how to repair a concrete driveway ?

The sewer line goes under my driveway. The ground has settled under the concrete leaving an 1' by 2' gap that runs all the way across the driveway. The concrete is still in task but I'm afraid that it will crack and fall in with the weight of my vehicle crossing. Does the concrete have to be cut out, the dirt packed and concrete poured in it's place or can I just pour concrete under the existing concrete? Is there another alternitive?