MX Metro Collapse Inspection Finds More Cracks
Officials have announced that inspectors found 21 more cracks on an elevated subway line in Mexico City while investigating the metro collapse from May last year.
According to reports from the Associated Press, the newly discovered cracks were found in welds or steel structural pieces on other parts of the line that didn’t collapse.
Studies from the collapsed line section have reportedly found that the failure was caused by construction defects, such as poor welds and missing connection studs, and bad design plans. The current inspection is about one-third complete.
Outside inspectors, including from the Autonomous University of Nuevo Leon, have been invited in an effort to reopen and reinforce the line.
Overpass Collapse Background
At 10:22 p.m. on May 3, one of the concrete beams stretching along Line 12 of the Mexico City Metro collapsed as a subway train passed over it, resulting in 26 fatalities and injuries to nearly 80 others.
An overpass carrying a subway train has collapsed in Mexico City, killing 15 people and injuring at least 70, according to local government officials. https://t.co/6YFQXr9FX9— CNN Breaking News (@cnnbrk) May 4, 2021
The overpass measures roughly five meters (16 feet) above the road below; however, the passenger trains were reported to have run above a concrete median strip, potentially saving additional casualties and motorists below.
In response to the tragedy, hundreds of police officers, firefighters and rescuers were on scene to help those involved in the crash, as well as cordon large groups of friends and relatives of people believed to be on the train. Throughout the night, a crane was issued to the scene to help stabilize the train cars amid concerns that they could collapse onto the road. This forced officials to temporarily halt rescue efforts.
The following morning, the search for survivors turned into a recovery operation, with four of the victims’ bodies still trapped in the wreckage, according to government officials.
Line 12 History
Having been inaugurated in 2012 after four years of construction by a consortium including Mexico’s Ingenieros Civiles Asociados and Carso Infraestructura y Construcción, and Alstom Transport of France, Line 12 has long been criticized. During its construction, the railway was plagued by delays and technical issues which were reported to have driven the cost of the project to $2 billion—approximately 50% above the original estimate.
Although the structure is reported to be one of the city’s newest metro tracks, just two years after its inauguration, Line 12 underwent an 18-month partial closure for track and structural repairs. The rehabilitation was brought on after a city investigation discovered a number of defective construction materials and questionable project supervision, which eventually lead to the criminal prosecution of several senior project officials.
Additional reports from the past indicated that the structural integrity of the overpass was damaged after an 7.1-magnitude earthquake rocked the area in September 2017.
Multiple residents expressed concerns over the cracking in the concrete, however, El Universal newspaper reported that transport authorities carried out repairs following those concerns. Additional allegations regarding the structure’s poor design and construction were also voiced when Mexican Foreign Relations Secretary Marcelo Ebrard’s term as Mayor ended in 2012. Servicing the city from 2006 to 2012, Ebrard oversaw the construction of the subway’s Line 12.
Like many other tracks within the city, the Line 12 runs underground through more central areas of the city of 9 million, but then runs on elevated concrete structures on the city’s outskirts. The subway system is reported to be the second-largest in the Americas—next to New York City—handling more than 4 million passengers a day.
Previously, the Mexico City Metro experienced two other serious accidents since its inauguration almost a decade ago. The first occurred in 2015 when a train that did not stop on time crashed into another at the Oceania station, injuring 12. And last year, in March, a collision between two trains at the Tacubaya station left one passenger dead and injured 41 people.
In more recent reports on the collapse, however, consulting engineer and veteran of many Mexico City construction projects Rozbeh Moghaddam pointed out to Engineering News-Record that investigators should not overlook Mexico City’s notoriously complex subsurface conditions. Specifically, where the metro collapse occurred, Moghaddam reported that the area’s upper layer of soft, “virgin” clay can occasionally define even the most precise calculations.
“Building in that soil is a big challenge, and it’s impossible to predict how it will behave,” he stated.
With the soil in mind, combined with the structure’s history and previous seismic stress, Moghaddam concluded that the structure should have undergone special attention and more periodic inspection to better understand how the infrastructure was being affected, given that all the previous factors in mind could have created a lot of their own issues.
Following the tragic event, Mayor Claudia Sheinbaum announced that DNV would be conducting an independent forensic investigation into the collapse. Although an independent forensic investigation, Sheinbaum reports that DNV’s work will parallel federal and agency probes that are currently being carried out with engineering faculty from the National Autonomous University of Mexico.
The DNV was selected by Sheinbaum for its work on the 2010 explosion of the Deepwater Horizon oil rig in the Gulf of Mexico for the U.S. Departments of Interior and Homeland Security and believes the firm will ensure the investigation is free of politics or other influences.
In June, early findings of DNV’s probe into the accident found deficiencies in building materials used on the subway project, including bolted and deformed structural supports—both of which were observed in the section of line that collapsed.
The initial report by DNV, an external auditor, found “six deficiencies in the construction process” that helped to bring about the accident. In addition to the bolts and structural supports, other identified deficiencies included the welding of bolts and their attachment to girders, missing bolts on some girders, different types of concrete used and unfinished or poorly executed welding.
In additional probes into Line 12, the College of Engineers studied 7.5 miles of the elevated track (excluding the section that collapsed) and found various cracks in columns and support beams, skewed braces, rainwater leaks and metal beams that may not meet standards because they were welded in the middle or weren’t resting properly on rubber shock absorbers.
Although some of the issue could have occurred from normal use or earthquakes, researchers believe that many of the visible issues appeared to date from initial construction. For example, some of the braces appeared to have been installed crooked or without the necessary cross-bracing bars, and various girders also appeared to be welded differently from one another.
While the defects would need to be more closely looked at, the college suggested that the line be investigated further prior to reopening. The remaining two-thirds of track checked out as having traditional wear and tear.
In a final report issued by DNV, the firm concluded that the May collapse was ultimately the result of poor installation. Due to bad welds, poor location and missing studs, the DNV wrote in its report in September that the elevated structure had been working as “two independent parallel beams, a concrete beam and steel beam, that experienced loading conditions for which they were not designed.”
As a result of the distorted framework, the transportation infrastructure later developed “fatigue cracks” that further reduced the structure’s ability to bear weight.
The DNV also noted in its report that other possible factors to the structure’s collapse included deficiencies in the beams themselves and the overall design of the framework.
DNV is expected to deliver a third report into the collapse; however, no timeline for the publication of the report had been announced at that time.
Following publication of DNV’s preliminary report, Sheinbaum has reportedly called on Grupo Carso to pay for the repairs. While Carso had agreed to fund the remediation work, the company continued to defend the original construction.
Carlos Slim, Mexico business magnate and owner of Grupo Carso, reached an agreement in November with Mexico City officials to pay for the costs of rebuilding the subway line that collapsed.
Slim, reportedly Mexico’s richest man, plans to cover the costs of rebuilding the collapsed span, as well as reinforcing other parts of the elevated line to meet higher construction standards. According to the Associated Press, Grupo Carso said in a statement that the spending did not constitute any admission of responsibility for the collapse and would not “materially affect” the company’s business.
Criminal charges were announced in October against “10 individuals and companies” for construction and design defects that reportedly caused the collapse. The Mexico City Attorney General’s Office reportedly are seeking criminal cases for negligent or involuntary homicide, injury and damage to property.
“This prosecutor’s office has the evidence to charge a number of people and companies who were in charge of ensuring there would never be cause for a collapse,” said Attorney General Ernestina Godoy at a press conference.
Mexico News Daily reported that Godoy added that companies could avoid legal proceedings if they reach agreements with Mexico City government and provide compensation to victims of the tragedy.
“Some companies that participated in Line 12 showed from the very beginning their interest in participating in the mitigation and repair of effects from the collapse,” she said.