66kV Electrical Connection, Hillingdon


JSM was appointed as Principal Contractor to deliver a new cable installation comprising of a single, dual and triple 66kV circuits and pilot cables on a 10km route in the London Borough of Hillingdon.


£6.2 million


16 Months

Date Completed

May 2019

The route went through busy public highways, industrial and residential areas of Uxbridge and West Drayton. The 3 x circuits were installed to Both SSE and the IDNO standards as the network was to be adopted by an IDNO connecting into SSE’s network at Ivor.

Delivery and Approach

JSM carried out their own site survey to fully appraise themselves of the scheme and its requirements along with verifying all surface types and identifying areas of traffic sensitivity and Special Engineering Difficulties (SEDS) in conjunction with the CAT Surveys Feasibility, JSM proposed an alternative route to over congested areas whenever possible.

The project required extensive stakeholder management with LB Hillington to agree traffic management including road closures and other measures while minimising whenever possible disruption to the road users.

The route was also massively congested with utilities on most roads, requiring liaison with owners and extreme care when digging. JSM’s competent and experienced civils teams implemented safe digging procedure in accordance with HSG47, used cable avoidance tools to scan the areas of the excavations, marked services accordingly inside and outside of the excavation areas and hand dug within 0.5metres of known/identified cables.

The Challenges

Such a long route presented 5 SEDS, including managing culvert crossings, overbridges and cable bridges. 2 SEDS in particular required specialist management.

SED 1 – River Colne Crossing – Temporary Dam

A trial hole revealed that there were not enough existing ducts in size or number to install the 66kV cables on the bridge. The alternatives were to HDD or a cable bridge. The solution was to appoint a specialist contractor to construct a temporary dam across the River Colne and Temporary Dam over River Colne and install a Frame Dam over 2 spans, dewater the area to be excavated, allowing JSM to carry out the civils works including shoring and the duct installation in the bed of the River Colne.

The temporary dam over the River Colne was such a major procedure, it presented the following risks among others:

  • Flooding;
  • Weil’s disease;
  • potential environmental damage; and
  • Disturbing wildlife habitat in particular Daubenton’s bat;

All these risks were properly mitigated, through respectively constant monitoring of the water levels, wearing adequate PPE and life jackets, creating segregated area for bat roosting, and ensuring no fish/fry were present.

SED 2 — River Pinn—Flume Pipe to temporarily divert the river

Following trial holes, it was ascertained that due to the presence of 132kV circuit and telecoms networks, it was too risky to install the dual triple 66kV circuit on the bridge over the River Pinn.

JSM therefore had to open cut through the River Pinn, using a flume pipe to temporarily divert the water, sandbagging and dewatering the areas either side of the flume pipe and then proceed to open cut, install ducts and backfill before removing the pipe and reinstating the area. The works were carried out successfully. Both SEDS required environmental permits to be in place before works started. JSM as Principal Contractor ensured the contractors held the right competencies and experience to deliver these works, managed their day to day output and ensured they delivered the works in line with all Health and Safety and Environmental regulations.