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Fears of soil liquefaction in Gyeongsang grow

Nov 22,2017
After finding some 100 locations with signs of soil liquefaction in the North Gyeongsang city of Pohang, which can cause buildings to collapse, experts say such liquefaction may have taken place during the Gyeongju quake last year as well.

“Near the epicenter in Gyeongju, we found water flowing some 3 meters [9.8 feet] under the surface, which is unusual because underground water is normally found deeper than that,” Kim Yong-shik, a researcher at the Korea Institute of Geoscience and Mineral Resources, told the JoongAng Ilbo in an exclusive interview on Tuesday.

“And we think this may be a sign that soil liquefaction took place after the 5.8-magnitude earthquake last year.”

Soil liquefaction occurs when three conditions are met, according to the U.S. Department of the Interior, which are “loose, granular sediment, saturation of the sediment by ground water and strong shaking.”

Shaking causes soil to lose its shear strength, which is the result of friction between soil particles, and underground water mixing with loosely-packed sediments could create a quicksand-like effect.

The process of soil liquefaction can lead to ground movements, sand boils (where a mix of sand and water is ejected above ground), the floating of pipelines and sewer pipes to the surface - and the collapse of buildings.

“Sewerage infrastructure is often damaged by the liquefaction process,” Sjoerd van Ballegooy, geotechnical technical director at Tonkin+Taylor, a New Zealand environmental and engineering consultancy, told the Korea JoongAng Daily. “If so, the ground water and soil in the area will be contaminated, which poses a serious public health risk.”

Soil liquefaction can be incredibly destructive and is said to have intensified the impact of the 1976 Tangshan quake, one of the deadliest earthquakes in modern history, which left up to 700,000 dead. Liquefaction is also reported to have worsened the impact of the 7.5-magnitude quake that hit Niigata, Japan, in 1964, and four quakes that hit Christchurch in New Zealand in 2010 and 2011.

Signs of liquefaction including sand boils were found in some 100 locations in Pohang since a 5.4-magnitude quake hit the city on Nov. 15.

The Korea Meteorological Administration has not confirmed that soil liquefaction took place in Pohang or Gyeongju. The administration and the Ministry of the Interior are inspecting signs of possible liquefaction in Pohang this week.

“We will announce the results of the government inspection as soon as it is completed,” said Ahn Young-gyu, head of the disaster management policy department of the National Disaster and Safety Control, which is run by the Ministry of Interior and Safety. “If liquefaction is confirmed, we will work on responding to it in affected regions.”

Some experts questioned Tuesday if a railway near the epicenter of the Pohang quake should continue to run trains. A newly constructed 44.1-kilometer (27.4-mile) railway from Pohang to Yeongdeok County in North Gyeongsang is set to open next month.

Trains were being test run on the line Tuesday, operating at the maximum speed of 150 kilometers per hour. The railway is elevated and supported by piers.

The Korea Rail Network Authority said it stopped test runs on the day of the quake in Pohang. Finding no problems during the day’s inspections, however, the authority continued test runs the next day. It said the railway is built to be quake-resistant.

“The piers that support the railway have been built to be quake-resistant,” said a spokesperson of the Korea Rail Network Authority. “The railway is safe. But we will conduct additional safety checks on possible liquefaction in the area.”

“In case liquefaction did take place in Pohang, the authority had better stop running trains on the railway because the railway could start to sink into the softer ground,” said Yoon Seong-hyo, a professor of earth science at Pusan National University in Busan.

The College Scholastic Ability Test, originally scheduled to be conducted nationwide on Nov. 16, was postponed by a week to Thursday due to safety concerns.

Authorities will be dispatching 2,372 firefighters to 1,180 test sites throughout the country on Thursday. There will be at least two firefighters to assist students’ evacuation from each test site in case a strong earthquake hits during the test.

The government as of Tuesday received 9,070 reports about private property damage from last week’s quakes, of which some 8,200 involved houses, and 617 reports of public property damages, including 234 schools.

Some 1,071 people remained homeless on Tuesday. The number of injured rose to 90, of which 15 are hospitalized. The rest were dismissed after being treated. Of the displaced, some 800 are staying at a gym in Heunghae town in Pohang.

But the JoongAng Ilbo found on Tuesday that the gym was not built to be quake-resistant.

Additionally, signs of fire or disaster hazards were found at the gym. Four out of five exits of the gym were blocked by either groups of people lying down near the exits or by piles of emergency kits and boxes, a JoongAng Ilbo reporter found on Sunday.

Central and local governments passed the buck back and forth.

“The local government is in charge of disaster management at the gym,” said an official of the National Disaster and Safety Control.

“The city’s disaster management center does not have a specific requirement on how many exits need to be secured in an evacuation center,” said an official of the Pohang city government’s disaster management center. “I think the city official managing things at the gym didn’t see this loophole.”

Additionally, Heunghae Elementary School, one of the government-designated evacuation centers in Heunghae, was shut down Monday after it was destroyed by the quake. Questions are arising as to how safe the other dozen or so evacuation centers in Pohang are if more quakes hit.

Pohang continues to experience aftershocks, which numbered 61 as of Tuesday.

BY SONG WOO-YEONG, KIM JUNG-SEOK AND ESTHER CHUNG [chung.juhee@joongang.co.kr]