Witnesses reported hearing scraping and grinding sounds coming from below the rig for about three minutes before the ground began to slip, and workers had completed installation of their 29th column of the morning in the area—out of 90 to be installed—just one minute before the landslide. They shut down the rig and moved to leave the area, but the one man, identified only as a man in his 30s employed by Danish engineering firm Niras, was caught in the slippage. It took nearly two weeks to recover his body. 

The slide displaced more than 72,000 cu yd of material, according to the report prepared by Norwegian engineering firm Dr.techn. Olav Olsen AS. In addition to killing the worker, more than 550 ft of existing rail line and 320 ft of the neighboring E6 highway were destroyed as material slipped down toward the nearby Nevastnet lake.

Report Looks Closer at Cement Slurry Injection

The project was planned to add a 3,126-ft-long passing siding to the railroad, upgrade two level crossings and replace four culverts. Ground stabilization was needed because of the area’s quick clay, which is prone to liquefaction.

The columns were formed by injecting a cement slurry and mixing it with ground material as part of the deep cement-mixing process. The investigating engineers found that the process weakened the disturbed clay before the columns were able to cure. The injection process also caused pressure to rapidly build up in the permeable sand and silt layer about 20 ft below the surface, displacing water faster than the quick clay could drain it. The material, already weakened, failed under the added stress.

Image courtesy Dr techn Olav Olsen AS

The initial clay block failure occurred in the area directly under and around the rig at the eastern end of the site, slipping south toward the lake, according to the report. That then triggered further movement south and west of the initial failure, which in turn removed support for the uphill areas containing the highway. The highway also collapsed and pushed the initial block forward into the lakebed. 

Quick clay is relatively common in Norway, but is also found in parts of North America and elsewhere. In the U.S., quick clay was involved in a 1964 landslide in Anchorage, Alaska, triggered by a major earthquake. 

Image courtesy Dr techn Olav Olsen AS

In this case, the permeable layer had been detected in bore holes during geotechnical investigations ahead of work, but had not been fully mapped out. Doing so, along with placing pore pressure monitors in that layer rather than just in the clay, and monitoring them in real time, could have helped identify the risk, according to the report. The authors recommended mapping such layers thoroughly as a standard practice in future deep cement-mixing work, and placing pressure monitors in those permeable layers.

Gunhild Hernes Synnestvedt, executive vice president for compliance and safety at Bane NOR, said in a statement that the railroad hopes the findings will lead to improvements in the industry that avoid similar incidents in the future.

While the engineers’ investigation for Bane NOR is complete, the incident is also being investigated by police and the Norwegian Labor Inspection Authority. Niras, the firm that employed the man killed in the landslide, expressed condolences in a statement and said that it is assisting police with their investigation.