What can be used to extend a caisson to ground grade?

The correct answer is sonotubes, which are cylindrical forms made from cardboard or plastic that are used to create concrete columns or caissons. When extending a caisson to ground grade, sonotubes can be placed in the desired location and filled with concrete, effectively creating a solid structure that reaches the required height. This method is practical because it helps to ensure a smooth finish and a uniform shape for the caisson, making it easier to work with during construction.

Concrete forms also provide structural support and can be used in various concrete pouring applications, but they do not specifically extend caissons to grade in the same way that sonotubes do. Steel reinforcements are vital for strengthening concrete structures but do not serve the purpose of extending the height of a caisson. Piling materials are used for driving into the ground to support structures but are not directly related to the process of extending the height of caissons. Thus, sonotubes are uniquely suited for this task in the context of creating a smooth and manageable extension for caissons.