Today, full face tunnel excavation is usually performed by TBMs and based on ground properties, different machines are used. In urban areas with soft ground, Earth Pressure Balance (EPB) machines are widely employed to excavate tunnel with a variety diameter from small to very large. One of the most important parameters in EPB tunneling is face pressure. In fact, in EPBs, the magnitude of advance rate and discharging of excavated material from chamber by screw cause a pressure in chamber that is called face pressure. Low face pressure leads to tunnel face instability and subsequently ground surface settlement. In contrast, high face pressure, especially in shallow tunnels cause blowout. There are several methods to calculate face pressure in which can be categorized as: empirical, analytical and numerical methods. Numerical methods are used to confirm the result of analytical methods as well as where tunnel influence on important structures. In numerical model, water table was assumed as a horizontal plain and natural and saturated unit weights were considered above and under the water table, respectively. In numerical model, all parts of excavation, shield drive, grout injection and segment installation were considered. In addition, some important items such as gap between shield and tunnel perimeter was modeled exactly as it is. The loads acting on the tunnel face were estimated according to the active and passive earth pressure principles. The optimal face pressures for EPB shield was proposed in different soil strength parameters and adjacent structure type. The result showed that, face pressure is the most parameter in the shield tunneling. Also the obtained results show that, decreasing and increasing face pressure cause the stress state to be closer to the soil mass failure envelope.
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