Reasonably, those members digging the pit weren't experts. The thought of the mountain troll escaping was enough a motivation for them to dig such an adequately deep pit. It was enough to completely trap the mountain troll within. However, when Qiao Gun exploded the tree club, it caused a big chunk of earth at the side to be blown away!
The pit originally had a jar shape. When a portion of the roof got blown up, the soil rolled down and became the mountain troll's footstool! Furthermore, the roof of this pit had a slight slope now, this undoubtedly provided a ray of hope for the mountain troll!