Thermal stress state of an underground powerhouse roof during construction and operation

Abstract

1. Forced ventilation accompanying the driving of underground workings leads to substantial temperature fluctuations of the roof surface and to the occurrence of thermal compressive stresses in the roof of the powerhouse. The maximum compressive stresses in the roof are observed at those times of the construction period when the values of the temperature gradients in the radial direction and values of the ratios of the height of the powerhouse to its width are maximum. This circumstance should be taken into account when designing and constructing underground powerhouses of hydrostations under conditions of the Far North. 2. A tectonic fracture passing near the roof at the initial time of thawing of the mass promotes the occurrence of stress concentration in the roof, increasing the maximum compressive stresses by 3 times in comparison with the case when the rock is solid. 3. The temperature regime of a perennially frozen rock mass around the powerhouse of a hydrostation during its operation stabilizes within 40–50 years of the constant thermal effect from the machine hall. In this case, a halo of thawed rocks forms. The temperature distribution in the rock mass after 15–20 years of operation of the hydrostation is close to steady. 4. Thawing of frozen rocks in the operating period of the hydrostation, i.e., the establishment of a steady temperature distribution in the mass, is favorable from the viewpoint of the stress state of the powerhouse. In this case, thawing of the mass leads to a decrease of the values of the thermal stresses in the concrete roof of the powerhouse in comparison with the values of these stresses in the construction period occurring as a consequence of forced ventilation.