The River Medway Cable Tunnels are a pair of tunnels carrying high-voltage electricity transmission lines beneath the lower

River Medway The River Medway is a river in South East England. It rises in the High Weald AONB, High Weald, East Sussex and flows through Tonbridge, Maidstone and the Medway conurbation in Kent, before emptying into the Thames Estuary near Sheerness, a to ...

between the

Isle of Grain Isle of Grain (Old English ''Greon'', meaning gravel) is a village and the easternmost point of the Hoo Peninsula within the district of Medway in Kent, south-east England. No longer an island and now forming part of the peninsula, the area is ...

and Chetney Marshes, Kent.

Specification

Each tunnel carries a 400 kV electricity circuit on the transmission line between the National Grid substations at Grain and Kemsley (Line Reference: 4TL). The oil-filled cables are designed to carry 3780 Amps in Winter and 3060 A in Summer. The tunnels are 1,700 metres long, 2.54 m in diameter and between 34 m and 47 m below

ordnance datum In the British Isles, an ordnance datum or OD is a vertical datum used by an ordnance survey as the basis for deriving altitudes on maps. A spot height may be expressed as AOD for "above ordnance datum". Usually mean sea level (MSL) is used fo ...

. The tunnels are connected to a common 6.1 m diameter vertical access shaft at each end. Grain sealing end compound location: 51°26'09.4"N 0°42'18.9"E Chetney Marsh sealing end compound location: 51°25'14.6"N 0°41'59.6"E

Design and construction

The tunnels were built in anticipation of the commissioning of

Grain power station Grain Power Station is a CCGT power station and former oil-fired power station in Kent, England, with operational capacity of owned by Uniper (formerly E.ON UK). Oil-fired power station Grain was built on a site for the nationalised Centr ...

(with a planned capacity of 3,300 MW). The crossing would allow the transmission of power to the 400 kV transmission system to the south. A feasibility report of 1967 addressed both overhead line and tunnel options for the crossing. Overhead lines would have been longer than the 1,382 metre Swanscombe to Thurrock 400kV crossing and therefore require taller pylons. The tunnels were designed by the Transmission Development and Construction Division of the

Central Electricity Generating Board The Central Electricity Generating Board (CEGB) was responsible for electricity generation, transmission and bulk sales in England and Wales from 1958 until privatisation of the electricity industry in the 1990s. It was established on 1 Januar ...

advised by Charles Haswell and Partners. They were constructed between 1973–6 by John Mowlem & Co. Ltd. and L.G. Mouchel and Partners were consulting engineers for the civil engineering work. The tunnels are constructed from concrete expanding wedge block linings. The tunnels were designed to be constructed through a layer of

London Clay The London Clay Formation is a marine geological formation of Ypresian (early Eocene Epoch, c. 56–49 million years ago) age which crops out in the southeast of England. The London Clay is well known for its fossil content. The fossils from t ...

. Unexpected beds of gravel and silt were encountered and entailed a diversion of the west tunnel.

Operation

Medway Cable Tunnel South Pylon - geograph

The cables are air cooled to maintain the temperature within design limits at high electricity loads. A forced ventilation system, comprising fans, dampers, acoustic enclosures, and control systems delivers 58 m³/s of air at 7,500 Pa through each tunnel. The tunnels are provided with Distributed Temperature Sensing fibre system mounted in the crown of each tunnel to detect abnormal temperature conditions.

References

{{Reflist Powerline river crossings Electric power infrastructure in England Electric power transmission in the United Kingdom

Specification

Design and construction

Operation

See also

References