Heron Tower- The First Steps

Non-intrusive utility tracing assists in the design of a future London landmark.

Heron Tower, to be built in the heart of the City of London, standing at 41 storeys and 594 feet high, will alter the already impressive London Skyline. In common with even the smallest project an accurate survey was the critical starting point for the design and construction.

In particular, with this project it was recognised that the buried services and utilities infrastructure around the Heron Tower site would be highly complex and require specialist survey expertise. The construction project managers, Mace Ltd, brought in market-leaders Met Surveys Ltd to undertake a utility tracing survey to accurately pinpoint their locations.
Artist's Impression of Heron Tower
Leeds-based Met Surveys Ltd was commissioned to undertake the survey of the footways and carriageways using non-intrusive techniques. The survey covered an area of approximately 0.7ha and was carried out over a three week period. The site is located near Liverpool Street Station which meant that there was a heavy volume of pedestrian and vehicular traffic. The majority of the work was therefore undertaken at night in conjunction with traffic management procedures to minimise disruption to the traffic flow.

Prior to commencing on site, record plans were obtained from Statutory Undertakers who might have plant in the area. This stage of a survey is vital as it gives an indication of the minimum amount of apparatus that should be present. The document on avoiding danger from underground services (HSG47- revised 2000) published by the Health and Safety Executive (HSE) requires that record plans should be used as an initial assessment but the use these alone is not sufficient as they are rarely complete and their accuracy can be questionable.

Section 60 of HSG47 sates that 'the position of any services in or near the proposed work area should be pinpointed as accurately as possible by means of a locating device'. The guidelines detail what sort of locating devices should be used and briefly summarises their strengths and limitations.

Met Surveys recommended carrying out a survey using two techniques, radio frequency location and ground penetrating radar, both of which are also recommended by HSG47.
The primary investigation technique used was radio frequency location (RFL). This is a sophisticated type of 'CAT' scan that can locate metallic apparatus and allow the tracing of drainage routes. A ground penetrating radar (GPR) survey was also undertaken as this can obtain information on sub-surface variations and features and can potentially locate all types of utility apparatus (including non-metallic pipes, cables and ducts).
Utility Tunnel Surveyed
The operatives that Met Surveys use to lead utility tracing teams are taken from its Geophysics Division and are qualified to graduate or post-graduate level. As such they have a thorough background in a range of geophysical techniques which helps them better understand the applications and limitations of RFL and GPR. This is backed up by a culture of excellence, rigorous in-house training and a detailed quality control procedure. Using the expertise of Met Surveys meant that the client could be sure that they were fully compliant with Health and Safety regulations as well as obtaining information critical to their infrastructure design.

It is important to recognise that HSG47 requires that a non-intrusive survey using a locating device should be undertaken by someone who has received 'thorough training in its use and limitations'.

A standard component of a full utility tracing survey should be the investigation of all inspection covers within a site. This obviously gives a further indication of what utility apparatus is present but more importantly if apparatus can be accessed then it may be possible to attach a signal generator to it. If the apparatus is metallic or has a metallic tracing wire then it can be possible to trace it and potentially obtain depth information using RFL. Over 320 inspection covers were investigated at the Heron Tower site, each cover was given a unique number, photographed and a drawn record of the chamber contents was produced as standard.

The routes of all of the apparatus that were traced by RFL were marked out on site using spray paint or wax crayon. GPR data can be examined on site and features of interest marked out immediately however this does not involve the collection of data and identification of features is more difficult in complex sites or sites that are 'noisy' in terms of data quality. The more reliable approach in such instances involves collecting the data on a grid system and processing and interpreting it back at the office. In the majority of cases Met Surveys usually adopt the latter option as this increases the chances of identifying features with relatively weak responses and it was this detailed strategy that was adopted at the Heron Tower site.

The GPR survey grids and the features identified by RFL were all tied in to an existing topographic survey using a total station. At the request of the client a full topographic survey of an adjacent shopping arcade was also undertaken by the utility tracing teams. All of this information was collated, along with the interpretation of the GPR data and the schedules of the inspection chambers, into a digital CAD plan.

The utility tracing survey highlighted a large service tunnel present on site which contained numerous pipes and cables. The tunnel was located too deep to be detected by GPR but knowing the location of such a major feature was critical to the project management team at Mace Ltd. Met Surveys therefore used a topographic survey team accurately locate the tunnel relative to the above ground features. Operatives qualified to enter confined spaces were used for the survey and they utilised gas monitors and escape breathing apparatus in compliance with Health and Safety regulations.

This survey was one of the most exacting utility tracing jobs that Met Surveys have undertaken but the results will enable the design and subsequent construction work of Heron Tower to proceed with more confidence. Neil Harvey, Director, Met Surveys said: "We are proud to have contributed to what will be a major landmark in the City of London for years to come."

Nigel Barker is the Project Geophysicist at Met Surveys and specialises in utility tracing and the applications of geophysical surveys to brownfield site assessments. He is happy to discuss in more detail the range of non-intrusive survey techniques that are available and how they can benefit projects and meet Health and Safety requirements.