Underground construction in urban areas that need more efficient transportation systems, energy and water supplies is a very challenging task, due to the complexity of the work itself and because surface disturbances and subsidences require to be minimized. In order to face the challenge and allow the excavation of tunnels even with low overburdens, Earth Pressure Balance Shields (EPBS) technology has been increasingly used in the recent years. EPBS is a type of tunnel boring machine (TBM) with earth pressure support. By transforming the excavated material into a soft paste that has plastic properties to be used as support medium, it is possible to balance the pressure conditions at the tunnel face allowing minimum settlement. In addition, uncontrolled inflow of soil and water into the machine is avoided and tunnelling process becomes more effective. In order to better understand soil behaviour and improve the performance of the EPBS machine, two variables are studied: soil conditioning and tools wear. During excavation with EPB machines the correct soil conditioning is a very important parameter to be controlled. For this reason, it is critical to perform preliminary tests with different conditioning agents in order to determine the most suitable reference dosage for an excavation project. Currently, soil conditioning is evaluated by performing slump tests and plasticity and homogeneity checks of the dough at laboratory scale. There is also the Extraction Test used to evaluate material extraction that is one of the most realistic tests to know the behaviour that soil could develop during excavation process. On the other hand, a very important but still less studied variable is the wear of EPBS metal parts like excavation tools, rotating head, shield and screw conveyor. Wear leads to a reduction in working yield due to the mentioned machine components lose their optimum properties and have to be replaced; consequently, downtime is required in order to execute proper maintenance, which is difficult and dangerous. Many aspects play key roles in the wear process like excavated medium, water content, applied pressure, soil conditioning and type of metal used for machine tools manufacturing. In fact, rocks and soil excavated during EPB work can be composed from any kind of minerals. Hard minerals increase the wear phenomenon that concerns all parts of the machine where there is friction between metallic part and the medium. For this reason, study of wear phenomenon is an extremely important issue for new projects with EPB excavation technology. Therefore, the objective of this work is to study of the influence of soil conditioning on tools wear and the relationship of the main factors associated to this phenomenon. In order to accomplish this goal, about 26150 kg of different soils were studied using 4 different wear test methods. These tests were based on the application of a test methodology already used in the Tunnels and Underground Works Laboratory of Politecnico di Torino and other new methodologies and equipment, developed with the aim of deepening the study of wear taking into account several variables that concern the construction of tunnels. Each methodology implemented has different benefits, limitations and scopes, but provides congruent technical results. Finally, prediction indexes were proposed in order to evaluate tool wear phenomenon and compare the effects of different soil conditioning. As a result, better decisions are made when choosing the ideal conditioning for EPB tunnelling, maximizing projects effectiveness and success.
|Titolo:||Influence of material conditioning on tools wear of EPBS machines using different laboratory test|
|Data di pubblicazione:||7-mag-2018|
|Appare nelle tipologie:||8.1 Doctoral thesis Polito|