if something is built on sand, it is not firmly established and is likely to fail.

They seem quite happy now but I have a feeling that this marriage is built on sand.

• There are no flies on .
• There are no flies on someone.
• There are plenty more where that came from.
• There are plenty more where they came from.
• There but for the grace of God .
• There but for the grace of God go I.
• Thereby hangs a tale.
• the received wisdom
• the red-carpet treatment
• the red-light district
• Therein hangs a tale.
• There is more to someone than meets the eye.
• There is more to something than meets the eye.
• there is never a dull moment
• there is no such thing
• there isn't a [mean, jealous, unkind etc.] bone in body
• there isn't a [mean, jealous, unkind etc.] bone in someone's body
• there is safety in numbers
• there'll be hell to pay
• there's a lot to be said for
• there's a lot to be said for doing something
• there's a lot to be said for something
• There's a time and a place.
• There's life in the old dog yet.
• There's many a slip twixt cup and lip.