Copyright of RSK

PAS 128: Specification for underground utility

detection, verification and location

What it requires and how to use it

14 November 2014 1

Gerwyn Leigh BSc, MSc, CSci, FGS

Principal Geophysical Engineer

Copyright of RSK

PAS 128 launched on 30 June 2014

Copyright of RSK

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.

14 November 2014 3

Copyright of RSK

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.

14 November 2014 4

Copyright of RSK

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.

14 November 2014 5

Copyright of RSK

Survey types

14 November 2014 6

ABCD

Type D

Desktop

utility

records

search

Type C

Site

reconnaissance

Type B

Detection

survey

Type A

Verification

Copyright of RSK

Survey output: Quality levels

Each segment of utility in the deliverables will be identified with a ‘quality level’.

14 November 2014 7

Copyright of RSK

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.

14 November 2014 8

Copyright of RSK

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.

14 November 2014 9

Copyright of RSK

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’.

14 November 2014 10

Copyright of RSK

Detection accuracy

14 November 2014 11

The positional accuracies for

QL-B1, QL-B1P, etc., and how

these vary with depth

Copyright of RSK

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.

14 November 2014 12

Copyright of RSK

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.

14 November 2014 13

Copyright of RSK

Detection – Levels of effort and return

14 November 2014 14

Copyright of RSK

Post-processing of data – When and why?

14 November 2014 15

Copyright of RSK

Some subsurface environments are too complex

to disentangle using real-time signals on-site

14 November 2014 16

Copyright of RSK

Recording data and post-processing provide

a more-accurate interpretation

14 November 2014 17

Survey grid baseline

Dire

ctio

n o

f GP

R s

urv

ey lin

es

Copyright of RSK

Post-processed data can deliver detail

unavailable from site mark outs

14 November 2014 18

Copyright of RSK

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.

14 November 2014 19

Copyright of RSK

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

14 November 2014 20

Copyright of RSK

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.

14 November 2014 21

Copyright of RSK 22

Why were these missed?

14 November 2014 22

Copyright of RSK

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.

14 November 2014 23

Copyright of RSK 14 November 2014 24

Survey quality/accuracy and accountability

Copyright of RSK

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.

14 November 2014 25

Copyright of RSK 14 November 2014 26

Evidence of detection work done

Survey company A

claims that GPR

did not work

GPR data acquired by

survey company B

Copyright of RSK

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.

14 November 2014 27

Copyright of RSK

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.

14 November 2014 28

Copyright of RSK