The candy pipes ran below at sub level beneath the streets of the colony of Terra Alpha.
The pipe network ran to various locations below at sub level beneath the streets of the colony of Terra Alpha. It was used by the Kandyman to direct sweet from the Kandy Kitchen to the execution area. But the pipes contained areas and patches of stalactites of crystallised sugar that were prone to sound vibrations, causing an avalanche of crystallised sugar.
The indigenous Pipe People who were driven to live underground would use the Pipe network to get from place to place without being seen. The Seventh Doctor and Earl escaped from the Kandy Kitchen through a manhole in to the candy pipe line where they came across the pipe people.
Helen sent Fifi down the candy pipes, pursuing a number of killjoys, the Doctor and Ace. She was killed when Earl Sigma caused an avalanche that crushed her. The Doctor returned to the Kandy Kitchen to confront the Kandyman, forcing it to retreat into the Candy pipes.
The Kandyman was killed shortly afterward when its external candy shell was dissolved in a pipe by a flow of its own strawberry fondant surprise, released by the oppressed Pipe People. (TV: The Happiness Patrol)