Foaming and polymeric agents used in tests for the conditioning of soils during the progress of excavation work for a tunnel with a mechanized shield of the TBM-EPB type by Lamanna Luigi Franco (*)
A) - Introduction When excavating a metropolitan or railway tunnel, especially in the subsoil of large cities, using a mechanized shield of the TBM-EPB or TBM-Slurry type (where large quantities of bentonite mud are used in the latter), a particular attention to geotechnical problems, because it is necessary to guarantee the stability of the excavation face by maintaining a pressure on it, to avoid the collapse of the face itself with the consequences of subsidence such as landslides or elastic-plastic deformations of the soil. This can only be done by controlling the balanced confinement pressure that can be carried out, using additives in front of the excavation face, whatever the geological and stratigraphic, hydrogeological and geotechnical conditions of the soil. All photos illustred are copied from the WEB
Photo 01 – TBM-EPB, a) schematic diagram of EPB machine used in Beijin, China Metro Xijiao line; b) in the figure of the TBM-EPB, bottom right, schematic of the header gate that connects the soil chamber and the screw conveyor. (Earth pressure balance shield tunneling in sandy gravel deposits: a case study of application of soil conditioning, Bulletin of Engineering Geologyand the Environment, Spring er-Verlag GmbH Germany, part of Springer Nature 2020)
Before starting the excavation with an EPB it is necessary to define in detail the working pressures along the entire construction line of a tunnel. This consists in setting the stability limit of the excavation face and the maximum working pressure that can produce settlements (subsidence) or lifting. To avoid this, it is necessary to evaluate and take into account some excavation parameters or, in more detail, the coefficients in order to define the pressure limits (upper and lower) to be respected during excavation for the good functioning of the cutter. Therefore, the only valid element known, up to now, to counter this phenomenon is to create a plastic paste with the excavation
All photos illustred are copied from the WEB
soil in the TBM-EPB excavation chamber, as well as helping to maintain the same pressure, also at the inside the auger where the latter, with the soil-vine interaction during the material extrusion process, allows a speed of unloading of the excavated material itself.
Photo 02 – Slurry Shield Machine (Herrenknecht)
All this takes place with the sole purpose of maintaining a constant magnitude of the pressure. For the successful operation of the TBM-EPB, everything depends on the TBM technician (driver), on his preparation and experience, because he is the only one who can choose to reduce the thrust on the shield and regulate, fundamental thing, also the angular velocity of the cochlea.
B) - Brief description of the operating principle of a TBM-EPB
All photos illustred are copied from the WEB
The operating principle of a shielded TBM-EPB is very simple. Shielded TBMs, those that keep the excavation face under pressure, commonly called EPBs, are used when very difficult stress-deformation conditions of the ground occur; that is, in those situations in which the mechanical contrast provided by the milling head alone cannot avoid the relaxation of the excavation face (collapse).
Photo 03 – Schematic view of producing and injecting foam in EPB-TBM Soil Conditioning, Herrenknecht & Rehm 2002 (ITA/AITES)
A feature of these types of cutters is that the front of the cutter head has openings to allow the excavated soil to enter the pressure chamber, these openings are of the order of 20-30% of the total surface. The advancement of the cutter allows the material and fine parts to enter this diaphragm that is created between the excavation face and the milling head, thus allowing the thrust of the hydraulic pistons to be transferred to the shield, exerting on the front of I dig the pressure necessary to counteract that exerted by the ground. The shield of an EPB type has the function of counteracting the thrusts of the ground, keeping the excavation intact and at the same time securing the personnel in charge; in addition, the EPB shield prevents, through a system of gaskets, continuously fed by special sealing greases, the entry of any water encountered (Main Bearing Greases). In addition, in the final part of the EPB shield, the final lining of the tunnel is mounted, generally consisting of a series of prefabricated segments made of reinforced concrete put in place by a rotating system placed inside the shield. The last ring (concrete segment) mounted is used as a contrast by the thrust pistons for the advancement of the TBM. Finally, the extrados of the cladding, again the one made of prefabricated segments, the annular space between the excavation profile and the extrados of the cladding, is injected with a mortar or mortar under pressure, commonly called PEA-GRAVEL, which is used to seal and contrast the tunnel against the ground.
I would like to point out that some operators in the sector, who use very advanced technologies and methodologies, instead of injecting Peal-Gravel behind the segments (in the annular space between the excavation profile and the extrados of the coating), inject special resinous silicate formulations mineral-organic which also have the task, in addition to contrasting the rock mass with the final lining of the tunnel, to avoid for many years (at least 50 years) the infiltration of water inside the tunnel, at the same time avoiding degradation of the reinforced concrete lining (always the one made of prefabricated segments), both internally and externally, guaranteeing its "durability" which, as we all know, can be influenced by numerous factors that are commonly classified as chemical, physical and mechanical . Remember that reinforced concrete can be damaged by expansion, water, physical corrosion, due to physical and chemical damage, with a rapid process of degradation because it is exposed to these stimuli.
C) - Illustration of our experimentation conducted As I have already said, in order to condition the ground it is necessary to create a plastic paste with this, in particular the one that ends up in the excavation chamber, therefore, during the normal excavation we carried out an experiment, directly in an excavation site, on the advancement of the construction of a service tunnel (pilot hole) using an EPB (EARTH PRESSURE BALANCE), using the same excavation material as a test.
During the execution of the tests, a foaming agent FOAM was injected on the excavation face, in different percentages, by means of special nozzles placed on the tiller head, with the function of fluidizing the soil that was excavated and at the same time the resulting soil, which ended up in the chamber excavation, so as to form the layer of fluidized soil in its entire mass forming the plastic paste.
All photos illustred are copied from the WEB
Furthermore, the excavated material was mixed and accumulated under pressure, not only in the clearing chamber (excavation chamber), but also in the auger which removed it in a fluid and controlled manner.
Photo 04 – Pressure of the supporting soil paste in the excavation chambre of anTBM-EPB counteracting the water and earth pressure the face. (Lisa Mori, 2016, Colorado School of Mines, DPT of Mining Engineering)
In fact, these FOAM foaming agents, used for the conditioning of the ground (such as surfactants and polymers), are all monocomponent and non-reactive because their use, in particular for mechanized excavation by TBM-EPB, this occurs directly
All photos illustred are copied from the WEB
through of the nozzles placed in front of the cutter head, in the excavation chamber and also present in the auger. In any case, the entire pumping system is placed inside the TBM.
Photo 05 - Illustration of the foam generation system inside a TBM-EPB
All this also allows the constant control of the confinement pressure exerted on the excavation face and the coordinating the thrust of the cutter in advance as well as the extraction speed of the auger. For the first time, we tried to carry out various tests on site, trying to simulate borderline cases, using mud and water jets, simulating an underground water circulation source, creating a composition of soil, similar to how it can be presented in a real limit execution phase, using strong jets of water, at high pressure, which had the ability to break up the most resistant soil; this to simulate operating in those terrains not suitable for the shield. Dear colleagues, as you can imagine, this simulation was not easy to solve, even having taken great care in modifying and equipping ourselves to simulate risks similar to those of reality, as if we were in the presence of difficult terrain, with large quantities of water, and without having to resort to particular known techniques.
D) - Why it is important to maintain the pressure inside the "excavation chamber" An excess of operating pressure can represent a destabilizing effect while, a pressure lower than the equilibrium limit, can cause the collapse of the face and cause surface subsidence (subsidence) of the ground following the excavation of tunnels, such as underground or underground tunnels underground mines. But I want to remind you that the effect of the pressure applied to the machine tends to reduce the effect of the extent of the subsidence (subsidence) because there is a continuous monitoring and control system inside the TBM-EPB.
AVERAGE COMPOSITION FOR A NORMALLY
USED FOAM:
- CHEMSURF-A-22 AND CHEMSURF-H-38 : -- WATER:
AIR
0.5 – 1 %
< 0.1 %
FOAMING AGENT CAN HAVE INSIDE A SMALL AMOUNT OF POLYMER TO STABILIZE THE FOAM BUBBLES
FIR
90 -95 %
-- CHEMCRYL (EVENTUAL):
FOAM INJECTED
5 – 10 %
-- AIR:
WATER
FER
- CHEMSURF-A-22 - CHEMSURF-H-38 - CHEMCRYL
ADDITIVE 1
CHEMSPERSE
ADDITIVE 2
SOLUTION
Photo 06 - Average composition for a normally used foam
On the other hand, too high a pressure, excessively increases the stresses on the machine, decreases the forward speed and can in turn cause instability as it can generate the lifting of the ground above the excavation trajectory in correspondence with the construction of a superficial line tunnel underground.
D.1) - APPLICABILITY OF THE EPB METHOD Therefore, for an EPB cutter to function optimally, the excavated material must have the following characteristics: - good plastic deformability; - low internal friction angle; - low permeability. In clayey-sandy and clayey-loamy soils, with a light consistency, the presence of this feature is optimal for the use of EPB type shields. However, it is difficult to find a material that is naturally only suitable for excavating in EPB mode, so steps are required to make the soil suitable for excavation: - the excavated materials are generally mixed with additives (chemical foams and polymers). Otherwise the fluid will not be able to have a uniform movement, and therefore can cause an arc effect and a porosity inside the diaphragm through which the water, usually present at the front, is able to flow freely, thus determining a possible collapse of the front itself. Therefore it is necessary to ensure a plastic excavation terrain, and this is made possible by using chemical agents (foams, stabilizers, anti-aggregating agents), this operation is called conditioning of the soil which is the test and the object of this report. Conditioning agents are usually injected from the shield, into the excavation chamber and into the cochlea.
Therefore, the correct behavior of the foaming agent must be evaluated in advance on the basis of two performance indicators: the foaming and stability of the foam, which are quantified by the FER (Foam Expansion Ratio) and by the "half-life" period, the time required for the foam to drain 50% of the generator liquid used (EFNARC, 2005). Based on the minimum FER required, a concentration (Cf = 1.5 - 5%) of the foaming agent is recommended to develop adequate conditioning of the soil with a consumption between 0.5 - 2 Kg / m3.
To further develop the soil conditioning scheme, further tests, such as Slump Tests and Hydraulic Permeability, must be conducted to evaluate the homogeneity, abrasiveness, workability and hydraulic permeability of the conditioned soil. On the basis of these further indicators, related to performance, a FIR (Foam Injection Ratio) injection ratio of the foaming agent is determined which usually, the latter indicator, is indicated by the TBM-EPB conductors between 30% and 60 %. Below, I want to briefly indicate how you get there you have FER and FIR values, of the two main properties of the foam, according to EFNARC 2005, which are: - FER: is the expansion ratio of the foam is the ratio between the volume of the foam at the operating pressure and the volume of the foam solution. FER = V foam / V foam solution The expansion ratio of the foam should be between 5-30. If the FER value is high; the foam generated must be drier. Drier foam should be used in moist soil and vice versa.
- FIR: the foam injection ratio is the ratio between the volume of the foam at the operating pressure and the volume of the soil in situ to be excavated. FIR = 100 x V foam / V soil The foam injection ratio should be between 20-80% but in most cases it is between 30-60%.
- However, in order to have information on the best FIR value, laboratory tests should be carried out; - while, I would like to point out that the water content of the soil and the water content of the injection are important.
In fact, the type of foam is determined by the type of excavation soil encountered during the excavation phase. Therefore foams must be classified into three types; A, B and C (EFNARC, 2005):
FOAM TYPE A: - The dispersion capacity is high (breaking of clay bonds) - Good coating ability (reduces swelling effects) FOAM TYPE B: - For general use - Medium stability FOAM TYPE C: - High stability - Anti-segregation properties (develop and keep the impermeable ground cohesive)
However, to ensure the effectiveness of the soil conditioning schemes, which are based on the foaming agents used, it is important to verify with in situ tests and observations (including slump tests and visual inspections), samples of conditioned soil, collected on site, for satisfy all the necessary geological and geotechnical conditions.
E) - Example of use of surfactants and a polymer
To enter into the merits of the tests, the following products were used: a foaming agent formulated with particular anionic, biodegradable surfactants called CHEM-SURF-H-38; a liquid dispersing agent formulated with particular polycarboxylates called CHEM-SPERSE; and a synthetic liquid polymer, formulated with special acrylamide / sodium acrylate copolymers called CHEMCRYL to be used separately or in combination.
Tab. 01 – Foam classification and application area (EFNARC, 2005)
The products that I take the liberty of suggesting and that can be used for ground conditioning, are those developed by the company CHEMIX s.r.l., of Golasecca (Varese), an Italian chemical industry, in collaboration with myself. Products designed with the aim of finding the right proportion to stabilize any type of soil under consideration. This solution, that of the two CHEM-SURF-H-38 + CHEM-CRYL or CHEM-SPERSE + CHEM-CRYL products, called FOAM agents, is the injection solution to be done at the excavation face, in the excavation chamber and in the auger. It can vary, depending on the ground conditions, even equal to 50% of the excavated volume.
However, the proof of use of the FOAM was based on three fundamental operations:
- the development of the percentages of product to create a soil treated to make it become quite fluid as mud (water-surfactantpolymer); - identify the pressure to be exerted on the mud to stabilize the excavation face; - identify the pressure to be applied on the ground that has become mud to control the excavation by operating both on the auger and on the thrust cylinders (in our case only one thrust cylinder is sufficient) to maintain balanced weights and volumes.
In general, the products used during the soil treatment tests produced the following effects:
- the Foams (type CHEM-SURF-H-38 and CHEM-SPERSE): they have maintained the stability of the face, with an increased capacity for plastic deformation of the ground (foam that is transformed into a homogeneous mousse), as well as a reduction in internal friction and the torque of the cutting head.
- the Polymer (type CHEM-CRYL): it has maintained the stabilization of the soil and of the foams themselves, and water retention (ideal to be used even in the presence of large quantities of water).
Finally, type CHEM-SPERSE has proved to be of interest as a ready-to-use product, and finds and can be used as:
NO YES
NOT ACCEPTABLE
OPTIMAL CONDITIONING
NO
NOT ACCEPTABLE CONDITIONING, TOO FLUID
CONDITIONING, TOO DRY
Photo 07 – Slump test - On the basis of numerous experiences developed in the laboratory with the execution of specific research aimed at defining of a "test protocol" of the "the conditioned medium" and on the basis of indications of "technical standards", in this study the "indicator, the drop test” to the Abrams cone (Slump Test) for the purpose of preliminarily characterize the “conditioned medium”. In test the following techni cal aspects were analyzed: - presence of a condition of the soil plasticity (qualitative observation); - falling to the cone of about 15-20cm; - reduced release of ”Water + Foam" from the mass of "the conditioned medium" after the test.
- Dispersing agent: that is, to disperse / break up the strongly cohesive tabular structure of well-consolidated clayey soils, with a consequent decrease in their viscosity. Furthermore, this agent facilitates the resumption of excavation following stops or prolonged interruptions. I would like to underline that the choice of the specific product or combination of products, the identification of the various application parameters in relation to the product (concentration of the additive in the aqueous solution, rate of expansion of the foam, rate of injection into the soil) and the method of advancement to be followed depends, as already mentioned, on the geo-mechanical characteristics of the ground crossed during the excavation. All photos illustred are copied from the WEB
FIR
FER
Photo 08 - Graphic scheme - Conditioning layout showing injection at cutter face, working chamber and screw conveyor
E.1) - TECHNICAL CHARACTERISTICS OF THE PRODUCTS USED DURING THE TEST E.1.1) - CHEM-SURF-H-38 - Density 1.04 0.02 kg / l - pH (for 1% solution) 7 - Total water solubility
The dosage of the product diluted in water varies from 0.5 to 2% by weight. E.1.2) - CHEM-SPERSE - Density 1.26 0.02 kg / l - pH 7.5 0.2 - viscosity 300. 50 mPas - Total water solubility The product is formulated to be ready for use and ranges from 0.05 to 1.5% of the volume of the excavated soil. E.1.3) - CHEM-CRIL - Density 1.05 0.02 kg / l - pH (for a 5% solution) 6 - 8 - viscosity 1.000 400 cPs Its dosage varies from 2 to 4 kg per 1,000 liters of water (the product must be added to the water under constant stirring 20-30 minutes before the introduction of the foaming agent). All photos illustred are copied from the WEB
Photo 09 - Spray test on the cutter head of foaming agent formulated with particular biodegradable anionic surfactants, S.E.L.I. Rome, Italy
F) - Conclusion The tests carried out on site, with the use of FOAM, had the purpose of letting us know which parameters are necessary between weight and volumes of earth excavated and removed to guarantee the non-occurrence of collapses or superficial subsidence (subsidence) to obtain the characteristics of impermeability and plasticity, which are strictly correlated to the porosity of the soil used during the excavation phase. In general, by carrying out various tests, and finding ourselves in the presence of different types of soil, we have seen that the theoretical average quantity of FOAM that must be injected is equal to 300 l / m3. Furthermore, we verified whether the effect deriving from the compressibility of the foam (FOAM) reduces the pressure fluctuations on the excavation face, a test that was positive and necessary for the operators to have total control of the TBM-EPB.
Finally, we also saw and evaluated that the foam not only allowed us to increase the workability of the excavated material and to reduce its permeability, because it was more fluid, but what is more interesting it prevents the adhesion of the soil-foam on the metal parts of the internal walls of the excavation chamber and auger eliminating the risk of obstructions.
And to finish, we verified that the use of products, such as those mentioned above, allowed us to create a stable foam over time due to the foaming power of their composition and molecular structure. Therefore, I would like to point out that the comments reported in this report refer to real tests carried out directly on a construction site of a tunnel and not comments on tests carried out in the laboratory, which are usually made on samples of reduced material, with the result of having indicative data and it becomes necessary, during the excavation phase, to be verified directly on the TBM-EPB.
(*) Luigi Franco, LAMANNA Independent Technical Consultant in the sector of Tunneling, Mining and Underground Technology Presidente of the Fondazione Internazionale di Centro Studi e Ricerche, ONG 132, via dei Serpenti, 00184 ROMA, Italia, U.E. E-mail: lamannaluigifranco1@gmail.com LinkedIn: Lamanna Luigi Franco
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