Numerous papers have been written by chemical suppliers, academics and others on the subject of soil conditioning: its importance during TBM EPB excavation is well-understood in the global tunnelling industry. What is perhaps less known, or at least, not as well investigated in-depth, is its inevitable impact on the environment: chemical agents used for soil conditioning come into contact with subsurface water and end up in the excavated muck.

When traditional foaming agents are used, such an impact may pose risks for the environment. For example, the product can slowly biodegrade in the excavated spoil, thus posing a certain degree of toxicity to watery and terrestrial organisms over an extended period of time. But thanks to the development of innovative chemical products, the situation has significantly improved in recent years.

This paper aims to show that for TBM soil conditioning, technical performance and environmental sustainability can work together when using innovative products that biodegrade quickly and have extremely low ecotoxicity.

WHY SOIL CONDITIONING IS SO IMPORTANT

Soil conditioning is a key factor for successful TBM advance, with significant influence over the machine’s performance and, more generally, over project costs. The cost of purchasing soil conditioning agents is very low compared to the overall costs of a TBM project, but their impact is huge because their quality and how they are applied have a strong influence on the machine’s performance, including on productivity, speed of advance, number of production stops, wear to the cutterhead (and other parts of the machine) and cutterhead torque (i.e. energy).

Furthermore, proper soil conditioning is directly connected to safe tunnelling: the right EPB pressure at the tunnel face can be applied and maintained during excavation only when the soil is well conditioned, otherwise several problems can occur, including water inflows and superficial subsidence. Such problems are amplified when tunnelling beneath urban areas.

WHAT IS SOIL CONDITIONING USED FOR?

The most commonly used chemical product for soil conditioning is the foaming agent. It is injected at the tunnel face into the excavation chamber and along the screw conveyor, and mixed with the excavated soil with the aim of improving its consistency.

This ‘improvement’ means different things:

? In granular soils (e.g. sands and gravels), the aim is to improve soil aggregation and create a plastic paste which is used to apply the required EPB pressure at the tunnel face and which can be easily extracted by the screw conveyor. When excavation is below the water table, another aim is that the plastic paste should be impermeable.

? In cohesive soils (e.g. clays), the typical problem is stickiness. In this case, the aim of soil conditioning is to make the soil fluid and to avoid soil particles sticking to the metallic parts of the TBM (cutterhead, chamber, screw).

Three main parameters regulate the use of foaming agents in TBMs:

? Cf – this is the concentration of the foaming agent in water; the higher the concentration, the stronger and more durable the foam bubbles.

? FER, the Foam Expansion Ratio, is the
Volume of generated foam / Volume of generating liquid

The greater the FER, the greater the amount of air added to generate foam.

? FIR – Foam Injection Ratio [%] is the

Volume of generated foam / Volume of excavated soil [%]

Figure 2 shows foam generated in the laboratory. Its consistency depends on the parameters of concentration and the FER selected.

Sometimes, particularly in difficult geological conditions, foams are simply not enough to achieve optimal soil conditioning. In these cases, a second product can be introduced in conjunction with the foam, the so-called ‘polymer’. A range of several types of polymers is now available on the market and they can be divided into two main categories:

? Polymers for water-control and soil aggregation;

? Polymers for reducing soil stickiness.

The correct choice of these products and the setting of their parameters are of paramount importance for the success of the soil conditioning. For this reason, soil conditioning should be considered as a complex process to be designed and applied with the cooperation of all parties involved: construction companies (with all the different personnel involved, from management to TBM drivers); chemical suppliers; external laboratories and academic departments.

Doing laboratory tests on representative soil samples is strongly recommended in order to gain a deeper knowledge of the soil to be excavated, and to get a first approximation of the soil conditioning parameters to be applied in the TBM.

HOW SOIL CONDITIONING AGENTS IMPACT THE ENVIRONMENT

The main constituents of foaming agents are surfactants, but they can also include other chemical compounds, such as polymers and stabilisers. Different chemical formulations result in different technical performances. This has been clearly observed and proven throughout the tunnelling industry.

What may be less known is that different chemical formulations also mean different environmental impacts. It is important to understand that the injection of chemical products during the TBM advance will involve a degree of environmental impact that is impossible to avoid, particularly in terms of excavated muck and subsurface water.

EXCAVATED MUCK

Foaming agents are mixed with the muck which is temporarily stored at the jobsite and later taken to a dedicated area for disposal. Considering the massive volumes of spoil produced during TBM excavation (for example, a 10m-diameter EPB produces around 100t-130t of muck per linear metre of tunnelling), a fundamental question for the project is how to classify, store and repurpose the excavated material.

Regulations covering the classification and reuse of tunnel spoil differ from county to country; sometimes, even within a single country, different rules may be applied to each individual project. What does not change is that the chemical products added to the muck influence this process: when the product is rapidly biodegradable and of low toxicity, the probability of reusing the tunnel muck increases drastically, as well as the time required to wait before the ‘zero impact’ of the soil conditioning occurs.

In several countries, there is a huge difference in terms of cost between the disposal of tunnel muck as ‘hazardous waste’ and as a ‘by-product’ or ‘non-hazardous material’.

SUBSURFACE WATER

Chemical soil-conditioning agents inevitably come into contact with subsurface water, and may be partially dispersed within aquifers. This situation becomes even more sensitive when tunnelling occurs in urban areas and the aquifer is used for civil purposes.

All foaming agents will have a certain degree of impact on water, but this will depend on their biodegradability and on the ecotoxicity of the chemical product. The variance between foaming agents available on the market is huge, especially in terms of toxicity for watery organisms such as fish, crustaceans and algae. When a comparison is made between an innovative foaming agent and a more traditional one, the difference can be a hundred-fold.

It is vital therefore, that the selection of soil conditioning products is based on the combined influence of technical performance, ecotoxicity and biodegradability.

ACHIEVING MORE SUSTAINABLE SOIL CONDITIONING

In recent years, Mapei has developed a new line of products which aim to reduce the environmental impact of soil conditioners on subsurface water and excavated muck. At the same time, the technical performance of traditional foaming agents is guaranteed, as proven by the several kilometres of tunnels already excavated using these products.

These products belong to the Polyfoamer ECO and Mapedisp ECO ranges and include a series of foaming agents and polymers, each one with its own characteristics in terms of biodegradability, ecotoxicity and technical performance.

TWO EXAMPLES

Polyfoamer ECO/1000 Plus is a foaming agent based on innovative surfactants, characterised by extremely low values of ecotoxicity against organisms typically used for these laboratory tests, such as fish, algae, crustaceans and mammals.

For example, EC50 is determined by OECD Guideline 203 as the concentration in water that is necessary to have a toxic effect on 50% of the zebrafish (Danio Rerio) population. Typical EC50 values for traditional foaming agents are in the range of 10-100mg/lit, while that for Polyfoamer ECO/1000 Plus is 3,500mg/lit. This means that Polyfoamer ECO/1000 Plus is between 35 and 250 times less toxic to fish than the traditional foaming agents.

REDUCING STICKINESS

Mapedisp 7030 ECO is specifically formulated to minimise the stickiness of clay and other cohesive formations. It differentiates from the other ‘anti-clay’ products available due to its faster biodegradability and lower ecotoxicity, which again mean lower impacts on subsurface water and on excavated muck.

The development of these products shows that efficient and safe soil conditioning can be combined with respect for the environment. This has been demonstrated on projects where these products have already been used for the soil conditioning. Details of the most significant projects in terms of TBM size and/or geological difficulty follow.

SANTA LUCIA TUNNEL

The 7.5km-long Santa Lucia tunnel on highway A1 in Barberino di Mugello, Italy, was excavated by Italian contractor Pavimental. With a diameter of 15.96m, this EPB is the largest TBM ever used in Europe, and one of the largest in the world.

Such a huge diameter presents additional difficulties for soil conditioning operations, as greater volumes of soil are maintained in the cutterhead chamber so as to apply the required pressure against the tunnel face.

Two main geologies were encountered: Monte Morello siltstone and Sillano claystone. Before TBM launch, the client and the contractor undertook a series of technical and environmental tests, carried out in the laboratories of Politecnico di Torino and of the National Research Centre (CNR) in Rome. The results of the tests saw the approval of foaming agent Polyfoamer ECO/100 as the product thought to combine optimal technical and environmental performance.

Environmental studies fixed a maximum dosage limit at which the foaming agent could be injected during each TBM stroke. By respecting this maximum dosage, no environmental impact to the muck in terms of ecotoxicity was anticipated.

TERZO VALICO HIGH-SPEED RAILWAY BETWEEN MILAN AND GENOA, ITALY

Three TBMs have already completed their excavation, while another two are currently mining to complete a series of tunnels which will connect Milan and Genoa. The first part of the excavation was carried out by Consorzio Cociv (a JV led by Webuild), while remaining works are currently being undertaken by Seli Overseas. Diameter of the five TBMs is around 9.7m.

Foaming agent Polyfoamer ECO/100 was chosen for the soil conditioning following technical and environmental tests carried out before the tunnel launches. It has already been used for more than 30km of tunnelling in a wide range of geological conditions (marlstones, clay, claystones, sand and limestones), and in each case has guaranteed fast spoil disposal to the final designated area, thanks to its fast biodegradability.

RIACHUELO OUTFALL TUNNEL, BUENOS AIRES, ARGENTINA

A 5.2m-diameter EPB TBM operated by Webuild completed a 12km-long tunnel in Buenos Aires. The geological conditions were extreme for soil conditioning: excavating under the Riachuelo River with high hydraulic pressures faced with an average earth pressure balance of 4.5bar. The soil mainly comprised a mono-granular and permeable sand (Puelchense formation), with a little fine material present (mainly lime). Despite this complicated geology, the Polyfoamer ECO/100 Plus was successfully used throughout the drive, ensuring good results in terms of TBM performance. Furthermore, muck was disposed of just outside the job-site area and, thanks to the rapid degradation and low toxicity of the foaming agent, vegetation was able to grow normally.