The impact of outdoor temperature on the beams while the tracks were being laid could have caused the pad to fall off from its position between two beams, the metro line’s contractor, a consortium comprising Vietnam’s Civil Engineering Construction Corporation No. 6 (Cienco 6) and Japan's Sumitomo Corporation, or SCC, says in a report sent recently to the investor, the HCMC Management Authority for Urban Railways (MAUR).

This is a preliminary conclusion that the SCC has reached after investigating why one of the Elastomeric Laminated Bearings pads had fallen out from its position between two beams on the line’s elevated section last October.

These pads are made from high purity elastomers that encapsulate layered internal steel reinforcing plates and are designed for use in bridge and building constructions for beam support.

Aside from temperature, the SCC has also forwarded another theory, saying the vibration of the U-beam on the tracks had been increased many times more than designed when the tracks were temporarily installed for testing.

Metro line No.1 runs 19.7 km (12.24 miles) from Ben Thanh Market in District 1 to Suoi Tien theme park in District 9. The elevated section where the incident occurred is in Thu Duc City.

A week ago, the investor, MAUR, had agreed that an independent consultant investigates why the supporting pad fell from its position. It made the decision after failing to figure out the reason on its own.

Now, in its latest report, SCC said the incident was an isolated one that did not occur anywhere else along the track. In another place, preventive action was taken before the pad fell.

In January, three months after the pad fell off, a team of independent inspectors found another pad that had moved from its position. The faulty pad had both sides not matching the position it was placed in, according to MAUR.

The second faulty pad is also on the elevated section of the line.

The project consultant, SYSTRA, a French firm, has concurred with the preliminary conclusion of the contractor but said the incident needs further inspection to come up with a more solid conclusion.

The investigation is still underway.

The State Council for Pre-Acceptance Test of Construction Works had in January concluded that the fault lay with the contractor. Since the project is still under construction and yet to be transferred back to the investor, the council decided that it will "respect all explanations of the contractor for the fault as well as its suggestions to handle the situation."

The metro route No.1 will have 14 stations – 11 on the elevated section and three underground.

Its construction is expected to cost VND43.7 trillion ($1.89 billion) in total.

Work on the much-delayed project, which began in August 2012, is around 83 percent complete.

The city had hoped to begin commercial operations by the end of this year, but the investor said in February that the service would be delayed yet again and can only commence next year.

HCMC plans to build eight metro lines running a total of 220 km.