Revision to PAS 128 standard for locating underground utilities available for public comment

A revision to the PAS 128 2014 standard for locating underground utilities is available for public comment through March 16.

PAS 128 defines a hierarchy of detection methods for detecting underground utilities in terms and the maximum quality levels that can be achieved using each detection method. In a PAS 128 survey, at a minimum two techniques (EML and GPR) are deployed. In addition to EML and GPR, other detection methods may be applied to the survey area. PAS 128 sets out the accuracy to which the data are captured, the quality expected of these data and a means by which to assess and indicate the confidence that can be placed in the data.  To improve accuracy and how data are exchanged and integrated, PAS 128 encourages capturing the absolute geospatial location of utilities georeferenced to 3D using a national coordinate grid system and datum such as the Ordnance Survey’s National Grid (OSGB36) and Ordnance Datum Newlyn (ODN) coordinates and the Irish Grid (IG) as used by Ordnance Survey Northern Ireland.

The quality level is a classification applied to each segment of utility surveyed based on survey method (D,C,B or A), location accuracy achieved (A, B1, B2, B3), and whether post-processing was performed (e.g. B2P).

A number of issues identified in the 2014 version of the standard are now being addressed in a revision. An academic study from the University of Birmingham investigated the effectiveness of the British PAS128 2014 standard for underground utilities. Test pits revealed that achieving a high vertical accuracy with remote detection techniques was problematic.  Furthermore, it was found that while using multiple technologies for detecting underground infrastructure significantly increased the confidence in the detected location of underground utilities it did not increase the accuracy. Other issues that have been identified and taken into account in the latest revision include; the current standard is addressed to practitioners but neglects owners/clients; the current standard assumed a 2D world, 3D and BIM need to be taken into account; whether to include post-processing of GPR scans needs to be revisited; and finally there were inconsistencies between PAS256 and PAS128.

As a reminder PAS 128 Bx quality levels were defined as

Quality level	Location accuracy Horizontal	Location accuracy Vertical	Comments
B4	Undefined	Undefined	Undetected
B3	+/- 500mm	Not determined	One geophysical technique
B2	+/- 250mm	+/- 40% of detected depth	One geophysical technique
B1	+/- 150mm	+/- 15% of detected depth	Two geophysical techniques

 

In revised standard quality levels D, C, and A do not differ materially from the 2014 version, but now all Bx levels require the application of at least two detection techniques.

 

Survey type	Quality level	Location accuracy Horizontal	Location accuracy Vertical	Comments
Desktop utility records search	D	Undefined	Undefined	As recorded in as-built record
Site reconnaissance	C	Undefined	Undefined	A segment of utility whose location is demonstrated by visual reference to street furniture, topographical features or evidence of previous street works
Detection	B4	Undefined	Undefined	a segment of a utility is shown as either a QL-D or QL-C on as-builts but was not detected using either GPR or EML
Detection	B3	± 500mm	Undefined	utility is only detected horizontally with one technique
Detection	B2	± 250mm	± 40% of detected depth	one or both techniques (EML and GPR) have detected the utility but  in different locations within the tolerance stated, or the utility was detected with just EML
Detection	B1	± 150mm	± 15% of detected depth	the utility has been detected using both GPR and EML in different locations within the tolerance stated.
Verification	A	±50 mm	±25 mm	Horizontal and vertical location of the top and/or bottom of the utility.

 

The 2014 version included guidelines targeted on the locate practitioner on how to use the standard.  In the revised version there are now Annexes B and C aimed specifically at the client/owner explaining how they should use this document.

By the way the new ASCE Standard Guideline for Recording and Exchanging Utility Infrastructure Data (also referred to as the utility as-built standard) will be available for public comment shortly.