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The benefits of a PAS128 survey | What are you paying for?

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To give it the full name, the current standard is called PAS 128:2022 Specification for underground utility detection, verification and location.

Prior to 2014 there was no fully recognised standard for delivering a utility survey. Even though The Survey Association published, and continues to publish, guide notes, specifications for the type and quality of a survey were rarely issued by clients and little was understood about the different options available. This resulted in a wide range of survey standards with bad actors being able to offer their services at lower costs than bonafide organisations, with little information for a client to distinguish between the quality of the different offers.

Industry leaders comprising clients, contractors and leading survey providers combined to publish a recognised standard for utility surveying in the UK. This became PAS 128:2014 upon publication, revised and updated in 2022.

The standard provides a framework for clients to specify their requirements and for survey practitioners to deliver a standardised product. This ensures good surveying practice is being followed and survey cost can be assessed on a like for like basis.

To summarise, the purposes of PAS 128 can be defined as:

list of bullet points of the benefits of a pas 128 survey

In addition to the standardisation of survey data, PAS provides a framework to cover the following planning elements of a survey:

  • Ensure that a survey practitioner can demonstrate competence through training and nationally recognised qualifications
  • Communication between surveyor and client to ensure the requirements are fully understood on both sides. This can be via in person meetings or other means
  • Health and safety requirements are met with comprehensive method statements and risk assessments
  • The underlying geology of a site is considered when selecting geophysical methods
  • The required base mapping for overlaying detected services onto has been considered
  • The relevant permits and licenses have been obtained
  • Any required pedestrian or traffic management has been agreed and put in place

What are the different types of PAS 128 survey?

There are four different types of PAS survey:

infographic showing the different levels of PAS128 survey available

Type D – Desktop Study

A survey Type D is a desktop search where available utility records are interrogated. These may be publicly available STATS records or privately held existing services drawings for an establishment.

The final deliverable would be a report containing any relevant records along with details of the process followed, statutory undertakers contacted and their responses. There is also an option to collate that information in a CAD format and issue a STATS site plan.

A Type D surveys a prerequisite prior to undertaking any further survey work.

Type C – Site Reconnaissance

A Type C survey takes the information collated from the desktop search and a site reconnaissance is undertaken to establish the validity of the services records. Existing surface features such as:

  • Trench scars
  • Chamber covers
  • Lamp columns
  • Traffic signals
  • Marker posts
  • Valves
  • Kiosks

These features are used to determine whether a service is as detailed on the records and their approximate location. It is important to note that no services locating is undertaken at this level as it is purely a visual inspection.

The deliverables from a Type C survey would typically include marked up and annotated drawings, a written report and site photographs.

Type B – Detection Survey

A PAS 128 Type B survey is the element of the standard that most people would recognise as the techniques used in a utility survey, often referred to as a GPR survey. As a minimum the equipment used for a PAS128 survey should include electromagnetic location (EM) and GPR.

This is where a standardised approach to surveying is utilised to locate the buried services within the work site. A Type D survey would have already been undertaken and a Type C survey is incorporated as part of the process.

As a minimum a Type B detection survey should include the following survey elements:

  • Type D records search
  • Type C reconnaissance which forms part of the wider survey
  • Lift all chambers and trace available pipes, ducts and cables with EM location, utilising a signal generator and accessories such as cable clamps, sondes and tracer wires
  • Create chamber record sheets to include sketches and photos
  • Connect the signal generator to  above ground apparatus to locate further services not located via chambers
  • Carry out passive and active EM sweeps of the work site
  • Carry out a GPR survey of the work site
  • Undertake a topographical survey to record all of the marked information

Further survey techniques which should be considered depending on their practical use in a given environment:

The deliverables for a detection survey would include:

  • A survey report
  • CAD & PDF copies of the site drawings
  • Quality levels assigned to each service
  • Identification of service type such as whether a gas low pressure, medium pressure or high pressure etc.
  • Copies of the record drawings
  • Copies of the chamber records

The different levels of a Type B detection survey

PAS provides a range of different survey levels to suit various types of site, these would have an increased cost as the complexity rises:

image showing how cost rises with complexity in a pas 128 survey

Data collection density of a PAS128 type B survey

As the survey level increases from M1 to M4, the density of the grid transects for data collection increases.

image showing the grid transects spacings of a pas 128 survey

It is easy to see that an M1 survey level would not be suitable for a city centre location with data collected at 5m centres. Likewise the M4 level would be considered overkill for a green field site where the desktop study indicates little or no buried infrastructure.

What is a multi antenna GPR array?

A multi antenna array is a GPR system that has multiple antennas with dual polarization. This means that the data can be collected in both longitudinal and transverse directions simultaneously. The close alignment of the antennas means there are no gaps in the data. The 0.5m grid transects of a regular GPR system becomes non-existent with an array. Providing significantly more data to process. It should be noted that all GPR equipment can be subject to geophysical limitations.

image indicating the benefits on a multi antenna array over other GPR methods

What are the benefits of post processing of GPR data?

Each of the types of survey can have the addition of post processing of the GPR data. This would make them M2P, M3P etc.

Traditionally a GPR system would be employed where the operator identifies features as they are surveying the site. These features are marked on the ground with no further processing of the data undertaken.

In more recent years post processing of the GPR data has become commonplace and has many benefits over mark on site surveys:

  • No longer relying on the operator to make decisions on site
  • Specialised geophysics staff can interrogate the data
  • Additional filters can be applied to the data that are not available or too slow to be used on site
  • Permanent record of the data and identified features for quality control purposes

The post processing of data makes an impact on the cost of a survey, but should be a prerequisite for certain locations and environments. In the 2022 version of PAS, post processing of GPR data is required for an M4 level of survey.

Detected services quality levels in a PAS128 survey

One of the most important elements of a PAS 128 survey from the perspective of the end user of the information is the quality level assigned to each service. This provides a positional accuracy with tolerances based on how the survey was identified. Often clients specify that a QL-B1P is a requirement of a survey. This is in fact not a matter of specification it is a matter of tolerance of the located apparatus. A client cannot specify required tolerances because geophysical methods cannot be guaranteed to deliver them.

The different survey levels are:

Positional Accuracy
QualityHorizontal AccuracyVertical AccuracyCriteria
QL-DUndefinedUndefinedService taken from records for during a Type D survey
QL-B4UndefinedUndefinedA service which has been assumed. This may be from records during a Type B survey
QL-B3+-500mmUndefinedHorizontal location only. Detected by one technique
QL-B2+-250mm or 40% of depth whichever is greater+- 40% of detected depthHorizontal and vertical location detected by one technique
QL-B1+-150mm or 15% of depth whichever is the greater+- 15% of detected depthHorizontal and vertical location detected by multiple techniques
The addition of a suffix of P means the feature was identified using GPR post processing. For example QL-B2P. This cannot be applied to the QL-D or QL-B4 quality levels

Type A – Verification

A Type A verification service is where a service is visibly exposed and the relevant information relating the service is recorded. This type of verification can be used where the location and type of a service is critical and non intrusive methods cannot provide the necessary accuracy.

This verification of a service could be through the access to chambers and pits as well as the excavation of trial holes. Any service identified in this manner can have the QL-A quality designation, which means the service is verified.

The tolerances of a QL-A designated service are +-50mm horizontal and +-50mm vertical.

Where excavations are taking place this should be performed by trained and competent personnel and thought should be given to the use of techniques such as vacuum excavation.

The benefits of a PAS128 survey

Put simply, specifying surveys to the PAS128 standard ensures that a number of key factors are included in the identification of underground services:

  • The survey is carried out to the industry recognised standard
  • Costs between practitioners can be compared on a like for like basis
  • Surveyors are trained and competent for the task
  • There is a traceability of the process and information collected

All utility surveys in the UK should be requested to this standard. The processes required contribute to safer working practices when dealing with design and excavation around live services. Which in turn contributes to a much safer industry. It then becomes the job of surveyors to ensure they are meeting the requirements.

Andrew Botterill