Nine labourers were trapped alive when part of the hillock fell in Ramgarh, Circle Officer Bhowali R S Hyanki said.
Eight of them were asphyxiated to death while one was rescued alive, he added.
Most of the workers belonged to Lakhimpur Khiri in Uttar Pradesh and Jharkhand.
The deceased labourers have been identified as Deshraj, Rahul and Ramesh Rana from Lahimpur Khiri besides Ajay Kumar, Mthun, Arun, Devindra and Ujjwal from Palamu in Jharkhand, he said.