ABSTRACT
During the excavation of a tunnel in urban environment, under water table, and in very complex geological conditions, maintaining the stability of the excavation face is essential for preventing excessive settlement and consequent damages. Face stability can be controlled by managing different excavation parameters: the face support pressure and the extracted volume are the key parameters for an EPB machine. Some innovative concepts to calculate, measure, and control these parameters, discussed in this paper, are: (1) how to calculate and measure the parameter values, in reference to the ground characteristics; (2) how to control these values during excavation; and (3) how to react when the threshold values are exceeded. The methodology for controlling the two key parameters is as follows:
Control of the support pressure is carried out by (a) measuring directly the earth pressure at the different levels of the excavation chamber, (b) calculating the apparent density of the material inside the excavation chamber, and (c) installing a secondary pressure circuit for injecting small quantities of bentonite slurry into the chamber to maintain the support pressure above the threshold. Control of the extracted volume is done through the correct installation of a balance device, combined with the estimation of the in-situ density of the material to be excavated. The results of application of the above methodology to an example problem are presented. It is expected that this methodology will become a part of the standard practice of tunneling by EPB Shield.
