The influence of cyclic loading frequency on the response of an unsaturated railway formation soil

This paper presents the results of cyclic triaxial tests conducted on unsaturated soil samples recovered from the railway embankment of a South African coal line to investigate the influence of cyclic loading frequency on a formation layer material, in this case a clayey sand. The testing involved suction-monitored repeated loading tests accounting for the conditions likely to be encountered in practice under varying train speed. The results of single-stage loading tests with a frequency applied individually in a range of 0.5–4 Hz were compared to the results of multistage loading tests with a frequency increasing from 1 to 4 Hz. The results of the single-stage loading tests indicated that the accumulated axial strain increased with an increase in the cyclic frequency, implying that a higher train speed would result in a higher level of the permanent deformation. The staged increase in the frequency of the multistage loading tests showed that the permanent deformation mainly took place during the first packet of cyclic loads whereas the deformations measured during the following repeated loads were negligible. Suction reduction and an increase in degree of saturation during cyclic loading was dependent on the water retention state of the sample. The results were then interpreted in terms of resilient modulus.