Is it a bridge or a causeway?

ONE of the joys of the new Bridgewater Bridge is actually seeing it being built, right before our eyes. The photo shows current waterside activity at Granton, on the southern side of the river. Here’s the story.

To construct the new bridge, a temporary crossing is first assembled over the river, a platform from which the permanent concrete piles and deck will be set in place. And to get that temporary structure erected, 100m of dry land is being formed from the Granton shore.

Think of it as a modern version of the 1830 convict-cut causeway upstream, this time using a contemporary construction technique known as grouting.

In construction terms, it’s not a huge step from using earth (and its many constituents) as a building material, as it has been since ancient times. Essentially, river mud is mixed with grout to form a concrete-like substrate capable of taking the weight of the trucks and cranes coming to build the bridge.

These naturally occurring materials, gravels and sands are bound together using the grout, now stored in the large red containers (rear of photo) sited opposite the Black Snake Inn. That excavator to the left has on the end of its boom a rotating head attachment which both turns the existing mud and pumps grout into it, mixing the materials together. This mixing process continues for about a month in total, and then standard earthworks for the temporary bridge will take over. That process is expected to last about two months.

The truck and trailer is delivering sand that’s used to form bunds, an embankment, around the mixing locations to ensure material does not escape into the river. An additional environmental control is the yellow silt curtain which sits in the river to stop any material floating into the water. In fact, water testing is undertaken regularly.

Three special buoys take water samples every 10 minutes, feeding that information back to the environmental team.

When the permanent bridge is complete at the end of 2024, this newly built 100m ‘causeway’ will remain in place.