Ground freezing at Forrestfield Airport in Perth

This new line consists of two twin-bored tunnels, connected by a total of 12 cross passages. In order to avoid surface deformation in the airport area, three cross passages were mined under favor of ground freezing. At our geot­ech­ni­cal laboratory, we executed freezing tests with the Australian ground samples from which we determined ther­motech­ni­cal and static parameters for the frost body sizing. As the cross passages would be located right underneath the runway, particular attention was turned to frost heave tests. These parameters formed the basis for our numeric design engineering regarding heat calculation, frost body statics and surface deformation. Through our heat calcu­la­tions we ensured a temperature of T ≤ 0°C in the area of the maximum dimensions of the cross passages’ centres near the tunnel ridges to prevent material from advancing during excavation. Thereby ensuring safe tunnelling. Owing to our ther­motech­ni­cal calcu­la­tions we were able to deviate the frost body’s dimensions as well as provide evidence of its sufficient performance.

We calculated the statics numerally for a freezing period of six weeks. Taking also into consid­er­a­tion the frozen frost body’s thicknesses derived from our ther­motech­ni­cal calcu­la­tions. For the decisive cross-sectional area adjoining the tunnel lining, we also precon­ceived the sufficient structural stability of the frozen frost body’s thickness with 1.5 meters. We were able to provide evidence for the frost body statics with safety factor 2 and 4 for this cross-section consid­er­a­tion. As the cross passages are located underneath very sensitive areas as the Forrest­field Airport, surface defor­ma­tions were limited to 1.5 cm at most. We verified complying with these deformation limits by means of bivariate numeric calcu­la­tions of frost heave. Addi­tion­ally, we evaluated temperature measuring data during execution, and monitored approval and opening of the cross passages by our experts on site. Train service is already operating on this new airport line.