Bridgewater causeway

The Bridgewater causeway is an artificial embankment that runs roughly 730 meters across the River Derwent from Granton, stopping just 340 metres from what is now the Hobart suburb of Bridgewater.

The planning for the Bridgewater causeway began in 1826 in order to provide a safe way to cross the river, which was previously only possible by ferry boarded close to the nearby colonial settlement of Black Snake, located in what is now Granton.

Use of these ferries came at considerable risk, with incidents like the 1818 capsizing of a ferry boat which resulted in 12 people drowning.

To reduce such incidents, a committee initially decided to construct a wooden bridge, but eventually decided on constructing a causeway. This consisted of an artificial embankment that would span most of the river from its southern bank.

The causeway was placed on top of existing mud flats that spanned roughly three quarter’s of the river, leaving the final quarter open due to the presence of a fast-flowing channel.

Sandstone arches located at the southern end of the causeway were intended to serve as a viaduct that would allow water flow, but these were abandoned after they sunk due to insufficient foundations.

This was a reoccurring problem due to the high levels of silt and clay, with the overall construction often being referred to as the ‘Bridgewater Folly’. The endless task of filling in the mud-hole was solved with still green willow branches as a base for the embankment.

Brief View of the Roads, Bridges, and other public buildings’ Launceston Advertiser, 17 Jan 1831, p.24

“An establishment has been formed at Bridgewater for a Chain Gang, which is employed in constructing that great work, the causeway over the Derwent…the bed of the river over the flats at this place is composed of soft mud, which the heavy mass of stone thus thrown upon it soon displaces, and in this manner a good foundation is obtained on which to raise the subsequent work. Five and twenty small abutments will then be built and covered with timber. From the piers to the edge of the deep channel a solid road of stone will be formed with a small basin at the end to haul the punt into. As the distance across is very trifling no delay can occur, because the punt instead of being towed by a boat will be made to swing backwards and forwards.” - ‘

Construction of the causeway began in 1830 using convict labour under the supervision of Roderic O’Connor, utilising sandstone quarried nearby at the Granton Quarry which was carted by convict gangs.

As many as 280 convicts at a time laboured on the causeway, using only shovels, picks and wheelbarrows to move and deposit at least 1.8 million tonnes of sandstone, soil and clay.

In 1835, just a year before the Causeway was complete, it was decided that a punt, a small ferry that was winched across the causeway’s northern end to the shores by cables, would be constructed to allow people to cross the remaining 340 metre gap.

This punt would eventually be replaced by a bridge in 1849, and the three more bridges being built in 1874, 1893, and 1942, all of were built using the causeway as their southern entrance.

At the time of its completion in 1836, the causeway was one of the largest and most expensive convict projects in Australia, costing the government roughly £52,000 pounds and using around 1,800,000 tonnes of stone and clay to provide a solid base.

“…We visited Bridgewater for the first time, last Saturday. It certainly is a stupendous work for this Colony, and will be considered so even in England. That it will be a useful work, no one can deny -- that its formation was a judicious application of the public labour, we cannot admit. We crossed the Derwent at Bridgewater, and saw where the causeway had sunk. It had, apparently, been filled up, and leveled a considerable way. We observed a beautiful specimen of the Colonial road engineering… We are convinced, that the tide will never rise over it ; indeed, there are on each side of the stone and gravel mound an immense mound of mud, which has been displaced, and forced up by the materials forming the causeway. It stands some feet above the level of the tide, and appears quite dry, getting green at the top.” - ‘Bridge Water’ The True Colonist Van Diemen’s Land Political Despatch, and Agricultural and Commercial Advertiser’ 15 January 1836. Accessed from Trove.

References:

• Bridgewater causeway, midland Hwy, Granton, TAS, Australia’ Australian Heritage Database. Ian Cooper 2018, ‘Bridgewater Bridge Tasmania’ Department of Infrastructure, Tasmania. P.2
• ‘Bridgewater causeway, midland Hwy, Granton, TAS, Australia’ Australian Heritage Database. Ian Cooper 2018, p2.