Monday, 7 November 2016

Dover Sea Wall


A year since my last blog!  How time flies...!

On 24th December 2015, a heavy storm damaged the sea wall protecting the railway running between Shakespeare cliff in Dover and the English Channel.

Damaged sea wall

The level of the shingle on the beach had been falling over a number of years and the sea had gotten underneath the foundations of the wall causing it to fail.  Additionally, sea water had penetrated the ground behind the wall causing it to subside.

Storm damage

Railway operations were stopped immediately and despite the festive period, a response team was mobilised to prevent the sea wall from collapsing into the sea.  I took up the role of Site Surveyor on 1st February.

As soon as the condition of the wall had been assessed, sheet piles were driven into the shingle in front of the wall to prevent any further undercutting whilst large boulders of rock were placed up against the wall to prevent it falling into the sea.


Emergency rock armour

A breakwater of granite was also placed down shore to dissipate the force of the waves.

The line between Dover and Folkestone is used heavily by commuters and so it was critical that the repairs to the sea wall and the railway line was re-opened as soon as possible.

Monitoring of the sea wall started as soon as it was safe to do so.  Retro targets were placed on the face of the wall and observations were taken on a daily basis, seven days a week at each low tide.

Surveying beach levels

At the same time, beach levels rose dramatically following the installation of the breakwater with over a metre of shingle being deposited in just 24 hours.  [Photo ITV]

Whilst the temporary works were being carried out, several different designs were put to the client Network Rail, ranging in speed and longevity of the permanent works.  

By March 2016, a design had been chosen that would see 132 piles driven into the ground behind the sea wall with a 235m concrete "raft" placed on top.  The advantage of this design was that it was independent of the existing sea wall and should the wall deteriorate in the future, the concrete viaduct would remain intact.  The positioning of the piles was hampered by the fact that the original railway sat on a timber trestle that had been back-filled behind the sea wall.  [Video of the proposed design]

Piling mat

Following the removal of the old track (and a badly salt-corroded concrete footbridge), the first operation involved making sure that the ground behind the wall was firm enough to withstand the weight of two piling rigs and associated traffic.  The top layer of ground was removed and a metre thick layer of fine aggregate material was compacted into place.

Piling operations

Working in a tight environment made plant movements difficult.  Operations were undertaken typically 24 hours a day, 6 days a week in order to complete the repairs as soon as possible.

As well as monitoring the wall for any further movement, the role of the site surveyor was to establish and maintain site control for as-builts and the setting-out of the new structure. Not easy in such a congested area where everything could be moving!

A timelapse of the temporary works can be seen here:  Network Rail Video 1 

In addition to the sea wall monitoring from the beach, a system of tilt monitors was installed onto the wall at 3m centres.  These "Senceive" nodes could detect very small tilts and by working 24/7 they were able to report the condition and state of the wall every 15 minutes.  The data from 180 nodes was sent to a central server via a GSM data-link and depending upon the degree of movement, text or email alerts could be sent immediately.

Senceive nodes installed on the sea wall
Surveys of the beach levels, monitoring of the sea wall retro targets and data supplied via the Senceive nodes continued to be analysed throughout the Summer to detect any movement of the sea wall.

Meanwhile, the construction of the raft continued...

Pile excavation
Concrete blinding

Steel mesh


Concrete raft

Click here to view a timelapse of the concrete pour:  Network Rail Video 2

Once all four slabs had been poured, the concrete spray walls were cast using a bespoke profiled steel shutter.

New Sea Wall

All that remained was to lay the ballast and new tracks.

Track installation

The new railway was opened on Monday 5th September 2016, ahead of schedule and to the delight of the passengers that had had to endure a bus replacement service for over 8 months.

Click here to see full story of the repairs:  Network Rail Video 3

A fly-by video of the new sea wall can be seen here:  Dover Sea Wall August 2016

Public consultations and planning permission approvals meant that permanent rock armour could not be placed onto the beach until mid-Summer.

Over 100,000 of Norwegian granite will have been placed onto the beach at the end of the project. This rock was transported by barge in loads of 25,000 tonnes and offloaded directly onto the beach via a smaller barge which could deposit up to 1,500 tonnes at each low tide.

Delivering rock armour

Two videos of the rock armour being delivered can be seen here:  Barge 1  Barge 2

The position of the rock armour against the old sea wall is critical to maximise the dissipation of the wave energy as it breaks on the shore.  Too densely packed and the waves will ride up the revetment, too loose and the waves break inside the rocks forcing them apart and continuing to damage the old sea wall.  Similarly, the level and angle of inclination of the revetment is a decisive design factor.

To guide the placement of the granite rocks - varying in size from 1 to 8 tonnes each - a 3D model of the rock armour was produced which could be used by GPS enabled excavators to place critical rocks to within a 0.2m tolerance.

As part of the process, it was necessary to verify the position and volume of the rock armour. Initially, it was proposed for a surveyor to obtain discrete GPS points on the rock surface. However, it became clear that walking on 8 tonne granite rocks was not practical!

Advances in technology means that it is now possible to carry out low level aerial photogrammetry from unmanned aerial vehicles (drones).

DJI Inspire 1

By taking a series of digital images from the air, these could then be "stitched" together using commercial software and GPS co-ordinated Ground Control Points to create a 3D virtual model of the rock armour.

Captured photograph

Textured 3D model

Point cloud

3D mesh

Using the GPS adjusted model, it is possible to compare the as-built survey of the rock armour to the design at any location rather than at any particular surveyed location.  The textured model is also a permanent record of the final product.  The volume and mass of the rock can also be calculated.

Note:  The use of drones for commercial purposes is monitored by the CAA and a PfCO (Permission for Commercial Operations) authorisation must be obtained before carrying out any aerial surveys.

It only remains to finish off the rock armour and install a replacement for the demolished bridge.

The next update will be in January 2017...

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