The minimum trench depth for a gas line is 12, and the same goes for water lines where freezing is not an issue. However, the electricity must be deeper (18 or 24 deep), so that sets the depth of the trench. The width is not critical, except to be able to properly install the pipe with due concern for expansion (especially for CPVC pipes and non-metallic rigid ducts). Most codes allow two-in-one tench as long as some mitigation measures are taken.
They are usually some vertical or horizontal separation, and usually the waterline has no joints or fittings in the trench. Acceptable materials generally match those that would be used anyway, and exclude older ones, such as clay tiles or corrugated plastic that, even if properly installed, allow some effluents to seep into the adjacent soil. If both pipes developed leaks in the same area, contamination of the water supply in a pressurized line is very difficult.
