Industry Perspective | Deep in the Oslo fjords, what challenges are faced in tunnel construction in Norway?
Editor's Note
At this year's British Tunneling Society (BTS) conference, many experts introduced tunnel construction in Norway to the guests. After TBM rises again in Northern Europe, what technical challenges will it face, and what star projects will emerge in the future?
Beneath Oslo – Geological Background
Under Oslo – The Geological Context
The geology of Oslo is mainly hard rock, including Precambrian crystalline basement (gneiss, granite and amphibolite), Cambrian and Silurian sedimentary rocks (limestone, sandstone, siltstone, hornfels, shale), Intrusive rocks and dykes (syenite, porphyry, basalt, diabase).
In general, Oslo's hard rock formations are homogeneous and solid, with fewer joints in them. The permeability of the rock mass is extremely low, and a single crack or a group of cracks will become a drainage channel in the rock mass. Since the Ice Age, hard rock has been subject to continuous erosion, especially in fault zones and weak areas of rock mass. These glacially eroded "troughs" are filled with thick layers of marine clay that are sensitive to subsidence.
Tunnels that pass through (cut off) fault zones will cause the groundwater level to drop quite far away from the tunnel, which may cause settlement or environmental problems. This situation has a particularly serious impact on buildings built on shallow, soft marine clay layers. Therefore, Whether there is precipitation during tunnel construction is extremely important. If precipitation occurs, designers need to determine the allowable amount of water seepage and calculate its impact on groundwater levels and settlement. Therefore, in local drill-and-blast tunnel construction, systematic pre-excavation grouting may be required to reduce water seepage to an acceptable level. If TBMs and segments are used to build tunnels, pre-excavation grouting is sufficient before TBM advancement.
Other geological risks
More Hazards
In addition, Norway also has various unpredictable geological risks——
Another geological hazard that is a specialty of Norway is "Quick Clay", also known as Leda Clay - an open-structured sea clay stabilized by salt. If the salt in it is lost from the clay, the clay will be extremely unstable and will quickly collapse like a house of cards upon slight disturbance, and then turn into a fluid, thus causing landslide events on extremely shallow longitudinal slopes. There have been many landslides in fast clay layers in the history of Norway:
■ The most serious one was the Verdals landslide in 1893, when 55,000,000m³ of clay turned into fluid and caused a landslide, killing 116 people;
■ The Bekkelags landslide in 1953 destroyed two rail lines and killed five people;
■In the 1978 Rissa landslide accident, 6,000,000m³ of clay dissolved and collapsed in an area of 330,000㎡, killing one person, damaging 20 buildings, and even triggering a 3m-high tsunami.
Hot spots and future projects
Future Projects
After entering the year 2000, Norway gradually began to apply TBM in tunnel engineering. Not only did it carry out technological innovations to overcome these geological problems, but it also built a number of tunnels that attracted worldwide attention. More innovative projects will be launched in the future——
Rogfast Tunnel
The world's deepest undersea tunnel - Rogfast Tunnel is a road tunnel between Boknafjorden Bay and Kvitsøy Fjord north of Stavanger in southwest Norway. The total length of the tunnel is 27km, the deepest is 392m below sea level, the maximum longitudinal slope is 5%, and there are two tunnel tubes , a parallel 10.5m wide x 27km long two-lane one-way tunnel.
Industry Perspective | 392m below sea level! Norway will build the world’s longest and deepest undersea tunnel
Currently, the construction methods of the project are compared between the drill and blast method and the TBM method:
■ The original plan of the project was to use the traditional drilling and blasting method, covering more than 50m of soil, carrying out pre-excavation grouting and drilling and blasting excavation, with anchor rods and sprayed concrete support, and an independent lining;
■ If TBM is used in the project, the cost will be 20-35% higher than the drill and blast method, the construction period will be shortened by 1.6 years, and the risk will be lower. If TBM is used, dual-mode TBM needs to be applied and equipped with pre-excavation grouting and advanced drilling capabilities.
■ Researchers evaluated the number of TBM applications: multiple TBMs can make construction schedule adjustments more flexible, but are not conducive to logistics and logistics; while using a single TBM costs less but takes longer.
If the project wants to use TBM for construction, there are still some problems that need to be solved, such as the 392m groundwater head and complex geological conditions. The final engineering construction plan will be evaluated after a more detailed investigation.
Undersea oil field tunnel
Norway plans to use TBMs to dig a tunnel to connect submarine oil fields to the mainland for transporting crude oil. At present, the builder has conducted exploration in the seabed area 30km offshore and 150m deep. According to the plan, the tunnel will be excavated using a double-shield TBM with a diameter of 6.6m. The challenges include:
■ Extremely unstable formation and rock mass environments, including mixed geology, flowing formations, expansion rocks, squeezed bottom layers, etc.;
■ The amount of water inflow is large, the groundwater head can reach 250m, and pre-excavation and grouting are required;
■The parallel excavation of three tunnels at the same time can achieve high efficiency and requires two transportation tunnels for personnel and equipment, as well as an oil and gas pipeline.
StadShipTunnel
The Stade Tunnel will be the first tunnel in the world specifically designed for ship traffic. Stade Bay is surrounded by plateau mountainous areas, with the highest point being 645m. The tunnel will be dug under the mountains to connect the coastal cities of Bergen and Alesund. Ships can safely travel in the tunnel all year round. The total length of the tunnel is 1.7km, with a central height of 33m above sea level and a depth of 12m below sea level. The ship width limit is 17m and the length limit is 20m. According to the design, the tunnel is 36 meters wide and 45 meters high, and can accommodate ships up to 16,000 tons.
Hot Spot Tracking | The budget of Norway's "ship-specific" tunnel project has been compressed and construction has been delayed. When will the "new tunnel concept" be launched?
The tunnel is not a complex project; the challenge lies in transportation and logistics.
Hyperloop tunnel
In addition, there are plans to build an undersea tunnel between Stockholm and Helsinki for the Hyperloop, which will shorten the travel time between the two places to 28 minutes. This will require digging a 170km long tunnel. The biggest challenge is evacuation and escape in the tunnel. It is currently planned to use an oil platform as an intermediate shaft during construction and operation.
