Tamping lifts the sleeper and packs stone under it. The stone must have crushed faces so the new pack holds. Flaky or elongated pieces snap and fill the voids they were meant to leave open.
Crushing, not only geology
The parent rock must be hard. The crusher must still produce a cubical product. A good stone badly crushed is not ballast.
Interlock versus breakage
Cubical pieces create several stable contacts and resist rolling when the sleeper is lifted. Flat and elongated pieces bridge poorly, orient under vibration and concentrate stress at thin edges. Those edges break into fines, so an apparently acceptable grading can deteriorate after only a few maintenance cycles.
Manage shape at the crusher
Feed consistency, closed-side setting, chamber choice and recirculation all influence shape. Shape testing should be repeated after a setting change and at the same sampling point used for the delivery certificate.
