Tunnels and shafts

Waterproof temporary support for tunnels and shafts.

Contracting services

Weakness zone

Figure: Stabilizing weakness zone by ground freezing.

Geofrost delivers 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.

Photo: Ground freeezing for drill and blast tunnel through weakness zone.
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

Figure: Ground freezing for inadequate rock cover.

Geofrost delivers 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.

Photo: Pilot tunnel under area with lack of rock overburden, stabilized by ground freezing.
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

Figure: Soil tunnel stabelized by ground freezing.

Geofrost delivers 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.

Photo: Soil tunnel stabelized by ground freezing.
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

Figure: Access to rock tunnel through soils stabilized by ground freezing.

Geofrost delivers 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.

Photo: Acess to rock tunnel through soil stabilized by ground freezing under retainnig wall.
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

Figure: Ground freezing to stavilize CP excavation in soil between two TBM excavated tunnels.

Geofrost delivers 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

Figure: Frozen block for TBM starting pit in soil.

Geofrost delivers 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.

Photo: TBM starting to run through frozen soil behind a diaphragm wall.
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

Figure: Freezing mixed face for TBM.

Geofrost delivers 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

Figure: Shaft for tunnel ventilation stabilized by ground freezing.

Geofrost delivers 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.

Photo: 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

Figure: Shaft sinking for mining, stailized by ground freezing.

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.

Benefits

Safe and predictable

Ground freezing is a reliable solution, well known for its safety. Both client and contractor can trust in predictable time and cost with low risk for surprises, even when crossing complicated ground conditions.

Ground freezing works in all types of soil and rock, independent of fracture size, permeability, water content, water pressure, degree of rock degradation, block or grain size.

The temporary support gives a robust and reliable stability globally and locally and provides a safe working environment. The frozen structure is monitored and has large inertia in case of power outage. A potential collapse will have a plastic failure mode with a long warning time.

Completely impermeable

The underground is stabilized by ground freezing and becomes completely impermeable, resulting in dry working conditions in the tunnel and no pore pressure drop in the surroundings. There will be no need for water wells and water infiltration to keep up the pore pressure. Grouting by cement or chemicals is avoided. The ground is kept frozen until the permanent structure is brought in place and by eliminating water leakages, there is no need for drains behind the concrete and a good concrete quality is secured.

Ground freezing is installed from a safe area, before any failure or water inrush has occurred. To keep the pore pressure unaffected, drilling from below groundwater level is performed through a blowout preventor (BOP). If the surroundings are highly sensitive, drilling is done with mass control as well.

Environmentally friendly

It is the ground itself that is used as the temporary supporting structure. No cement og chemicals are used in the ground. Coolant and refrigerant are natural media, without any effect on the climate (0 GWP). The CO2 footprint is low compared with other steel and concrete based methods.

Efficient tunneling

Geofrost's ground freezing structure is designed for full face excavation. Typical blast rounds are 2 m full face, which will include less supporting materials and a faster excavation than a solution with more faces in the cross section. Concreting directly against the frozen walls, without any leakages, further contributes to predictable time schedule and quality.

The frozen structure takes the load until a permanent structure is ready to take over. By interaction between frost and sprayed concrete with lattice girders put up as work support after blasting, necessary thickness of the frozen structure can be reduced and thus also initial freezing time and cost. Permanent lining is achieved by additional layers of sprayed concrete, or cast in place concrete, or concrete elements put in place and grouted behind.

When installing ground freezing from surface, the frozen structure is established in advance such that it is ready when tunneling reaches the zone. Drilling may be done vertically or inclined from the best locations. Freezing pipes may be insulated so only the needed volume is frozen. By drilling from above groundwater level, all issues connected with pore pressure reduction and settlements in the surroundings are avoided.

What we deliver

We are a total supplier of temporary support by ground freezing for tunnels and shafts and offer all related services within consulting and contracting.

We deliver designed and engineered solutions for tunneling by drill and blast or TBM, whether it is for weakness zones, lack of rock overburden and mixed face situations or pure soil tunnels. We also provide designed and engineered solutions for shafts, either as waterproofing only or combined waterproofing and support.

Consulting services

  • Project management
  • Sampling
  • Lab testing
  • Design and engineering
  • Numerical analyses
  • Design report, drawings and specifications

Contracting services

  • Planning and procurement
  • Site management
  • Drilling and surveying
  • Pipe installation
  • Plant and equipment for brine freezing and nitrogen freezing
  • Instrumentation
  • Monitoring

Where we operate

We offer our services worldwide.

Contact

Contact person

Anne-Lise Berggren, Ph.D., M.Sc
Senior specialist geotechnics
 (+47) 95 29 29 30
 Anne-Lise.Berggren@geofrost.no

Enquiry

If you have questions or would like more information, please contact us. With project info, we quickly provide price estimates.

Ground freezing services since 1986.

Clients