In one project we plan to build over 400 km of new sewers in two large cities in eastern India. The good news is we are in the coastal plain and don’t have to deal with rock excavation. In our case when you dig trenches deeper that 2 meters (6 feet), and sometimes less, the key issues are groundwater and soils. The worst conditions are lots of water and sandy soils. Unfortunately that’s exactly the condition we face in most of the coastal plain of eastern India.
The theory is easy, de-water the trench with well points and pumps and shore up the sides so you can work safely.
The trick of course is execution in narrow crowded streets with construction crews that aren’t always well experienced in dealing with the unexpected, or for that matter the expected.