Construction Flaws Led to Mexico City Metro Collapse, Independent Inquiry Shows

MEXICO CITY — Construction errors led to the May collapse of an overpass in Mexico City’s subway system that killed 26 people and injured scores more, according to the preliminary results of an independent investigation released by the city’s government on Wednesday.

The report, produced by the Norwegian risk management company DNV, suggests that deficient welding of metal studs, which were not properly fused to a steel beam that held up the train tracks, was among several mistakes that contributed to the collapse.

The results support the findings of a New York Times investigation that highlighted shoddy construction on the metro line. Some of the studs holding the structure together appeared to have failed because of bad welds, The Times found, a crucial mistake that probably caused the overpass to give way.

Engineers consulted by The Times pointed to the presence of ceramic rings, or ferrules, left in place after the welding process as evidence of subpar workmanship, a finding that was confirmed by the DNV investigation.

In a statement, DNV said its report was based on “the field investigation and the laboratory testing of samples from the accident” and that it “only contains DNV’s hypothesis at this point.” The full investigation will be finished later this year, the company said. Mexico City’s government, which hired DNV to examine the causes of the crash, is also conducting its own investigation into the accident.

The results of the independent inquiry could spell trouble for two of Mexico’s most powerful figures: Marcelo Ebrard, the foreign secretary, and Carlos Slim, one of the world’s richest businessmen.

Mr. Ebrard, the mayor of Mexico City when the line was built, wanted it completed before he left office in 2012, according to multiple people who worked on the project. He is seen as a powerful contender to succeed President Andrés Manuel López Obrador in Mexico’s next presidential elections in 2024.

Mr. Slim’s conglomerate, Group Carso, built Line 12, the part of the metro that collapsed. Line 12, Carso’s first rail project, was intended to expand the company into the lucrative sector.

Carso is now building a significant part of Tren Maya, a 950-mile railway meant to bolster the economy of southern Mexico — one of the country’s poorest regions — and stand as Mr. López Obrador’s legacy project.

Some engineers and architects working on the Tren Maya have complained of problems similar to those faced when they built the subway: a rushed, disorganized process that has no master plan to guide construction. And Mr. López Obrador has insisted that he wants the Tren Maya to be finished before he leaves office in 2024.

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