21 - 26 April 2018 Dubai International Convention

& Exhibition Centre, UAE

ITA - AITES WORLDTUNNEL CONGRESS

POSTER PAPERPROCEEDINGS

Effectiveness of Foam Injection during Mechanized Excavation of Tunnels with TBM-EPB Technology

Diego Sebastiani1, Salvatore Miliziano2 Giuseppe Zanetto3 and Roberto Ginanneschi4 1 SAPIENZA University of Rome, via Eudossiana 18 00184 Rome, Italy.

diego.sebastiani@uniroma1.it

2 SAPIENZA University of Rome, via Eudossiana 18 00184 Rome, Italy.

salvatore.miliziano@uniroma1.it

3 SELI Technologies S.r.l., via Achille Campanile 73 00144 Rome, Italy.

g.zanetto@selitech.net

4 SELI Technologies S.r.l., via Achille Campanile 73 00144 Rome, Italy.

r.ginanneschi@selitech.net

ABSTRACT

In the mechanized excavation of tunnels with Tunnel Boring Machines, an ever more

frequently used technique worldwide is the Earth Pressure Balance (EPB) technology;

during the excavation phases this technology requires continuous injection of chemicals

under the form of foam to be mixed with the excavated soil.

Since the features of the foam influence significantly the excavation performances and the

propagation of induced effects such as settlements in the environment, the studies of the

parameters affecting the quality of the foam have great importance for practical purposes.

This study summarizes the results of several years of research performed in laboratories of

Sapienza University of Rome with the contribution of the expertise provided by SELI

Technologies and months of data acquisition derived from actual excavation projects.

Results show the influence on the quality and stability of the injected foam for several factors

such as: a) typology of foaming agents, b) dosage, c) pressure and flow of air and foaming

solution, d) typology and geometry of the foam generator, and e) features of the injection

plant.

The experimental activity was performed using a particularly arranged test apparatus, a

large number of chemical products from the main European suppliers commonly used in

tunnelling projects and a series of mechanical elements originated directly from injection

plants of several real tunnel-boring machines combined into the laboratories in different

configurations.

In parallel with the experimental activity, a series of field observations were collected and

processed to take into account several practical details that influence the design and

operation of the injection plant and on the quality of the foam injected. These data, from

various projects worldwide, provide general indications on the elements that cannot be

considered directly in the laboratory.

The results obtained from laboratory tests combined with the on-site observations, provide

both a useful input for an informed design of the mechanical elements of the plant and their

arrangement as well as indications on the choice of chemicals and management of the

conditioning parameters during the excavation process.

Key Words: Tunnelling; Soil-Conditioning; Foam Injection; EPB; TBM.

1. INTRODUCTION

The technology currently most used in urban environments involves the use of Tunnel

Boring Machines (TBM) and the Earth Pressure Balance (EPB) technology; its rapid

development is mainly due to economic, technical and practical aspects.

As has been described in detail by several authors as Merritt (2003) and Milligan (2001), this

technology has provided significant improvement over other technologies in the possibility to

control the pressure at the front face; this control is carried out by careful maintenance of a

balance between the excavated soil that enters the excavation chamber and the soil that is

extracted through the screw conveyor.

The pressure control to the front face is applied by means of the excavated soil appropriately

treated with chemicals in the form of foam. By controlling the rotation speed of the cutter-

head, the pushing force and the rotation speed of the auger, it is possible to balance the

volume, and therefore the pressure, of soil in the excavation chamber at any time

(Anagnostou and Kovari, 1996). These operations are possible only if the soil has the proper

consistency and its features do not vary too rapidly over time.

It is commonly known (Thewes et al. 2012) that from the dosage and the features of the

foam will depend on the characteristics of the soil present inside the excavation chamber

and, consequently, the ability to apply correctly a pressure to the front face.

Having understood the importance of injected foam characteristics during excavation,

several studies were carried out to control and measure the foam quality. The main

considered factors are a) chemical composition and dosage of the product used to generate

the foam; b) the foam generation modes c) system configuration of the plant and d) the

boundary conditions (flows and pressures) of the injection.

In few exhaustive studies and laboratory activities performed by Mori, Mooney and Cha

(2018), Parikh (2017) and by Thewes, Budach and Bezuijen (2011) the authors listed which

parameters are most closely related to the stability of the foam generated.

This paper resumes the results of a research activity aimed at quantifying the contribution of

the parameters involved in the quality of the generated foam. To consider more

combinations of the factors listed above, an industrial equipment was developed in the

Geotechnical laboratory of the Sapienza University of Rome to generate foam in controlled

mode; 25 different chemicals were considered, an extremely wide range of generation

parameters, 5 different foam generation cylinders (lance), different conformations of the

foam injection system, and different injector pressure values were considered. Finally, some

practical information from the site was taken into account to enrich the evaluations coming

from the laboratory activities, with information coming directly from the real on-site

experience.

2. THE FOAM GENERATION

During tunnel excavation with TBM-EPB technology, a continuous flow of foam has to be

injected to the front face through a series of injection point (nozzles); the system provides for

each point of injection a dedicated line for the generation of the foam, as described in

EFNARC (2005).

The generation of the foam requires some inlet lines providing a continuous flow of air, water

and foaming agent and takes place in a dynamic way, conveying these flow into a metal

cylinder in which some elements are placed to generate a turbulent motion.

The foam quality for mechanized excavation applications is commonly defined through some

of its features: bubble size and foam stability are the main two.

There are, of course, significant differences between the behaviour of the foam before and

after being mixed with the soil but, to isolate the effect of some characteristic parameters of

foam generation, we will then begin to evaluate the characteristics of the foam before the

injection into the soil.

2.1. Description of the phenomenon

The foam is composed by individual air bubbles, volumes of air kept separate by a fluid film.

The dimension and the features of its walls play a key role in guaranteeing the stability of a

single bubble; the thickness of the bubble walls and the typology of chemical bonds between

the molecules in the film determinate the strength in counteracting shocks and the ability to

accommodate variations in bubble size due to pressure variations.

In general, for a given chemical product, small and spherical bubbles (Fig. 1a) provide the

higher stability of the foam over the time.

In any case due to the combination of a series of effects, the liquid that forms the walls of the

bubbles will tend to drain, the walls will become thinner and micellar interaction phenomena

will tend to modify the shape from a spherical to a polyhedral (Fig. 1b). The bubbles in the

final configuration (Fig. 1c) for geometric and chemical reasons tend to collapse and in

general are more unstable if compared to the spherical configuration with considerably

thicker walls.

a ) b ) c )

Figure 1. Foam bubbles in their starting form (a), the evolution of the form over the

time (b) and the final form just before the collapse (c).

Foam generation occurs with a water dilution of chemicals essentially composed of

surfactants; these compounds are extremely soluble, by their nature, so they have no

difficulty in distributing themselves homogeneously in water.

Surfactants can be described as chains composed by a hydrophobic head and a hydrophilic

tail; the air and fluid injection under pressure will lead to the creation of so-called "micelles"

or particular forms of aggregation of molecules: the bubbles.

The cylinder in which turbulence is generated is called foam lance or foam generator, and it

is filled with elements of various shape. The diameter and the length of the cylinder and the

geometry of the elements inserted inside it, significantly affect the quality of the foam

generated.

The cylinder geometry influences both the quality of the foam and the range of flows (of

water, chemical and air) that can be used to generate a foam with characteristics suitable for

excavation. Finally, fundamental parameters are also the water and air injection pressure

and the pressure of the environment in which the foam is injected.

The bubbles during the advancement phase of the TBM explode due to the contact with the

ground, due to excessively sudden pressure variation, to the mixing process of the soil

inside the excavation chamber and in the screw conveyor. To produce the lubricating effect

and the change in the consistency of the ground necessary for the EPB technology are

required foam bubbles able to resist in the spherical shape inside the soil.

3. LABORATORY FOAM GENERATION PLANT, TEST APPARATUS AND

STANDARD PROCEDURE

3.1. Laboratory foam generation

The quality and reproducibility of testing is strongly reliant on the quality of the foam

produced for the tests. In poorly produced foam the bubbles combine and break more

rapidly, and the foam quality varies from test to test. Generating foam with a laboratory foam

generation system similar in all its parts to those installed inside a TBM, ensures that the

quality of the foam will be the same each time and, moreover, will be the same as that used

during excavation.

3.2. Foam generation system

The laboratory foam generation system used in this research is described in figure 2. It is

composed by four main lines with their respective links, measurement system and control

point of flow and pressure: air line, water line, foaming agent line and polymer line.

The selected chemicals, used to generate foam for the tests described in the presented

paper, are named as Foaming Agent (FA) and numbered from FA1 to FA25.

Figure 2. Main components of the laboratory foam generation system of Sapienza

University of Rome.

Water and air pressure, as well as the water, air and foaming agent flow can be controlled by

pressure gauges and flow meters on each line and their values can be modified in real time

during foam generation. All these parameters are constantly monitored through the HMI

(Human Machine Interface) unit.

3.3. – Foam generation parameters

Properties of the foam generated are described by selected parameters, synthetically listed

below:

. .

.

100f ag

sol

mCf

m (Concentration Factor of the chemical in the solution) (1)

.

f

sol

VFER

V

(Foam Expansion Ratio of the foam) (2)

100f

s

VFIR

V (Foam Injecion Ratio of the foam into the soil) (3)

where mf.ag. is the mass of foaming agent used, msol. is the mass of foaming solution, Vf is

the volume of foam, Vsol is the volume of foaming solution and Vs is the volume of the soil.

3.4. Test apparatus and standard procedure

As proposed by EFNARC (2005), to evaluate the durability of a foam under atmospheric

pressure a considerable number of half-life test was performed. The half-life time (hlt) of a

foam is defined as the time required by a foam to drain 50% of the weight of the initial

conditioning liquid used in foam generation.

The test apparatus consists of a cylinder with a funnel lower basis and a graduate cylinder to

collect the liquid drained from the foam over time (Fig. 3). According to the test procedure,

foam is prepared to the required FER, then the filter-funnel cylinder is filled with 80 g of the

foam and, finally, the volume of liquid collected in the lower cylinder is measured to define

the time required to collect 40 ml of the liquid used to generate the foam, the half-life time hlt.

Figure 3. Main components of the half-life laboratory test apparatus.

Tests were performed, using the 25 chemical products selected, on foam generated at very

typical FER values from 8 to 18. For the concentration factor, following the information

provided on technical datasheet of each product, typical values of Cf, in the range 1.5% -

2.5%, where generally adopted.

4. EXPERIMENTAL RESULTS

4.1. Effect of FER

The physical characteristics of the foam are first and foremost a factor that has to be

considered; to give an example, to appropriately condition coarse-grained soils it will be

necessary to inject large volumes of air to fill the relative high volume of the voids between

particles, therefore it will be necessary to inject a foam with higher FER values; vice versa, to

adequately condition fine-grained soils, it will be necessary to substantially change the

consistency index by injecting water and therefore it will be necessary to inject a more liquid

foam with a lower FER.

In the following graph in figure 4a are reported the results of several half-life tests performed

on the foam generated using several foaming agents at the same Cf of 2.0% and FER of 15

to underline the effect of the chemical composition of the product on the stability of the foam.

In the graph in figure 4b are presented the results of half-life test on foam generated at

various FER values using FA13 dosed at Cf 2.0%; it is visible that as the FER increases the

drainage time increases significantly.

Even if, on the basis of what has been said above, in the real application it is not always

possible to drastically modify the FER value to obtain an increase in the stability of the foam,

these results are useful to understand the possible effect that the increase or decrease of

the FER produces in the stability of the generated foam.

a ) b )

Figure 4. Results of the half-life test performed on foam samples generated using

FA13 dosed at Cf 2.0% and different FER values.

4.2. Effect of Concentration factor Cf

The effect of the amount of surfactant present in the liquid on the stability of the foam is well

known in the literature (Rosen, 2012); the concentration of surfactant affects the number and

strength of bonds within the foam walls and consequently the durability of the foam. Over a

certain concentration value, on the other hand, it is possible to arrive at the presence of

excess surfactant molecules that instead of contributing positively to the stability of the foam,

constitute a harmful element.

In the graphs in figure 5 (a) are reported the results of several half-life tests performed on

FA2 at increasingly value of Cf to verify the existence of a limit beyond which foam stability

decreases with increasing Cf. In figure 5 (b) the results of half-life tests performed on foam

samples generated using FA8 clearly prove that as the Cf increases the drainage time

increases.

a ) b )

Figure 5. Results of the half-life test performed on foam samples generated using

FA13 dosed at Cf 2.0% and different FER values.

4.3. Effect of Injection Pressure

The influence of air and water injection pressure on the quality of generated foam has been

addressed with a specially arranged equipment by Mori, Mooney and Cha (2018) and Parikh

(2017).

Having no further elements to add to that extremely complete study, for the sake of brevity it

will be specified only that for each configuration of the plant, included the variation in the

foam generation lance, has been verified the effect of water and air injection pressure on the

stability of the generated foam.

In figure 6 are presented the results of the half-life test performed on foam samples

generated using FA5, FA6, FA14 and FA18 dosed at Cf 2.0% and FER 12 generated using

the plant with the FG1 installed and different air injection pressure values. The water

injection pressure for each test has fixed 1 bar less than the air injection pressure, the water

flow has been constantly maintained at 6 l/min and the air flow consequently regulated to

reach the fixed FER values of 12.

Figure 6. Half-life time of foam samples generated using FG1, different air injection

pressure values and several foaming agents FA dosed at Cf 2.0% and FER 12.

4.4. Effect of foam lance diameter and filling elements

Inside the Geotechnical Laboratory of Sapienza University of Rome a series of generation

elements directly from several real TBMs were assembled.

It was thus possible to compare the effectiveness of the lance originally present in the

laboratory plant (FG1) particularly reproduced in scale and visible in figure 2, with different

cylinders actually used in TBMs (FG2 and FG3 are visible in figure 2).

In table 1 are listed the features of the five foam generation lance, numbered from FG1 to

FG5, considered in this study together with the operative range.

A remarkable effect is the relation between the diameter of the lance and the flow range of

water and air required to generate a suitable foam and consequently the difference between

the generators with a smaller diameter than the one with a larger diameter, as listed in Table

1. These values of water flow and FER must be understood not as absolute maximum and

minimum values but as the values beyond which the quality and homogeneity of the foam is

progressively compromised.

The generation lances of medium diameter (FG2 and FG4) have their optimal range

between 6 and 8 l/min of water; while FG2 is able to cover a wide range of FER that goes

from 6 to 20 in case of suitably dosed foaming agent, for FG4 the range is definitely reduced.

The generation cylinder with a larger diameter (FG3 and FG5) have ranges of flow shifted to

higher values, between 8 and 15 l/min and also in this case the range of FER values is wide

and goes from FER 6 to over FER 20.

Table 1. List of generation lance configuration tested.

Foam lance

diameter length filling Water flow Foam Expansion

Ratio

(-) (mm) (mm) (-) min

(l/min) max

(l/min) min (l/min) max (l/min)

FG1 60 150 filters 4 7 6 20

FG2 60 250 rivets 5 10 6 20

FG3 80 400 glass balls 8 >15 6 16

FG4 60 250 metal

sponges 5 8 8 14

FG5 80 400 rivets 8 >15 6 20

Figure 7. Details of the FG5 and FG3 filling elements.

In figure 8 are presented the results of several half-life test performed on foam samples

generated using the foam lances described, considering various FER values and a single

foaming agent dosed at the fixed value of Cf of 2.0%. Air and water injection pressure and

flow values have been calibrated for each configuration to guarantee the most effective

conditions in terms of stability of the generated foam.

The first visible effect is the difference between the laboratory scaled generation lance

(Foam Lance 1) and the real lances taken directly from TBMs: considering in fact different

values of FER the hlt for FG1 is always definitely lower if compared to the others. This effect

has been verified for different chemical products and two of them are presented in figure 8,

where each point is the average value of hlt relative to a number of tests variable from 3 to 5.

This suggest that the scaled-down generators for laboratory activities produce commonly a

less stable foam than the generators installed in the TBMs.

The same graphs in figure 8 provide some additional information on the effect of the

diameter and length of the lance in the stability of the foam. From figure 8a it is possible to

note that results for FG2 and FG5 are extremely similar, this suggest that for the same filling

element typology, the diameter (60 mm for FG2 and 80 mm for FG5) and the length (250

mm for FG2 and 400 mm for FG5) of the lance have a very low effect on the stability of the

generated foam.

It also seems that, despite having a more limited range of usage modes, the foam lance FG4

with metal sponges as filling elements is able to generate a foam with a slightly higher

average half-life time for the considered FER values.

a ) b )

Figure 8. Half-life time of foam samples generated using different Foam lances with

FA16 and FA3 at Cf 2.0% and different FER values.

In general, it is necessary to consider that the stability of the foam is only one of the

parameters to be taken into account and that the choice of the type of generation lance is

extremely complex and involves wider technical evaluations.

In this regard a phenomenon that often causes some operational difficulties during the

excavation and that must be considered to choose the proper system configuration, it is the

malfunction of the generation plant caused by the clogging of the filling elements cylinder.

This is caused by a series of factors, including, among the others, the hardness of the water,

the characteristics of the chemical products used, the use of additive polymers, the methods

of injecting the foams, unplanned stops, malfunctioning of the filter system. The result is an

increase in the time and frequency of interruptions required for maintenance the plant.

From a series of site observations, it has emerged that metal sponges tend to clog rather

quickly if compared with the other filling elements; for spheres, sporadic cases of clogging of

the foam generation cylinder are recorded, resolved by dismantling the plant and leaving the

contents of the cylinder in a solvent, while there are no registered reports of problems

connected to the use of rivets.

4.5. Effect of other elements composing the foam line

Even if, several attempts have been made to bring the foam injection lance close to the

cutterhead in real TBM applications, there is a minimum distance that the generated foam

will have to face before reaching the front through the injection points (nozzles). In its path

the foam have to cross the rotary, the node that separates the fix body of the TBM and the

cutter-head that instead rotates and this lead to a sharp reduction in the pipe section

typically from 5 cm up to even less than a 0.5 cm of diameter.

To make the experimental observations more detailed and the test apparatus more similar to

the real configuration, it has been reproduced in the laboratory all the elements of the foam

line located downstream of the foam generator using parts directly from the real TBM plant:

pipe of variable length, the section variation required to cross the rotary and the nozzle

element (Fig. 9).

The figure 9a is the photo of one of the nozzles particularly designed for this experimental

activity, realized to be in all its part equivalent to the one inserted in the TBM cutter-head; the

figure 8b is the pipe, taken directly from a TBM injection plant, added to the laboratory foam

generation system for this second part of the experimental activity.

The tests aim was to verify if the sum of the effects induced by the pipe length, the variation

of pipe diameter and the presence of the injection nozzle could have some influence on the

stability of the injected foam.

a ) b )

Figure 9. One of the typologies of nozzle considered and a section of the tube

inserted in the foam line.

For this reason, the foam generation plant was modified by adding the pipe used in the TBM

at the end of which a series of progressive section reductions were placed, until reaching a

diameter of 1 cm.

In figure 10 are presented the results of the half-life tests performed on foam samples

generated using the chemical product FA1 at the fixed Cf of 2.0% and the foam lance FG2 in

the original configuration, in the configuration with the pipe and the progressive diameter

reduction and finally with the configuration complete with also the nozzle element.

For several FER values can be noted a reduction of the hlt measured on the foam related to

the addition in the plant of the pipe and the diameter reduction.

From a comparison of these results with those obtained with the addition of the nozzle

element, vice versa a positive effect was found and an increase of the hlt recorded.

Overall, however, the effect of adding all the elements actually present in the foam injection

plant of a TBM produced a reduction in the stability of the generated foam; however, the

variation in hlf is lower than the variation caused by changes in the generation cylinder

configuration.

Figure 10. Result of the half-life tests performed on foam samples generated using the

plant in the complete configuration.

5. CONCLUSIONS AND FUTURE DEVELOPMENTS

An intense experimental activity has been developed, based on the use of a particularly

developed laboratory foam generation system arranged with components directly from TBM

injection plant. The activity allowed to obtain information on the effect of several components

of the foam injection system on the stability of the generated foam as the diameter and the

filling elements of the generation lance and the effects of the injection nozzles.

Results provides information also on the differences between the laboratory scaled-down

generation lance and the real lances taken directly from TBMs, this scale-effect should be

considered in the interpretation of the tests usually performed in the laboratory before the

beginning of the tunnel excavation.

In the future, since two different injectors and a series of different protective membranes

have been prepared the evaluation of the effect of the geometry of the nozzle on the TBM

head will be enriched with further tests.

In order to simulate the foam injection inside the working chamber, in the geotechnical

laboratory has been arranged a cell to be connected directly to the plant for the injection of

the foam in a pressurized environment. The creation of an “equivalent” half-life test in a

controlled environment at variable air pressure will allow to have information on the effects of

pressure variation in the foam stability.

Finally, the results obtained will be directly correlated with the TBM performances, to

understand as the expected excavation parameters could affect the design of the foam

injection plant.

REFERENCES

Anagnostou G., Kovari K., (1996). Face stability conditions with earth-pressure balanced

shields, Tunnelling and Underground Space Technology, vol. 11, No. 2, Pergamon – Oxford,

pp. 165–173.

EFNARC (2005). Specification and guidelines for the use of specialist products for

Mechanized Tunnelling (TBM) in Soft Ground and Hard Rock, Recommendation of

European Federation of Producers and Contractors of Specialist Products for Structures.

Merritt A.S., (2003). Soil Conditioning for Earth Pressure Balance Machines. PhD Thesis,

University of Cambridge.

Milligan G., (2001). Lubrication and soil conditioning in tunnelling, pipe jacking and

microtunnelling state of the art review. Geotechnical consulting group – London.

Mori, L., Mooney, M., & Cha, M. (2018). Characterizing the influence of stress on foam

conditioned sand for EPB tunneling. Tunnelling and Underground Space Technology, 71,

454-465.

Parikh, D. (2017) Experimental study of pressure drop and bubble size in a laboratory scale

compressed air foam generation system (Doctoral dissertation, Colorado School of Mines.

Arthur Lakes Library).

Rosen, M. J., & Kunjappu, J. T. (2012). Surfactants and interfacial phenomena. John Wiley

& Sons.

Thewes M., Budach C., Bezuijen A. (2011). Foam conditioning in EPB tunneling. 7th Int.

TC28 Conf. on Geotech. Aspects of Underground construction in soft ground, Rome.

Thewes, M., Budach, C., & Bezuijen, A. (2012). Foam conditioning in EPB tunnelling.

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