Out of the trenches

Accurate topographical information is key to Severn Trent Water's £2.3M program of capital investment. Surveying business 40Seven was called in to provide the data with the minimum of disruption.

Severn Trent Water (STW) provides sewerage services to 8.2 million people, and water services to 7.3 million, in an area stretching from the Severn Estuary to the mouth of the Humber.

The company faces a vast programme of works, and an investment of around £2.3B, to meet its AMP4 contract strategy. Not surprisingly, accurate site plans are an essential part of deciding what to build and where.

Using non-destructive technology (NDT), surveying business 40Seven is working for STW as part of its Underground Services Survey Framework (USSF). It says it is helping deliver the information and data necessary to enable the development of accurate proposals, so that greater cost certainty can be achieved before solutions are progressed to detailed design.

The five-year framework, which began in Autumn 2005, is for 40Seven to provide utility location, mapping and topographical surveying services within four geographical areas – Leicestershire and Warwickshire, Staffordshire and Derbyshire, Yorkshire and Lincolnshire, and Shropshire, West Midlands and Nottinghamshire.

40Seven says it has two teams of surveyors – one utility surveying and one topographical – using the most up-to-date NDT to provide detailed information on what lies above and below the ground. STW is then using this data to design feasibility studies proposing a variety of works, including:

  • Where existing services need to be diverted
  • Where it is possible to connect to an existing service
  • Where a new service can be installed or constructed

Having been bought by IETG earlier this year, 40Seven says it now offers a complete surveying solution to clients, including gas hazard surveying and measured building surveys.

STW delivery manager Mike Fairlamb says: “As we had already appointed 40Seven as a framework contractor, the additional services that can now be supplied means that they can use the most appropriate surveying technique to enable the provision of the data we require.

“It’s also a really good example of joined-up thinking and best practice, as we can be confident that there is no repetition in the work being carried out. And the resultant data is all plotted onto one drawing, resulting in a comprehensive overview of what is above and below the ground. It is also cost-effective, as there is only one contractor doing the work.”

Pipeline routes

40Seven is using a variety of NDT equipment on the framework, including radio frequency receivers and transmitters, total stations, and GPS and GPR operating different frequency antennae, depending on the nature of the site and specifics of the survey.

By using the latest reflectorless total stations, 40Seven can collect measurement data in awkward situations. The system uses a laser to range onto existing structures without the need for a conventional prism/reflector.

40Seven says this gives the benefit of being able to map out inaccessible areas, as surveys can be conducted from a distance, while still allowing for the fast and accurate data collection.

Add the use of GPR and GPS into the mix and data that was difficult or impossible to calculate can be surveyed and plotted without having to dig a hole in the ground.

The type of utility and location mapping services being provided to STW ranges from a blanket survey of an area onto existing topographical drawings, through to 10m-wide corridor topographical and utility surveys over proposed and existing pipeline routes.

Flexible solutions

Where budgets are tight, 40Seven is flexible enough to supply electronic slit trench surveys – a 3m-wide survey that incorporates only the carriageway and footpaths. This gives the client a big enough picture of the underground services without the cost of carrying out a comprehensive blanket survey.

Results are supplied to Severn Trent as two-dimensional AutoCAD.dwg files and hard copy plots, with depths and surface features mapped out.

Stewart Randall, project manager for 40Seven, says: “Adopting non-invasive surveying techniques has a massive benefit to everyone and reduces the risk of injury to employees and inconvenience to the public.

“Information derived from our surveys can be critical to ensuring that Severn Trent’s services remain uninterrupted during the early works and construction stages and that all risks are minimised.

“By employing our services at an early stage, cascading the derived information through all relevant parties involved, and then updating our surveys with newly laid or changed services information, Severn Trent is able to achieve absolute best value for money from the initial expenditure and, in many cases, the surveys quickly pay for themselves.”

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