When work on London's Heron Tower is complete in 2010, the 202m- high building will dominate the London skyline. Before reaching for the skies, Project Manager Mace has first had to make sure it knows exactly what is below the ground around the site. B&E hears from Met surveys about how it used tracing techniques to help in the design of a future London landmark.

City Centre construction projects can provide a minefield of buried pipes and cables, which can potentially cause serious delays to a scheme before work has really got underway. The City of London’s planned tallest building, the Heron Tower, has provided its own logistical problems with mapping the utilities services around the site while keeping disruption in this hectic part of the capital down to a minimum.

Construction project manager Mace recognised this project could cause problems when it came to buried services and utilities infrastructure around the site. The company decided the Heron Tower’s location meant the site would be highly complex and require specialist survey expertise. The company brought in Met Surveys to carry out a utility tracing survey to accurately pinpoint their locations. 

Mace commissioned Met to survey 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 traffic and a lot of people travelling through the area. Met carried out most of the work at night, and also used traffic management procedures to reduce disruption to the traffic flow.

Before starting work, 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 Health and Safety Executive (HSE) requires that record plans should be used as an initial assessment. However, to use these alone is not sufficient as they are rarely complete and their accuracy can be questionable. The HSE recommends using a locating device to pinpoint services in or near the construction site.

What devices does the organisation recommend, and what are their strengths and weaknesses?
Surveying companies can use two techniques; radio frequency location and ground penetrating radar, both of which are also recommended by the HSE to avoid danger from underground services.

The primary investigation technique used for the Heron Tower site was radio frequency location (RFL). This is a sophisticated type of scan that can create a three-dimensional image to locate metallic apparatus and trace drainage routes. Met surveys also carried out a ground penetrating radar (GPR) survey. This obtained information on sub-surface variations and features. The survey can potentially locate all types of utility apparatus, such as non-metallic pipes, tables and ducts.

Met had to inspect all manholes and inspection covers around the site to carry out a full utility tracing survey. This gave the company some idea of what utility equipment was around the site. More importantly, if apparatus could be accessed Met could 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.

Met investigated more than 320 inspection covers at the Heron Tower site. Each cover was given a unique number, photographed and a Met produced a drawing of the chamber contents.The routes of all of the apparatus that were traced by RFL on the Heron Tower site were marked out onsite using spray paint or wax crayon. GPR data can be examined onsite 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 projects that are “noisy” in terms of the quality of data.

Met says a more reliable approach in such instances is to collect the data on a grid system and process and interpret it back at the office. The company says it adopted this option on the Heron Tower, as this increased the chances of identifying features with relatively weak responses.

The GPR survey grids, and the features identified by RFL, were all tied in to an existing topographic survey. Mace requested a full topographic survey of an adjacent shopping arcade, 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 hambers, 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. Met used a topographic survey team to accurately locate the tunnel relative to the above-ground features. The company used operatives who were ualified to enter confined spaces. They utilised gas monitors and escape breathing apparatus to meet health and safety regulations.

Met says Heron Tower was one of the most exacting utility tracing jobs it has undertaken, but he results will enable the design and construction work of Heron Tower to proceed with ore confidence.