Influence of Water Content on the Behavior of Partially Saturated Fouled Ballast

The water content of fouled ballast is important when considering the shear strength and deformability of the ballast, and therefore critical in evaluating whether the track is at risk of excessive deformations warranting a speed restriction order. Fouled ballast from northeastern United States was tested in the laboratory to assess changes in shear strength and deformability as a function of water content. X-ray fluorescence analysis determined that the fouling material was 95% by weight basalt in origin. No more than 5% of the fouling material could be attributed to the abraded concrete ties. The field capacity of the fouled ballast was measured to be at a water content of 10%. Freezing and thawing tests indicated that approximately 4% of mass loss could be expected as a result of 25 freeze/thaw cycles. 6-inch triaxial tests, TX-CIDC, were conducted on the ballast at water contents between dry and field capacity (10%). As the ballast was partially saturated, volume change was measured using circumferential string potentiometers. The water content had an influence on the shear strength and the modulus of elasticity of the fouled ballast. The Mohr-Coulomb friction angle decreased from 47.3° for the dry ballast to 42.5° for the field capacity ballast. The Mohr-Coulomb cohesion decreased from 3.38 psi to nearly zero with initial addition of water, but increased to 6.18 psi as the water content reached field capacity. This is likely attributable to changes in capillary tension of the partially saturated fouling material. The average shear strength, Mohr-Coulomb friction angle, Mohr-Coulomb cohesion, modulus of elasticity and Poisson’s Ratio all showed weakening and strengthening effect by addition of water.