‘Ten cars dropped from a ten story building’ for harbour entrance

If you have heard a distant thumping sound or seen a very large crane operating out at the Harbour project site, you may have wondered about the activity and what is involved. The work is a vital piece of the build process called “dynamic compaction”.

David Wyeth, HEB construction’s Project Manager on the site explained the process.

“An essential part of construction of the Opotiki Harbour breakwaters is the foundation they will sit on.

“Investigations of the seabed foundation have demonstrated that the breakwater foundation is loosely compacted sands that may be susceptible to liquefaction in the event of a significant earthquake. Liquefaction can occur when loosely compacted saturated sands are shaken, making the sand act more like a liquid than a solid. This means any loads on the top of the sand (buildings or in this case seawalls) can sink.

“To combat this, we can improve the sand foundation using dynamic compaction. In very simple terms, this means hammering the top surface, compressing the underlying sand layers making the ground less prone to liquefaction and the seawalls will be more stable during an earthquake in the future,” Mr Wyeth said.

To evaluate the impact of dynamic compaction on the specific ground conditions out at the harbour entrance, HEB Construction, the head contractors on site, are carrying out dynamic compaction trials.

“We are trialing various patterns of drops, drop heights and weights on a constructed section of proposed breakwater to determine the best foundation improvement.

“Drop heights from the crane range from 20m to 30m and the drop weights range from 20 tonnes to 30 tonnes. At its maximum, this is the equivalent to 10 family cars being dropped from a 10 storey building.

“We then measure the crater produced so we know the point at which we have the maximum compaction and use a drill machine to send a probe through the ground the test ground improvement and increase our certainty further,” Mr Wyeth said.

Dynamic compaction trials are expected to be complete before the end of November.