KEEPING THE CHAMBER FULL: MANAGING THE 'AIR BUBBLE' IN EPB TUNNELING

摘要

Soil conditioners mixed with compressed air to create foam are often added to the confined pressurized environment of EPB TBMs in order to reduce cutterhead and screw torque and properly condition the excavated material. An air "bubble" at the top of the TBM excavation chamber can form if the compressed air is not entrained in the EPB Muck. The formation of such an air bubble can lead to a variety of problems such as: excessive drop in EPB pressures between tunnel advances, less stable face support and possibility of water and material flowing into the excavation chamber, over excavation and possibility of surface settlement, potential blowouts through the screw or to the surface, an increase in the chance of methane explosion by providing oxygen into the mix etc. In this paper, several key parameters such as muck apparent density, EPB pressures, geology, foam expansion and injection ratios are used as tools to identify the presence of a "bubble" in the chamber, in addition to practical procedures for eliminating "bubbles" and the conditions which contribute to their formation are presented.