Kolkata: Kolkata Metro Rail Corporation Limited (KMRCL) overlooking the fact that along with clay there may be sand pockets in the subsurface layer of the city, might have led to the disaster in Bowbazar in which several buildings developed cracks and portions of some old structures collapsed.
 

Also read this for background of what happened in Kolkata Tunnel Boring Machine:
http://www.millenniumpost.in/kolkata/overlooking-subsurface-layer-composition-might-have-led-to-bowbazar-mishap-experts-372361