Case Study - Blackwall Tunnel, London

3D high definition scans of the northbound tunnel, before and after its refurbishment. The Blackwall Tunnel provides a critically important road link between the north and south banks of the river Thames in east London. There are actually two separate tunnels at Blackwall, the first of which opened in 1897 and now carries northbound traffic along its 1,364m length. (The second tunnel was built some 70 years later and carries southbound traffic.) Recent refurbishment works to the old, northbound tunnel were designed to bring it into line with modern safety standards, and reduce the number of incidents that in the past had required frequent closures. (Since the tunnel typically carries around 50,000 vehicles per day, such closures were enormously disruptive and expensive.) The works included refurbishment of the four main ventilation shafts and installation of new lighting, CCTV, ventilation fans and communications systems. An additional lane also had to be built to take heavy goods vehicles on the northbound approach, allowing over-height vehicles to be diverted away from the tunnel before they could reach the entrance and cause an obstruction. The tunnel was fully re-opened in November 2011, more than a year ahead of schedule. Scope of Works Apex Engineering Service’ role was to carry out a survey of the whole length of the tunnel – including the ventilation shafts and the 239m northern and 263m southern approaches – using 3D high definition laser scanning technology. Our client required surveys both before commencement of the works and after their completion. Methodology The initial survey was undertaken by a team of three engineers, who established control over the entire length of the tunnel and co-ordinated the key positioning of the ventilation shafts in relation to one another. For the return visit, the tunnel closures for the survey took place over the course of five nights. A team of four surveyors worked for the first two nights to establish control and survey approximately 50% of the tunnel. For the remaining nights, a team of two surveyors completed the tunnel and ventilation shafts. Deliverables Our CAD team delivered grayscale Truviews as well as a 3D point-cloud on an Ordnanace Survey grid with a tolerance of 5mm for interrogation by the client. Taking into consideration the 1,866m length of the tunnel (including the northern and southern approaches), employing 3D high definition laser scanning technology for these surveys enabled us not only to turn both jobs around very quickly but to deliver a highly cost effective package too.