Composite Shell Lining for Tunnels and Mined Stations

Amit Kaul, BASF India Limited (Construction Chemicals Division), India, tells about the innovative tunnel lining system and related design aspects, and also describes the main advantages of this lining system in comparison to the traditional double shell lining system.

During the last two decades, significant progress was made in sprayed concrete technology, with advanced admixtures, as well as in the application of sprayed concrete, with sophisticated spraying robots and in waterproofing, with spray applied membranes. Also numerical design techniques have improved. All these factors have enabled designers to use sprayed concrete linings increasingly for long-term service life. In many cases, the traditional double-shell lining system was replaced by the composite shell lining system, which consists of two concrete linings with a sprayed waterproofing membrane between linings.

Typical tunnel lining configurations are the double-shell lining (DSL), the single shell lining (SSL) and the composite shell lining (CSL), as schematically shown in Figure 1.

Composite Shell Lining for Tunnels and Mined StationsFigure 1: Typical tunnel lining configurations (Thomas, 2009)

Usually, the lining of tunnels excavated by conventional methods has been designed and built based on the double-shell lining (DSL) approach. Initially a temporary sprayed concrete lining (primary lining) is built to stabilize the opening after excavation and to contain only short to medium-term loads. Later on a permanent cast in situ concrete lining (secondary lining) is installed to contain long-term loads, and attend the requirements of serviceability and durability. Before the secondary lining is installed, a pre-fabricated waterproof sheet membrane is installed against the primary lining, separating the primary lining from the secondary lining. This approach does not consider any structural contribution of the primary lining, although loads are carried by the primary lining for a long time after construction. However, under some project conditions, e.g. deep tunnels with anticipated high water pressure and required fully drained conditions, this design approach is the only possible approach to build underground structures.

Composite Shell Lining for Tunnels and Mined StationsFigure 2: Change in the design of tunnel linings and related key change drivers

Single shell linings (SSL) have been built for decades as permanent concrete linings, mainly in impermeable ground or ground with minor water inflow, to construct different underground structures. They may consist of a single layer or several layers of concrete placed at different times, without a waterproofing membrane. The key design issues are related to the structural interaction between primary (outer) lining and secondary (inner) lining, since they are usually built at different times and thus submitted to different stresses and strains, as well as to water tightness of the lining.

During the last two decades, significant progress was made in concrete technology (mix-design), with advanced admixtures (e.g. water reduction, alkali-free accelerators), as well as in the application of sprayed concrete, with sophisticated spraying robots, and in waterproofing of tunnel linings with spray applied membranes. All these factors have enabled designers to use sprayed concrete increasingly for long term service life.

Using a spray applied waterproofing membrane with high bonding strength to both primary and secondary linings, the integration of the primary lining in the final tunnel lining has become viable under groundwater conditions that previously did not allow the use of single shell lining. The composite shell lining system was born. Design engineers have now more options for the lining design and can optimize it according to the specific project conditions.

The Composite Shell Lining Approach
The composite shell lining system is a further development of the single shell lining system (see Figure 3). It consists of two concrete linings with a double-bonded spray applied waterproofing membrane embedded between them. It is suitable for tunnels in permeable ground with limited, manageable water ingress or treated by means of pre-grouting or groundwater lowering, since these conditions allow the use of spray applied waterproofing membranes. While the primary lining consists of sprayed concrete, the secondary lining may consist of sprayed concrete or cast in-situ concrete. Both concrete and waterproofing membrane are relevant functional parts of the composite shell lining system.

Composite Shell Lining for Tunnels and Mined StationsFigure 3: The composite shell lining system

Design options and related benefits
The following options are available for the design of composite shell linings (see Figure 4):
Composite Shell Lining for Tunnels and Mined StationsFigure 4: Design options for composite shell linings and related benefits
Design options and related benefits
  • Option 1: Temporary primary lining / Cast in-situ permanent concrete secondary lining
  • Option 2: Temporary primary lining / Permanent sprayed concrete secondary lining
  • Option 3: Primary and secondary linings made of permanent sprayed concrete
The benefits resulting from option 1 are mainly related to the bonding properties of the waterproofing membrane and its fast application (shorter construction program). Additional benefits resulting from option 2 are faster construction of the sprayed concrete secondary lining, with no need for formworks.

In the case, the structural contribution of the permanent primary lining can be ensured, option 3 enables additional savings through substantial reduction of the lining thickness of the secondary lining, leading consequently to a reduced tunnel cross section, which requires less excavation and reduced volumes of construction materials.

Composite Shell Lining for Tunnels and Mined StationsFigure 5: View of the Cathedral of Lausanne, Switzerland

Besides of the above mentioned advantages, the use of a spray applied waterproofing membrane also leads to higher flexibility of work programming and sequencing. Also long-term maintenance costs can be reduced, because with the use of a double-bonded waterproofing membrane any leak through the tunnel lining can be easily located and treated locally with a small volume of injection materials.

Project reference: Tunnel de Viret, Metro Lausanne, Switzerland
The final concrete lining of the tunnel de Viret, part of the metro line M2 in Lausanne, Switzerland, was designed based on the composite shell lining approach, with permanent fiber reinforced sprayed concrete and the spray applied waterproofing membrane MasterSeal 345 installed between primary and secondary linings. The tunnel is about 275 meters long and passes under the Cathedral of Lausanne.

The tunnel is entirely located in weak molasse rock, partially with very low rock overburden. The overlying soil consists of water saturated strata of sand, gravel and moraine. Because the cathedral of Lausanne is founded on these sensitive soils, one of the critical issues for the design and construction of this tunnel was to maintain the groundwater level, avoiding any drainage of groundwater during tunnel excavation or operation, in order to reduce risks of settlements of the cathedral. The following Figure 6 shows the critical vertical section where the tunnel passes close to the cathedral. This situation required a proper technical solution to maintain the groundwater level during construction. For that purpose, pre-injection of the ground, surrounding the tunnel was done in combination with groundwater infiltration through wells. In order to avoid any drainage of groundwater during tunnel operation, the tunnel lining was planned with a fully-tanked (submarine) waterproofing system.

Composite Shell Lining for Tunnels and Mined StationsFigure 6: Geological cross section with the tunnel de Viret below the cathedral of Lausanne

As shown in Figure 7, the original tendered design called for a double shell lining system, with a cast-in-place secondary lining with a thickness of 30 cm, and traditional waterproofing with a pre-fabricated polyethylene waterproofing sheet membrane over the entire tunnel perimeter. This design was reviewed during construction, and an alternative technical solution based on the use of permanent fiber reinforced sprayed concrete and the spray applied waterproofing membrane MasterSeal 345 was adopted, creating a composite shell lining system.

A risk analysis was carried out in order to verify long-term durability issues, as well as possible repair and maintenance of the tunnel lining. This analysis concluded that the adopted technical solution met the requirements for long-term durability. Different technical and financial aspects were analyzed and considered favorable for this alternative technical solution, including:
  • Reduced total lining thickness, from 56 cm to 43 cm (about 23% reduction)
  • Shorter construction time
  • Reduced total construction cost (no formworks, faster construction)
  • A more reliable technical solution due to the properties of the waterproofing membrane (no migration of water, easy repair of eventual leaks)
The construction of the tunnel lining was done in the following sequence (see Figure 7 and Figure 8):

Composite Shell Lining for Tunnels and Mined Stations
  1. Installation of temporary excavation support
  2. Construction of the permanent fiber reinforced sprayed concrete primary lining
  3. Smoothening of irregular areas on the primary sprayed concrete lining
  4. Manual spray-application of the waterproofing membrane against the primary lining (on the crown, bench and invert)
  5. Treatment of localized seepage points (temporary drainage, injection)
  6. Construction of the cast-in situ concrete invert
  7. Construction of the permanent fiber reinforced sprayed concrete secondary lining in the bench and crown of the tunnel
  8. Construction of an unreinforced sprayed concrete finishing layer (4 cm thick) in the bench and crown.
The tunnel de Viret is in operation since 2008 (see Figure 9). In addition to the successful design change with this innovative technical solution, significant construction cost and time savings were achieved, including

Composite Shell Lining for Tunnels and Mined StationsFigure 9: Tunnel de Viret in operation since 2008
  • Reduction of the excavated section due to the reduction of the lining thickness
  • Significantly reduced amounts of construction and excavation materials
  • Elimination of cost of formworks through the use of permanent sprayed concrete for the secondary lining
  • Reduction of the original construction program by two months
  • Total cost savings of approximately CHF 700,000 for the whole tunnel (about CHF 2,500 per tunnel meter)
Since taking into operation in 2008, the tunnel lining has shown neither defects nor leaks.

Waterproofing Membrane for Composite Shell Lining
MasterSeal 345 is a cementitious polymer-based spray applied waterproofing membrane for underground structures with a successful track record since 2005. The membrane is produced by means of a non-reactive system. It is applied against the primary sprayed concrete lining (substrate), typically with a thickness of 3 mm, in only one stage, and covered later on by a secondary concrete lining or a protective non-structural concrete layer, building a composite shell lining.

The outstanding feature of this membrane is its bond strength. The membrane adheres equally to primary and secondary concrete linings (double-bonded) with significant bond strength, giving the tunnel lining system unique mechanical properties and waterproofing features. The bond is unaltered by the concrete placement technique, be it sprayed or cast in place, or by the presence of fibers.

In a composite shell lining with this double-bonded waterproofing membrane an eventual groundwater inflow through the primary lining stops as it reaches the membrane and it cannot migrate along the membrane-concrete interface. Thus, potential groundwater paths can be eliminated, mitigating considerably the risk of water ingress into the tunnel, as shown in Figure 11. Additionally, the strong bond between the membrane and the secondary lining provides a further barrier against water ingress into the tunnel.

Composite Shell Lining for Tunnels and Mined Stations

The use of MasterSeal 345 is particularly advantageous in geometrically complex areas such as in lay-by niches, cross passages, turn-outs and crossover caverns, where installation of conventional waterproofing membranes is inherently difficult and locating of possible leaks is challenging. Additional advantages result from the sprayed nature of the membrane and its bonding properties.

Composite Shell Lining for Tunnels and Mined StationsFigure 12: Application of MasterSeal 345 in a cross passage of a tunnel
Application of this waterproofing membrane is fast. It is done by means of a dry-mix process (dry powder and water). Typically 50 to 100 m2 per hour can be manually applied by 3 operatives, while mechanized /robotic spraying can reach up to 180 m2 per hour. These rates are far higher than typical installation rates of sheet membranes (approx. 25 m2 per hour). Resulting time and cost savings for both owners and contractors have contributed to spread out this waterproofing concept worldwide.

It is suitable with all types of inner lining including steel fiber reinforced sprayed concrete and cast in-situ concrete. It is appropriate for application on all types of concrete substrates which provide proper bonding and allow building of a continuous membrane. It does not require surface evenness. However, the smoothness (maximal aggregate size of the concrete substrate) and quality of the substrate influence the substrate preparation works and the consumption of material.

This waterproofing membrane is compatible with conventional waterproofing systems, i.e. sheet membranes. It can be sprayed onto properly installed and cleaned sheet membranes. It is also compatible with all types of reinforcement, including steel fibers, i.e. the membrane can be applied over reinforcement, steel insertions and bolt plates. Steel fiber reinforced sprayed concrete can be applied onto the membrane.

Different underground structures, e.g. tunnels, cross-passages, metro stations, shafts and caverns have been successfully completed using MasterSeal 345, under quite different conditions and design requirements. It has offered a viable, cost-effective solution for regularly found ground and hydrological tunneling conditions. It strongly contributes to the functionality of the tunnel lining, and enhances the durability and serviceability of the tunnel lining system.

Bibliography
Holter, K., Bridge, R., & Tappy, O. (2010). Design and Construction of Permanent Waterproof Tunnel Linings Based on Sprayed Concrete and Spray-Applied Double-Bonded Membrane. Proceedings of the 11th International Conference of Underground Construction “Transportation and City Tunnels”, (S. 571-574). Prague.

Thomas, A. (2009). Sprayed Concrete Lined Tunnels, An Introduction. London and New York: Taylor & Francis.

NBM&CW June 2020

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