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Accessing the inaccessible

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In May 2010, surveying specialist Severn Partnership was approached to provide a proposal for track movement monitoring during piling works for a new footbridge adjacent to the railway fence line. This needed to adhere to Network Rail Standard NR/BS/LI/045 – Monitoring track over or adjacent to civil engineering works: procedure and intervention levels.

Severn Partnership is a well-established firm of chartered land surveyors and geospatial engineers with an excellent reputation for providing bespoke measurement solutions. The company was established in 1983 and now employs over 20 well-qualified surveyors combining to form experienced and accredited teams. Based in Shrewsbury, their expertise – particularly in railway surveying, laser scanning, and 3D modelling – draws opportunities to manage projects throughout the UK and overseas.

The company has been a user of instrumentation from Leica Geosystems for many years. It has eight Geodetic GPS receivers, three Leica Digital Levels and two Leica High Definition Surveying™ laser scanners.

Gearing up

The site of the new footbridge is adjacent to the Great Western Main Line just outside Swindon where the linespeed is 120mph. Working with lookouts is prohibited in this area and opportunities to take line blockages are very limited. Given that the monitoring period was expected to last at least two months, Severn Partnership proposed an automated monitoring system using Leica instrumentation and software. The extent of the monitoring covered 90m of track and four rails, with Leica GMP104 glass prisms located at 3m centres (124 prisms in total).

Subsequent to the award of the contract, a site meeting was arranged with Severn Partnership and Network Rail. Following the acceptance of several method statements, prisms were installed, trackside equipment established, and both cant and baseline surveys completed.

Whys and wherefores

Two, stable reinforced pillars for Leica prism/target assemblies were constructed within adjacent fields and two no-dig temporary instrument pillars were installed trackside for prism/target assemblies. All were clear of the zone of influence. A further no-dig pillar was constructed within the zone of influence, but such that lines of site to all others were maintained throughout the contract period. This facilitated constant free-station observations to ‘correct’ the instrument position and height should any movements occur during the piling works. The instrument was housed in a purpose-built shelter to protect against the elements.

A Leica TS30 precision monitoring total station was chosen for the project for its high accuracy, Automatic Target Recognition (ATR), fast action and reliability. With an angular accuracy of 0.5” and a distance accuracy of 0.6mm + 1ppm, the TS30 was ideal for the job.

A UHF radio link was established between the instrument and a laptop over 500 metres away in a nearby contractors’ cabin using Leica TSPS28 radios. A Leica meteorological sensor was connected to provide readings and corrections for temperature and pressure. A wireless dongle enabled Severn Partnership to remotely access the laptop from the office where its data was pushed into the Leica GeoMOS dedicated web interface. This allowed Severn and the client to graphically analyse the data securely online.

Software benefits

With the TS30 operating 24/7, observations were taken of the prisms remotely using the GeoMoS software. This provides a flexible automatic deformation monitoring system that allows connectivity and data acquisition of various geodetic and geotechnical sensors (total stations, GNSS receivers, Digital Levels, meteorological sensors, inclination sensors, strain gauges) via various communication devices and protocols including cable (RS232, RS485, fibre optic, LAN), radio (WLAN, Bluetooth, UHF, VHF), TCP/IP, mobile internet, GPRS and 3G.

GeoMoS is able to collect data from these geodetic sensors, then control and compute various parameters, and automatically store all measurements and results in an open SQL database. This can be accessed online both locally and remotely by Severn Partnership as well as the client.

It is not only possible to use the values obtained directly from the measurement sensors, but also to derive so-called ‘virtual sensors’ from these observations. Therefore cant can easily be computed in real-time within the software and treated as an independent measurement in its own right. Once these values have been obtained, a variety of limit checks/tolerances on these observations can be applied and automatic alert messages triggered via various means such as fax, email, SMS text messages, alarm bells. External applications such as batch files (.bat) and executables (.exe) can be delivered once the limits have been exceeded. Network Rail’s guidelines for track monitoring NRS/BS/LI/045 specify these trigger levels and necessary actions should they be exceeded.

Validated accuracy

GeoMos was set up to automatically email a report on vertical movement, cant and twist to Severn twice a day. These were analysed and forwarded daily to Network Rail, with comments. The software also offered the facility to assess lateral and longitudinal movements, and effectively the gauge. Additional results and trends were obtained and analysed by accessing the site laptop remotely and downloading the results.

Warnings of cant and twist exceeding trigger levels would have been sent by text and email had the situation arisen. The facility to receive this information and view the data on site was available to Network Rail, the contractors and any other interested party.

As was expected, the equipment performed reliably and the results validated the implied accuracy. The system provided not only a cost effective solution but enabled an accurate monitoring programme to be established within an area that was effectively inaccessible during the monitoring period.


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