In his PhD thesis defended at the UPV/EHU-University of the Basque Country, Javier Gallo has presented an innovative model that allows the excavation rate of tunnel excavation machines in rocks and soils to be predicted by means of simple variables that can be measured in real time. The work is entitled Definición de un modelo para la estimación de la fuerza de arranque en la excavación mecánica de túneles en suelos y rocas (Specification of a model to estimate the breakout force in the mechanical excavation of tunnels in soil and rocks).
The model developed by Gallo has come about to solve a real problem facing tunnelling and micro-tunnelling machines: "When the tunnels were being dug, breakages unexpectedly used to occur in the cutting tools which could not be explained with the existing models," says the author of the thesis. To solve the problem, and taking advantage of the fact that the modern machines gather many more data than the previous ones, he decided to analyse these data. Gallo has analysed the drilling records of 6.6 km of tunnels. The data were obtained from various sources, but mainly from the research project "Reduction in mains water consumption in the micro-tunnelling process" run by Grupo Mecanotubo, S.A. with the participation of the Centre for Technological and Industrial Development (CDTI) that reports to the Spanish Ministry of Science and Innovation. The author participated in this project as a Project Engineer.
"There is a genuine need because at the company where I work, for example, there were problems when it came to predicting the breakages of the cutting tools or to knowing what the machine's operating limit was: How far can we go with the machine?" explains Gallo. Thanks to the mathematical model developed by the author, it is possible to know what the machine's operating limit is, or whether it would be better to use one kind of cutter head or another.
The importance of predicting
"Imagine a hammer is hitting the rock," explains Gallo. "You have to know the rate at which you can work with that hammer and when it is going to break. These are the two things that are relevant." The model in fact helps to predict questions like this: when the hammer is going to break, how much it can be tightened, etc. "And since moving forward is in proportion to the force you apply to the blow, it allows you to know when applying a blow of a certain bore, whether the hammer will withstand it, and how far you are going to move forward. That way, I can say I am going to need three hundred hammers, for example, and that I can finish the work in, say, two months."
Predictions of this kind used to be made with previous models, but the results did not coincide with the actual observations. According to Gallo, "this is partly because the machines only began to record the data in real time from the year 2000 onwards. In the old days, the machines did not gather so many data and laboratory tests used to be carried out. But in the absence of real data, the models were not so reliable."
Gallo has also developed some software which provides the operator with information about the resistance of the rock. The software picks up the data gathered by the machine, applies the model and gives the operator information about the resistance of the terrain in real time.
On the other hand, the model can be of great use when designing new tools, since on the basis of the estimates provided by the model, manufacturers could try out new alloys or modify the mechanisms. And it will also be of great use for construction companies to provide them with a more reliable equation with which to estimate excavation performance and carry out quality control on the excavation process.
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