What is symbolic about what happens to the foundation for the Glass Castle?

In Part III of the novel, the dream of the Glass Castle is destroyed. Brian and Lori begin digging a foundation for the home Rex had designed for his family, but the foundation instead becomes a ditch for garbage that the family cannot afford to have picked up. The idea and dream of the Glass Castle as a physical structure is also destroyed when this happens. Jeannette realizes that it will never be built. Yet, the Glass Castle as a symbol and an abstract hope lives on.