Influence of aging on hydro-mechanical behavior of unsaturated ballast

Abstract

To better understand adverse impacts of aging on track maintenance, the hydro-mechanical behavior of unsaturated fresh and aged ballasts was investigated by means of saturated permeability tests, water retention tests, and monotonic loading triaxial compression tests. Firstly, the aged ballast was prepared through the Los Angeles abrasion (LAA) test to reproduce the gradation and particle shape for in-situ aged ballast extracted from actual railway tracks in Japan. Moreover, the results of saturated permeability tests and water retention tests prove that aging induces a remarkable decrease in the saturated coefficient of permeability and a significant increase in water retentivity compared with fresh ballast, due to the presence of fine fraction content. For the shear strength, the effective shear strength parameters of fresh ballast under fully drained conditions (CD test) are close to those under fully undrained conditions (CU test), regardless of water content. However, for the aged ballast, due to the low permeability, it is difficult to simulate fully drained conditions with laboratory tests, even at a very low axial strain rate (0.01%/min). In this case, the effective shear strength parameters of aged ballast are more suitable to be determined by the CU test. Besides, the results of triaxial compression tests prove that aging reduces the effective internal friction angle of ballast, while it increases the total cohesion compared with fresh ballast, especially under the high saturation condition. Lastly, the shear strength of both materials has a decreasing tendency with increasing water content, and the declining trend of shear strength is more noticeable for aged ballast. Therefore, the effects of aging and moisture content on the hydro-mechanical behavior of the railroad ballast should be synergistically considered in the railway track design.