What is the minimum depth of excavation for the curbs below the sub grade?

• A. 6 inches
• B. 4 inches
• C. 8 inches
• D. 12 inches

Answer: (A)

The main idea is to provide a solid footing for the curb by giving it a base well below the finished subgrade. Excavating at least six inches below the subgrade creates space for the curb footing and the required bedding/base material, and it allows these materials to be properly compacted. This extra depth helps ensure the curb rests on stable, well-compacted material rather than on potentially soft or shifting soil, which reduces the risk of movement, cracking, or settlement under traffic loads or frost conditions. If the trench is shallower, there isn’t enough room for the base and bedding, compromising support. Deeper excavation isn’t usually necessary unless site conditions demand it, but six inches below subgrade is the standard minimum to establish a reliable curb foundation.