A few years ago the government of Buenos Aires commissioned the Arroyo Maldonado tunnel project with the aim of reducing the inundations that periodically hit the Argentinean capital, causing incalculable damage to people and structures.

The project involves the construction of two 7.9m-diameter tunnels, with the function of catching and conveying rain water to Rio de Plata River. The client is Gobierno de la Ciudad de Buenos Aires, with Geodata as designer and Ghella as the contractor. Two Lovat EPBMs were used.

In September 2009 the first TBM was launched on the approximately 5km-long ‘short tunnel’ and finished it after about 350 working days. Construction on the ‘long tunnel’ started a few months after the first and was completed in December 2011.

Along the alignment the TBM mined through varying geological conditions with low overburdens, generally 15 to 30m with particularly challenging passages less than 10m under existing structures including old hydraulic channels and buildings.

Choice of a suitable ground conditioning system
The choice of the most appropriate products and parameters for the ground conditioning is of paramount importance to achieve satisfactory TBM performances.

In particular, the attention that had to be given to ground conditioning in the Arroyo Maldonado tunnels was even greater than in standard tunnelling projects, because of the continuous variations of geological profiles, as well as the low and sometimes very low overburdens.

Sometimes ground conditioning is described as a ‘black art’, however some parameters can be taken in consideration in order to rationalise it. In particular the following steps were observed for the Maldonado tunnels:

– Significant laboratory tests: a series of tests were carried out in the laboratories located at the Technical University in Turin, Italy with representative samples of the different types of soil to be excavated. This first step identified the most appropriate foaming agent and gave the contractor a first approximation of the ground conditioning parameters and consumptions. The laboratory tests gave indications that the product Polyfoamer FP (biodegradable foaming agent in combination with a lubricating polymer, produced by Mapei) was totally compatible with the types of soils to be excavated on the project.

– Optimisation of the ground conditioning system directly on the TBM: the parameters calculated in the laboratory were used as reference and modified with the aim to achieve the most appropriate solutions from technical and economic points of view.

– Adjustments of the ground conditioning parameters during the advance, focusing the attentions on geological variations.

– Continuous controls of the consistency of the muck extracted by the screw conveyor and of the TBM parameters, such as cutting wheel and screw torques, advance speed, and also working pressures in order to understand as early as possible eventual problems or difficulties in the EPB operations which can be solved (partially or totally) by changing the ground conditioning system.

During all four steps the cooperation between the contractor and the supplier was proficient and useful to solve the eventual problems encountered during the TBM operations.

A rational ground conditioning system used during the whole tunnel alignment and the choice of appropriate foaming agents allowed the contractor to achieve satisfactory advance performances in both short and long tunnels.

The appropriate use of conditioning products allowed EPB control during the whole advance within the design ranges of minimum and maximum pressures, including the most challenging sections.

In the long tunnel excavated under the Rio Subterraneo, overburden reduced to a minimum of just 6m. It was clear that in such conditions the EPB pressure must be controlled perfectly: the result was that no movements were registered.

A proper addition of foam gives the soil a consistency able to transmit pressures to the face and decrease its permeability significantly, and better controlling the water ingress inside the chamber. As a result, the surface settlements can be better controlled.

Two-component backfill grout injection for a complete filling of the annular void
The filling of the annulus void created during the shield advance was carried out in Maldonado with a two-component grout mixture made of:
– A very fluid and stable cementitiuos grout, with long workability properties due to the addition of the retarding agent Mapequick CBS System One, produced by Mapei.
– An accelerator admixture, the Mapequick CBS System Two, added to the cementitious grout and able to react, resulting in a thixotropic and nonwashable gel.

Several tests carried out, firstly in the laboratory (using the raw materials received from Buenos Aires) and later at the job-site, were useful in finding the most appropriate mix-design according to all the specific requests coming from the project, in terms of pumpability, volumetric stability, jellification and mechanical strength development.

Two factors were considered by the contractor during the injection to control the real complete filling:
– The volume: knowing the theoretical empty volume left by the shield behind the segments, a slightly higher amount of grout must be injected.
– The pressure: the injection was carried out during the advance with average pressures slightly higher than the EPB ones, until achieving a maximum pressure at the end of every advance.

These two parameters were continuously checked and allowed the contractor to properly fill the annulus space in both short and long tunnels.

Graphs showing the grout volume injected per ring allowed the contractor to control and confirm the complete filling of the empty spaces behind the tunnel segments. Figure 2 clearly shows a homogeneous grout volume injected with an average of approximately 6.1m3 per ring.

The theoretical volume to be injected per ring is 5.7m3.

By using a proper mix-design and specifically designed equipment, the risk of clogging the pipes was minimised, ensuring a constant and continuous injection of the mix. If the injection lines are often clogged, the injection cannot be constant and it is more probable that empty spaces will be left behind the lining.

The durability of the two-component backfill grout is guaranteed in the natural conditions of humidity present in the underground excavations.

In the short tunnel it was possible to directly observe the backfill grouting material due to the removal of superficial ground down to the tunnel depth a few months after the TBM passage. As shown in figure 3, the two-component material was found in perfect conditions, hardened and perfectly filling the space between the tunnel lining and the surrounding ground.

Conclusions
Ground conditioning and backfill grouting carried out in the Arroyo Maldonado project were both described in this paper. The operations are of paramount importance to minimise the surface settlements, particularly under difficult conditions such as in Buenos Aires: very urbanised areas,reduced overburden and the presence of underground water with an average pressure of two bars.

Appropriate use of the foaming agent Polyfoamer FP allowed for correct ground conditioning, improving the pressure transmission through the soil mass, which lead to better face stability and guaranteeing satisfactory TBM advances and performance.

The two-component backfill grouting injection allowed a complete and immediate fill of the annular void behind the tunnel lining, minimising volume loss around the tunnel and therefore minimising associated settlement.