Their love was built on the sand,that is why their marriage has landed on the rocks.
他們的愛(ài)情缺乏堅(jiān)實(shí)的基礎(chǔ)，這就是他們的婚姻瀕臨破裂的原因所在。

built on the sand 字面意思是“把......建筑在沙土地上”，喻指“基礎(chǔ)不牢固”或“不切實(shí)際”，常用于口語(yǔ)之中。

比如：

I hear he decided to invest in a film,it is built on the sand. 

我聽(tīng)說(shuō)他要投資拍電影，這太不切實(shí)際了。