Behavior of structural tunnel elements exposed to fire and mechanical loading

Abstract

Purpose Incidents like the fire in the Channel Tunnel, where severe concrete spalling was determined, have led to requirements in limiting the spalling depth and involved zone to local and compatible magnitudes. Because the prevention of critical concrete spalling was also significant for the validity of the load-carrying capacity calculation for an existing railway tunnel, this paper aimed to investigate the spalling behavior of two contemplable concrete mixtures. The large-scale tests should show the load-carrying capacity over the whole duration of the fire exposure respecting all thermal and mechanical loads considered in the calculations. Design/methodology/approach In this paper, the fire behavior of two concrete mixtures for an existing railway tunnel are investigated. Small-scale tests prior to the main tests were conducted to identify an appropriate concrete mixture for the large-scale tests. During the large-scale tests, a tunnel segment is loaded with horizontal and vertical loads derived from a calculation taking into account the existing boundary conditions. Resulting restraint forces were calculated using the soil stiffness and tunnel fire design curve as fire scenario and applied via hydraulic jacks. To avoid additional restraint forces during the experiment, thermal strains due to fire exposure were allowed. Findings The results of the small-scale tests did not allow for a clear statement whether one concrete mixture would perform better regarding the spalling behavior. The two large-scale tests showed different results regarding the spalling behavior. Over the whole duration of fire exposure, the first test specimen remains nearly undamaged. During the test of the second specimen, spalling started about 3 min after burner activation. Because of the results, a suggestion for the concrete mixture of the first test was made, and this mixture was then used for the redevelopment of the existing railway tunnel. Originality/value The test setup was capable of incorporating all relevant boundary conditions for the analysis of an existing railway tunnel as part of an important north – south connection. The results have shown that a fire-proof construction is possible by adding polypropylene fibers to the concrete mixture. Additionally, it was possible to avoid the mounting of expensive and time-consuming fire protection measures like the installation of thermal insulation boards.

Design of the metro tunnel project tunnel linings for fire testing

“…To achieve the desired loading, three loading systems were required, horizontal loading, vertical center loading, and vertically offset loading (left and right of center). A mortar bed was required to be laid on each strip area of loading to ensure the loading was transferred equally to the segment which has been required for similar tests 5 . The load was applied using Enerpac jacks.…”

“…These incidents have led to requirements in limiting the spalling depth and the magnitude of temperatures of the in situ reinforcement 5 for both rail and road tunnels. While road tunnel fires are more frequent, the number of fatalities in railway tunnel systems generally seems to be far greater 4,6 .…”

“…Apart from real tunnel accidents including the well‐known cases that were discussed earlier, limited large scale experimental research programs have been carried out on the behavior of tunnel linings in a fire 5,19–27 including the Runehamar fire tests (Norway, 2003, under as part of the UPTUN project) 28 . However, only limited works 5,20,21,25,27 have conducted full scale tunnel lining structurally loaded fire testing incorporating real life boundary conditions, external loading and conditioning corresponding to design assumptions. This is due to the fact there are only a limited number of test facilities capable of carrying out these tests, mainly in Europe and the tests are complicated and expensive.…”

Effect of uniaxial loading on the spalling of concrete panels for Melbourne's Metro Tunnel Project

“…Lessons from past accidents have led to the development of stricter practices assigned by different authorities and various research activities. 6 These incidents have led to requirements in limiting the spalling depth and the magnitude of temperatures of the in situ reinforcement 7 for both rail and road tunnels. Throughout the previous decade, the quantity of underground tunnels has significantly increased globally 8 and as a result of COVID- 19 Australia, like many countries, are fast tracking tunnel transportation projects to revitalize the economy.…”

“…These tests are essential to be conducted on a large scale to reproduce the identical boundary conditions, stress conditions, and material properties of the real structure, all of which are dependent on the spalling behavior. Limited large-scale experimental studies have been carried out on the performance of tunnel linings in fire 7,[31][32][33][34][35][36][37][38][39] including the Runehamar fire tests. 40 Nevertheless, only limited works 7,18,31,33,37,39 have been conducted on full-scale tunnel lining segments that were tested under combined structural and fire loading.…”