Larger bridges over canons and various water ways without disrupting such things as marine traffic, fish migration and so-on can be quite a challenge. Building huge structures on land has it's problems, just think of the problems of building them in water.
For example; concrete piers must rest on bedrock. Before reaching bedrock there is water and sand to get through. Piles are often driven into the bottom to support a temporary work platform. They may also be driven to be used as anchorage for the piers. This is done by hoisting a very large heavy weight called a pile driver and dropping it on the pile. The effect is to literally hammer the pile into the ground.