Motorway metrology

Motorway Metrology

The M25 motorway, or London Orbital, encircles the UK’s capital city and was officially opened in 1986. It is big enough to be seen from space as the picture from NASA below illustrates.

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The M25 motorway circling London (image: NASA)

The 117 mile long ring road provides an essential route for travellers and was originally built as a 3 lane motorway. The M25 is constantly being expanded to accommodate increasing levels of traffic, but despite this, the motorway was officially recorded as the slowest in the UK with an average speed of just 25 mph in 2016.

In 2012, Optical Metrology Services Ltd (OMS) were invited to assist with the commissioning of the widening of the motorway to 4 lanes in some areas. It’s encouraging to be part of such ground-breaking projects, even if our work failed to provide much of a boost to the average travelling speed in this case. The image below is a typical of the motorway where it has been widened to four lanes.

A widened 4-lane section of the M25 motorway

The problem we were appointed to solve was related to the accumulation tanks that were originally designed to absorb the extra water flow from the new lane, passing it into the local drainage system at a rate that it was able to cope with. To achieve this, the drains along the motorway were fed into a number of holding tanks which would gradually fill up during a downpour and then slowly release the water at a slower rate. A rather novel design was commissioned for these holding tanks consisting of a set of upright vanes, arranged together such that they would be load-bearing for the road above. The tanks were then encased in plastic covers with a pipe running through the centre. The arrangement is illustrated in the following figure of a semi-dismantled tank.

Water holding tanks with central waste pipe

The main contractor on the project faced an issue around the shape of the pipes - was the rather obvious ovality (which was visually evident when looking into the pipe of the buried tank) stable or gradually getting worse? If the latter was found to be true, the tank could quickly become unfit for purpose. In other words a potentially very expensive legal and contractual case could have rested on the outcome.

The task of measuring the pipe was made more difficult by having to be undertaken whilst the M25 was in operation, and having to do so in a number of different locations over several months. The plan was to measure the pipes once and then return to the same locations another three times to check whether there was any statistical change in the shape of the pipe. Those that know OMS will understand that such a project would play to our strengths, and that our experience in challenging locations added to our reporting capabilities would be put to good use. The following image shows one of our tools being lowered down the manhole to gain access to the pipe.

OMS measurement and inspection technology being lowered into position

As is typical with such projects, the ‘clean and dry’ pipes with minimal water which were reported at the project specification stage by the client, turned out to be a third full of debris with a considerable amount of water in some cases. In order to carry out the project we used a specially adapted crawler system with forward and rearward-looking cameras for video capture.

The ‘clean and dry’ pipe

The objective was to measure a specified number of times in each section of pipe in the tank runs with a maximum length of 100 metres to survey. The location of each measurement was carefully noted and the tool was brought back to the same location on each occasion to obtain the check measurements.

In order to ensure that the tool was traceable to National Standards for length so that that the measurements would be usable in a contractual dispute, we calibrated the laser system before use and verified that it was still within calibration after use. The calibration arrangement on site can be seen in the following figure.

The inspection robot and calibration ring positioned ready for action

In accordance with good practice, the thermometer to measure the temperature of the calibration ring can also be seen in the image.

Following each measurement graphical reports were provided in a PDF format to facilitate an easy reference to the shape of the pipe at any location. Two of these show a reasonably round and quite oval section of pipe are provided in the following illustrations.

A relatively round pipe with custom dimensions displayed.

A deformed pipe with considerable ovality which illustrates why the client was intent of having these pipes fully checked

The outcome of the project was a statistical comparison between the pipe ovalities over a period of time which provided the necessary proof that the client required. While this project location may have appeared to be one where short cuts could be taken, the rigorousness of the OMS work proved invaluable as a gold standard against which the multi-million pound element of this project could be assessed.

For more information about OMS and the services we offer, contact: +44(0)1279 656038.

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