As a shielded TBM advances, an annular void is inevitably created behind the concrete lining. This void has to be completely filled with an appropriate material as soon as operationally possible. This is to minimize ground movement, lock the ring in its design position and guarantee a homogeneous strength, leading to evenly distributed transmission of pressures between the soil and concrete.

Different solutions for filling the annular space have been tested. The most traditional material, especially in Europe, is a cementitious grout—made up of water, cement and chemical admixtures, and able to improve the mix pumpability and to regulate its setting time. Bentonite can be added to stabilise the material and improve its waterproofing properties. Sometimes, particularly with French constructors, cement is not used.

In recent years, both tunnel designers and TBM manufacturers are introducing more and more the two-component backfilling system, composed of:
• Component A: a super-fluid grout, comprising cement, water and bentonite; dosed with a retarding agent able to guarantee its workability and pumpability even over long periods and distances.
• Component B: an accelerator admixture, added to Component A immediately prior to its injection, enabling an almost immediate jellification of the mix.

The main advantages of this system over other filling materials are:
• Its super-fluid consistency and workability maintenance that minimises the risks of clogging transport lines and pumping pipes. Pumping the grout over several kilometres obviates the need for grout cars.
• Its ability to completely fill the annular space behind the ring, minimising ground movement and consequently risks of subsidence.
• The very quick hardening, even in presence of underground water, which allows a rapid development of early-stage mechanical strengths, locking the ring in its design position. The fast gelling action also reduces losses through the tail-brushes.

Grouting Line C
A joint venture of Astaldi, Vianini Lavori, Ansaldo Trasporti-Sistemi Ferroviari, Cooperativa Muratori e Braccianti di Carpi, Consorzio Cooperative Costruzioni is constructing Metro Line C in Rome.

Four EPBMs, manufactured by Herrenknecht, have excavated approximately 12km of tunnels up to date. Most of the alignment is through pozzolanic soil with the presence of ground water.

The EPBMs are equipped with a backfill grouting system (pumps, lines, etc.) designed for use with two-component materials.

The two-component ingredients and their respective dosages have to be carefully studied in order to achieve the most appropriate solution, both from a technical and economical point of view. Every project has its peculiarities; therefore it is necessary to adapt the mix-design of the two-component mix to the specific requests of each job site.

The first step is the choice of the raw materials. In particular, bentonite is the most important material to be selected. Technically, the bentonite chosen should be able to guarantee the quickest activation/hydration with the available water supply. This means that bentonite, after being mixed in a high speed colloidal mixer with water, starts hydrating in only a few minutes. This mitigates the pre-hydration operation that is common when using bentonite in other fields (for example, diaphragm wall slurries) that require large storage facilities.

The choice of the cement and its dosage mainly depends on the mechanical strength development requested in the project specification. In the Metro Line C project the specifications for compressive strengths relate only to the very short term and short term stages (until 24 hours from batching).

Another important parameter to consider is how the mix is to be transported to the TBM. In Rome, two different ways have been used: in the first two tunnels excavated (T6 alignment) a mortar car was used, while in the T5 lines the mix (Component A) is pumped from the batching plant directly to the TBMs. The second way requires a better volumetric stability of the Component A, where the separation between water and solid components (“bleeding”) has to be almost zero. This avoids the settlement of solid materials in the pumping lines, which would cause their clogging and consequent long production breaks.

Also the choice of the batching plant to be used in the job site is of paramount importance. Its productivity and its storage capacity have to be carefully chosen according to the expected productivity of the TBMs. The mixer has to guarantee enough turbulence/shear in order to efficiently activate the bentonite and disperse the cement particles.

Keeping in mind all the specific requests, a number of tests have been carried out by Mapei’s research and development laboratories in Milan. The aim was to test different raw materials, supplied by the contractor, their compatibility and the dosage of every ingredient.

Once a mix-design was established in the laboratory, the technical assistance of the Mapei underground technology team (UTT) continuously tested the backfilling material produced in the job site batching plant during the first weeks of the TBMs advances. The aim was to check if the results are comparable to those achieved in the laboratory.

Characteristics of the two-component material
The campaign of laboratory and in-situ tests lead to a mix-design that is able to guarantee all the requests that the backfill material has to achieve:
• Fluidity of Component A able to ensure easy pumping even for long distances.
• Workability maintenance of Component A that allows storage of the material over long periods (up to 72 hours from batching) mitigating cleaning over extended TBM breaks. This is possible due to the Mapequick CBS System 1, a highlyefficient liquid retarding agent with a plasticising effect. It is automatically added into the turbo-mixer after all the other ingredients and then mixed for a few minutes.
• Volumetric stability of Component A, which does not “bleed” and therefore minimises risks of clogging pipes. The right kind of bentonite and its appropriate dosage facilitated this.
• Almost immediate jellification after the addition of Component B to Component A. The time between the additions of the accelerator admixture (Mapequick CBS System 2) to the complete loss of workability has to be balanced. The time for gelling is balanced between avoidance of clogging the injection pipes, travelling around the annulus and gelling to avoid being lost through the TBM tail brushes.
• Early and very-early mechanical characteristics that avoid surface subsidence and lock the ring in its design position. Long-stage compressive strengths do not represent a significant parameter, as the main aim of backfilling material is to perfectly fill up the annular space left empty behind the ring while excavating.
• Low permeability that makes the filling material a barrier against ground water passing into the tunnel (a large benefit in ensuring the tunnel water-tightness is guaranteed in combination with gaskets and concrete quality).
• Stability and durability against eventual washing-out actions or chemical attacks from ground water, and vibrations due to train passages.

Injection
The Component A is batched in a plant located close to the tunnels entrance, which is connected with a mixing tank with capacity of approximately 5m3.

Then the Component A is transported in the TBM and stored in a mixer tank with capacity of 7m3, connected with n.6 screw pumps.

Component B (accelerator admixture) is stored in the TBMs in a 1,000l tank, which feeds n.6 screw pumps.

The TBMs are equipped with six injection lines, regularly distributed around the shield circumference. Normally only the upper four lines are used, because the flow that they can ensure is enough to fill the annular space left empty by the shield advance (average speed 30-35mm/min). Furthermore, the two lower lines are the most likely to choke.

Component A is normally injected at a pressure 0.1 – 0.2 bar higher than the EPB pressure. It is important not to use a pressure much in order not to submit the shield brushes to elevated strains. Elevated grouting pressures also often lead to ring deformation.

The injection is started when the TBM has already excavated the first centimeters of the advance, so that there is already an empty space to be filled. There are two criteria to stop the injection: the volume of injected material should achieve the desired one (normally the theoretical empty volume plus 15-20 per cent), and in every line the preset pressure should be reached. The achievement of the above proves the complete filling of the annular space behind the segments.

Regular cleaning of every injection line is carried out. After every advance, water at a pressure of 30 bar is injected, and approximately once a week every component of the circuit is checked and eventually replaced. The cleaning is important especially for the nozzle of the accelerator lines and to completely remove any hardened particles of Component B. Any undesired contact between Component A and Component B due to leakages in the nozzle would cause at least an impairment to the system.

Controls
Some relevant facts and controls are necessary to prove the efficient backfilling. This proves the procedures used for the injection are appropriate, as well as the chosen materials and their dosages.

The main properties of the twocomponent system are checked daily on the job-site. Characteristics of the Component A, such as its fluidity and volumetric stability, are measured, as well as the properties of the hardened material (after the addition of Component B), such as gel time, mechanical strength development up to 24 hours from batching, permeability, etc.

The effective 360 degree filling of the annular space is periodically inspected by extracting cores. To date all the cores have always shown the presence of the hardened two-component material behind the ring with the expected length (approximately 130-150mm).

An experimental test has been carried out. Several sensors have been inserted in the segments before the backfilling injection in order to measure the effective pressure of the two-component material. The measured pressures almost correspond to those set by the operator.

Furthermore, during some stages of the metro line execution, such as TBMs arrival in the final shaft, execution of stations, etc., some concrete segments had to be removed. This allowed a visual check of the backfilling material. In all the cases, the presence of the two-component mix could be noticed for the whole thickness of the annular space between concrete and surrounding ground.

Finally, it should be underlined that the tunnels are excavated in densely urbanized areas, below several roads, buildings and historical monuments. Therefore not to properly fill the annular space would be a huge risk. To date no significant subsidence has been measured on the surface.

Conclusions
It is possible to affirm that in the first 12km of excavated tunnels no significant problems associated with the twocomponent backfilling material have arisen.

The volumetric stability of Component A has been proven by the absence of blockages in the transportation lines from the batching plant to the TBMs. Also the injection lines of Component A and Component B are very seldom subject to problems, due to appropriate cleaning system.

Therefore downtime due to pipes choking is minimised or almost nullified. This is of paramount importance because sometimes, especially when using traditional cementitious mortars, the delays associated with pipe cleaning represent a significant loss of time and money.

Another advantage is that all the raw materials for the two- component systems used in Line C (cement, bentonite, Mapequick CBS System 1 retarding agent and Mapequick CBS System 2 accelerator admixture) are industrialised and therefore their origin and properties constantly controlled. Sand, which is always used in traditional cementitious grouts, can be subject to variations in its natural humidity content, which could lead problems in the final grout properties.

The almost immediate jellification of the material allows a very quick development of mechanical strength. The very early and early mechanical resistances, in combination with a complete filling of the annular space, minimise risk of surface subsidence. It’s important to underline that the long stage strengths aren’t significant for the backfilling material because the structural resistance of the tunnel must be guaranteed by the concrete segments.

Finally, the very fast gelling and the complete void filling reduce significantly the ring deformation, which could be due to the TBM shove forces.


Final tank where the Component A is stocked and then pumped to the tunnels Views of the batching plant in the Line C job-site Views of the batching plant in the Line C job-site Length of the cores extracted in the tunnels The whole annular space has been filled with the two-component material Extraction of a segment Presence of the two component material in the annular space