The phrase "built on sand" signifies something that is precarious or unstable, hence it might be useful to have alternative ways of conveying this idea. Synonyms for this phrase include "unreliable," "unsound," "insecure," "precarious," "shaky," "vacillating," "unsteady," "tentative," and "weak." Each word subtly connotes the idea of something unstable and unsustainable, whether it's physical, emotional, or structural. By using these synonyms interchangeably, you can develop a more nuanced language, which can convey the finer points of the topic you are discussing.