Provincial government spokesman Sediqullah Azizi today said that 20 more were injured.
The accident occurred yesterday in Ruyi Du Ab district of Samangan province, one of the safer parts of the country from the standpoint of insurgency-related violence.
Aminullah, a police official who like many Afghans goes by one name, says more than 1,000 area residents rushed to help, using shovels to try to rescue the workers.
Workplace safety standards are poor in Afghanistan as in many developing nations, and such accidents are common.