Optimisation of ground conditioning is one of the key parameters to achieve a successful tunnel drive with an EPB shield machine.

The conditioning products and parameters therefore have to:

•Generate uniform face pressure
•Minimise inflow/over-excavation
•Create a homogenous flow path from the tunnel face to the belt conveyor
•Minimise torque and friction

Before the TBM launch, it is strongly recommended to carry out a series of laboratory tests. These should be able to characterise the soil to be excavated and to give a good estimate of the parameters to be used during tunnelling.

For this reason, over the last 10 years, there has been a strong cooperation in research between the laboratory of Tunnelling and Underground Space at Politecnico di Torino (Turin Technical University) and the UTT (Underground Technology Team) of Mapei. This has led to the creation of a laboratory fully-dedicated to ground conditioning tests as well as the development of new laboratory devices and procedures specially designed for use in the tunnelling industry.

Tests
Some of the most important laboratory tests for characterisation of conditioned soil are presented and discussed as follows.

Foam half-life
The half-life of foam (t1/2) is the time required for 50 per cent of the water used to generate the foam to drain away. The half-life depends upon the foaming agent concentration in water and the foam expansion ratio (FER). The longer the foam half-life, the higher its durability and efficiency when mixed with soil.

Slump tests for conditioned soils
In order to quickly characterise the conditioned ground, it is possible to carry out standard concrete-type slump tests on the material. In a standard mixer, the proportioned amounts of ground and foam solution are mixed together in order to achieve the desired FIR (foam injection ratio). The slump tests allow ta visible check and measurement of the consistency of the conditioned ground and to determine whether it is appropriate to EPB operation.

Permeability test
Soil permeability is a key parameter for good conditioning, particularly when working below the water table. Only conditioned ground with low permeability facilitates the correct management of the pressure to the face in the presence of water and prevents possible water inflow into the excavation chamber.

For this assessment it is not possible to use a standard permeability test since this procedure requires a constant water flow through the sample. In the case of conditioned soils, the flow washes away the bubbles thus changing the results that can be obtained and that are significant for the technological application of an EPB machine. To get significant results a modified permeability test should be carried out. In the laboratory of Politecnico di Torino a specific procedure with the measurement of the time required by a predefined amount water to pass through a sample has been set. A series of tests have shown the feasibility of the procedure.

Wear test
To ascertain if the adopted conditioning is able to reduce both the wear on the metallic parts of the machines and the friction of the muck, a test device, where a disk is turned inside the soil at a constant speed for a pre-defined time, has been developed in the laboratory and the test procedure has been defined. The test is carried out with the measurement of the loss of material of the test disk (made of brass or aluminium) before and after the test and the measurement of the applied torque. This test procedure has been shown to be feasible and to permit a good comparison between different conditioning sets.

Simulation of the ground extraction by a screw conveyor
This test allows evaluation of the suitability of conditioned materials to be extracted from a chamber under pressure by means of a screw conveyor. The experimental apparatus available at the laboratory (the only one in Italy, and among the few available in the world of this type) consists of a metallic cylinder of nominal diameter 600mm that is coupled to a 1.5m-long screw conveyor. The apparatus is equipped with several sensors to measure the pressure in the tank, along the screw conveyor and the torque on the screw.

Basically this test allows checking of the efficiency of the ground conditioning parameters found during the slump test on a larger scale with a procedure closer to real action of an EPB machine.

The ground is considered to be conditioned correctly if the screw conveyor is able to extract soil constantly, and the pressures registered along the screw decrease regularly from the cell closest to the chamber to the last one. Furthermore this test assesses the suitability of the conditioned soil to apply a regular pressure and to traverse the screw conveyor and measure the reduction in torque.

Some case histories
During the co-operation between Mapei UTT and Politecnico di Torino, several tests have been carried out with samples of soil received from mechanised tunnelling job sites located all around the world.

From these tests it is possible to establish a number of correlations between the results achieved in the laboratory and what is actually used on the EPB on site. In the following projects, initial trials were carried out in the laboratory in Turin and subsequently it was possible to check directly on the TBM if the parameters were appropriate for real EPB operation.

Maldonado water tunnel in Buenos Aires
Client: Gobierno de la Ciudad de Buenos Aires
Contractor: Ghella Costruzioni About 10km of tunnel have been excavated with two EPB shield machines. Several different and severe geological conditions have been excavated, including both very fine and sticky soils and gravels with highpressure water present.

After the laboratory tests, the product Polyfoamer FP was chosen, with optimal parameters including concentration at 2.0 per cent, FER 12 and FIR 40 per cent. The use on the EPB has completely confirmed the parameters established in Turin: FER varied from 10 to 14 and FIR between 35 and 60 per cent, dependant on the geological conditions at that point in time.

These subtle variations allowed the optimisation of the EPB operations in terms of advance speed, torque, wear reduction, and to minimise the foaming agent consumption.

Metro Line C, Rome
Client: Roma Metropolitane
Contractor: Metro C Spa (joint venture of Astaldi, Vianini Lavori, Ansaldo STS, CMB and Consorzio Cooperative Costruzioni). Two parallel tunnels are under construction in Rome using four EPB machines. The geological conditions include generally clay and silty ground with underground water.

The laboratory tests were carried out with the foaming agent Polyfoamer FP by Mapei, and provided useful information to set the optimal conditioning on the job site.

More than 18km of tunnels have been successfully excavated proving the compatibility between the ground and Polyfoamer FP. The parameters established in the laboratory tests were optimised on the EPB (in particular the distribution of the foaming solution among the injection lines) and promoted very high production (up to 25-30m/day), with consumptions of foaming agent lower than 1l/m3 of excavated soil.

Conclusions
The experience accumulated during the recent years of co-operation between the UTT of Mapei and Politecnico di Torino, proves the need for a series of laboratory technical tests to be carried out before the launch of an EPB machine. This allows us to get basic information about:

•The products to be used for the ground conditioning (foaming agents and possibly polymers)

•The parameters relating to concentration, FER and FIR to be set in the machine

The laboratory tests also give the designer and or the contractor the possibility to estimate the costs to be considered for the ground conditioning operations during tunnelling before the TBM starts.

The use of the appropriate products and parameters minimise the possibility of facing unexpected problems during tunnelling operations.

These could otherwise lead to expensive and dangerous (for workers) delays due to, for example, the replacement of tools. When no intermediate shafts are scheduled in the design, the previous knowledge of the soil and its conditioning is even more vital to success.

This has been confirmed now by several TBM projects. It has been possible to show a strong correlation between the products and conditioning parameters established in the laboratory tests and their ultimate use on the TBM.


Laboratory equipment for simulating EPB operations The wear test; it is possible to see the position of the torque-meter and the tank where the soil is inserted Figure 1, results obtained from half-life tests using the same foaming agent liquid for differing foam expansion ratios or FERs Figure 2, example of measured torque during the wear test without conditioning and with conditioning Two images of the ground from the Buenos Aires job site, before and after the conditioning with the foaming agent Polyfoamer FP (FIR 40 per cent, FER 12) The two image show the results of conditioning testing on Metro C Spa’s Rome Metro Line C (FIR 40 per cent and FER 12) The modified permeability test layout and equipment