Network Rail: Harley Road

The site is located adjacent to Harley Road and is approximately 70m deep by 42m wide. Two existing buildings on the site (26 &26A) needed to be demolished, although no.26A was completely demolished no.26 was only partly demolished with the roof, front façade and flank wall retained. A Network Rail express tunnel runs diagonally under the corner of no.26A.

  • Project Cost £12 million
  • Architect Darling Associates
  • Images Fluid Structures

The structure comprised a steel framed building with timber roofs and 2nd floor and concrete ground and 1st floors.  The foundations of the main building between gridlines comprise a piled raft supported on piles located to avoid existing foundations.  The remainder of the foundations close to the tunnel and the boundary with no.24 comprise reinforced concrete ground floor slabs supported on ground beams that are in turn supported by piles.


  • Possibility of damaging major tunnel and halting or damaging trains and causing fatalities.
  • £6 million per day cost if tunnel is out of action.
  • Possibility of tunnel damage and collapse causing road collapse.

Design Procedures

  • We contacted Network Rail at an early stage in the project before any design work had commenced and agreed methods of construction that would be acceptable close to the tunnel.
  • A microgravity survey was carried out to establish the location of the tunnel on site. A pile exclusion zone and influence zone were identified based on this information. All piles within the influence zone were sleeved to prevent them surcharging the tunnel.
  • A finite element model of the tunnel was analysed with various overburden/surcharge conditions including allowance for construction plant and crane outrigger loads.
  • Ground beams and ground floor slabs adjacent to the tunnel were cast on Cellcore so that only the self weight of the beams and slabs would be supported by the clay soil adjacent to the tunnel. The ground beams were designed to cantilever from the piles located outside the pile exclusion zone so that any surcharge approximately matches the existing situation. As a result, the loads on the tunnel during construction and when the proposed structure was complete are approximately the same as the originally existing situation.
  • We carried out a detailed site investigation and Network Rail carried out a condition survey prior to any work starting.
  • A vibration study was carried out to establish if it would be necessary to isolate the foundations to minimise any structure borne vibration. This established that it wasn’t necessary to isolate the foundations.
  • Based on our calculations, meetings, Network Rail meetings and drawings the feasibility of the proposed works was established and agreed with Network Rail.
  • An Approval In Principle (AIP) document was submitted to Network Rail.
  • Once this was approved the work on site could proceed.