The major breakdowns on the North-East Line in January this year and, March and August last year were caused by stress corrosion cracking of steel wires and bolts in the tunnel, a joint team by the Land Transport Authority and SBS Transit has found.

In a statement Friday morning, the joint team said chloride from tunnel water seepage had corroded the balance weight anchor wires that broke in the March breakdown last year that affected 90,000 commuters.

Separately, chloramimes were the corrosive agent that caused the u-bolts to break in both last August's and January's breakdown. The bolts breaking caused the line's power supply to fail.

In the interim, SBS Transit has embarked on steps to prevent the incidents from happening again by changing out all 1,600 u-bolts along the problematic stretch between Outram and HarbourFront stations. Longer term preventive measures will also be rolled out, said the statement.