A supporting arch being built for the new Pink Line commuter railway fell to the ground in Nonthaburi province, injuring five workers, early on Thursday morning.
The Mass Rapid Transit Authority blamed it on the rain-softened substratum.
The accident happened on Chaeng Watthana Road in Pak Kret district about 1.30am. Debris blocked the road, which was cleared and reopened to traffic about 6.30am
Thai News Agency reported five workers were injured - an engineer and four labourers. Most injuries were minor, but one person remained under medical care.
The MRTA said the five-metre high arch was being built to support a prefabricated beam. Support scaffolding collapsed while concrete was being poured. Persistent rain had softened the ground.
Sino-Thai Engineering and Construction Plc is the contractor.