sunken monoblock driveway near mains drains access. Possible causes?

Typically sunken areas near drain access are a result of the blocks directly under the access lid has collapsed or crushed (perhaps from heavy vehicle). This is more common than you think and you'll find hundreds of examples on the roads in the UK. Other reasons could be the soil around the manhole was not properly compacted or at the worse case, the pipe run has collapsed (not likely). Its hard to tell without seeing a picture or providing any more details but the easiest way is to lift the lid and have a look under there. Are you after possible solutions to fix?