Thursday 19 January 2017

Installing a plastic bridge

On Sunday 1st January 2017, whilst many were still celebrating the New Year, two bridge spans of a new pedestrian footbridge were installed over the Folkestone to Dover mainline railway line in Kent.

Constructed of Glass Fibre Reinforced Plastic by PipexPx in Plymouth, the spans weighed just 6 tonnes each and were lifted into place by a 40 tonne RoadRail crane.

Although the landings were also installed during the same possession, the three flights of stairs had to be installed on a subsequent visit the following week.

The bridge will not be open to the public until Easter as work is still being carried out on the beach to complete the installation of 100,000 tonnes of rock armour following damage to the sea wall in December 2015.

Given a working design brief, the bridge was designed and fabricated by PipexPx and has become a model for future installations.

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...

Wednesday 4 November 2015

Steel, Website & UAVs

With the flyover at Hammersmith now complete, I have been fortunate to start work as a site engineer with a steel erection company.  28-32 Chancery Lane is a nine storey office development in the heart of London's legal district.

Arriving on site in mid-September, the demolition of the original building to occupy the site was complete and the basement slab was almost finished.

Steel frame erection
With a retained fa├žade and existing adjacent buildings just millimetres away, the installation of the perimeter steelwork has been a challenge.  It is apparent that previous surveys have been carried out to determine the location of these walls and only minor adjustments of the designed structure have been necessary (so far).

Is it bad luck to walk under a tripod?
The website is now complete and is now live.  This is a showcase of my own Limited Company and the services that I am able to provide as a freelance engineering surveyor.

In October, I successfully passed my Unmanned Aerial Vehicle (UAV) flight examination, which means that I am now able to apply for a Permit for Aerial Work to fly "drones" commercially in the UK.  This is the culmination of nearly seven months work involving a two-day aviation ground school (with exam), writing an Operations Manual which covers all aspects of the procedures and safety whilst flying the UAV and also acquiring sufficient flying hours and practice.

DJI Inspire 1 during training

Thursday 13 August 2015

The End is Nigh

After nearly 20 months of working on Hammersmith Flyover in West London, it only remains to tidy up and leave the bridge "as we found it". Although, hopefully in a better state than it was in 2012 when it was in danger of collapse following years of neglect and had to be closed to traffic!

Today, the flyover has 34 new sliding bearings beneath its piers, over 150 km of steel cables holding it together (the distance from London to Calais - rather apt given that they were installed by Freyssinet, a French company) and a new waterproofed road surface.

Access to the bearings has been improved to allow future inspections to take place and five new door openings have been cut into some of the piers for access into the bridge deck above.

New door opening (without door)
One of the final tasks is to re-instate the covering over the pits surrounding the piers.  These were removed to enable access to the old and rusting bearings upon which the piers sat.

Threaded couplers were cast into the pit wall during their strengthening.  Following a detailed survey of these, bespoke seating cleats were made so that the bolt holes exactly matched the coupler locations.

One of four seating cleats per pit
Next, galvanised (rust proof) steel beams are installed around each pier and a shelf angle is attached to the pier itself.

Steel beams installed
The pit is then covered with a frame and pre-fabricated trench panels - provided by Fibrelite.

Panel installation
The final steps are to replace the tarmac and paving slabs around the piers to blend into the covers. At the pier, another overlapping cover will be fixed to the pier to allow it to move freely.

[to be continued]

Tuesday 28 July 2015

All quiet on the Western front

The temporary ramps that have been in place at the centre of Hammersmith Flyover for over a year have finally been removed.

The tarmac ramps and hinged covers were put in place to allow work to be carried out at night on the replacement of the expansion joint - before being lowered again in the morning for the rush hour traffic to pass over.

The only problem with this system was the noise created by the cars, lorries and coaches as they clattered over the joint at often more than the regulation 30mph.  Residents in nearby flats overlooking the flyover have had to endure months of disturbance as the works have progressed.

Over the last few weekends, the new expansion joint has been installed - during a full closure of the flyover to allow the concrete to set without any undue vibrations.  Working throughout the 48 hour weekend closures, day and night teams worked continuously to ensure that the road was ready for Monday morning.

Installing the expansion joint - at night
First of all the Eastbound joint was installed and then the Westbound joint was set in place a week later.  At just under a ton each, each metre long segment of the joints had to be aligned and levelled to a high accuracy as cars would be speeding over the joint in all weathers and conditions.

Aligning and levelling the joint
Each steel joint must slope from the verge at the edge of the bridge towards the central reservation at the middle to follow the alignment of the road surface.  This is to allow any rainwater to be collected and removed along drainage channels in the kerb.  In addition, the joint also slopes from East to West to follow the natural "hump" of the brdige.

The specialist joints allow the bridge to expand and contract with changes in temperature and are made up of two interlocking "combs" that are expected to move by 100mm on each side of the bridge between a cold Winter and hot Summers' day.  Movements of 20mm are not uncommon during a single 24 hour period.  The teeth maintain a road surface whatever the gap in the concrete bridge underneath.

Interlocking teeth
Over the last weekend, the ramps and temporary covers were finally removed and a fresh layer of tarmac was laid flat at the same level as the rest of the bridge allowing vehicles to travel over the bridge without even realising that they were "jumping" across a gap between the two halves of the flyover.
The new joint in place
There is still work to be done installing the drainage channels and replacing the central crash barrier, but the local residents can sleep peacefully once again!

Tuesday 30 June 2015

Expansion Joint Repairs

Hammersmith Flyover is over 600m long and built in two halves with a thermal joint in the middle - actually 6/15th's of the way along from West to East. The expansion joint allows the concrete bridge to "grow" and "shrink" as the bridge temperature changes. During a 24 hour period, the gap at the centre can be 10 to 20mm smaller at night than during the day and over 100mm different between Summer and Winter.

Anyone who has driven over the flyover recently, and especially anyone living nearby, will know that there is a temporary plate covering the joint. This causes cars to have to slow down and makes a loud noise each time a car or lorry passes over it.

Covering over the old expansion joint
Temporary cover and ramp
The temporary covers, which are on hinges, allow works to progress underneath by enabling the plates to be lifted at night, the old concrete to be removed and then being lowered again for normal daytime traffic.

Old and broken out concrete
Surveying the expansion gap
The remedial works are now almost complete and the new expansion joint (a large steel plate with interlocking teeth) to ready to be set into position in the next two weeks.

Once the "steel combs" have been concreted into position, the old covers and temporary Tarmac ramps will be removed and some fresh Tarmac can be laid at the final road level. Job done.

Thursday 11 June 2015

Awards and UAVs

Another award for Hammersmith

My current project, the Hammersmith Flyover in West London was awarded the CIHT (Chartered Institution for Highways and Transportation Innovation Award on Tuesday. The award was in recognition of the enterprise, ingenuity, design, performance and quality of the project.

Award presented by Robert Llewellyn
Not least were factors such as best-value and cost-benefit.

The re-strengthening of the flyover, working around the existing structure and keeping traffic disruption to a minimum has been a challenge.  Bespoke machinery and techniques have been used to install kilometers of new internal cabling into the bridge to replace the original tendons that have become corroded through use of winter salt and poor maintenance.

Installation of the external "blisters"

French sub-contractor "Freyssinet" was commended for its technological expertise.

UAV Aerial Modelling

As an exercise to determine the method, procedure and results of 3D modelling by use of an unmanned aerial vehicle (UAV), I flew my trusty DJI F550 hexacopter around an old stone building near Thetford, a rabbit Warreners Lodge dating back to about 1400.

DJI F550 with GoPro 4

Approximately 80 photographs were taken from all angles around the lodge and stitched together using Agisoft Photoscan.

Some of the aerial photographs
The photographs are automatically aligned to its neighbour using common points and although there was no positional information, such as GPS data, the software was able to produce a photo-realistic model of the building.

3D model created in Agisoft Photoscan
Once created the program could then export the results in 3DS format (3DS max), Wavefront OBJ (3D data modelling transfer format) or even generate a point-cloud which can be imported into AutoCAD via Autodesks ReCap software.

Point cloud in AutoCAD 2014
The finished model is not orientated or scaled.  This must be done from ground dimensions taken independantly.

The quality of the final product reflects the quality of the screen captured stills taken from the original GoPro video.  My next project will use high resolution photographs taken at 3 to 4 second intervals...