The TBM is an integral tunnelling machine. The TBMS require strong economic investment to dealing with equipment manufactured virtually tailor-made for each work the tunnelling machines of last generation allow yields relatively high and in good condition of re-entry for workers. Today it is excavated with acceptable yields, all kinds of difficulty of mechanical excavation of hard rocks hard rock comes from its resistance to cutting and high abrasiveness of the majority of the siliceous hard rocks. The Mole is a machine designed to do only the excavation of the tunnel along its entire length, then building cladding. Therefore, Mole is a machine without shield so support of the roof of the tunnel systems are essential. The development of shields to excavate soil and soft rock was older than the rock machines, since the metal shield that surrounds the entire side of the machine protected staff before dorsal field detachments.
The land with unstable fronts, most of the time has the added difficulty of the presence of water. The first technology applied to these cases consisted of the pressurization of the tunnel with compressed air. Fatal accidents by subsidence with flooding of tunnels disappeared. It was little by little, by checking that the operations of daily depressurization which had to pass the operators before leaving the tunnel, were causes of progressive and severe deterioration of health. On the other hand, labour safety regulations ended making wasteful system, so this technique is practically obsolete. Thus arose the modern technologies of coats of front in pressure.
Front pressure badges are coats of arms where they work at atmospheric pressure. Only put in pressure head or cutting wheel of the machine and the front camera is placed where the excavated ground, camera from feeding the extraction system. The problem in not cohesive or sandy land with shields front in sludge, also known as hidroescudos pressure was dominated in the early 1990s. The hidroescudo technology has been employed in unstable terrain, even under load of water, and its evolution from the past few years has consisted of the refinement of the constructive process that today is almost exclusively made with bentonite sludges. The only difficulty with the hidroescudo resides in that, as the excavated ground is mixed with the drilling mud for its extraction for pumping, mixing treatment requires separation plants that are costly. Front in land pressure shields or EPB (Earth Pressure Balance) shields are building machines of tunnels that we have today for cohesive or clayey soils.