The excavation of any underground work alters the initial tension state of the earth, which means displacement of the land surrounding the excavation site. The magnitude of that movement depends on various factors associated to the nature of the materials, the excavation process and the rigidity of the earth support system.

The purpose of the monitoring systems is two-fold:

• To check that the structural elements constructed are in accordance with the calculations made for them.

• To ensure that any effects produced by the work on the surrounding area is maintained within the predefined limits. It is essential that any deformations or land settling transmitted to the surface area or infrastructure and buildings on top of or near the route of the line are controlled.

A series of instruments which periodically measure the tension and deformation parameters are available for this purpose.

There are two types of devices and equipment normally used in the monitoring of underground work, Electronic Sensors and Topographic Control Points.