GEOTECHNOLOGIEN aktuell

Ice as a building material

Subway tunnel DOE
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It is rumbling under the Marienplatz in Munich: the underground station is being expanded to make 2006 more smoothly for the new football stadium in Fröttmaning. At the same time, scientists are using the tunnel construction site as a geotechnical laboratory: at a depth of around 15 meters, they observe the deformation behavior of frozen soil - a research project that is being carried out in Germany for the first time in this form.

Under the New Town Hall, two escort tunnels will be built, which run parallel to the existing subway tubes. To build these tunnels, the ground is iced to give it more strength. The icing lances required for this purpose were drilled from pilot galleries, which had previously been laid laterally offset over the escort tunnels. The icing causes the pore water contained in the soil to freeze, which gives the soil approximately the strength of lean concrete and makes it watertight.

In order to be able to determine the stability of the frozen soil bodies, the strength properties of the frozen soil are examined. If one applies the ground freezing under subsidence-sensitive structures, the measure of frost heaves must be limited. In the worst case, they can lead to considerable damage. The forecast of such uplift due to soil freezing is the central aim of a current research project at the Geotechnical Center of the Technical University of Munich.

Freezing can be attributed essentially to two mechanisms. Unless there is free access to free pore spaces (gravel) without resistance, homogenous frost heaves occur due to the approximately nine percent volume increase during the transition from water to ice. Their dimension is proportional to the thickness of the frozen layer. In addition, especially in fine-grained soils on the border of frozen to unfrozen soil of pure ice crystals growing bodies, so-called "ice lentils". At constant temperature they also cause gradual uplift.

In order to be able to record the different parts of the lift separately, the TUM scientists developed a laboratory test that simulates essential boundary conditions - such as the presence of drained conditions or the on-site load on the icing body. In a two-chamber technique, a soil sample is frozen one-dimensionally from above, but it can simultaneously absorb water from the bottom. Laboratory calculated lift rates and speeds serve as input to finite element calculations simulating the effects of frost heave up to the terrain surface. display

Finally, in the tunnel construction site under the Munich town hall, the actual deformation behavior was measured directly on site with a tube scale measuring system, which measures settlement differences of up to 1/100 millimeters at selected measuring points according to the principle of communicating tubes. In addition, indirect measurements were carried out with strain gauges on the reinforcing steels of the tunnel shell. So far, the measurements have confirmed the predicted frost heave. The TUM researchers have thus made an important step in researching coupled heat and mass transport in the soil. In the future, uplift can be better estimated in advance.

(Technical University Munich, 21.12.2004 - NPO)