Tunnels and shafts

Contracting services

Weakness zone

Geofrost provides temporary support for tunnel excavation through weakness zones or zones with potentially large water leakages in rock of otherwise high quality. With ground freezing these zones can be excavated in a predictable way.

The distance between the freezing pipes determines required freezing time. It is beneficial to install the freezing pipes as parallel as possible with the tunnel. It is also easier to protect the pipes against the blasting if the bore holes start outside the tunnel cross section.

Required thickness of the frozen structure is calculated based on load from acting rock and water pressure, cross section geometry and length of excavation section.

Inadequate rock cover

Geofrost provides ground improvement above the rock surface which can be established from the tunnel face or from the ground surface. The soils are frozen to a strong and impermeable temporary supporting structure. By freezing from ground surface the tunnel excavation can continue without stop, either by traditional drilling and blasting or by tunnel bore machine.

We install freezing pipes around the whole tunnel, or part of the tunnel circumference, depending on the situation. Often the whole circumference must be frozen when sealing against water ingress or pore pressure reduction. In cases of local support against break in of blocks or poor ground it may be sufficient to freeze a smaller arch, for example above the crown or on one side.

Soil tunnel

Geofrost provides temporary support to enable soil tunneling efficiently with full face excavation. Keeping the water away also ensures good quality concreting. Ground freezing provides dry and safe excavation.

Required thickness is determined on the basis of sectionwise tunneling with alternating excavation and support for given lengths. The frozen structure is maintained until the load is transferred to other load carrying support.

Tunnel portal

Geofrost provides stabilization by freezing so tunnel excavation can start directly in the soils outside the rock, minimizing environmental footprint. The freezing pipes are installed from the surface into the rock where normal tunnel excavation can continue.

Issues avoided:

• Impacting landscape and natural groundwater levels.
• Removal of buildings in affected zone.
• Slope stability problems.
• Excavation of large volumes that need to be transported, deposited and potentially brought back and laid out on top of the cut and cover portal.
• Rerouting of roads and other infrastructure crossing the area.

Cross passage

Geofrost provides waterproof temporary support for construction of cross passages. The whole cross section is frozen and excavation length is adapted to load and capacity in the support.

In urban areas groundwater control may be cruicial. Waterproof support by ground freezing avoids potential problems and provides safe excavation. The freezing pipes can be drilled from one or both tunnels, depending on access. If the geometry allows it, the freezing pipes can be drilled through from one tunnel to the other, but geometric limitations and potential pump sumps in the passages may require several blind holes.

TBM start block

Geofrost provides frozen blocks for launch and reception pits for TBMs. The start block ensures stability and waterproofness toward the pit until the wall is sealed against the closest element ring.

The freezing pipes can be installed vertically or inclined from ground level, leaving a small footprint, or horizontally from the pit.

TBM with mixed face

Geofrost provides ground freezing for TBMs to make the ground conditions more uniform for parts of the tunnel with significant deviating properties compared to the rest of the alignment.

It is an advantage that TBMs intended for rock can get resistance across the whole face if parts of it consist of soils. For EPB machines, which are designed to excavate soft soil below ground water level, the ground freezing of loose frictional soils eliminate the risk for sink holes by large variation in layers and permeability.

Shaft

Geofrost provides waterproof temporary support for shafts. Shafts for infrastructure can be several tens of meters deep and have to pass through soil and weathered rock before entering good rock. The ground freezing ensures waterproof support down through all the various ground conditions. With a waterproof support there is no need for water handling, neither inside nor outside the shaft.

The purpose of the ground freezing can be groundwater cut-off only, as a thin shell outside the shaft itself, or a thicker wall which provides strength and earth support during the excavation. This depends on the method used to construct the shaft.

Deep shaft

Geofrost provides temporary waterproof support for deep shafts. Mining shafts can be several hundred meters deep and need to cross water-bearing strata and challenging layers. Ground freezing is a highly reliable method to handle these challenges.

The purpose of the freezing might vary from groundwater control alone to temporary support and groundwater control combined. The long holes for the freezing pipes are drilled by directional drilling to ensure sufficient drilling accuracy. Sectionwise ground freezing is an alternative for large depths.