In the earth-pressure balance (EPB) system, the excavation face is supported by matching the spoil removal rate from the face with the advance rate of the boring machine. If the advance rate is too slow, over excavation may occur - allowing ground settlement. If the advance rate is too high, then under excavation may occur - potentially leading to ground heaving. To better control the pressure balance and smooth movement of excavation spoil, foam may be added to the face chamber in the boring machine.