In the case of installing a lining inside of an existing pipe, there are robotic methods available, such as spin lining, where the cementitious material is cast from a rotating head as the carrier is moved along the pipe. For pipe smaller than 42 in. (1067 mm) diameter, the spin lining is likely the best method.
For pipe larger than 42 in. (1067 mm) and up to 6 or 8 ft (1.8 or 2.4 m) diameter, either hand shotcrete nozzling or spin lining are applicable. For pipes much larger than 6 to 8 ft (1.8 to 2.4 m), hand nozzling is likely the best solution.
In the case of clearance between a wall to be shotcreted and an obstruction, 3 ft (1 m) is a good rule of thumb, but a qualified and experienced shotcrete contractor can use modified equipment to place quality shotcrete in tighter spaces. A recent article on shotcreting in confined spaces can be found here.