Experimental study of vibrations in underground structures

Vibrations produced on the ground surface by engineering construction processes can damage underground structures. At present there is little knowledge of the level of surface vibrations that could cause such damage. The relevant British Standards, BS 5228 and BS 7385, have little relevance to underground structures. This paper presents experimental investigations on small model tunnels that were instrumented with miniature accelerometers. Impulsive and vibratory loadings were produced on the soil surface by a drop hammer mechanism and an electric eccentricmass motor respectively to simulate dynamic loading from construction activity. It was found that tunnels with different lining materials absorb different amounts of vibration energy. Power spectrum analysis of the acceleration signals showed that the vibrations are transferred into a brass model tunnel at lower frequencies than for a plastic model tunnel. A relationship has been proposed for the ratio of energy transferred and the ratio of impedance mismatch between soil and tunnel lining. Similar experimental studies can be carried out to understand the energy transfer from ground into underground tunnels with various linings (precast concrete lining, shotcrete or steel).