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Introduction to GDMS Surveying

GDMS is National Highways’ technical information about the location and condition of all drainage infrastructure on their network.

It includes data on all drainage assets, not just below-ground pipes.  These include surface items like ditches, kerbs and ponds as well as below ground pipes and manholes.  GDMS investigation requirements are defined in CS551 and can be:

• Validation survey

  • Verifies whether the inventory information held on the system, previously derived from drawings, is a true record of the visible surface point assets or not.

• Priority asset survey

  • Carried out in areas where the highway drainage system is unknown or uncertain, in order to locate the outfall or soakaway point of each drainage catchment, and any culverts passing under the carriageway, to facilitate management of pollution and flooding risk.

• Filter drain condition survey by GPR (Ground Penetrating Radar)

  • Determines the service condition due to sediment contamination of the filter medium of combined surface and sub-surface drains, and other forms of filter drain, including the use of GPR.

• All assets condition and connectivity survey

  • To determine the inventory, asset level condition and connectivity of all drainage assets in a drainage system.

• Pipework and chambers defect survey by CCTV

  • Determines the inventory, condition and detailed defects of pipework and the associated chambers and gullies, including the use of a closed-circuit television (CCTV) camera.

• Pipework geometric survey by laser profiler

  • A pipework geometric survey shall be carried out to determine the pipework internal circumferential geometry, including the use of a laser profiler in association with a pipework and chambers defect survey by CCTV.

• Pipework inclination survey

  • Carried out to determine pipework inclination in association with a pipework and chambers defect survey by CCTV.

• Chambers defect and geometric survey by laser scanner

  • Determines the inventory, condition, detailed defects and internal geometry of chambers, including the use of a laser scanner with inbuilt panoramic still camera(s) and a closed-circuit television (CCTV) camera.

• Ditch profile survey

  • Carried out to determine the longitudinal and cross-sectional profile of ditches in association with an all assets condition and connectivity survey, and including the determination of potential soil contamination.

• Soil characterisation sampling and testing

  • Soil characterisation sampling and testing shall be carried out in association with a drainage survey to determine whether the spoil arising from potential drainage works should be characterised as hazardous or non-hazardous for the purposes of off-site disposal.

• All assets defect survey

  • Determines the inventory, condition and detailed defects of pipework and the associated chambers and gullies and the inventory, asset level condition and connectivity of all other drainage assets in a drainage system.

Common mistake that are made by CCTV contractors in this area include assuming that this type of inspection is only about drains and pipes, where as in truth the scope is much bigger than this and involves a lot of GIS supporting data collection and validation and other surveying techniques.

The work is usually tendered by the National Highways Areas, also sometimes known as the Managing Agents and information for the current managing agent for any area can be found online.  There are 13 Areas across England numbered from 1 to 14 (there is no Area 11) and the areas are now grouped into regions under National Highways.

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Herein, we will refer to ‘The Area’ or similar as any of these operational areas.

Given that GDMS inspection work is often carried out at night due to the high risks associated with working on busy roads, common phrases that are often used by the Areas in tender negotiations with contractors refer to how the contractor should behave when they discover that an asset cannot be inspected dues to its current operational condition (i.e. a pipe is full of silt, so how do we inspect it, but it’s the middle of the night so there is nobody I can call at the Area to ask the question regarding what to do next).

The following is a guide only and must not be taken for granted.  The details of these kinds of terminology must always be negotiated and agreed between the client (usually the Area) and the contractor, or between the contractor and the sub-contractor.

• Do Nothing – in very simple terms, these means that the contractor will not make any reasonable attempt to clean the pipe.  They will just record in the data that an inspection has been attempted and the current condition of the asset.

• Do Something – the contractor will agree with the client that they will make a considered attempt to clear the asset and to get a good CCTV inspection.  Maybe they will give the pipe 3 passes with an appropriately sized jetting machine (this should also be clearly specified for different pipe sizes).

• Do Everything – the contractor will agree to keep on cleaning the pipe until it is at an agreed cleanliness level (this is usually specified as something like ‘less than 10% debris’), and then complete the CCTV inspection.

Clearly, the requirements of the tender can have a significant effect on the price that the contractor submits to complete the work.

Whatever the scope of work or the scheme extents, the process of delivering GDMS inspection data to the client using the specified standard always begins with the existing known data of the drainage network within the scheme.  This is the beginning of the round-tripping process (described later in this document)'

Other questions that the contractor should always fully understand before entering into GDMS works include:

• Scheme Extents – on motorways, this is usually described by the road description (i.e. M25, Junction 3 to 4), or by Marker Post IDs which are the small black, red and white posts every 100 metres along the side of all motorways.  On non-motorways and all projects, this should always be accompanied by drawings supplied by the client outlining the extent of the scheme of work by an outline on a plan.

  • Then, the contractor should understand which parts of the road are included in the requested inspection data.  Terms like ‘verges only’, ‘central reservation only’, ‘inner ring of the roundabout’ etc are common terms when understanding these needs.

• Traffic Management – all work on the highways network always requires traffic management, and in most cases, this is provided by sub-contractors.  The key here is to understand who is going to organise and fund this resource.  It is often the contractor’s (or the sub-contractor’s) responsibility, but sometimes maybe the Area is already doing some other work on the carriageway and so they already have TM (Traffic Management) in place which the contractor can also utilise.

• Hours of Work – these describe the start and end times of the permit to work on the road and caries across different areas.  Area 5 (the M25 around London) has the shortest road space times because of the sheer volume of traffic on that road.  An example of the hours might be 8pm to 6am, but this does not mean that the contractor will be able to start work at 8pm and go straight through to 6am.  This means that the TM crews will start putting their management precautions (signs and cones etc) out from 8pm, and they may have to be all off the road by 6am, so the result to the CCTV contractor is that they are only likely to actually get on site from 9:30pm until 4:30am, because their access to the site is controlled and monitored by the TM.

• Waste Disposal and Water – where jetting and suction machines are used to clean pipes (usually this is the case), there may be requirements like; ‘The jetting machine is not allowed to leave site to fill up with water’.  So, the contractor must consider how they are going to keep the machine working for the whole shift, maybe with bowsers on site.  Additionally, it is common for highways drains to have significant quantities of silt inside, and large combination jetting machine can comfortably suck up 10 tonnes of silt in a shift.  This silt must be disposed of safely to a registered facility and this of course costs money, and the truck operators need reasonable time in their shift to complete this task before returning home so that the truck is ready for the next shift.

• Welfare – the operators on site always need welfare facilities because there are no toilets dotted along the sides of the motorway, and they should always be provided with a safe-haven for breaks etc.

• Deliverables – the standard deliverables required for GDMS are not particularly complicated although they do require a high level of validation and post-processing, but it is most often the case that the Area requires a significant amount of additional data on top of the standard deliverables, usually because they are using the data collected for design and modelling.  The contractor should understand exactly what these requirements are.  Not all Areas have the same needs here.

• Delivery Date – when does the client expect to receive their data?  GDMS data processing is tough work and takes a long time after all works have completed on site, and the client’s needs here may dictate how many vans and/or trucks are needed on site at any one time to complete the works.

• Training – it is usual for Areas to specify levels of training that is required by operators on site.  These will follow national standards and codes of practice and are likely to be higher than those required for non-GDMS inspections.

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