Use of shock mats for enhanced stability of railroad track foundation

Increasing demand for High Speed Rail (HSR) and fast heavy haul poses a serious challenge for stability of tracks on problematic ground. Ballast is a key track foundation material placed underneath the sleepers which provides structural support against high cyclic and impact stresses caused by moving trains. Degradation of ballast contributes to a large percentage of track maintenance costs apart from affecting longevity and stability. In recent years, use of elastometric soft pads underneath sleepers has become increasingly popular as means of reducing track damage. The ‘shock mat’ placed under the sleeper is traditionally called Under Sleeper Pad (USP), and when it is placed under ballast, the term Under Ballast Mat (UBM) is often used. Currently there is lack of comprehensive assessment on the geotechnical behaviour of ballast using these artificial inclusions under impact and cyclic loading. In this study, a series of large-scale laboratory tests were conducted to understand the performance of these energy absorbing ‘shock mats’ in the attenuation of impact and cyclic stresses and subsequent mitigation of ballast degradation. Impact loads were simulated using a high-capacity drop-weight impact testing equipment, while the cyclic loads were simulated using a large-scale prismoidal process simulation test apparatus. This paper presents a state-of-the-art review of laboratory studies and field trials demonstrating the benefits of USPs and UBMs in rail industry.