Performance of Two-Component Back-filling Grout in TBM

To ensure a successful backfilling operation, the following are essential:

• Lock the segmental lining into its final position, avoiding movement of both the segmental self-weight and the thrust forces

• Bear the loads transmitted by the TBM’s backup weight

• Ensure immediate uniform contact between the ground and the lining

• Avoid puncture loads by ensuring the application of symmetrical, homogeneous loading along the lining

• Complement the waterproofing of the tunnel with the concrete lining (i.e., if the lining has cracked due to improper installation, back-fill grout can help to mitigate any water)

To achieve these demands, the simultaneous back-filling system and the injected material should meet the following technical, operational, and performance characteristics:

• The backfilling must be instantaneous to avoid creating voids in the annulus while the TBM is advancing. This is why backfilling is typically carried out through a network of pipes located in the TBM’s tail skin
• The annulus must be regular and filled so that the lining is continuously linked to the surrounding ground (the system becomes monolithic)
• The reliability of the system must be guaranteed in terms of the transportability of the mix
• The injected material must gel very quickly after injection (which occurs progressively as the annulus is created)
• The injection process must continue until the maximum pressure is reached

The 2K system injection for back-filling while excavating with shielded TBMs is progressively replacing the traditional use of 1K cementitious mortars, for two main reasons:

1. It reduces the risks of choking pipes and pumps (typical when pumping cementitious systems

2. It guarantees the complete filling of all annular voids created after the TBM’s tail passage, thus avoiding surrounding movements 

2K annulus grout  - total thickness of the annular space.

The main features of two-component grouts

The main features of two-component grouts are:

• Ability to provide a super-fluid initial consistency (Component A)
• Development of a gel a few seconds after injection, giving an early compressive strength >0.1 MPa (Component A+B)

Consequently, according to international methods, it is vital to verify that the mix develops into gel even before it is injected into the annulus space to confine the segment ring homogeneously. Since it is impossible to verify the event inside the annulus, this must be simulated by preparing samples in the laboratory to determine the consistency achieved by the gel in its early stages. The later stages are less critical because the gel’s mechanical strength does not influence the structural behavior of the tunnel lining if the annulus is filled. As the injected material for a 2K system is an ultra-fluid liquid that reaches a thixotropic consistency in a few seconds thanks to the addition of an accelerator admixture just before injection, and as it consists of a vast amount of water (approximately 800 liters per cubic meter of material), it is undoubtedly an incompressible fluid, just like water.

As a result, the annulus void created can be considered a closed annular bubble that is eventually filled with an incompressible fluid. So every movement of the surrounding ground entering the bubble or any movement of the concrete lining reducing the bubble volume instantaneously leads to the creation of another reaction pressure in the ball. This is uniform throughout the volume and especially the surfaces of the volume, thus avoiding any deformation. To achieve this, we need the following conditions:

• The injected material must remain uncompressible
• The fluid cannot escape from the bubble:
   
  • It cannot permeate through the surrounding ground (this is avoided by underground water exerting a hydrostatic pressure on the injected material)
  • It cannot escape through the space between the tail and the excavation profile, avoided by maintaining a correct balance between the tunnel face pressure and the injection pressure (which must be approx. 0.2 bar, not more)
  • If the surrounding ground condition is poor and tends to close toward the bubble, it cannot be allowed to move with excessive pressure, as the pressure needed to advance the machine would increase to much. 

These conditions must be balanced and controlled with the correct equilibrium between the pressure in the excavation chamber and the injection pressure, assisted by lubricating the extrados of the tail skin with a bentonite slurry. The bentonite slurry injection should take place exactly where the tail is blocked and weighs on the ground that leans behind the invert of the lining and in the final part of the tail. Therefore, the segment ring being installed cannot have deformations such as ovalisation due to its own weight, which could lead to the irregular installation of the rings or insufficient pressure on the upper segments, and the gel cannot leach into any groundwater present.

Bearing all of these criteria in mind, it is necessary to inject a fluid that does not harden instantaneously, but that becomes a gel quickly and progressively without avoiding the formation of an incompressible ball at constant volume. The long-term mechanical strength of the backfill material does not matter because it does not give any structural contribution to bearing the hydrostatic and geostatic loads (the concrete lining supports these completely).

The gel must be as homogeneous as possible to mitigate the external loads. To this end, it is essential that the gel cannot decompose after its injection: its durability must guarantee that the incompressible annular ball is permanent.

Therefore, attention should be paid to the behavior of the injected material at early stages (from the first seconds to several hours), which includes the control of the installation of 2-3 segment rings.