Business profile
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
Reading Station 2012 - Project example
March 2013 Page 107
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