Question

In Railroad Geotechnical Engineering, please give a correct answer and good explanation

1. Define “Critical Speed” related to a train inducing a large ground-borne vibration.

Answer 1

lncreasing speed brings new challenge to conventional railway engineering which include rail system control passenger safety and comfort and noise and vibration hazard.Ground born vibration induced by high speed rails.According to previous research it can induce a significant increase in vibration level when train move at the critical speed.Theoretical modeling of a rail as a beam supported by track structure and ground reveal that the dynamic amplification will occur when train reaches a certain speed this has been studied.Hence for a long period the train load have been assumed to be quasi static moving load however krylov(1994) considered that a train will encounter something equivalent to sound barrier when reaching the velocity of Rayleigh wave travelling into soft Sandy soil is 320 to 470 km/hr.

The critical speed is the velocity of moving train that leads to higher amplification of the dynamic response of the track structure and subgrade .For conventional railway track the critical speed is close to Rayleigh wave velocity of subgrade soil it have high speed slightly less than shear wave it needs more attention because it is susceptible to increase vibration when Rayleigh wave speed are approached .Another phenomena of critical speed effect is the cone shaped wave motion of ground surface.

For example in sweden a new railway line with design speed of 200km/hr encounter large deflection of track on soft cohesive soil.The speed of train has been decreased to 160 km/hr then to130km/hr.The reason for large deflection is track turned out to be the approaching the critical speed.