Start with the local building department.
To reduce the cost of multiple trenches and minimize the potential for damage to existing facilities, the telco, CATV provider, and electric utility may choose to dig one trench and share it with one or several parties. If you plan to joint trench, make sure you refer to the National Electrical Safety Code (NESC) Part 3 for rules on separation of cables.
NESC RULE 354A: RADIAL SEPARATION OF SUPPLY OR COMMUNICATION CABLES OR CONDUCTORS FROM GAS AND OTHER FUEL LINES SHALL BE NOT LESS THAN 12".
TEL/CATV companies adhere to NESC requirements.
Maintain 12" minimum separation between TEL/CATV & electric lines. Band all telco/CATV conduits together.
Backfill & compaction of all trenches shall meet the requirements of the City and other applicable State, Federal, DOT or railroad requirements.
After you've completed the design, but before beginning construction, obtain all required permits from the local building inspector. If construction is planned on a public right of way, obtain permits from state, county, and city agencies. If you're working on private right of way, negotiate rights with the landowner.