Assessment of the Potential for Rock Spalling in the Technical Rooms of the ONKALO: Work Report 2011-35

It is important to be able to predict the rock spalling in the ONKALO while the excavation advances deeper. When stresses at the excavation boundary reach the rock mass spalling strength, a brittle failure occurs that is often called “spalling”. The spalling phenomenon occurs as a strong compressive stress induces crack growth behind the excavated surface. Spalling is, expressly, an event that can create problems in the ONKALO, not so much for the overall stability of all of the excavations, but rather in particular areas that can cause unnecessary and unintended over-excavations and hazards.

For rock engineering and layout design purposes, the knowledge of the predicted spalling in the excavation surface is crucial. Optimization of the design is mainly done by directing the tunnels parallel to the major principal stress direction. However, due to the complex forms and crossing tunnels, especially at the shaft access drift area, sophisticated methods are required in order to minimize spalling and to support the unavoidable spalling that occurs.

The complex tunnels require three-dimensional analysis. The software used for the main calculation has been MIDAS/GTS, a geotechnical 3-D FEM that is able to calculate complex geometries rather easily. Most of the models have also been verified with Roc-science Examine3D, which returns the results with a high precision at boundary. The area to model is large, and due to the computational limits, it is divided into six blocks. This analysis, carried out step by step for each block, permitted to draw a map of the spalling depth prevision in the whole tunnel contract 5 (TU5) area.

The dominating rock types in the area are migmatitic gneiss and pegmatitic granite. The strength of these rocks has been broadly tested with point load and uniaxial compressive strength tests. The test results show a deviation of the UCS as well as other parameters. Due to this large deviation, a Monte Carlo has been used as an auxiliary analysis method.

The results of the Monte Carlo analysis indicate that in the ONKALO 23 percent of the local rock strength and in situ stress combination cases coupled together result in spalling with a mean depth of 0.28 metres. In the three-dimensional models with con-servative parameters, there is spalling in the crown of the tunnels in an unfavourable direction, but moreover, in the shaft access drifts, the tunnel crown is highly stressed with large spalling. At the worst occasions the spalling depth is over one metre. By using average parameters, spalling is predicted only near the shafts.

The prediction should be taken into account when designing the rock support for the zones with spalling. Especially in the shaft access drifts, reinforcements that are able to withstand the rock weight of the predicted spalling area, are recommended. Such sup-port measures could be, for example, pre-tensioned anchors together with steel wire mesh and shotcrete. This work underlined how important it is to evaluate the stability of an excavation, even if carried out in very hard rock with low fracture intensity.