PAS 128: Specification for underground utility detection, verification and location. What it requires and how to use it. Gerwyn Leigh BSc, MSc, CSci, FGS RSK Principal Geophysical Engineer

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PAS 128: Specification for underground utility
detection, verification and location
What it requires and how to use it
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Gerwyn Leigh BSc, MSc, CSci, FGS
Principal Geophysical Engineer

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PAS 128 launched on 30 June 2014

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What is a PAS?
 A publicly available specification (PAS) is a sponsored fast-track standard.
 It is written by a drafting panel and supported by a steering committee.
 After two years, the PAS is reviewed and a decision is made as to whether it
should be taken forward to become a formal British Standard.
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Aims of this PAS
PAS 128 aims to provide
 Clarity on the service offered and the
methods employed
 Consistency in the approach to data
capture
 Classification of the results and the
confidence that can be associated
with them
 Standardisation of the format of the
deliverables
 Accountability for the work
undertaken.
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Specification of minimum requirements
versus best practice guidance
Specification
The normative text sets out the minimum that must be done.
Best practice
The informative text provides guidance and also points to best practice.
This enables a range of responses to any request from a client. The client needs
to decide what specification they want and what contractual conditions they want
to put in place.
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Survey types
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ABCD
Type D
Desktop
utility
records
search
Type C
Site
reconnaissance
Type B
Detection
survey
Type A
Verification

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Survey output: Quality levels
Each segment of utility in the deliverables will be identified with a ‘quality level’.
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QL-D – Desk study
 Identify the known utility owners
within the specified survey area.
 Request asset information from the
identified utility owners.
 Collate all the data on the utility
owners and their assets.
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QL-C – Site reconnaissance
On-site checks shall document the
 Presence, type and markings of utility-related
surface features
 Measurements between known surface
features on the ground compared with those
depicted on the plan
 Differences between the map/drawing or
digital features supplied and those extant on
the ground.
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QL-B – Detection surveys
 Use geophysical techniques to detect and identify the utilities
within the survey area.
 The quality level (accuracy) achieved shall be documented as
QL-B1, QL-B2, QLB3 or QL-B4.
 If post-processing has been used to improve the confidence
of the data, the quality level shall be suffixed with the letter ‘P’.
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Detection accuracy
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The positional accuracies for
QL-B1, QL-B1P, etc., and how
these vary with depth

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QL-A – Exposure and verification
Data shall be obtained through visual
inspection of the utility
 At access points such as in a manhole
or inspection chamber, and/or
 By its excavation and exposure.
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Detection – Levels of effort and return
 PAS 128 sets out guidance on the minimum effort required for different
situations.
 For example, an urban area likely to be congested with multiple surveys would
be expected to require a more dense detection grid in order to detect utility
segments accurately.
 It also makes clear that additional confidence and accuracy can be obtained by
post-processing the data:
 This is done by taking the recorded geophysical data off-site, processing it in
3D volumes to enhance and isolate signals from buried utilities, and
transferring the results of that analysis onto detailed CAD drawings.
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Detection – Levels of effort and return
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Post-processing of data – When and why?
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Some subsurface environments are too complex
to disentangle using real-time signals on-site
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Recording data and post-processing provide
a more-accurate interpretation
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Survey grid baseline
DirectionofGPRsurveylines

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Post-processed data can deliver detail
unavailable from site mark outs
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Comparing apples with apples at the tender
stage
It is required for the practitioner to submit
 The survey type(s) to be deployed, including the extent for each type
 For survey type B, the detection methods to be deployed, as specified in
Table 2, including the estimated extent for each method
 Comment on these survey type(s) and, for survey type B, the detection
methods, with regard to satisfying the client’s requirements
 Comments on the expected achievable quality level
 The names and experience of the project team
 How the survey area is to be managed to maximise the area available for
survey and ensure the safe execution of the works.
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Transparency of work done – Deliverables
A detailed report, including
 Detailed survey outcomes, including how successful each detection
methodology proved to be and a plan showing any areas where these detection
methodologies were unsuccessful
 The utility segments identified with the quality level achieved
 Recommendations for any further survey work required to meet the client’s
requirements
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Transparency of work done – Deliverables
Retention of survey data/records
 All recorded and processed data, site notes, metadata and intermediate-stage
processing files shall be retained and shall be available to the client on request
 It is recommended as best practice for all data to be recorded as evidence of
detections and of work undertaken
 This is required where post-processing has been specified
 This is optional for other detection surveys – unless the client chooses to
make it a requirement.
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Why were these missed?
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Accountability of results
In the deliverables, the practitioner must state
 The quality level (level of accuracy) achieved for each segment of utility
identified and
 Which detection techniques were used in all surveyed areas of the site,
including areas where no utilities have been detected.
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Survey quality/accuracy and accountability

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Accountability of results
The client gets further protection if they specify that
 All geophysical data are to be recorded and retained
 Evidence is to be provided of the detections achieved and areas where
detections were impossible as part of the deliverables.
Benefits
 Evidence is retained of the work done.
 Data can be reviewed at a later date in the light of subsequent findings or as
part of a routine audit by the practitioner, the client or a third party.
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Evidence of detection work done
Survey company A
claims that GPR
did not work
GPR data acquired by
survey company B

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Protection for the client
The practitioner needs to set out
 What they did and where
 What the outcome was, i.e.,
 What accuracy and confidence were achieved
 What areas of uncertainty and risk remain
 What could additionally be done to reduce risk.
 The practitioner should then stand by its product.
 If PAS 128 is used properly by the client, the practitioner can no longer hide
behind a ‘black box’ of complex geophysical equipment to explain away why
something was missed or inaccurate in its survey output.
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Take up
If adopted by the industry, PAS 128
could enable
 Clear definition for a minimum
standard of utility verification and
location
 More control for the client
 More comeback for the client when
issues arise
 Fewer incidents related to service
strikes
 Fewer delays caused by unknown
buried services.
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