How does the advance rate affect the pressure maintained by an EPB TBM?

The advance rate and the pressure maintained by an EPB (Earth Pressure Balance) TBM are closely linked. The EPB TBM's primary function is to maintain face stability by balancing the pressure exerted by the machine with the earth and water pressure acting on the tunnel face. The rate at which the TBM advances directly influences the volume of soil being excavated and the pressure required to support the face. If the advance rate increases, the TBM needs to excavate more soil per unit of time. To maintain face stability, the pressure inside the excavation chamber must be increased to counteract the increased soil removal rate. This is achieved by controlling the discharge rate of the excavated soil through the screw conveyor. Conversely, if the advance rate decreases, the pressure inside the excavation chamber can be reduced. Reducing the pressure too much, however, could lead to face instability or collapse. The TBM's control system continuously monitors the earth pressure and adjusts the screw conveyor speed to maintain the desired pressure balance. The optimum pressure is determined by the soil type, groundwater conditions, and the depth of the tunnel. Therefore, the advance rate is a key factor in determining the pressure required to maintain face stability in an EPB TBM. The 'advance rate' is the distance the TBM progresses in a given unit of time. 'Earth Pressure Balance (EPB)' refers to a TBM technique that uses excavated soil as a support medium. 'Excavation chamber' is the space inside the TBM where the soil is excavated. 'Screw conveyor' is a rotating helical screw that transports the excavated soil from the cutterhead to the rear of the TBM.