system
Given the following post, respond as if you're commenting on Reddit:
human
How are tunnels maintained? How do are they kept from collapsing over time?

If a tunnel cracks then how is it fixed? Do they have to be repaired from above?
gpt
The arch shaped crown of a tunnel matches the shape of the stress pattern induced naturally by a cavity within the ground, so a tunnel tends to be stable other than for spalling of small chunks from the roof.  Spalling causes progressive deterioration of the tunnel, and needs to be controlled, such as with bricks, cast concrete, reinforced shotcrete or other tunnel liner system.  A tunnel through hard, strong, weathering-resistant and unfractured bedrock might not need to be lined.  

Cracks alone do not hurt a liner system;  cracks normally occur in most concrete and brickwork due to shrinkage or during local stress redistribution.  However, physical displacement of cracked portions of the liner would be a more serious symptom.  

The other main purpose of a tunnel liner system is to control groundwater seepage.  Some liners include a drainage layer that conducts seepage down to drains near the floor.  A crack that occasionally drips would likely be considered normal.  A crack that is spraying or gushing groundwater into the tunnel would become problematic due to erosion and serviceability issues. 

Cracks can be repaired by low-pressure grouting of cement mortar into the cavity behind the liner, with care to avoid interfering with drainage.  Another method is to bolt steel plates onto the cracked liner.  However, a liner that is merely cracked (but not displaced) might well be stable and functioning in a satisfactory manner in accordance with its design.  Tunnels should be routinely inspected by a tunnelling engineer.