Mobile Mapping at Oxhey Cutting for J.Murphy & Sons supplements conventional survey methods on difficult access site
Murphy & Sons Limited (Murphy) required a topographical survey to produce a Digital Terrain Model (DTM) of the embankment, including structures, drainage features, boundary features, mature trees, adjacent rails and track infrastructure.
Access along the embankment was severely limited due to steep slopes, uneven ground and overgrown vegetation. At the time of survey (August 2017), seasonal vegetation cover was at its most severe and access to survey detail by direct methods was limited, to the extent that only partial coverage of the site area was possible.
Laying adjacent to six busy lines, no direct track access was available during the works and all activities were to maintain a clearance of >2m from open lines. Some access to the opposite embankment was possible, although line of sights to the site area were obscured, due to the vegetation cover present.
Give the complexities of the site and further complications through the limited access, it was decided that a combination of both conventional surveys (by total station observations) and mobile mapping (ROBIN) be utilised to provide the most comprehensive coverage.
The initial total station observations provided accurate (albeit sparse) survey coverage of accessible areas, which were free from vegetation. Targeted survey transects were surveyed to provide accurate profiles of the embankment slope at key locations, although incomplete in places due to the site conditions.
The ROBIN survey was undertaken in backpack mode (walking) from both the site embankment and the opposite rail cess (>2m from open lines with site warden) to provide coverage of the site area. Ground control points established by total station were used to verify and adjust the point cloud data to tie into conventional survey results to <15mm accuracy.
Ground levels from the total station survey were used to validate ground level data, with false ground readings (distance returns from vegetation cover) being removed from the dataset.
- Vast improvement in data capture and modelling capability – through the ROBIN’s ability to gather data quickly (without the need for terrestrial set-ups) and to penetrate light vegetation, it was possible to significantly supplement the conventional total station survey results with full site coverage of not only the site but also adjacent NR infrastructure and opposite embankment. This would not have been possible by alternative methods, given the challenges presented by the site.
- Reduction in time required on site – to achieve similar comprehensive coverage by terrestrial laser scanning methods would have required a significant number of set-ups and establishment of associated reference points, to allow accurate registration of the point cloud. A further site duration of 1 day would have been required to complete (as opposed to 90 minutes of ROBIN survey) and tie into the conventional survey data. Even then, the completeness of data would have been limited due to the restricted number of locations suitable for terrestrial set-up whilst maintaining line of sign to established reference points.
- Less manpower required means safer working practice – the team consisted of only one surveyor operating the ROBIN in back pack mode and one assistant coordinating ground control with GNSS (to increase the accuracy of the point cloud.
“As a trusted pre-construction partner for Murphy and with a reputation as an innovator in methods of data capture, Central Alliance were the first port of call for a surveying solution for this difficult to access site. We are really impressed by the fast data capture and the completeness of the information provided in such a short timeframe” Adrian Deery, Murphy
Murphy has a long and distinguished record of delivering varied rail projects and a fundamental understanding of the complexities involved in all types of railway construction. Murphy has experience in working on large infrastructure projects for clients such as Crossrail, Network Rail and Transport for London.
Murphy’s skills in project management, pre-construction, supply chain management, and plant and transport management enable the company to successfully deliver key infrastructure projects. Offering pre-construction and project management capabilities means that Murphy can offer clients end-to-end solutions, and are highly regarded as industry leading by both our clients and peer group.
Working closely with clients from the early stages of a project means we get it right first time, every time, and continue to deliver excellence on site.