I wasn't able to find specific documentation on the recommended safe angle of repose for a 5-foot trench in pipeline construction in our current safety knowledge base, but here are industry-standard safety guidelines:

For trenches less than 20 feet deep, OSHA requires a protective system unless the excavation is made entirely in stable rock. Protective systems include sloping or shoring. Sloping involves cutting back the trench wall at an angle inclined away from the work area, and the angle of slope depends on the soil conditions. Always check the legislative requirements in your jurisdiction related to the use of protective systems.