Stad Ship Tunnel investigation reveals encouraging results

13 February 2024

Core drilling for the world’s first full-scale tunnel for ships has revealed positive results for rock quality.

Norway’s Stad Ship Tunnel, which will be 1.7km long, 37m high and 26.5m wide, will connect to fjords to allow passenger and cargo ships to more safely navigate the Stadhavet Sea – regarded as the most exposed and dangerous section of the Norwegian coastline.

The Norwegian Coastal Administration conducted the core drilling from the Kjøde side, having earlier drilled from the Moldefjord side.

“Our goal was to drill 1,050m into the mountain to achieve a complete infiltration of the ship tunnel route,” said construction manager Per Åge Havnegjerde. “The remaining 650m has already been documented through drilling from the Moldefjord side.”

He added that the drilling took longer than expected because of water infiltration. Equipment fractures, such as rod breakages several hundred metres into the drilling, also created challenges.

By late January, just over 500m had been drilled.

“We are surprised that we are still struggling with significant water infiltration, 5-600 litres a minute at the most, with a pressure of over 20 bar,” said Havnegjerde.

He said some water in the route from Kjøde was expected, but it was thought the volume would decrease as drilling progressed into the mountain.

Positive rock quality has been discovered, however.

“We have encountered some weak zones and a few minor zones with clay. The clay zones have been analysed, and it is concluded that it is not expansive clay, which could have given us some challenges. The rock is of good quality, with normal fracture formations,” said Havnegjerde. “The rock type on the core samples is primarily various types of gneiss, with inclusions of quartz and other rocks. In other words, it is typical Norwegian mountain rock.”

Tunnel excavation is expected to be by drill and blast, removing 5.4 million m3 of stone.