Why did Stanley and zero fill the bottom of the hole with rocks?

Stanley and Zero filled the bottom of the hole with rocks to create a solid foundation, which made it easier for them to dig and also helped to prevent the walls from collapsing. This was part of their strategy to survive and make their escape from the harsh conditions of Camp Green Lake. Additionally, it was a way to conceal the true purpose of the hole they were digging, as they were trying to avoid detection by the warden.