Large Direct Shear Tests Performed with Fresh Ballast

Abstract

This study evaluates the ballast shear strength from the testing of 20 specimens, performed in a very large direct shear box that has a shearing plane of 1 m by 1 m and can hold specimens with a thickness up to 80 cm. Tests comprise two different types of ballast (mylonitic and andesitic) and two different specimen preparation techniques (with and without compaction). The normal stresses used cover a wide range from 10 kPa to 400 kPa. Geometrical and physical properties of both ballasts were obtained according to EN-13450, Aggregates for Railway Ballast, showing that both ballasts are similar, can be ranked as high-quality class ballast and fulfill the recommended limiting values set by the American Railway Engineering and Maintenance-of-Way Association. The values obtained for the Mohr–Coulomb failure criterion were: (a) for noncompacted ballast—a friction angle of 39° combined with an “apparent cohesion” of 25 kPa, and (b) for compacted ballast—a friction angle of 50° and cohesion between 10 kPa and 20 kPa. Test results were also interpreted according to a parabolic model that proved the nonlinear nature of ballast shear envelope. The results also showed the decrease of secant friction angles with an increase of normal stress. It is worth remarking that, for very low normal stress (10 kPa), secant friction angles are around 70°, while for large stresses (400 kPa), they decrease to values around 40°. Finally, the use of direct shear boxes of two different sizes (30 cm by 30 cm and 1 m by 1 m) made it possible to analyze the scale effect in these tests. Test results show that the use of the smallest direct shear boxes produces an increase in the shear strength between 40 % and 60 %. Results obtained in this study agree well with those in the literature.

Shear Strength Properties of Coarse-Grained Soils Determined Using Large-Size Direct Shear Test

“…Similar results were published by, e.g. Estaire and Santana [20]. This effect is mainly caused by the crushing of grains, which can be significant, especially in poorly graded materials, see, e.g.…”

Steel Slag Aggregate Characteristics Evaluation as Railway Ballast

“…The shear stress-displacement curve of basalt and SSA samples is presented in Fig. 4, which are approximately in the range of results reported by former studies [29,37]. As shown in Fig.…”

“…As already reported by Estaire, J [37], parabolic curves could be precisely fitted on the variation of shear resistance and friction angle.…”

“…To compare the dilation of basalt and SSA, at 100 kPa normal stress, the dilatancy is higher for SSA specimens, in which the measured vertical displacement was about 20 mm, while in the specimen contained basalt aggregates, the vertical displacement was about 15 mm. It is noteworthy that the dilation of basalt samples was smaller than that reported by Estaire, J [37], which can be attributed to the application of the large size of the shear box (1m × 1m × 1.2 m).…”

“…4(b)). Since the recorded shear stress of SSA is in the range of ballast shear resistance reported by Estaire, J [37], SSA could be used as the substitution of rock material in ballasted railway tracks. As shown in Fig.…”