An oil transmission line contractor is excavating for a valve box to connect transmission lines. To determine the proper depth of excavation he records the following: A. Benchmark = 30.0` B. Grade @ trench (frontsight) = 10.35` C . Backsight = 4.10` D. Final trench grade = 17.25` How deep must the contractor dig?

An oil transmission line contractor is excavating for a valve box to connect transmission lines. To determine the proper depth of excavation he records the following: A. Benchmark = 30.0` B. Grade @ trench (frontsight) = 10.35` C . Backsight = 4.10` D. Final trench grade = 17.25` How deep must the contractor dig?