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Maintaining safe passage through the Medway Tunnel
Technology is a key element in making sure every journey through the Medway Tunnel in southern England is a safe one.
Regular commuters who pass beneath the Medway River in London’s satellite county of Kent on their journeys between historic Chatham and nearby Rochester could be forgiven for not giving the neatly landscaped, twin-bore submerged tunnel that they pass through a second thought. There’s no reason why they should; it isn’t exciting, it isn’t an eyesore and it isn’t even very long. Taken in clear traffic at the speed limit of 50mph (80kph), it feels more like a very wide bridge than the kind of tunnels that pierce an Alpine precipice and plunge into the depths of the earth only to emerge a mile or so later at the apex of a sharp bend. However, behind its rather under-stated public facade, there’s a lot of magic going on beneath the Medway!
Opened in 1996, the tunnel provides an alternative crossing point of the Medway River to the busy Rochester Bridge carrying the A2 across the river further upstream.
The tunnel is subject to an overnight closure by Medway Council once a quarter for preventive maintenance and TrafficSafe was invited to its July closure where we met the Head of Highways, Phil Moore.
Technology beneath the Medway
The first impression you get of the tunnel on the approach road is that it doesn’t look like a conventional tunnel at all as it more resembles a dark bridge. The reason for this is the square cross section. It’s very difficult to drill a hole in the ground with any other cross section than a circle yet this tunnel has two large gaping rectangular holes facing the traffic. This is the first aspect with which the Medway tunnel breaks convention and is strikingly visible. The tunnel wasn’t drilled at all – it’s one of only two submersed tube tunnels in the UK. The huge rectangular section tubes were laid on the river bed to form the tunnel which doesn’t sink below the ground anywhere along its length.
Each tube (or bore) carries two lanes of traffic in a westbound or eastbound direction, the two being connected periodically along their length by emergency doors to enable vehicle occupants to escape from one bore to the other in case of an incident threatening to trap them. The complex construction of the bores themselves involves several sections sealed against each other with compression joints which prevent any leakage of river water into the tunnel.
When asked about the dangers of flooding from the river or structural collapse, Phil explained that despite these things being a popular concern among people, it never happens. The least thing that anyone should worry about when travelling through a tunnel is whether the structure will fail in some way. The dangers of collisions, smoke and fire are much more real and significant in any tunnel.
However, a system does need to be in place to deal with surface water and leakages from vehicles and so the tunnel bores are equipped with three giant sumps, one mid-river and two under the carriageways. Left to fester, these would become very unpleasant fairly quickly so one of the tasks of Phil’s team at each quarterly shutdown is to pump out the sumps, flush them through and wash them down. A lot of importance is placed on keeping the pumps and sumps as clean as possible.
Cleanliness in general is an important aspect of keeping the tunnel operating efficiently and reliably. The carriageways themselves stay surprisingly free of litter, being cleared naturally by the aerodynamic effects of the passing traffic dragging it through the tunnel and out of the ends in their wake. Unfortunately, the same can’t be said about airborne particulates from dust, brakes and exhaust fumes which cling to the tunnel walls like sand in Marmite. Contractors are therefore hired once a quarter to use a bespoke tunnel wall-washing truck to remove all the surface grime.
Watchkeepers: The eyes and nose of the tunnel
When passing through the tunnel, there are few clues that the motorist can detect concerning the control and operation of the tunnel or the management of traffic. The overhead fluorescent lighting, large industrial looking cowled fans rotating lazily and an ambient orange background glow are almost the only signs of electrical activity but the tunnel bores are also equipped with sensors and video surveillance cameras that are essential to the control of the tunnel but tucked away out of view. The heavy iron-clad Bosch “Metal Mickey” cameras hang in the shadows off the ceiling and cover the whole length of the tunnel in their field of vision using Pan-Tilt-Zoom (PTZ) facilities.
Other cameras are set for incident detection. Each covering a patch of the tunnel in a plan view, the camera analytics can detect accidents, shed loads, objects falling from vehicles, cars travelling in the wrong direction, pedestrians, sudden speed decreases and other incidents that would result in an alarm condition. Other sensors in the tunnel detect heat, fire, smoke and bad air quality.
All of these sensors and camera systems are fed into a SCADA (Supervisory Control and Data Acquisition) system. Alarm conditions are prioritized based on severity (eg a defective lamp has low priority and fire detection has high priority). The system for how to respond to various alarm conditions is very procedural and responses are planned for every possible condition or combination of conditions. The SCADA system was recently upgraded by Vital Technology Ltd. (part of Vital Services Group) to provide the control room operators with the highest level of reliability with low false alarm rates and the latest security against cyber attacks on industrial control systems.
Since the Medway Council is a local Government authority, its computer systems all need to be “Code of Connection” (CoCo) compliant and this includes industrial control systems that are network connected. The Medway tunnel systems meet CoCo compliance regulations. However, the risks associated with the computer system being compromised are low since there are no traffic safety implications in the event of a total system failure.
Behind the scenes
Coming out of the tunnel itself and getting behind the landscaping by way of the discreet service access road at the western end of the bores reveals the true complexity of tunnel operations combining high technology with power engineering at its brutal best. Each bore has its own independent power sub-station with two feeds from two national grid networks plus their own generator and a humming switchroom creating seemingly as much heat as electrical power.
To keep everything ticking over in the event of a mains power failure, uninterruptible power supplies (UPS) cut in for long enough to maintain the load until the diesel generator cuts in. The modern UPS facility was recently installed and the neat grey cabinets are equipped with the latest in battery technology which has done away with the need for the previous facility which was a room full of rows and rows of lead-acid accumulators stacked up in banks.
The tunnel’s control centre is tucked neatly away from the switches, generators, power feeds and UPS equipment and is furnished only with the necessary communications equipment, computer console and video wall installed by Vital Technology earlier this year. With backup facilities being important for continuity of operations, a second feed from the computers and sensors supplies information to the Medway Council Control Centre where operations are monitored around the clock.
From either Medway Council or the tunnel’s local control centre, actions can be put in place regardless of whatever alarm condition is encountered. This procedure is occasionally put to the test in disaster response exercises involving the tunnel staff and the local emergency services to ensure that all the systems operate in practice as they’re expected to do.
The video wall shows images from all of the surveillance cameras operating within the bores and at the tunnel entrances whilst the computer console shows the status of the SCADA system, the sensor condition and alarm status. It also shows the exact position on the roadway which can be matched to one of the video surveillance camera displays on the video wall.
Arguably the most critical emergency that could take place in the tunnel is fire. Surprisingly, petrol fires are not at the top of the list of Phil’s worries since liquid spillages are the most easy to deal with. In the event that a petrol tanker cistern is breached in an accident, sensors will detect the presence of the leaking fuel in the sumps and will flood them with foam. Of greater concern is a fire involving solids such as a flat-bed truck carrying pallets. In such emergencies, one innovation at the Medway tunnel could prove to be a life-saver. The tunnel is equipped with wires across which information can be broadcast to drivers across 6 pre-set channels enabling controllers to address vehicle occupants directly and advising what they should do.
It isn’t just emergencies that the control room staff has to deal with though. Simple things that are often taken for granted require extra effort in a tunnel environment such as ventilation and lighting.
The lungs of the tunnel
Controlled mainly automatically via sensors which detect bad air quality, the twelve ventilation fans in the tunnel bores usually operate at about 25% capacity. Logic controllers sequence the operation and power of the fans to prevent any unwelcome aerodynamic effects being generated from these powerful units. To the passing driver, they don’t appear to do very much with their fan blades visibly rotating at a lazy pace as if they’re idling. However, the fans are designed to shift air at a colossal rate and can quickly remove toxic air from the tunnel when required as well as dissipating accumulated heat from the soffit in case of fire.
Able to direct the flow of air in either direction, the fans usually run in the same direction as the flow of traffic. If an incident occurs in the tunnel and traffic is stationary, such an airflow direction will ensure that fresh air is pulled from the rear of the tunnel over the stationary cars behind the incident. Any smoke from the incident itself is pulled to the front of the tunnel which is clear of traffic.
The lighting levels within the tunnel are similarly sophisticated. Photometers outside the bore entrances measure external levels with a 20 minute sensitivity. Measuring over such a period prevents lighting adjustments being made on the basis of a passing cloud or shower. The ambient lighting level is then passed to the controller for the tunnel lighting which is adjusted to match ambient levels as closely as possible. The tunnel lighting is therefore brighter in daytime and darker at night which is the opposite of what would intuitively be expected but is designed to make the transition from one environment to the other as closely matched as possible so that drivers don’t have to adjust their vision so rapidly.
When this photograph was taken during tunnel maintenance, the eastbound bore (on the left of the picture) was set to daytime lighting levels with high pressure sodium lights whilst the westbound bore (on the right of the picture) is set to night time levels. It’s easy to see just from the photograph that at night, it would be easier on the driver to enter the bore with lower lighting levels than to encounter the dazzling sodium lights.
A tunnel of local importance
The Medway tunnel is of immense value to businesses and residents around the Chatham and Rochester area in Kent and even the quarterly preventive night maintenance closures are met with inconvenience by some of the people in the area. However, the tunnel isn’t regarded as being of strategic national importance and it isn’t connected to the Trans-European Road Network despite its proximity to London and the south east of England. The nearby trunk routes of the M2 and M20 motorways are the main transport arteries of the area and if they close, their traffic isn’t diverted through the Medway tunnel.
Despite being in the backwaters of the nation’s road transport infrastructure, there’s a lot to be learnt from the experiences of the Medway Tunnel, its technology and its approach to keeping its users safe. Operating quietly and unobtrusively in this pleasant corner of England, it serves its purpose best by being almost unnoticeable whilst its systems work their magic.
Studied Engineering at Loughborough University and now involved in broadcast and technical journalism. Jonathan is based in London and Almaty.
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