Measurement of formation stiffness supports line upgrading: Fid-Bau Portal

Measurement of formation stiffness supports line upgrading

Increasing speeds on existing main lines can lead to deterioration in the condition of the underlying trackbed, especially in soft ground. Over the past 15 years, Scott Wilson has developed the application of the FWD (Falling Weight Deflectometer) in the rail environment. This rail-mounted device can measure both critical velocity and trackbed stiffness to a high degree of accuracy. FWD works by simulating the loading regime of an axle and measuring the response of the trackform. The device drops a heavy weight onto a loading beam placed on a sleeper which has been undipped from the rails. The resulting impulse simulates the loading experienced under the passage of a train. An array of nine geophones is placed on the loaded sleeper and along the ballast at intervals along the track. The outputs of the geophones are then double integrated to give displacements. The test sequence takes less than 2 min and the results are available immediately. Typically the testing is done at 10 m intervals, allowing 2 km of track to be investigated in an 8 h possession. FWD has proven a highly effective, versatile and efficient way of assessing the performance and limitations of a trackbed.