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What lies beneath

When it comes to utility mapping picking the right company with the correct tools for the job is crucial. On site investigation company Zetica offers some guidance.

Utility surveys are used to build accurate asset registers and minimise the risk of damaging assets during construction. More importantly they reduce the risk of personal injury or environmental incident caused by striking a utility such as a high pressure gas main, sewer, fuel pipeline or electricity cable.

In the UK, the health and safety requirement to locate utilities prior to any intrusive work is driven by regulations such as CDM (Construction (Design and Management) Regulations 2007) and general health and safety legislation.

Further guidance is provided from the UK’s Health & Safety Executive (HSE), principally through their HSG47 publication Avoiding danger from underground services.

Positioning

Confirmation of the position and depth of existing utilities allows for the planned diversion of essential services and ensures that excavations, construction or site investigation exploratory points can be safely positioned to avoid damaging existing utilities.

A plethora of above- and below-ground site mapping tools exist to address the issue. Advances in survey technologies designed to make life easier, have unfortunately increased confusion.

For example, a discussion with a specialised topography survey company, may require an appreciation of the resolution/accuracy trade-off between 3D laser scanning and traditional point and shoot methods.

A specialised geophysical survey company deploying the latest radio frequency location (RFL) or multichannel ground penetrating radar (GPR) systems will highlight the improvements these systems may bring in terms of productivity and resolution but they may not mention fundamental intrinsic limitations of detection that relate to target size, depth of burial and in-ground materials.

Dazzle factor

Buyers should be wary of the dazzle factor and the predictable tendency to ‘cut your coat to suit the cloth’.

The detection and tracing of utilities presents a challenge for geophysics due to the range of materials and target sizes that can be encountered. For this reason it is common to approach utilities detection with a broad suite of both passive and active techniques. Those most commonly used include continuous-wave electromagnetic profiling, passive electromagnetic methods such as the cable avoidance tool (CAT), magnetic profiling and ground penetrating radar (GPR).

A benefit of an integrated approach is that the survey company can also report on other in-ground hazards such as buried tanks, piles, infilled ground, and depth to bedrock.

There are two standout guideline publications which anybody procuring buried utilities surveys is advised to take a look at in order to understand which might best serve their purpose.

The Survey Association’s (TSA’s) guidance refers to the location, positioning and identification of buried pipes and cables and offers six levels of survey that may be required depending on the accuracy and detail required.

US guidance

The American Society of Civil Engineers (ASCE) guidance refers to the collection and depiction of subsurface utility information, and more broadly references geophysical methods. It puts the onus on the survey engineer to, ‘Furnish the desired utility quality level to the owner in accordance with the standard of care’.

Unfortunately, not everything can be taken at face value even in such guidance. TSA and the ASCE suggest that for depths up to 2m in ‘good ground conditions’, the diameter of detectability of a utility reduces by 1mm for each 10mm of depth. So a 200mm pipe can in theory be detected at 2m depth and a 25mm pipe at 0.25m depth.

However, the results of work carried out on site by investigation company Zetica’s test site confirm that these estimates are best case scenarios only applicable in homogenous dry soils.

Burial setting

The detectability of utilities is also very dependent on burial setting and the material type of the utilities being mapped. For example, an inaccessible concrete waste pipe of 30cm diameter, buried at greater than 2m depth in clayey soils could be undetectable with any geophysical method.

Utilities running beneath reinforced concrete slabs may only be detectable within 0.5m of the surface with GPR as the presence of reinforcing will preclude the use of electromagnetic methods.

Exaggerated claims that ‘95% of all buried utilities will be mapped’ should be weighed against the limitations of geophysical methods in general.

Best value from a utilities survey can be achieved by considering the following:

  • Ensure that the technology used is fit for purpose and can achieve the required detection and accuracy specification.
  • Ensure that the survey design offers a cost effective approach utilising BATNEEC principles and that the survey company has considered a broad range of survey options.
  • Understand the limitations of the recommended survey options in the light of actual site specific conditions.
  • Verify that the survey company is experienced in both the design and implementation of a utility survey.
  • To maximise the value of time on site, check if the survey company can report on other in-ground hazards such as buried tanks, voids, infilled ground, contamination pathways, depth to bedrock etc.
  • If a utilities survey is carried out at the same time as a topography survey the completeness of salient features mapped by either method could be significantly enhanced.
  • Check that the deliverables (scaled maps) are compatible with your design systems.

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