M J Rees & Company is a chartered land surveying business formed in 1972 and regulated by the Royal Institute of Chartered Surveyors (RICS). Its services are externally audited under accreditation to OHSAS 18001, ISO 9001 and ISO 14001.

The company is Link-up approved and its rail staff are qualified and experienced land surveyors holding current Personal Track Safety (PTS) certificates. Work is carried out across the UK rail network on all aspects of its infrastructure including the track, platforms, station buildings and open areas, depots, yards and ticket halls.

For many years, M J Rees has completed surveys in the rail environment for: setting out and monitoring, including embankments; utility and drainage investigation; tunnel profiling; gauging; OLE; cable routes; surveys to record the condition of assets and for recording information on structures.

A significant part of the business is in locating and mapping utilities. This involves supporting the work of the Utility Mapping Association (UMA) in promoting best practice and guidance on techniques and methods for utility mapping, and to that end the company welcomes the forthcoming British Standards Institute (BSI) specification (PAS) 128.

Reduct used for buried assets

While some utilities are relatively easily traced, there are limitations to all traditional non-intrusive locating methods including electromagnetic location (EML) and ground penetrating radar (GPR), such that data will be issued with appropriate disclaimers , advising against reliance on the ‘surveyed’ position of buried assets, without further (intrusive) investigations.

In 2012, to complement its EML and GPR capability, the company introduced the gyro technology – Reduct – to assist its work in high risk environments. This system records the 3D profile and size of buried assets to the centimetre level of accuracy, regardless of the depth of asset, condition of surrounding ground or pipe material /condition.

How does the technology work?

Reduct DR-HDD-4.2 is a unique gyroscopic inertial pipeline measurement and mapping system designed to provide accurate 3D positional information for pipeline installations including: traditional trenching; horizontal directional drilling; pipe ramming/bursting; tunnelling and the rehabilitation and/or maintenance of existing pipelines.

Using a range of interchangeable wheel sets, the system is adjustable to fit internal pipe diameters from 125mm – 1500mm.

Inertial measurement systems contain a range of inertial sensors including gyroscopes, accelerometers and magnetometers – together these form the Orientation Measurement Unit (OMU).

The inertial measurement system in Reduct samples at up to 100 Hz, the sensor data is then calibrated with precise start and end co-ordinates to calculate gyro alignment and changes in x, y and z directions.

In the two images above, the system is shown configured with centralised wheel sets; in this mode of operation the gyro is kept aligned to the pipe centreline by the wheels which are ‘sprung loaded’ to deliver accurate detection and measurement of changes in alignment and variations in pipe diameters.

As an alternative and typically used in sewers where man-entry isn’t possible, the systems wheel sets are configured to run along the bottom of the pipes.

Reading station 2012

Reading is one of the busiest parts of country’s rail network. Redevelopment work to decongest and improve passenger journeys began in 2010 and included the construction of new track and platforms, a new passenger footbridge, a viaduct to the west of Reading and the widening of the rail bridges over Cow Lane. The project is scheduled for completion in summer 2015.

In spring 2012, M J Rees was appointed by Costain/Hochtief JV to locate a section of sewer running underneath the track and platforms. It was thought the sewer followed a straight line and varied in size from 1050mmdia to 1100mm x 800mm (oval). There were issues with the flow. The red line on the survey below shows the line of the sewer from service records.

M J Rees prepared the risk assessment and method statement (RAMS) and liaised with the asset owners to put in place all necessary permits to work. The sewer was cleaned, after which M J Rees’ surveyors transferred coordinates from the existing Reading station redevelopment survey control from the surface into both storm chambers (6853 and 6704).

The alignment survey was undertaken at night time working between the coordinated points using the Reduct DR-HDD-4.2 Survey System.

The water level in the sewer was reduced prior to cleaning and with a safe system of work established and approved the survey was completed to specification in a couple of hours

The sewer run was measured four times, twice in each direction. All runs were valid and showed a high degree of repeatability. XYZ results were checked and then plotted in CAD onto the existing topographical survey and checked against the piling design – shown as a blue line on the CAD extract below.

Departures from the expected sewer alignment in xy and z resulted in the need to make adjustments to the piling locations. This phase of the construction then went ahead, with confidence and without infringing on the sewer easement.

It was also found that the sewer was a 640mm diameter structure, not variable as had been historically recorded.

Repeatability is the level of precision measured by the difference between multiple runs; this gives confidence in the results, but is not a guarantee of accuracy.

The OMU’s are assembled and calibrated to obtain a unique finger-print of the assembled sensors. The calibrated accuracy of the OMU is 15cm in xyz over a 500m distance between two coordinated start and end points providing temperature change is < 5°, pulling speed is approx. 1.25m/s and acceleration / shocks are < 2G.

The standard tolerance is the sum of the calibrated accuracy of the OMU, the quality of the surveyors coordinated start and end points, the state of the pipeline (dirt and debris, heavy internal welds, deformations and cracks all have the potential to add noise to the results) and a safety margin.

In numbers the standard tolerance is given as:

• Horizontal plane (XY) 0.25% of the distance between coordinates (i.e. 25cm per 100m)
• Vertical plane (XZ) 0.10% of the distance between coordinates (i.e. 10cm per 100m).

The process of detecting, verifying and locating buried assets is a highly skilled task, one that should be entrusted to experienced and qualified surveyors.

M J Rees’s reputation is built on advising its clients on what can and can’t be achieved and on promoting best practice in the utility survey market. The forthcoming British Standards Institute (BSI) publically available specification (PAS) 128 may also help reduce the confusion.

Right now, with its experience of proven gyro technology M J Rees is helping numerous clients avoid the risks of uncertainty and the delays this cause to their projects.

For more information contact Stephen Rixon Tel: 01454 252930
survey@mjrees.co.uk
www.mjrees.co.uk