What is PAS128: Underground Utility Detection, Verification and Location

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    PAS128:2014 is a publicly available specification issued by the British Standards Institution to provide guidance for: Underground Utility Detection, Verification and Location.

    Prior to PAS128, there was no standard in the UK for the way in which a utility survey should be performed, as a result, one companies ‘full utility survey’ could be significantly different from another’s. After a lengthy consultation within the industry (including representatives from service providers and customers), PAS128 was written and issued to overcome this.

    In brief, PAS128 allows a buyer to specify the type of survey they would like and the methodology by which it should be performed. It also provides some description of the deliverables which should be produced.

    PAS128:2014 Survey Types and Quality Levels

    There are four utility survey types specified within PAS128:2014 which are each assigned a letter from D to A, with A being the highest quality:

    D – Desktop utility records search

    A desktop search is undertaken by identifying known utility owners within the specified survey area and contacting them to request a copy of their asset information and then collating that information together onto one drawing which can be provided as a deliverable.

    The accuracy of records held by different asset owners is highly variable, therefor, results from a desktop search should be regarded as indicative only and as such they are ascribed the lowest quality level: D (from records).

    C – Site reconnaissance

    In the site reconnaissance phase, a surveyor can attend site and identify physical features on the surface which support the information provided by the desktop survey, some examples include: the presence of manholes, overhead cables, and lampposts etc.

    The existence of these features will provide some confidence that a utility is present and to some extent identify its location. On site reconnaissance will therefor increase the quality level of the survey and as such, services identified in this way can have their quality level improved from D (from records) to C (identified on site).

    B – Detection

    PAS128 specifies two geophysical techniques as a minimum to be used when detecting utilities: EML (Electro Magnetic Location) and GPR (Ground Penetrating Radar):

    • EML is performed on site by detecting electromagnetic signals emitted by utilities either due to the passage of electrical currents (in the case of live electricity cables), or induced onto the service by the surveyor and then marking the locations of these emissions on the ground.
    • GPR works by transmitting a signal into the ground which is reflected by changes in the below ground environment and receiving and interpreting those reflections, GPR results can be interpreted on site or saved for post processing in the office. Post processed results are usually assumed to have a higher quality than on site detection, although this is not always the case.

    The results obtained using geophysical techniques should be recorded and compiled with the data from the records search and the on-site reconnaissance. Results obtained using geophysical techniques should be labelled as quality level: B.

    Within PAS128, a quality level of B is further broken down into the following levels of increasing confidence:

    B4

    • Segments of utility which are suspected to exist but have not been detected by a geophysical technique and therefor ‘assumed’ to be in the position identified. For example connections in between detection’s or services marked on records which were not detected on site.

    B3

    • Utility identified by one of the geophysical methods applied to the survey only, horizontal location is shown but no accurate vertical position could be recovered. An example could be a trench scar detected by GPR, but the actual utility is not visible in the data (perhaps too deep).

    B3P

    • As above but identified during post processing rather than on-site.

    B2

    • The utility is identified by only one of the geophysical methods applied to the survey, a horizontal and vertical location is identified. For example a plastic pipe located by GPR but not the EML, or a small diameter cable located using EML, that cannot be detected using GPR.

    B2P

    • As above but identified during post processing rather than on-site (applies to GPR only).

    B1

    • The utilities horizontal and vertical location is identified by multiple geophysical techniques. For example an electricity cable identified using GPR and EML.

    B1P

    • As above but identified during post processing phase than on site (GPR only).

    A – Verification

    Verification refers to the process of exposing a utility and subsequently measuring its accurate location, this can be achieved by lifting manhole covers or by excavating the utility through a trial hole. Any service which has been verified in this way can be afforded the highest level of confidence, quality level: A.

    It is not uncommon to receive a request for a ‘B1P’ survey: it should be noted that quality levels are an output from the survey and cannot be specified in advance. A ‘B1P’ survey is therefor an aspiration, rather than a specification (i.e. we will try to detect all the utilities using both techniques, but of course it cannot be guaranteed and is unlikely to be the case). In order to specify a utility survey properly, please specify based on the Detection Methods as described below.

    PAS128:2014 Detection Methods

    PAS128 assumes the use of both GPR and EML as standard in the detection phase of all utility surveys (although at present they are not always both used by many companies: in particular GPR is not always included, and in some cases it is genuinely unnecessary to use both to achieve the survey objective).

    PAS128 incorporates a series of increasingly comprehensive methodologies from M1 to M4P which can be specified when requesting a survey:

    M1

    • EML at orthogonal search transects with not greater than 10m intervals and once a utility is detected, it should be traced it with transects of not greater than 5m.
    • GPR survey to be performed in an orthogonal grid with not greater than 5m between scans.

    M1P

    • As M1 with GPR data to be post processed.

    M2

    • EML at orthogonal search transects with not greater than 5m intervals and once a utility is detected, it should be traced with transects of not greater than 2m.
    • GPR survey to be performed in an orthogonal grid with not greater than 2m between scans.

    M2P

    • As M2 with GPR data to be post processed.

    M3

    • EML at orthogonal search transects with not greater than 2m intervals and once a utility is detected, it should be traced with transects of not greater than 1m.
    • GPR survey to be performed in an orthogonal grid with not greater than 1m between scans.

    M3P

    • As M3 with GPR data to be post processed.

    M4

    • EML at orthogonal search transects with not greater than 2m intervals and once a utility is detected, it should be traced with transects of not greater than 0.5m.
    • GPR survey to be performed in an orthogonal grid with not greater than 0.5m between scans.

    M4P

    • As M4 with GPR data to be post processed.

    This page provides only a brief overview of PAS128:2014, for more details, a copy of PAS128:2014 is available from BSI for £84 at the following link: https://shop.bsigroup.com/ProductDetail/?pid=000000000030267400

    To go back to Utility Surveys click here.

    Our Most Common Questions

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    What can a Ground Penetrating Radar survey detect?

    With GPR, you can detect a wide range of objects below ground level, including both metallic and non-metallic objects such as plastic pipework. GPR will also identify and map any voids below the surface, such as air pockets or mine shafts, as well as any other irregularities including concrete and previously excavated or back-filled areas.

    Will GPR compromise safety on my site?

    GPR equipment emits an electromagnetic pulse into the ground and records the reflected signals from subsurface structures and voids. It is entirely non-destructive and will not break the ground’s surface or affect any objects below. What’s more, it doesn’t emit any harmful levels of radiation, nor are there any other by-products created throughout the process. This means it’s entirely safe to use by its operators, and on sites of any type, including those open to the public.

    Is a GPR survey 100% accurate?

    While GPR is one of the most effective methods of non-destructive testing available, it can never be 100% accurate. One factor that can adversely affect the accuracy levels include the type of soil being surveyed. Clay soils and soils that contain high levels of salt or minerals can obstruct the GPR reading. Another factor is the experience of the equipment’s operator: interpreting the data collected can be complex, which is why it’s beneficial to commission surveys from an expert firm.

    Is GPR equipment difficult to use?

    The equipment itself is not difficult to use, but the interpretation of the data recorded tends to be complicated. The results of a GPR survey aren’t automatically translated into an easy-to-understand picture of what lies below the surface; instead, it’s a series of lines and waves and it can take both training and years of practice to master the art of correctly reading the output. Often, it is the experience of the equipment’s operator that plays the most significant role in the accuracy of the results GPR can achieve.

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