Installation and determination of the surface network

1. Installation and determination of the surface network • Establishment of a geodetic network on the surface based on GPS and astronomic measurements. • This network is materialized by pillars near the pits. Transfer of reference into tunnel • In each pit, the primary points of the GRN (Geodetic Reference Network) are defined from the surface on the tunnel ground.

2. Installation and determination of the tunnel networks • From the primary points of the GRN, a socket every 25 m is defined. • This geodetic network is necessary for the implantation of all the general services and the marking of all the components. It must be determined prior to any installation of general services. • In order to reach micrometric uncertainties, the MRN (Metrological Reference Network) takes over the GRN. It is composed by overlapping straight alignment references, measured with redundancy. The most achieved measurement systems are the stretched wires measured by WPS sensors.

3. Absolute alignment of the elements • Order of operations of the absolute alignment: • Marking of the position of the elements on the floor with respect to the GRN. • If needed, a concrete block is installed every beginning of a module or of a main beam quadrupole (in order to compensate for the ground difference of heights and provide a stability for the support). • Then an adjustable plate, on which are pre-installed the actuators is positioned with respect to the MRN (this plate is adjustable in order to allow the actuators to work in the middle of their range and compensate manually if necessary for important long term ground motions). • Installation and alignment of the elements with respect to the MRN . • To conclude, when a module will arrive on site, the adjustable plates, the actuators, the cam movers and stretched wire system will already be on place.