The Wikipedia article on the Siege of Tuyen Quang contains the following:
"...the Chinese sapped relentlessly towards the French perimeter wall. If they could mine the walls and collapse sections of the defence [sic] perimeter, they could bring their superior numbers to bear by assaulting the French through open breaches.... Both the French and the Chinese feverishly dug galleries and counter-galleries.... On the evening of 12 February the first Chinese mine was exploded beneath the perimeter wall, but its effect was weakened by the counter-galleries..." (emphasis added)
What are counter-galleries, and how would they limit a mine's effectiveness?
Counter-galleries are tunnels dug by the defending side. These would be dug as close as possible to the tunnels dug by the attacking force. They could then be used to destroy the enemy tunnel - this was done by undermining, or detonating camouflets. These were small mines, large enough to damage nearby tunnels, but not to disturb the surface. When tunnels met, then fights between opposing groups of miners would occur. If these methods were unable to prevent a mine being detonated, then counter-galleries weakened the surface effect of the explosion by providing a lateral path for the explosive force - explosions follow the path of least resistance and it's easier to flow through air than rock or soil.