Not a civil engineer but have done some work in construction. What you're describing is referred to as differential settlement, and can be a problem for any str

Not a civil engineer but have done some work in construction. What you're describing is referred to as differential settlement, and can be a problem for any structure (house, skyscraper, bridge, etc). I don't know enough to guess what they're doing here but usually there's some work with the underlying foundation (ie dig and fill back in with something uniform, put piers or piles to reach a more stable surface, etc). The other things you might worry about is expansion and contraction due to temperature changes, and even just minor size differences in manufacturing (for instance when using moulds to manufacture concrete, those moulds themselves might not all be perfectly the same size or they might change over time). So often in concrete bridges (or even concrete/steel pipes) there will be "joints" designed to absorb changes within some tolerance (eg with pipes, there's typically a rubber gasket that keeps things air/water tight while also allowing some compression/expansion).