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Clachnaharry Sea Lock, Caledonian Canal
Clachnaharry, Inverness, Highland, Scotland, UK
Clachnaharry Sea Lock, Caledonian Canal
associated engineer
Thomas Telford
William Jessop
date  November 1808 - August 1812
era  Georgian  |  category  Locks  |  reference  NH644466
ICE reference number  HEW 84/01
photo  © and licensed for reuse under this
Clachnaharry Sea Lock lies at the east end of Telford and Jessop's Caledonian Canal. It's tidal, and is one of 29 locks along the length of the canal. The challenging ground conditions led the engineers to use innovative soil mechanics techniques to construct it, and it remains in use to this day.
The Caledonian Canal links the North Sea with the Irish Sea via Scotland's Great Glen, and the whole 97km long waterway opened in 1822. Thomas Telford (1757-1834) was its principal engineer, with William Jessop (1745-1814) as consulting engineer.
Clachnaharry Sea Lock opens into the Beauly Firth north west of Inverness. It is sited on mudflats and its construction required outstanding engineering ability in its achievement as it's founded on pre-consolidated fill. As Telford remarked, "It became necessary to adopt a new method, one at least which I had not known to have been elsewhere practised”.
To take the canal into sufficient depth of water — 6.1m was the required water depth over the lock sill at ordinary neap tides — it had to be contained within twin artificial embankments stretching 366m out over the flats. The embankments were constructed using a mixture of boulder clay and quarry waste, both of which were brought in on a purpose-built iron plateway. Each one also has a narrow vertical core of impervious puddle clay.
It was found impracticable to use a cofferdam to hold back the sea for the construction of the lock, as the elasticity of the mud (soft silty clay in fact) made pile driving impossible. A way had to be found to consolidate the underlying clay. The solution used was to join the two embankments into one large mound and add a kentledge of stones on top. This was left for six months during which time the mound sank some 3.4m. Settlement had almost ceased by May 1810.
Between June 1810 and June 1811 the lock chamber pit was excavated through the mound and into the consolidated clay beneath, down to 9.1m below high water spring tide level. The mud was 16.8m deep at this spot. The excavation was dewatered, first using a chain pump driven by six horses, and below 4.5m using a 6.7kW (9hp) Boulton & Watt steam-powered pump.
Once the pit was dry, rubble stone masonry was laid across its floor, 610mm thick in the centre and 1.5m thick on each side. The rubble is set in hydraulic lime mortar. The walls and inverted arch at the foot of the lock are founded on this base, constructed in squared stonework. To make sure that the consolidated soil/mud didn't soften again, only short sections of about 5.5m long were exposed at any one time.
Telford thriftily reused the kentledge stones for the masonry of the lock, which is 51.8m long and 12.2m wide. During construction, blocking material was used at the seaward end. This was removed in August 1812, allowing the sea to flood the lock.
The sea lock raises vessels some 2.4m from firth level to canal level. There is a second lock at Clachnaharry, further inland and founded on firm clay, with a basin 884m long and 148m wide.
The original lock gates were designed by Thomas Rhodes (1789-1868) and were made of oak with cast iron framing. They were replaced in 1890-1906 by gates of oak and steel. Repairs to the lock were carried out in the 1840s and 1920s. The gates were mechanised in the 1960s. They were also fitted with flotation tanks to make them easier to open and close.
The canal is open in daylight hours and the locks are available for up to four hours either side of high water. At low water and during spring tides, the locks are closed for at least two hours either side of low tide.
Resident engineer: Matthew Davidson
Contractor: John Simpson and John Cargill
Original lock gates: Thomas Rhodes
Iron castings: Butterly Iron Works
Research: ECPK
"The Caledonian Canal" by Thomas Telford, in Life of Telford, Vol.1, pp.49-67, ICE virtual library, London, January 1838
"Landmarks in Early Soil Mechanics" by A.W. Skempton, 1979,
in Selected Papers on Soil Mechanics, Thomas Telford Ltd, London, 1984
"Skipper’s Guide: Caledonian Canal” by Scottish Canals,
British Waterways Scotland, Glasgow, 2008
reference sources   CEH SHIBDCE1

Clachnaharry Sea Lock, Caledonian Canal