I was watching this engineering video here and it talks about a technique used to prevent the pillars of a bridge from falling over in an earthquake. The pillars of the bridge are partially submerged under water, but instead of resting on sand at the bottom, they actually rest on a bed of gravel just above the sand. The rationale for this is that gravel has a larger particle size than the sand below it and therefore the pillars are less likely to fall over when they shake and move.
So my question is, why is it easier for pillars to move on gravel than sand? What role does particle size play in this? Does it reduce the friction when the pillars slides and if so why? It feels like this question should be obvious but I can not seem to fully understand why.