THE DESIGN OF CONDITIONING PROCESS FOR MECHANIZED TUNNELLING WITH TBM-EPB

The growing demands for mobility in large urban areas and the need of expansion of large rail and road infrastructures have led, in recent decades, to a notable boost in the world of tunnels, made evident, among other things, by a number of realized projects in exponential growth, by an unprecedented technological development and by the consequent growth of professionalism involved and jobs, investments and incomes generated. In the mechanized excavation of tunnels, the EPB-TBM excavation technology can rightly be considered one of the latest innovations in terms of time and certainly one of the most widespread methodologies in the world. Much of the diffusion of this technology is due to its flexibility and the ability to quickly adapt to the variable conditions, making its strong point in the possibility to control the pressures applied to the excavation front-face by soil conditioning and to the considerable advantages linked to the possibility of avoiding the complex process of separation and re-use of bentonite mud from the spoil. For a successful excavation, conditioning is a decisive process and its good management leads to the reduction of risks, the reduction of a series of undesired effects (surface induced failures/settlements, wear of TBM metal parts in the case of coarse grained soils, clogging in the case of fine-grained soils), the increase in excavation performance and the possibility of optimizing the re-use of the excavated soil; in extreme synthesis to the reduction of risks, times, costs and impact of realization of the work. This process, determinant for the success of the excavation activities and in general of the realization of the work, is not substantially the object of design. In the preliminary and definitive design phase, soil conditioning is currently only considered for the environmental aspects necessary for the definition of the plan for the use of excavated earth and rocks. The study of soil conditioning is entrusted in small part to the executive design phase of the work, in which the characteristics of the TBM (and therefore also of the injection systems) are defined, and almost entirely directly in the construction phase for the choices of products, dosages and conditioning parameters. The dosage definition is made on the basis of the indications provided by the producers of conditioning agents or by the TBM manufacturers, refined directly during the first section of the TBM excavation (learning phase) and occasionally confirmed by conditioning tests carried out in the laboratory close to the beginning of the excavation activities. The operative procedures for managing and modifying the conditioning parameters are agreed on site by TBM staff with professional figures such as the TBM engineer. Occasionally, accidents due to errors in the conditioning occur, the greater consumption of conditioning agents is systematic up to 3 times the estimated values and there is almost never certainty about the possibility of improving the performance of the TBM by optimizing the conditioning process. This process is determined by the widespread belief that each construction site and each tunnel is a work "stand-alone" for which the conditioning has to be defined directly in the course of work; this conviction matured over time due to the differences related to the formulation of the products, very similar but never exactly the same, due to the variability of the soil encountered during the excavation, predictable but never accurately, due to the scale effect that naturally exists between the evaluations made in the laboratory and what happens on site, due to the remarkable effect that the subjective evaluations and decisions of the workers involved in the excavation of the tunnels (foreman / TBM chief) and in the lack of studies and in general of the a clear understanding of the conditioning process from the chemical, physical, geotechnical and mechanical point of view. After years of experimental study and comparison with real cases of tunnelling, I certainly cannot claim to have a clear knowledge about every aspect of the conditioning, but I am absolutely convinced that this aspect can and must be the object of a design process like so many other aspects of excavation of a tunnel, once entrusted to the experience of the miners and which today are the subject of in-depth analyses developed by engineers. These engineers, to make a good job, must have listened to the miners' stories, seen the mechanics at work in a TBM, studied the scientific evidence produced by different research groups in the world, make use of the calculation tools and engineering judgement that are proper to them and, above all, they must resist the temptation to say "in the jobsite, somehow, they will handle it". The layout of this thesis stems from the belief that, while considering all the uncertainties and peculiarities typical of each tunnel project, in a general perspective the conditioning is a process that responds to precise and always valid rules, of which the typology of soil, the chemical composition of conditioning agents, the characteristics of the TBM and the injection system are simply variables of a system that is decidedly complex but substantially responsive to laws and relationships like everything we know as engineers. More in detail, I have gained the conviction that all the effects of the conditioning of any type of soil can be seen as the combination of three effects: i) the effect of the chemical interaction between the conditioning agents and the soil; ii) the effect of the variation of the water content caused by the injection of water and the liquid part of the foam and iii) the effect of the introduction of air into the soil and consequently the lowering of its saturation degree. Consequently, during the course of the PhD, one of the first aims was certainly the study and measurement of the three effects listed on a very wide combination of soils and conditioning agents necessary to understand how the chemical composition of conditioning agents and the characteristics of the soil affect these effects. On the basis of the existing literature and the first experimental evidences obtained in the laboratory, I have therefore begun to think of a conditioning design process that can be integrated into the entire project from the preliminary stages up to the executive design that it was able to define, with subsequent investigations, costs, consumption, dosages and features of the conditioning process during the excavation, to foresee the correct maintenance of the pressures at the front during excavation and during the standstill and to foresee the geotechnical and environmental characteristics of the soil after extraction from the excavation chamber to define the most suitable reuse methods. This process responds to rules and concepts of general validity to be declined, for each tunnel project, based on the boundary conditions; in this design process characteristics of the soils, TBMs features, environmental and economic constraints, interactions with the external environment are all parameters input of a clear process developed according to well-defined and always valid rules that will lead to a solution that is always different for each gallery to be built; I am indeed deeply convinced that every tunnel is a separate project, but that instead the rules with which every aspect must be designed, including conditioning, are those of physics, chemistry and geotechnics. The definition of this design process required the development of new specially designed laboratory tests, standards and interpretation and classification systems based on the results of a decidedly high number of combinations of types of soil (8 fine-grained and 8 coarse grained) and more than 30 different conditioning agents including foaming agents and polymers. The process I have conceived involves three successive phases, the first to be developed in the context of the Technical and Economic Feasibility Study and of the Definitive Project, the second to be developed in the context of the Executive Project and the third to be developed together with the excavation of the gallery. Each phase involves the use of a series of data and information (inputs - what I have) commonly available for each design level, provides a series of objectives (targets - what I need) necessary for the correct design of the work, foresees the execution of analyses and laboratory tests purposely conceived of which the standards are defined (actions - what I do), foresees the obtaining of a series of indications (results - what I get) and finally it foresees the verification (controls - what I check) of the hypotheses made to acquire information on residual uncertainties and to perform parametric analyses necessary to define the ranges of variability of the results. The integration of this process within the process of design and canonical execution of a tunnel excavation could allow the reduction of accidents, risks, unforeseen events, excessive consumption, maximize excavation performance, optimize the reuse of conditioned land, reduce the duration and problems of the learning phase. Furthermore, the development of all the conditioning study activities in a single process will bring order, avoiding overlaps and repetitions in the different phases of the design, reducing burdens and timing associated with these activities, which are often out of control. This thesis proposes a method, based on experimental evidence, the result of continuous comparisons with real projects of tunnels excavated in recent years and a significant number of combinations of products and soils analysed. Certainly, like every Research product, it should not be considered a finished and defined product, but I hope in the future it will be integrated and modified, even by myself, on the basis of new experimental evidences, of new requirements and boundary conditions (characteristics of TBM and conditioning agents) that will certainly have changed in a few years. It is important to underline that the contents of this design process are based on widely shared approaches from the international literature, that only some laboratory tests and some classification systems have been developed and proposed by me during the last three years and that the overall proposed design process, in a nutshell, can be considered a reorganization and an organic collection of a series of information that the scientific community, in which I undeservedly include myself, has collected in the last decades and an attempt to transmit it to those who they are engaged daily in the design and construction of tunnels with EPB-TBM. The highest and certainly ambitious goal, which I would be satisfied to have achieved even in small part, is to modify the perception of studies on soil conditioning to give them the level of attention usually reserved for the most relevant aspects of tunnel excavation; the aspects that must be carefully designed, the aspects that determine the success of an excavation, the aspects that if undervalued can cause risks and considerable damages, the aspects that can substantially determine the time, cost, durability and environmental impact of an excavation work. I am convinced that the most recent history is showing that soil conditioning, in the mechanized excavation of tunnels, deserves this level.