The extent of damage to building foundations caused by the 2011 Great East Japan Earthquake could go much deeper than previously thought, highlighting the need for new quake-resistant design standards, researchers say.
The extent of damage to building foundations caused by the 2011 Great East Japan Earthquake could go much deeper than previously thought, highlighting the need for new quake-resistant design standards, researchers say.
A research team, including specialists from Chiba University and the Building Research Institute, a national research and development agency, inspected the foundation piles of public buildings such as housing blocks, schools and municipal offices in six prefectures from 2012 to 2013.
They examined 61 buildings suspected of damage in Miyagi, Fukushima, Ibaraki, Tochigi, Saitama and Chiba prefectures, and found that the foundation piles of 42 had been affected by the magnitude-9.0 earthquake.
As the assessment was conducted mainly at public facilities suspected of having problems, the study results do not show the possible full extent of the problem in privately owned buildings.
Reacting to the findings, Shoichi Nakai, a professor at the department of Urban Environment Systems, Chiba University, said, “We understand what we have found is only a small tip of the iceberg, as it lacks the data on privately owned buildings.
“When we consider the significant financial burden the owners of buildings would face after a large earthquake in the future, it would be advisable to raise quake-resistant building standards.”
An underground camera revealed crushed and ruptured concrete piles. The damage is thought to be the result of extraordinary stress applied on the piles from buildings shaking above ground, or movement of the earth underneath.
The affected buildings were mostly built on poor quality ground with 23 located on low land and nine on reclaimed or raised land. Seventeen buildings were found to be tilting, although suffering no structural damage.
In some cases, the inconsistent make-up of a plot of land was suspected to be a cause. In at least four cases, only one building out of several similar structures in the same location was found to be damaged. All such cases were found on the low-laying grounds wedged between tablelands.
In severe cases, one building was found subsiding more than 80 centimeters and tilting, and another building was tilting by more than two degrees.
A house is deemed uninhabitable if it becomes slanted by more than a certain degree. In insurance policies, a property would be written off with a tilt of more than one degree or a land subsidence of more than 30 cm.
Damage to foundation piles may not pose an immediate danger or cause structural damage to the buildings above them, yet most buildings with compromised foundations will eventually be mandated for overhaul or reconstruction from scratch.
In cases where structures can be kept as they are, extensive repair work will still be necessary.
The experts fear that in either case the impact on people’s livelihoods and the financial burdens would be too great. Rebuilding and repair would be only possible for public facilities that can access large sums of money from recovery budgets.
For example, in a public housing block in Ichihara, Chiba Prefecture, the prefectural government found multiple damaged piles in a survey, and it decided to condemn the apartments in 2013.