Influence of heat on the physical and mechanical properties of selected rock types

Sammanfattning: Impelled by the increase in the number of tunnel fires in the last decade alone, wide spread attention has been drawn towards tunnel fire safety studies. Many of these fires occurred in road and railway tunnels involving vehicles and trains. These fire incidents have claimed lives, caused structural damages to the tunnel infrastructure and even economic losses to the government, businesses and communities concerned. When there is a fire in a tunnel, the temperature inside the tunnel increases rapidly to magnitudes as high as 1500°C. At such high temperatures costly damages to the tunnel structure is inevitable. Having an understanding of the detrimental effects of such high temperatures is essential and valuable when carrying out preliminary assessment of the type and extent of damage in the tunnel. This would in turn provide useful information in determining the appropriate remedial measures required to make the tunnel safe and usable again in the aftermath of a tunnel fire. In most tunnel fire safety studies, the focus has been on the behaviour of concrete, since of course concrete is one of the major support elements in tunnels. However, in several cases, such as in Scandinavia for example, where the rock mass is competent enough to support itself only a thin layer of shotcrete is used usually on tunnel walls. In such cases the rock will be fully exposed to heat in an instance of fire. In this case, whether it is prevention or maintenance of the tunnel, it would require knowledge on the effect of elevated heat on the rock mass. Hence, it is line with this thinking that a study was initiated by the Swedish Transport Administration (Trafikverket), Kärnbränslehantering AB, SKB and Vattenfall to study the effect of heat on the physical and mechanical properties of some common rock types, and hence the focus of this thesis. This thesis presents the results of a series of laboratory studies which was carried out to investigate the effect of heat on the physical and mechanical properties of selectedrock types, namely; diabase, granite and quartzitic schist. Samples from these rock types were heat treated at temperature levels of 400°C, 750°C and 1100°C, before investigating their mechanical and physical properties through mechanical testing and microscopic investigations of thin sections. Because the effect of heat on rock can be affected by the heating rate and exposure time, the test were conducted under controlled conditions in order to avoid significant variation in the results. The results clearly show that the rock types behave differently at different temperature levels, which tend to depend on the mineral composition and micro cracks distribution. As the temperature increases the rock forming minerals undergo changes in their chemical structure thus causing them to alter from the original phase they had existed in. With these phase changes different reactions take place such as re-crystallization, the loss of crystal bound water, thermal expansion and micro cracking of mineral grains as well asthe development of voids. These microscopic changes were manifested in the macro-scale by the variations observed in the behaviour of strength and stiffness of the samples in the mechanical tests.