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Civil Engineering heritage: Scotland - Highlands and Islands

Date 2007

Event ID 931732

Category Descriptive Accounts

Type Publication Account


(Institute Civil Engineers Historic Engineering Works no. HEW 0084/01)

Clachnaharry Sea-lock

This sea-lock, sited on the mudflats of Beauly Firth, called for outstanding engineering ability in its achievement – a classic case of founding on pre-consolidated fill. It was necessary to build twin artificial embankments, 400 yards long over the mudflats, into a 20 ft depth of water (lock sill level) at ordinary neap tide, to contain the canal, and then to construct the lock, 170 ft long 40 ft wide, where the mud was 55 ft deep. It was impracticable to use a coffer-dam as the elasticity of the mud would have

prevented pile driving.

Based on experience gained in forming the embankments nearby, clay, transported by plateway, was formed into a mound at the site of the lock. This was then surcharged with a kentledge of stones and allowed to consolidate for six months during which it sank 11 ft. The lock chamber

pit was then excavated in the consolidated clay and, despite the need to de-water the excavation, at first with a six-horse driven chain pump and, below 15 ft, by means of a 9 hp Boulton & Watt steam-powered pump, the masonry of the inverted arch at the base and the side walls of the lock were successfully completed in August 1812.

Telford and Jessop were the engineers, Matthew Davidson

was the resident engineer, and John Simpson and John

Cargill were the contractors.

R Paxton and J Shipway, 2007.

Reproduced from 'Civil Engineering heritage: Scotland - Highlands and Islands' with kind permission from Thomas Telford Publishers.

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