Apparatus and Techniques for Static Triaxial Testing of Ballast

Railroad tracks are constructed on a layer of ballast overlying the roadbed subgrade. The ballast is a coarse-aggregate-sized, noncohesive, granular material with angular particles and usually with a uniform gradation. In order to evaluate the geotechnical performance of railroad tracks, representative stress-strain-volume change and strength properties of the ballast are needed. Because ballast consists primarily of large [up to 76.2 mm (3-in.)] angular particles, conventional triaxial testing equipment is not appropriate. Therefore a large triaxial cell was designed and constructed. The apparatus used in the static ballast testing, and the sample preparation techniques developed, are presented. Triaxial compression test results on the granite ballast for two different density states are shown. Mohr-Coulomb strength parameters are presented, and the typical stress-strain-volume change behavior is shown. The hyperbolic parameters calculated from the static tests are also shown. The static test results using the new apparatus and sample preparation techniques indicate that railroad ballast possesses many characteristics common to granular materials.