Blackfriars Bridge over the Thames is one of a number of major re-modelling projects in the programme and requires a completely new steelwork design to carry the re-aligned permanent way over the original steel arched river spans below.

The project content is yet to be finalised but will include a new ticket hall on the south embankment with a new passenger walkway on the western side of the bridge. The existing platforms 1-3 will become redundant leaving the through trains to be served by a remodelling of platforms 4-5 and a realignment of the through tracks. All the supporting steelwork from the arched spans upwards is going to be replaced to support the realigned trackwork.

Blackfriars bridge comprises five arched spans supported on four stone piers with stone abutments on the north and south banks.

The bridge has the benefit of the Blackfriars road bridge running parallel and at a distance of some 50m to the west.

The southern three arches are of the same width while the northern two arches widen to accommodate the five platforms at Blackfriars station.

The worksite therefore has an operational railway above and an operational river below with all spans being classed as navigable channels.

The proposed plan of survey would therefore have to satisfy both Network Rail and The Port of London Authority.

The Leica Scanstation has the range but not the speed. The HDS 4500 has the speed and resolution but not the range. The solution we decided on was to use both and thereby harness the benefits of each.

The Scanstation was used for scans from the neighbouring road bridge and embankments. These locations offered relatively stable platforms for the hour or so needed to scan at high resolution at ranges generally between 50-125m. From the road bridge the scanner was positioned to see through the lattice of steelwork above the arched span beams.

The HDS 4500s were used for all other work. Under the bridge itself the scan locations were chosen to see up between the arched span beams collecting detail of the underside of the bridge deck. Additional scan locations we used to record detail (services mainly) that would ordinarily have been hidden.

We did briefly consider scaffolding down from the bridge parapet but the railway possession time needed to erect and then dismantle coupled with the fact that, while in place, the river navigation could be severely restricted at high tide meant that this possibility was really a non-starter from the outset.

The solution adopted was to fabricate a steel ladder with a T-bar at the top that would hang from and between the arched beams and rest against the stone piers.

An initial scan from the embankments enabled us to ascertain the precise length of T-bar that would be needed.

We then constructed a special frame table to hook over the ladder rungs at just above water level.

Speed of the operation was going to be a critical factor with the water level changing by up to a metre in one hour. Too long on position on a falling tide and the scanner might be unreachable from the boat. Too long on a rising tide and the boat would be unable to get close enough underneath the bracket to recover the scanner before the tide engulfed it.

On the table was a mounting for a battery column on top of which was the mounting for the HDS 4500 scanner. To complete the equipment a Panasonic Toughbook was connected to the scanner and secured to the frame. The whole mounting was designed to keep the scanner more than 0.8m from the nearest object to avoid the safety cut-off operating.

The final addition to the mounting was a length of rope to enable us to recover the scanner should we drop it in the Thames. All part of our method statement approved by the PLA to ensure that it would not present a hazard to shipping!!!

On the first day the Thames had waves of around 500mm that looked calm until we left the jetty. The workboat aft of the cabin had an open work area with a gunwhale of some 800mm high.

The second part of the procedure involved hooking the bracket with battery, scanner and laptop attached onto the appropriate rung of the ladder. This was the most critical part of the mounting and involved keeping the boat away from the ladder/bracket yet close enough to enable us to engage the rungs without falling or dropping the scanner into the Thames.

Now came the easy part – pull the boat away and control the scanner using a second Panasonic and the WIFI link. The chance for a well-earned rest while the scanner does its job.

Six minutes and forty seconds later and it was time to move the scanner to the next location.

The first scan took over one hour much to the amusement of a select audience on the bank during which time a lot of nautical terms were learnt while the team moulded itself into a well oiled machine. Subsequent scans took an average of thirty minutes to complete including moving the scan location. In total some twenty four scans were made of the underside of the bridge over a period of three days.

For this earlier work a control network had been established over the bridge and this was extended to the north and south embankments and to points on Blackfriars road bridge. The points on the road bridge were positioned over the bridge piers to minimise vibration effects from the traffic.

Points of detail were coordinated from this control using Leica 1201 total stations in reflectorless mode.

Scanstation scans were made from the road bridge and the embankments and these were first registered together (cloud to cloud) and then to the control points and to detail common with the bridge deck scanning.

For the bridge substructure the six scans comprising each span were first registered together using cloud to cloud registration. Each of the spans was then registered to the Scanstation scans by selected points and then by cloud to cloud.

The cloud to cloud registration using Cyclone software worked faultlessly. Because of the size of the datasets (an average HDS4500 scan has more than 30m points) most of the registration was left to run overnight even using workstations with twin dual core processors.

It was gratifying to see the way that the data from both types of scanners merged seamlessly together and produced a natural synergy.