The Lune Aqueduct was one of John Rennie’s earliest bridge designs, built between 1793 and 1797. It carries the Lancaster Canal (1792-1803) over the River Lune, Lancaster in the county of Lancashire.
This magnificent 600ft (183m) long structure boasts five semi-circular arches which span 70ft (21.3m) each. The arches extend to the springer (founding level).
A unique detail introduced by Rennie was the use of inverted arches, these sit between the main archers as illustrated in Figure 2 below. Rennie stated that this was detail would improve the strength of the structure significantly.
The bridge piers sit in 10ft (3m) depth of water, with the foundations sited a further 10ft (3m) below bed level. They were constructed using timber sheet pile cofferdams.
The cofferdams were dewatered using a innovative steam driven pump. A description by a man who was probably an eye witness reads “A fire engine [i.e steam engine] was erected in line of the piers a little behind where the south east abutment is built this worked on an iron balance wheel to the axis of which were fixed cranks at each end one communicated with the engine beam the other worked a horizontal piece of wood that communicated with the pumps when they were fixed in the cofferdams.“ 
The original design of the structure shows bearing piles, comprising 30ft Baltic pine piles imported from Russia. It is know from written records that inspections were made on bearing stratum to decide how many piles if any were needed. Some records suggest that only sheet piles were driven around the edge of the foundations and the foundations bear upon a three layer lattice timber raft where the voids were filled with masonry and the first course of stones of the pier proper laid on this surface. For the pier bases, volcanic pozzolana powder was brought from Italy and mixed with lime in the mortar, which enabled it to set under water.
Rennie also designed the temporary works to install the arches. His original design was for the timber centres to span from pier to pier, however the Figure 4 details a centre with a support at midspan. These centres would have been set at a higher level than the designed crown soffit with an allowance for the dropping of the crown when the centres were struck.
The spandrel walls were designed to have three longitudinal voids covered by pointed arches and the masonry was then to be made up to a level surface over which a 3ft bed of puddle clay would be laid on the whole bridge. When this was well bedded and compacted a concave canal bottom of stone was to be laid on it and the side walls of the canal and the exterior walls then built up with puddle packed between them
Rennie’s first estimate was £16,647 (£1.3m in 2021), probably excluding the cost of foundations.
The foundations alone cost £15,000 and the superstructure more than double that sum estimated. Rennie had advocated brick to save money, but the committee wanted stone, it was traditional building material for the district and brick was considered inferior. It was finally costed at £48,000 (£3. 65m – 2021).
The Lune Aqueduct cost stalled the progress of the Lancaster canal, with a report written in 1819 stating that “too much had been wasted ‘in ornamenting the town of Lancaster with a grand aqueduct upon which the water has lain stagnant for over 20 years”. Overspend led to another viaduct across the Ribble being scrapped along with a length of canal from Preston to link with the Leeds Liverpool Canal near Chorley. Overspend led to a cheaper wooden bridge and horse led tramway being built to as a link instead.
Today the Grade I listed aqueduct remains very much in its original form. It bears two inscriptions: “To Public Prosperity” (North Elevation) and “QUAE DEERANT ADEUNT: SOCIANTUR DISSITA: MERCES FLUMINA CONVENIUNT ARTE DATURA NOVAS. A.D. MDCCXCVII. ING. I. RENNIE EXTRUX. A. STEVENS. P. ET F.” (South Elevation) which can be translated as: “Things that are wanting are brought together / Things remote are connected / Rivers themselves meet by the assistance of art / To afford new objects of commerce. AD 1797. Engineer J Rennie. Built A Stevens father and son”
References: A bibliographical dictionary of Civil Engineers in Great Britain – W Skempton, Mike Chrimes Arch Bridges and Their Builders 1735-1835 – Book by Ted Ruddock John Rennie 1761-1821 – Cyril T G Boucher 200 Years of the Lancaster Canal – Gordon Biddle Railway Practice – S.C. Brees Lives of Engineers: Volume 2 – Samuel Smiles 1862
All images courtesy of the Institution of Civil Engineers unless stated otherwise