On site but out of sight

IF you often drive the Midland or Lyell highways, you’re probably asking what’s going on with the new Bridgewater Bridge.

At the Derwent Valley Gazette, we’re watching construction too, so that we can report to you on progress. Sometimes, the headway being made on this three-quarter-billion dollar project is where you can’t see it, on site but out of sight. And that includes the work being done underwater, down below one of those barges you’ve seen in the water over the last few months.

The new bridge is designed with two rows of 21 concrete piles, each pair spanning 51m to 65m, and each pile 2.5m in diameter. Those piles create the strong foundation needed to support the weight of the bridge itself, in addition to the traffic, wind loads and even flooding.

So that those concrete columns sit in solid rock, engineers have been boring deep holes between the shores and beyond. Eighty-five of these boreholes have now been completed.

The process requires drilling as much as 100m below ground, then removing the core to analyse what’s down there. The cores, by the way, are then stored.

With all that data, the bridge engineers and designers have determined that some piles will need to go down some 50m to penetrate existing silt and sand, cobbles and soft rock to the hard stuff below.

First though, the pile will need to get to the bottom of the Derwent River, which over its main channel along its northern edge is up to 10m deep. Out of sight, perhaps, but for the bridge builders, it’s another number to keep in mind.

Picture: Matthew Bisset