‘Liquefied soil’ sweeps away entire neighbourhoods in Indonesian quake

Thursday, October 4, 2018

Following a powerful earthquake last week, rivers of soil swept away entire neighbourhoods in Indonesia that also generated a tsunami leading to a disaster that has already killed at least, 1,407 people.

 
Experts say the destruction is due to soil liquefaction, a process occurring when soil becomes saturated with water causing it to erupt into tumultuous torrents, as it did on Friday, shifting and tumbling buildings.

 
Footage has emerged from the stricken city of Palu showing people running to find solid ground as structures were swept away and destroyed by waves of undulating earth.
In Petobo town the ground under the length of an entire main road was torn up, with the tide of soil leaving broken stretches of tarmac-topped debris in wave-shaped mounds and troughs.

 
“Liquefaction occurs when loose sandy soils with shallow groundwater are subjected to sudden loading such as shaking from an earthquake,” said Jonathan Stewart, a professor in the Civil and Environmental Engineering Department at the University of California, Los Angeles.

 
“During the earthquake, water pressure is generated in the soil, which causes a dramatic loss of strength,” said Stewart. “The strength loss can be so great that the soil behaves almost like a liquid.”

 
Soil running downhill, known as “flow failure,” is one of the most severe effects of liquefaction, and prone to occur in areas with a particular soil structure.

 
“The soils in the slide area are likely geologically young, loose, sandy materials with shallow groundwater,” said Stewart.

 
Residents of Petrobo spoke with horror about the nightmare that unfolded when the ground beneath their feet suddenly began to run following the earthquake and tsunami on Friday.

 
Several techniques are used to stop flow slides. The easiest is the construction of a strengthened zone in the soil near the lower end of the slope, said Stewart.

 
Approaches to this in geotechnical engineering include densification of the soil by the injection of materials, or the compaction of sediments and soils using weights or vibration that mimic earthquake processes, said Adam Switzer, principal investigator at the Earth Observatory of Singapore.

 
Another method, he added, is the use of “earthquake drains'” that aim to mitigate liquefaction by releasing pressure before it reaches critical levels.

 
Synolakis said that other ways to treat liquefaction are to avoid building multi-story structures and regulate to require deep foundations.

 
But in poorer countries these techniques might be too costly for government and individual budgets.

 
“Many of the mitigation techniques are likely to be well beyond the means of the average homeowner in southeast Asia,” said Switzer.

 
“The only way to completely mitigate the problem would be to move developments off the liquefied soils,” said Stewart.

 
The process is thought to have played a key role in previous disasters, such as the Japan earthquake in 2011, where the extent of the liquefaction over hundreds of miles was daunting to experienced engineers.

 
A detailed study has found that liquefaction wrought more devastation than shaking in the earthquake which occurred in Christchurch, New Zealand, in 2011.

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