CNIT underground station jacking

Load transfer under (pre)tension

In order to simultaneously transfer the load from the 120 existing structural columns, the Freyssinet teams implemented an exceptional system. This consisted in transferring the loads, previously taken up by the foundation sole plates of each column, to a falsework structure. To achieve this, each column was “waled” (i.e. “tightened”) by two reinforced concrete corsets flanged with Freyssibar + 1,200 prestressing bars. These prestressed structures were supported on four sets of micropiles by means of steel frames. Once this temporary structure was assembled, the stresses could be shifted to it and the columns sawed off.

Computer-aided precision jacking

Millimetre displacements associated with the release of tension when transferring loads onto the falsework were managed by a set of servo-controlled hydraulic jacks. The scope of the servo control system was unparalleled: a computer-aided lifting station (CAL) enabled to control all jacks of the 24 columns simultaneously. Thus, if a distant area of a column undergoing load transfer experienced higher than expected settlements, the control station was able to correct these deformations instantly. Subsequently, the CAL system was used to transfer the loads from the transfer slab to the final pillars. 360 jacks were then coordinated simultaneously around 56 pillars!

A restricted operation under strict monitoring

The safety of the load transfer operations was constantly ensured by a set of ultra-sensitive sensors, enabling the detection of displacement variations in the order of 1/10 mm and pressure variations in the order of one bar. In the event of a loss of pressure in the jacks or an unexpected displacement, an alarm was triggered and the system was designed to instantly compensate for the failure.

In addition to the great complexity of this unusual phasing, there was also the complexity of the logistics: the first phase took place in a confined area (2.2 m high ceiling), complicating the access of teams as well as the procurement of materials and equipment, the dimensions of which had to be adapted to the confined space.