Spotlight - Precise knowledge of what lies deep underground is essential to the successful construction of one of London’s new skyscrapers.
The 230m tall Heron Tower, to be built in the heart of the City of London, will dramatically alter the already impressive London skyline. But in common with even the smallest project, an accurate survey was the critical starting point for the design and construction.
Buried services and utilities infrastructure around the Heron Tower site are highly complex and required specialist survey expertise. Construction project manager Mace enlisted the help of surveying specialist Met Surveys to carry out a utility tracing survey to pinpoint their locations.
Met Surveys surveyed 0.7ha of footways and carriageways using non-intrusive techniques over a three-week period of night shifts, because of the high level of pedestrian traffic heading to Liverpool street station a short distance away.
Two techniques were used to carry out the survey: Radio Frequency Location (RFL) and Ground Penetrating Radar (GPR).
RFL is a sophisticated type of CT scan that can locate metallic apparatus and allow the tracing of drainage routes. GPR was also used as it can reveal subsurface variations and features, and potentially locate all types of utility apparatus including nonmetallic pipes, cables and ducts.
Over 320 inspection covers were also investigated at the Heron Tower site, with each given a number, photographed and a drawn record of the chamber contents produced as standard. This gives a further indication of what utility apparatus is present. 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 traced and depth information obtained using RFL.
This was vital at Heron Tower.
‘The utility tracing survey revealed a large service tunnel on site that 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, ’ explains Met Surveys geophysics manager Mark Whittingham.
The GPR survey and the features identified by RFL were all tied in to an existing topographic survey carried out using a total station and collated into a CAD plan.