Fig.: to start, create something without providing a stable foundation

How can you think of starting a new business without proper financing. It's like building on sand: at the first difficulty, everything will collapse

Come puoi pensare di avviare una nuova attività senza un'adeguata copertura finanziaria. È come costruire sulla sabbia: alla prima difficoltà crolla tutto

I should have seen it coming: our relationship couldn't last long; it was built on sand since we had nothing in common, to begin with

Dovevo aspettarmelo: la nostra relazione non poteva durare. Era costruita sulla sabbia dato che, tanto per cominciare, non avevamo niente in comune

It is taken from a parable in the Bible (Matthew 7:24-27), where Jesus, at the end of the Sermon of the Mount, says:
24 "Therefore everyone who hears these words of mine and puts them into practice is like a wise man who built his house on the rock. 25 The rain came down, the streams rose, and the winds blew and beat against that house; yet it did not fall, because it had its foundation on the rock. 26 But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand. 27 The rain came down, the streams rose, and the winds blew and beat against that house, and it fell with a great crash."