Railway ballast thickness and evaluation surveys

GPR railway ballast thickness and evaluation

Quality control of the railway ballast condition and its monitoring are key factors for an efficient maintenance policy. GPR can provide objective and measurable information about railway ballast thickness and anomalies.

GPR for ballast evaluation surveys

The properties and thickness of the ballast layer comprising the rail-track structural support largely determine the quality of the substructure and its ability to withstand the various loading intensities imposed by rail traffic. Minimising track deflections and long term track settlements are desired goals for track design and performance. It is known that the proper operation of railway ballast can be compromised by the presence of fine-grained material which can progressively fill the voids in the ballast and affect performance.

Ground Penetrating Radar (GPR) can evaluate railway ballast condition rapidly and non-destructively and enables the measurement of ballast thickness and the detection of changes in material properties. The fines within the ballast retain more moisture, increasing the permittivity of fouled ballast, therefore areas of fouled ballast can be identified.

Survey method

For railway ballast surveys, Sandberg typically use a GSSI GPR control unit with a digital dual frequency antenna with central frequencies of 300MHz and 800MHz. GPR scans can be conducted using antennas with different central frequencies. A higher frequency antenna provides better resolution and more detail, however penetration depth is limited. By using a dual frequency system, the best of both is achieved; the 800Mhz providing high resolution data to approximately 0.9m depth, while 300Mhz provides greater depth penetration. Penetration depth depends on site conditions, but under favourable conditions, penetration depths of up to 3m can be achieved.

For comparison, GPR scan lines can be collected in the 6-foot, 4-foot and cess.

Survey results

Cross section plots are produced, typically with 0.1m horizontal sampling density and 0.01m vertical resolution showing variation in ballast thickness. Areas of fouled ballast, or ballast deterioration are also identified.

Railway ballast thickness plot
Railway ballast thickness plot detected by GPR