My concrete driveway has a ditch that runs underneath the entrance. It appears that alot of the dirt underneath the driveway is washing away, leaving empty space and I am concerned that it will eventually cause the concrete to cave. There is already evidence of some cracking along the side wall and this house and drive are less than 2 years old. What is the best method to fix this problem?

Being an all round builder I came up against this problem a long time ago.

If the stream can be diverted whilst you work this will make the operation easier, if not then you are going to have to work amoungst it.

You need to fully excavate the stream area and dig down approx 8" below the stream bed.
You will then need to concrete this are using sulphate resisting cement, use a dry mix if you can't divert the stream.
Once dry (approx 24hrs) Build up to retaining walls using solid concrete blocks or engineering bricks and build to a minimum of 9" thick and to the required height.
From then on you can run concrete behind the blocks to the top of your drive giving it a lot greater strenght