The TBM industry is fast paced. Technological developments have enabled larger diameter machines to bore longer tunnels for either deep or shallow projects. But to support these developments in earth pressure balance (EPB) tunnelling, more is coming to be needed than standard foaming agents for soil conditioning.

A soil conditioner is a chemical product that is added to a soil to improve its physical and mechanical properties. Soil conditioning is the process to achieve this, and heavily depends upon the generation of foam and the quality of the foaming system used.

The use of such foaming agents is important for EPB tunnelling in heterogeneous ground with a large range of permeability, and often cohesionless soils, where the face stabilisation is the greatest challenge to control safety and performance. Contractors need robust foaming agents, able to control the rheology of the soil in a planned manner over time, and do so with significant increase in foam consumption.

Time, indeed, today represents a new vital factor to be taken in consideration when conditioning the soil during TBM boring. The longer the foam remains stable, the higher the quality of soil conditioning, and consequently the less the consumption of the material.

Standard foaming agents are based on the surfactant sodium lauryl ether sulfate (commercial name SLES). To support EPB machines as project challenges grow, new and readily biodegradable foaming agents have been developed that can manage the plasticity of the soil thanks to a richer chemistry with more durable surface tension on the bubbles, which ensures longer stability over time.

The new biodegradable foam agents help to ensure safety of the excavation but also to optimise performance and reduce consumption, thereby positively impacting on the overall TBM excavation cycle and contributing to achieving the construction schedule. It is now possible to work with a higher foam expansion rate (FER) and a lower foam injection rate (FIR) without reducing the quality of the foam, and so increase its stability.

SOIL CONDITIONING – OLDER TECH

The generation of foam, whether done in laboratory or in TBM, is in two separate steps:

1. the surfactant is mixed with water at a certain concentration rate (Cf), creating the foaming agent

2. the foaming agent is mixed with air to create the foam

Standard foams in the TBM industry are based SLES, which is ionic tensio-active, which, in contact with water, has its molecules work like an emulsifier, bonding the water to its chemical structure.

SLES works on the surface of the bubbles, favouring the stabilisation of the air-liquid interface, developing the necessary surface tension to create air bubbles. Initially, once the foam is generated, the bubbles are small but, with time, they tend to increase in size while, simultaneously, the surface tension reduces on the external layer of the air bubbles – but will pop at a cetain point and is detrimental to foam stability to work when mixed with soil.

Therefore, the concept of time, or durability, until now not properly addressed by the industry, is a vital consideration when conditioning a soil in a TBM.

Foam parameters

Foams are generated and used through the control of these three essential parameters:

i. 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

ii. FER, which is 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 solution foamed at FER 10 means that it expands its volume ten times

iii. FIR, expressed in percentage, is 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

Quality control 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, the standard technology of SLES as main chemical constituent plus water has shown its limitation.

After non-stability has been reached (FER above 10), the performance of the foaming agents flattens, thus forcing contractors to increase consumption to ensure workability of the excavated soil and to control of its rheologic behaviour inside the TBM excavation chamber.

This is also demonstrated by quality control of foams in the laboratory, where the internationally-recognized standard test of 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 laboratory test measures the drainage time of half a standard sample of 80g of foam, made at a certain Cf and FER. The sample is placed into a beaker over a synthetic fleece and a digital scale, located vertically on top of a graduated cylinder, and the test measures how long it takes for half the liquid content, or 40ml, to drain below. The foam is generated with a foam gun like those available on TBMs.

With the traditional foam agents of SLES and water, and depending on Cf and FER, the drainage time is approximately 5 to 6 minutes.

The test determines the drainage stability of the foam. The HLT results of the traditional system and the new, rapidly biodegradable technology for foam agents. Generally, the results indicates that with the established (‘Old Technology’), the drainage stability does not improve but flattens for values of FER above 10. However, improvement is seen at higher FER levels for the new biodegradable agents, such as MasterRoc products SLF 325, SLF 419 and also SLF 543, from Master Builders Solutions.

Another interesting non-standardised 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 taken for a water-filled ping-pong ball of 18g total mass to penetrate (top to bottom) through a foam placed into a vertical cylinder. Again, the foam is generated with a foam gun like those available on TBMs. The time of passage gives an indication of the mechanical resistance applied by the foam applies, at a certain Cf and FER, and therefore the mechanical structure and strength of the foam itself. The tests results of Mechanical Resistance Time for the ‘Old’ and ‘New’ technologies.

SOIL CONDITIONING – NEW TECH

MasterRoc SLF 325, SLF 419, SLF 543

The innovation in soil conditioning foaming agents from Master Builders Solutions broken the FER 10 psychological barrier.

The MasterRoc SLF 325, for example, while still based on having SLES as its main constituent, is now chemically enriched to help make a stronger foaming agent, more stable over time, and yet ensuring it is readily biodegradable as per norms of the Organisation for Economic Co-operation and Development (OECD).

Working with the very same steps for foam generation mentioned as noted before, and the same standard parameters (Cf, FER and FIR), and the same foaming systems as installed in TBMs, the ‘New Technology’ can generate highly stable foams above FER 10 with excellent results in terms of drainage stability and overall performance.

Foam stability increases with FER and 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 boring.

In comparison to the conventional, standard ‘Old Technology’ SLES-based system, there are multiple technical and operational benefits from the ‘New Technology’, such as:

a) Increased drainage stability over prolonged time

b) High foaming capacity makes it possible to work with high FER values, avoiding collapse of the foam in comparison to standard foaming agents

c) The desired soil rheology is achievable with lower FIR values, compared to standard foaming agents, therefore contributing to a substantial reduction of the overall foam consumption

d) Creation of a highly stable foam to hold the face pressure and maintain the rheology of the excavated ground for longer than provided by conventional foaming agents

e) 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

f) Improved soil behaviour

g) Reduced soil permeability

h) Creation of plastic deformation properties in the soil, which increase face stability

i) Lower inner friction and lower abrasiveness of the soil at the cutterhead and shield

j) Reduction of the stickiness of soils with high fines contents (i.e., <0.063mm), which would lead otherwise to clogging, blockage problems

INTERPRETATION OF RESULTS

The most effective way to provide a reliable scientific interpretation of the HLT and Mechanical Resistance tests carried out on the new foaming agents is to perform high-level technical benchmarking for performance and stability against other, similar, added foaming agents available in the TBM industry.

In 2020, the departments of Structural and Geotechnical Engineering and Chemical Engineering Materials Environment at La Sapienza University of Rome, in Italy, developed a new classification based on the stability of generated foams. The classification system has five classes of foaming agents, which range in decreasing order of stability and performance durability from Class I, the best, to Class V.

The data were arranged for benchmarking with FER as the x-axis independent variable and HLT (at Cf 2%) as the y-axis dependent variable.

In 2021, the university departments added the Master Builders Solutions’ new foaming agents (MasterRoc SLF 325, SLF 419 and SLF 543) to the benchmarking data. Their placements, for FER 10 levels and higher, were at the superior end of Class I, and higher still. Many of the competitors are placed across Classes I to IV for FER 10 levels and higher.

The company’s long-time established ‘Old Technology’ foaming agent MasterRoc SLF 32 was benchmarked in Class III over FER 10-15.

The university’s benchmarking results are reinforced by Master Builders Solutions’ internal benchmarking, where the HLT and Mechanical Resistance, respectively, of the new foam agents were tested against two other agents available on the market, at three different concentration levels (Cf 1% – 3%).

The data for the new foaming agents show higher drainage stability as FER increases. Our view is that no other foaming agents currently in the TBM market perform equal.

ENVIRONMENTAL ASSESSMENT

Compliance to International Environmental Standards

The OECD regulates international standards for a range of social, economic and environmental challenges. According to its guidelines for ecotoxicity of foaming agents for soil conditioning, levels of toxicity of chemical products, including foaming agents, are determination according OECD guidelines on risk posed to water and land organisms. There are three categories of risk:

a) WGK 1 = Low Risk

b) WGK 2 = Medium Risk

c) WGK 3 = High Risk

All Master Builders Solutions’ TBM Portfolio of soil conditioners – the ‘Old’ and ‘New’ technology products – stand in the WGK 1, low risk, category. Similarly, according to OECD 301, the products and their leachable fractions are readily biodegradable.

Biodegradability, as defined as products able to disintegrate/decompose by the action of microorganisms, such as bacteria or fungi (with or without oxygen), and get assimilated into the natural environment. This is determined by:

a) Measuring the addition of organic material added to the soil as a result of conditioning with foaming agents during TBM mining

b) The length of time that the added organic material remains in the soil (OECD 301)

In virtue of the above:

a) All our foaming agents are readily biodegradable

b) All our entire products pass the criteria for biodegradability (‘Old’ and ‘New’ Technology)

c) The leachable fraction of the products is readily biodegradable

CASE STUDY

todz Railway Node project, Poland-November 2021

The project has involved an EPB TBM with excavation diameter of 8.78m excavating 4 x 1.15km-long tunnels. Geology is sand and clay with mixt areas containing gravel and large boulders. There is little groundwater.

Challenges encountered for tunnelling included: very high torque (9.2MNm); excessive temperature in central plate of the cutterhead (picks over 50°C) and at the screw conveyor; EPB loss during the pause between excavation sessions (over 0.2 bar variation); high water absorption in the soil and low conditioning grade of the soil; the main chamber could not be filled properly with homogeneous muck; the foaming agent used, from another industry supplier, required consumption of 145 litres/ring; foaming with Cf 1.5%, FER 10, FIR 100.

A proposed solution to improve performance for tunnelling involved a trial with one of the ‘New’ products MasterRoc SLF 419, in 2 steps:

  • Step 1: aggressive approach to resolve the issue of high torque and temperature by starting foaming with MasterRoc SLF 419 with Cf 2%, FER 6-8, FIR 60-80%
  • Step 2: move then to a more normal soil conditioning regime with consumption and optimisation of parameters

In more detail, the two steps were undertaken:

Step 1

Used recommended soil conditioning parameters for two rings (3.8m) until changes in torque and temperature were recorded. The achievements were:

  • Foam consumption reduced to 120 litres/ring
  • Reduction of torque to 4.6MNm
  • Reduction of temperature from >50°C to 31°C in central plate and screw conveyor
  • No EPB losses

Step 2

Applied less aggressive parameters with lower quality of foam. Achievements over the following nine rings were:

  • Foam consumption was reduced to 75 litres/ring
  • Torque remained reduced by about half to 4.6MNm
  • No EPB losses
  • Soil much better conditioned
  • Improved face stability and change in rheology
  • Stable EPB reduced settlement risk

The site trial confirmed the lab test data. Also important, there was good feedback from the TBM drivers.

CONCLUSIONS

The constant increase of the diameter of final tunnels 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 stabilisation of the excavation face during mining and ring building. Benefits available from the new types of foaming agents are, therefore, particularly advantageous for TBMs of at least 6m-diameter and excavating in heterogeneous soils with variable range of permeability.

TBM excavation cycles benefit from improved, constant stabilisation of the face and consequent reduced downtimes. It is the time factor, indeed, that eases control of the plasticity of the excavated soil as a supporting medium for EPB, while operating with higher FER and lower FIR compared to previous agents.