Mike A. Sposetti, Global Technical Manager TBM, Underground Construction, Master Builders Solution, Malta, and Manish Gautam, Product Segment Manager - UGC TBM Tunneling Projects, Master Builders Solution, Mumbai, discuss how readily biodegradable foaming agents not only ensure safety of excavation but also optimize performance and reduce consumption, to positively impact the overall TBM excavation cycle.
With the new extended borders of engineering, the TBM industry is fast paced, and moving towards a direction where the excavation diameter of the modern machines and the length of tunnels is constantly increasing, making it possible to excavate deeper but also shallower tunnels than a few years back.
To cope with this technological development, it becomes clear that conventional foaming agents for soil conditioning in EPB machines, based only on SLES (Old Technology), do not represent a reliable solution anymore. This is particularly applicable in heterogeneous grounds with large range of permeability, where the face stabilization is the greatest challenge to control safety and performance. This is the reason why a new technology of readily biodegradable foaming agents has been developed; it not only ensures safety of the excavation but also optimizes performance and reduces consumption, impacting positively on the overall TBM excavation cycle.
The industry of EPB TBMs experiences an unprecedent exploit in projects with cohesionless grounds below the groundwater table. Foaming agents encompassing a higher stability are needed to contribute to stabilize the excavation pressure, increase front face stability and favor the extraction of the spoil outside the tunnel.
In such conditions, contractors need more robust foaming agents able to control the rheology of the soil in a planned manner over time, without increasing the consumption of foam significantly.
Time is a vital factor that must be taken in consideration when conditioning the soil during TBM mining. The longer the foam remains stable, the higher the quality of soil conditioning, the less the consumption.
The New Technology of foaming agents, that are based not only on SLES, represents a new way to manage the plasticity of the soil, ensure safety and proper extraction; thanks to a richer chemistry that makes them stronger than conventional foams (old technology), these new agents boast a more durable surface tension on the bubbles that ensures longer stability over time. In this way, not only will the foam work longer but consumption will be reduced when compared to conventional foams.
So, it is now possible to work with higher FER and lower FIR without reducing the quality of the foam and, at the same time, increase its stability over time. With the New Technology, the TBM excavation cycle is optimized, contributing to the overall schedule to complete a tunnel.
Basics of Soil Conditioning – Old Technology
A soil conditioner is a chemical product which is added to a soil to improve the soil’s physical and mechanical properties. Soil conditioning is the process to achieve this, and heavily depends from the generation of foam and the quality of the foaming system used.
The generation of foam, whether done in laboratory or in TBM, is divisible in two separate steps:
Step 1: the surfactant is mixed with water at a certain concentration rate (Cf), creating the foaming agent;
Step 2: the foaming agent is then mixed with air to create the foam.
The standard foams in the TBM industry are based on a surfactant, or tensioactive, called Sodium Lauryl Ether Sulfate (commercial name is SLES). SLES is an ionic tensioactive that, when enters in contact with water, its molecules work like an emulsifier, bonding the water with its chemical structure.
SLES works on the surface of the bubbles favoring the stabilization of the air-liquid interface by developing the surface tension that allows the air bubbles to be generated. Initially once the foam is generated, bubbles are in small size but, with the passing of time, these tend to increase in their size in a simultaneous effect with the reduction of the surface tension applied on the external layer of the air bubbles.
The reduction of the surface tension on the external layer of the air bubble results at a certain point in the air bubble to pop, which effect is detrimental to the stability of the foam and its ability to work when mixed with a soil.
Therefore, the concept of time, until now not properly addressed by the industry, is now a vital parameter that must be taken in consideration when we condition a soil in a TBM.
Foams are generated and used through the control of these three essential parameters:
- Concentration (Cf), expressed in percentage as the rate of SLES inside the foaming agent; the higher the Cf, the stronger the foam from the chemical and mechanical point of view;
- Foam Expansion Rate (FER), as the ratio between the volume of the foam and the volume of the liquid; the higher the FER, the drier the foam; FER is the indication of quality of the foam and the indication of how much air is trapped inside the bubbles; as a matter of example, a foaming agent foamed at FER 10 means that it expands its volume ten times.
- Foam Injection Rate (FIR), expressed in percentage as the ratio between the volume of the foam and the volume of the soil; the higher the FIR, the higher the volume of the foam injected into the ground and, therefore, higher the consumption.
Since the beginning of the EPB TBM tunnelling, the industry has followed the use of soil conditioners based only on SLES as main chemical constituent plus water (Old Technology), which has now become anachronistic for modern TBM operational standards.
When it comes to the TBM operating parameters for an optimum soil conditioning (Cf, FER & FIR) through heterogeneous soft ground conditions with variable range of permeability, this technology alone has shown its limitation.
The Old Technology has been stretched to a point that, after the non-stability has been reached (FER above 10), the performance of these agents flattens, thus forcing contractors to increase foam consumption to ensure workability of the excavated soil and control of its rheology inside the excavation chamber.
This is also demonstrated by the quality control carried out on foams in laboratory. The internationally-recognized standard test named as Half Life Time (HLT) follows the provisions of the “Specification and Guidelines for the use of specialist products for Mechanised Tunnelling (TBM) in Soft Ground and hard Rock” by EFNARC (April 2005) to determine the drainage stability of a foam.
The test measures the time when 80gr of foam, generated at a certain Cf and FER, placed into a beaker over a synthetic fleece and a digital scale, located vertically on top of a graduated cylinder, drains 40ml, which are equal to half of its liquid content. The foam is generated with a foam gun that is like the ones available in TBMs. The test determines the drainage stability of the foam, named as Half-Life Time (HLT).
With the Old Technology of soil conditioners, based only on SLES and water, depending on Cf and FER values the measured time is approximately ranging in 5 to 6 minutes. This indicates that with the Old Technology, the drainage stability above FER 10 is flattened out and non-performant; the optimum benefit of a foam can be utilised up to FER 10, limiting the control of the performance of the foam but also highlighting the inability to use foaming agents to their full potential and, ultimately, ease tunnelling operations in large variety of soft ground strata. The results of Old and New Technology Half-Life Time are displayed in Figure 6.
Another interesting non-standardized laboratory test used by Master Builders Solutions is the Mechanical Resistance test, where the concept of time is again fundamental.
The test measures the time a water-filled ping-pong ball needs to penetrate, from top to bottom, through a foam placed into a vertical cylinder. Also in this case, the foam is generated with a foam gun that is like the ones available in TBMs.
The test measures the time when a ping pong ball 18g weight vertically crosses a foam placed into a plastic cylinder, from top to bottom. The measure of time gives the indication of the mechanical resistance the foam applies to the vertical scroll of the ball, at a certain Cf and FER, indicating the mechanical structure & strength of the foam itself. The results of Old and New Technology Mechanical Resistance Time are displayed in Figure 7.
MasterRoc SLF 325, SLF 419, SLF 543
With our innovation in soil conditioning foaming agents, we have broken the FER 10 psychological barrier, providing more flexibility in terms of handling the foam generation and obtain the most effective soil conditioning result.
The New Technology MasterRoc SLF 325 for example, still based on SLES as main constituents, is now chemically enriched with some new other constituents that contribute to a stronger foaming agent, more stable over time, ensuring at the same time to be readily biodegradable as per OECD norms.
Working with the very same steps for foam generation mentioned in Chapter 1 (old technology), and the very same standard parameters Cf, FER and FIR, and with the very same foaming systems installed in TBMs, with the New Technology is now possible to generate highly stable foams above FER 10 with excellent results in terms of drainage stability and overall performance. The foam stability increases with FER, the air bubbles maintain a durable surface tension on the external layer over a prolonged period, thus ensuring a longer control of the rheology of the soil during TBM mining.
When compared to conventional Old Technology, the technical and operational benefits of New Technology are multiple:
- Increased drainage stability over prolonged time;
- Due to the high foaming capacity, it is now possible to work with high FER values avoiding the collapse of the foam, compared to standard foaming agents;
- Work with lower FIR values to obtain the desired soil rheology, compared to standard foaming agents;
- Creation of a highly stable foam to hold the face pressure and maintain the rheology of the excavated ground longer than conventional foaming agents;
- MasterRoc SLF 325 guarantees a new improved foam stability that guarantees proper EPB counter pressure; its improved foam properties ensure better control of the ground pressure, reducing the risk of additional ground settlement at the surface and over excavation.
- Improved soil behavior;
- Reduced soil permeability;
- Creation of plastic deformation properties in the soil, which increases face stability;
- Lower inner friction and lower abrasiveness of the soil at the cutterhead and shield;
- Reduction of the stickiness of soils with high fines contents (i.e. <0.063 mm), which would lead otherwise to clogging, blockage problems.
The most effective way to provide a reliable scientific interpretation of the Half-Life Time and Mechanical Resistance tests carried out on our New Technology of foaming agents is the execution of high-level technical benchmarks that include other similar foaming agents available in the TBM industry.
This benchmark allows to verify the validity of the New Technology of foaming agents, erecting an unbridgeable void with the Old Technology in terms of performance and stability.
In 2020, the Department of Structural and Geotechnical Engineering and the Department of Chemical Engineering Materials Environment, La Sapienza University of Rome (Italy), have developed a new classification of foams based on the stability of the generated foam; the new classification has arisen five classes of foaming agents, where Class I is the strongest (more stable, more performing and durable over time) and Class V is the weakest. In this scientific work, they have plotted all the Old Technology of foaming agents available on the market by displaying HLT (at Cf 2% - Y axis) vs FER (FER 10 – X axis), as shown in Figure 8 with dark dots.
of the generated foam
The same departments in 2021 have added Master Builders Solutions’ New Technology of foaming agents, MasterRoc SLF 325, SLF 419 and SLF 543, de facto being obliged to create a new superior class. The results of this scientific work are displayed in Figure 8.
It’s worth mentioning that also our Old Technology MasterRoc SLF 32 is displayed, belonging to Class III “Considerable Stability.”
The results displayed in Figure 8 are reinforced by our own internal benchmarking, where Half-Life Time and Mechanical Resistance tests results of our MasterRoc SLF New Technology (in orange) and Old Technology (in blue) are shown with two other foaming agents available on the market (in red) in Figures 9 and 10 at three different concentration levels (Cf 1-3%).
The conclusion we can derive is that, not only our New Technology has higher drainage stability, but the same stability increases with the increase of FER; plus, no other foaming agents currently in the industry perform equally.
Compliance to International Environmental Standards
The Organisation for Economic Co-operation and Development (OECD) is the international organisation that regulates the establishing of international standards to a range of social, economic and environmental challenges.
According to OECD guidelines for ecotoxicity of foaming agents for soil conditioning, the determination of the level of toxicity of chemical products, including foaming agents, is carried out in accordance with the OECD Guidelines, which rank the level of risk that a chemical product poses to water and land organisms, broken into three categories:
- WGK 1 = Low Risk
- WGK 2 = Medium Risk
- WGK 3 = High Risk
Similarly, according to OECD 301, all our products and the leachable fraction are readily biodegradable.
Biodegradability, as defined as the ability of things to get disintegrated (decomposed) by the action of micro-organisms such as bacteria or fungi biological (with or without oxygen) while getting assimilated into the natural environment, is determined by:
- Measuring the addition of organic material added to the soil as a result of conditioning with foaming agents during TBM mining;
- The length of time that the added organic material remains in the soil (OECD 301).
- All our foaming agents are readily biodegradable;
- All our entire products pass the criteria for biodegradability (Old and New Technology);
- The leachable fraction of the products is readily biodegradable.
The New Technology of foaming agents presented in this article is a brand-new revolutionary series of products, designed specifically for the TBM industry in matter of soil conditioning during EPB TBM mining.
With these new foaming agents, it is now possible to control the rheology of the soil over longer time, avoiding the continuing pumping of extra low-HLT foams in the excavation chamber, thus resulting in a drastic reduction of foam consumption carried with all its economic benefits that it entails.
The benefits are particularly advantageous in heterogeneous soils with variable range of permeability in TBMs of diameters above six meters.
Also, the overall TBM excavation cycle strongly benefits from it, ensuring constant stabilization of the face during mining and reducing downtimes; the muck extracted from the TBM contains now less readily biodegradable chemicals and therefore is less harmful to the environment when compared to the Old Technology of foams.
The advantages of the New Technology of foaming agents have been described in Chapter 2 above; the constant increase of the diameter of final tunnels (tunnel ID) leads to a corresponding increase of TBM excavation diameters, the volume of soil to be excavated for each stroke and, therefore, to the time the TBM needs to build up the required EPB pressure to ensure stabilization of the excavation face during mining and ring building.
It is the time factor indeed, the new parameter to take in consideration to eases the control of the plasticity of the excavated soil in its function of supporting medium for EPB counterpressure, by drastically reducing the overall consumption of foams than traditional Old Technology.
Contractors have now a new way to approach the soil conditioning: with the New Technology of foaming agents they are now able to plan the soil conditioning in advance, estimate performance and consumption of foams and properly evaluate the overall cost of the work by operating with higher FER and lower FIR when compared to conventional Old Technology.
EFNARC, (April 2005) - “Specification and Guidelines for the use of specialist products for Mechanised Tunnelling (TBM) in Soft Ground and hard Rock”.
Diego Sebastiani, Giorgio Vilardi, Irene Bavasso, Luca Di Palma, Salvatore Miliziano, Department of Structural and Geotechnical Engineering, La Sapienza University of Rome and Department of Chemical Engineering Materials Environment, La Sapienza University of Rome (2020) – “Classification of foam and foaming products for EPB mechanized tunnelling based on half-life time”.