The objective of this study was to experimentally investigate the effect of the initial compressive stress and a circular opening boundary on thermal spalling and determine the mechanism causing thermal damage (spalling and fracturing) in rock observed in laboratory experiments. This was done by testing seven blocks of oven-dried and water-stored granite, gabbro and schist. The rock blocks had the dimensions 600 mm  500 mm  300 mm and a 75 mm borehole was drilled through the centre of each block to resemble a tunnel. The blocks were heated up approximately following a hydrocarbon fire curve. During the tests, both temperature and acoustic emissions were recorded.It is concluded that the tested rocks presented fairly different behaviours during fire due to different mineralization and thermal expansion coefficients. The granite suffered explosive spalling under confined conditions for both oven-dried and water-stored samples. The spalling was a continuous process and was repeated as long as the conditions were met. The gabbro suffered only relative small damage as small pieces flew off the fire exposed surface. Since the layers of the schist were located perpendicular to borehole axis, no obvious spalling was observed but the layers separated after testing due to expansion along the borehole axis. Compared to oven-dried samples, water-stored samples displayed more thermal spalling on granite and gabbro. The tests also showed that the initial compressive stress has facilitated spalling by means of reducing tensile stresses within the rock block and increasing the compressive stress concentration near the opening boundary.