Railway Vibration Testing

Vibration is a complex mechanism by which mechanical movement from a source is transmitted to a receiver. The source of vibration is either construction plant or interaction between the rail and wheel of the moving train that passes through the track and into the ground. As it travels through the ground, vibration is reduced before reaching a building foundation but the degree of attenuation depends on soil and geological formations. Vibration enters buildings through their foundations where there can be attenuation or amplification of the level, particularly to upper floors.

 

The prediction of vibration through the soil at distances removed from the source is difficult as the soil/subsoil structure can vary considerably from one site to another. It is therefore normal practice to assess vibration propagation characteristics by on site measurements of similar track structures, if possible within similar ground conditions to those for the proposed new structure.

 

When vibration reaches the foundation of a building the proportion of energy entering the building depends on the nature of the foundations. For slab floors there is little attenuation as the slab is in close contact with the surrounding soil so that the vibration of such floor slabs are virtually the same as that which would exist without the slab. At buildings with spread or piled foundations vibration levels can be reduced by a factor of 2-4 when compared to the predicted ground vibration.

 

Vibration is carried within a building structure through walls and floors and is usually reduced at upper floor levels. However, at buildings with timber floors, such as many residential houses, vibration can be amplified, especially if the natural floor resonance frequency coincides with peaks in the ground borne vibration range. Increases by factors of 2-3 have been measured when compared to the foundation vibration level.

 

Criteria for evaluating the effect of vibration on buildings have been summarised in British Standard, BS7385 (BSI 1993). These levels tend to be very conservative and give an indication of safe limits to prevent the onset of superficial damage such as surface cracking. These guidelines would be used for assessment of construction vibration and for the protection of building structures, which is 5 mm/s PPV for standard buildings and 3 mm/s PPV for listed buildings or potentially sensitive buildings.

 

British Standard BS6472 deals with the assessment of the response to vibration of humans in buildings and recommends the use of an index known as Vibration Dose Value (VDV) when evaluating the effect of intermittent vibration such as that caused by a series of trains passing a given location. The standard provides a method of estimating the VDV from measured values of vibration and gives a set of curves relating PPV and acceleration to levels likely to cause annoyance.