any one time, with up to 50 per cent misalignment in the tunnel lining to a known as a cathodic protection system
of those coming from the Humberside full tunnel collapse. will also be installed in the tunnel to
area. Over the 18 months of tunnel To resolve this, the final section of the protect the pipeline.
drive, we’ve had 40 people directly ground leading up to the shaft was treated
involved in tunneling around the clock. using a technique called soil mixing. This Inserting the pipeline
involved creating piles of the existing The safest way to install the
Tunnelling trials and tribulations ground using a drilling rig and binding replacement pipeline is by welding up
The geology of the tunnel drive was them with a specially designed cement- the pipe sections onshore and then
especially challenging due to the large like material. This ground treatment pushing those into the tunnel under
variation in the mass and material resulted in significantly improved the river. Before we do that, in spring
properties of the soil and rock. The ground bearing capacity and reduced the 2020, the tunnel will be flooded to give
variation in the type of material we likelihood of settlement, which ensured some buoyancy and aid pipe insertion.
encountered meant we dug through our tunnel was built safely. We will then have two thrust machines

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HumberCrossingProject.indd 2 14/11/2019 16:56