Faults and their associated structures are, in many cases, the most important features to be assessed in many engineering site investigations. They cut, deform and displace the original fabric of the rocks (Figure 8.1a and b), thereby causing them to become, in general, weaker and more fissile, increasing the susceptibility of the ground to weathering and influencing the hydrogeological regimes. Faults range from a few centimetres to several hundreds of metres in width across a fault zone. However, due to the influx of hydrothermal fluids, some fault zones may be re-cemented by quartz, calcite and other secondary minerals (see Case Study 8.1). In these cases, parts of the fault zone can form extremely strong rock masses.