New test stand for annular gap mortar
Effective component optimization
A correctly executed annular gap compression in the tubbing tunnel is a basic prerequisite for a high quality tunnel construction. But it also provides a decisive contribution to a high propulsive power in mechanical propulsion. In order to further optimize the annular gap compression, the STUVA in Cologne has developed a complex and ingenious system for tests with annular gap mortars.
Annular gap mortar fills the annulus between segmental lining and surrounding rock, which is created by the necessary overcut of the tunnel boring machine. It usually consists of aggregate, cement, water and any additives. The right mixture is based on empirical values and is usually experimentally improved on the construction site using the trial-and-error procedure.
With the help of the new STUVA test rig for the annular gap grouting, not only conventional annular gap mortars can be examined, but also the influence of the individual components for innovative developments can be systematically analyzed. Thus, targeted optimization is possible, which would be too extensive or too time-consuming in the context of construction site tests. In the system, for example, all mortar components can be matched in terms of volume and pressure and monitored metrologically.
At present, the STUVA test facility is designed for four components: up to two additive streams as well as pressure and volume metered compressed air can be added to the mortar or cement paste component.
The test stand has a modular design so that components can be easily exchanged for one another. These include, for example, the injection pumps, the cross sections of the delivery line or the additive addition devices. Also, the cross-section and the length of the pilasters, through which the mortar is pressed during the propulsion into the annulus, are easy to vary. These pilaster stripes end in a pressure vessel in the pilot plant in order to be able to actively influence the back pressure in the real annulus by an artificial pressure in the boiler.
In addition, filter elements can be incorporated into the boiler to model the mountain permeability and to investigate the effects of mortar properties on drainage behavior with the aim of early strength development. Another special feature is the possibility of producing test specimens: these can be removed not only under atmospheric pressure conditions but also under counter pressures of up to 10 bar.
Currently, with the financial support of the Federal Ministry for Economic Affairs and Energy (BMWi), a water-permeable annular gap mortar is being developed on the new plant. The use of such a special mortar is always useful in tunneling in hard rock when high mountain water pressures occur and thus a pressure-tight design of the tunnel shell would lead to very thick tunnel shells because of the high loads from water pressure.