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.

• at once
• at one fell swoop
• at one sitting
• at one time
• at one time or another
• a tongue-lashing
• a toss-up
• a tough act to follow
• a tough cookie
• a tough nut
• a tough nut to crack
• a tough row to hoe
• a tour de force
• a tower of strength
• at own game
• a toy boy
• at pains to
• at pains to do something
• at peace with
• at peace with something
• at peace with yourself
• at point-blank range
• at present
• a track record
• a trail-blazer