Striking a buried service is one of the most serious risks on any site. It can cause injury, disconnect homes and businesses, and bring a project to a halt. Yet "utility survey" can mean anything from a quick look at the records to a full geophysical survey with trial holes. PAS 128 exists to remove that uncertainty.

What is PAS 128?

PAS 128 is the British Standards Institution's specification for the detection, verification and location of underground utilities. First published in 2014 and revised in 2022, it gives the industry a common language for survey quality. Every service shown on a PAS 128 drawing is labelled with a Quality Level (QL), so anyone reading it knows exactly how that service was found and how much confidence they can place in its position.

The four Quality Levels

QL-D: Desktop utility records search

Records are gathered from the statutory undertakers (water, gas, electricity, telecoms and others) and compiled onto one plan. There's no site work. QL-D is useful for early feasibility and design, but records are often schematic, out of date or incomplete, and should never be relied on for digging.

QL-C: Site reconnaissance

The records are checked against visible features on site, such as manhole covers, valve boxes, cabinets and street furniture. This helps spot where the records are obviously wrong or missing something, but nothing below ground is actually detected.

QL-B: Detection

This is where buried services are actively located using geophysical techniques, usually electromagnetic locators (EML) and ground penetrating radar (GPR). QL-B is split into sub-levels that show how confidently each service was located:

• QL-B4: a service is suspected to exist but couldn't be detected, so it is shown as an assumed route
• QL-B3: the service was detected, but only its horizontal position could be established
• QL-B2: both horizontal position and depth were established using one technique
• QL-B1: horizontal position and depth were established with the highest confidence, using more than one technique

A "P" suffix, such as QL-B2P, shows that the GPR data was post-processed after the survey.

QL-A: Verification

Services are physically exposed, usually by vacuum excavation or hand-dug trial holes, and their position and depth are measured directly. This gives the highest confidence of all and is typically used at critical locations such as crossing points and pinch points.

Which level do you need?

Each level builds on the one before, and the right choice depends on your project stage and risk:

• Feasibility and early design: QL-D, and often QL-C
• Detailed design and planning of excavation: QL-B, which is where most projects need to be before breaking ground
• Critical locations, congested areas or where design depends on exact positions: QL-A at targeted points

A common mistake is commissioning a lower level than the job actually needs and discovering the gaps on site, when changes are expensive. Another is assuming the whole drawing has the same confidence. Always check the Quality Level of each service, not just the title of the survey.

What PAS 128 doesn't do

A PAS 128 survey greatly reduces risk, but it can't guarantee every service has been found. Plastic pipes, deep services and congested or poor ground conditions can all be difficult to detect. A survey should always be used alongside safe digging practices in line with HSE guidance HSG47, with services treated as live until proven otherwise.

PAS 128 utility surveys in London and across the UK

ACUO Structural Insights carries out PAS 128 utility mapping, borehole clearance and non-PAS 128 utility scans for contractors, designers and ground investigation teams across London, Kent, Essex and the South East.

For full details on what the service covers, visit our PAS 128 utility survey service page. Where concrete scanning is also required on the same project, our GPR concrete scanning service can be mobilised alongside utility mapping.

Planning excavation or breaking ground? Call 07340 549892 or email [email protected].