New Scotland Yard, London
(on behalf of BL Development)
- Ground Investigation in Confined Spaces
- Structural Investigation
- Vault/Void Investigation
- Gas and Groundwater Monitoring
- Factual and Interpretative Reporting
- Stakeholder Liaison
Restricted access ground, foundation and vault investigation
New Scotland Yard, the previous headquarters of the Metropolitan Police, is undergoing a £1bn redevelopment into a mixed-use
development of six towers, 14-20 storeys high including 295 residential units. In addition, the existing single storey basement is to be extended to three basement levels for parking and plant space.
A detailed phased ground investigation was required to provide geotechnical design information on the underlying soils to allow foundation and basement design for the new development. Phase 1 needed to happen while the existing building was still fully operational, highly visible to the public and media, without hindering access for building users and within stringent security controls. The main aim was to find the condition and position of the existing piles, confirm the underlying deep ground conditions, and investigate the presence and structural condition of historic vaults surrounding the existing basement.
The ground investigation was undertaken through a series of cable percussive boreholes with planned depths between 46m and 68m. Some boreholes were made from ground level, drilling through the basement and requiring complex planning and propping of the rig loadings, while others utilised a reduced height drilling rig setup in the basement itself. Upon completion gas and groundwater monitoring standpipes were installed complete with automated DIVER groundwater monitoring devices.
Structural investigations by CGL involving openings in the basement walls and floor slab identified the as-built foundation drawings were not accurate. Openings were made using diamond drilling and coring which found the slab was significantly thicker than expected, between 0.8m and 1.0m thickness across the site. Critically the locations of the existing piles proved the as-built drawings to be unreliable and allowed early redesign of the proposed new structures, avoiding later delays and costs.
A new technique for remote investigation of unknown basement vaults was developed that is significantly safer, quicker, less disruptive and lower cost than conventional techniques. The technique could be adapted for other situations such as investigation of collapsed voids or abandoned tunnels. As-built plans showed a series of blocked-off vaults of unknown condition outside the existing basement perimeter. To investigate the vaults CGL made 18 horizontal concrete cores through the basement wall, then probed through the core holes using a video borescope.
If a vault void was discovered, the original investigation scope called for an opening to be made into the vault from the public highway on Victoria Street. However, there were significant safety risks of making sizeable openings in a public highway over a void of unknown structural condition as well as operatives working within the voids.
To avoid the hazards, CGL developed a technique using a 3D LIDAR scanner mounted to a boom arm and inserted into the void. The device could produce a fully dimensioned and accurate 3D model of the void interior within approximately 30 minutes of insertion, complete with a 360° image and video.
Despite tight operational constraints and unexpected problems, CGL’s investigations were completed to deadline and to budget so demolition of the unoccupied buildings could commence on programme.
CGL’s work on this project was selected as a finalist for the Ground Investigation Project of the Year at the Ground Engineering 2017 Awards.