Fire resistant tunnel construction: results of fire behaviour tests and criteria of application

Abstract

There is an increased public interest in the safety of tunnels in recent years, due to the huge number of fatalities and material damage from recent fires in tunnels. This is reflected in the actual standards and codes of practice for construction and operation of road and railroad tunnels. Back analysis of real tunnel fires, like the Euro Channel tunnel fire, underline the importance of structural fire protection. In the Channel tunnel fire the structural lining was reduced by explosive spalling, locally to one-third of the original 40 cm thickness. Within the scope of research and development, HOCHTIEF has been consistently pursuing the objective of developing fire resistant construction, especially for application in underground works and in tunnelling. In close cooperation with the technical university of Brunswick a fire resistant structural concrete 'System HOCHTIEF' using synthetic fibres and special aggregates has successfully been developed. This concrete shows no significant spalling under extreme fire. Due to the characteristics of burning vehicles the fire impact in tunnels differs significantly from fires in buildings. Apart from higher maximum temperatures, the rapid rise of temperature at the beginning of the fire leads to an increased propensity to explosive spalling especially when concrete with high compressive strengths are utilized. Different fire protection concepts have been assessed considering the criteria: fire protection suitability of the system, durability and maintenance demand over the lifetime of the tunnel. The substantial advantages of fire resistant structural concrete are evident. In 2003 full scale fire behaviour tests on highly loaded precast tunnel lining segments were conducted using the temperature time curve given by German railway authorities. The test set up simulates the stresses of the tunnel lining in the circumferential direction. External loads like earth and water pressure are applied by horizontal and vertical hydraulic jacks to a curved segment. Prior to the tests the actual state of stress in the tunnel lining during fire was theoretically analyzed by means of improved numerical methods taking into account the nonlinear behaviour of the concrete exposed to fire, as well as typical nonlinearities of the joints. With compressive stresses in the 'cold' initial state of approx. 21 MN/m at the inner face (exposed to the fire during the test), an upper limit for the stress level was chosen in comparison to other tunnelling projects. Although the segments were exposed for about 60 min with temperatures of 1200 degrees C no spalling occurred on the side that was exposed to fire. Even the surface of the concrete structure revealed a very good level of consistency after the test. Except for a thin layer close to the surface, concrete compressive strengths measured subsequent to the fire behaviour test clearly exceeded the reference values of a C35/45. The highly comprehensive test program has revealed that concrete as a building material may attain a high level of resistance to fire exposure. This newly developed and trendsetting fire protection system equips developers and planners with an important and novel tool for making tunnel construction more economic and safer. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.