Palu dual disaster – tsunami and earthquake liquefaction
I know I am not an engineer, nor a soil specialist, but I am trying to the best of my understanding to give the impression to my readers that “nature itself prepares the ground that man occupies” ready for devastation by natural forces, which man cannot control.
( October 5, 2018, London, Sri Lanka Guardian) It was a dual disaster on late Thursday 29 September 2018 when the tsunami followed by an earthquake, or was it the other way round, which struck the city of Palu, on the coast of the island of Sulawesi, Indonesia.
Entire neighbourhoods were devastated in and around this city. Part of the damage is the effect of the “earthquake liquefaction,” which made the soil flow like water.
More than 1 million people live in the area affected by the dual disaster. As of now over 2000 people have died including some 150 foreigners. We are told that in some places the tsunami reached a height of more than 20 feet.
More than 6400 personnel from Indonesian governmental agencies including the military, police, national search and rescue teams are involved “in efforts to find survivors, recover bodies and evacuate people”.
Many countries, at least 25 countries have offered assistance. UK will send military experts and a transport aircraft RAF A400 stationed in the Middle East with relief effort according the UK Defence Secretary. UK has also initially made £2 million available for relief effort. In addition, UK is supporting relief efforts through the International Red Cross and Red Crescent societies.
We are told some 200,000 survivors need urgent help, from tents to jerry cans for supply of fresh water.
There is debris and rubbish everywhere and according to reports hygiene standards are difficult to maintain, quite apart from getting access to food and clean drinking water for children and babies.
What went wrong with the Indonesian Early Warning System?
No one was able to predict or give an early warning of the scale of the tsunami.
Palu has a narrow, deep and long bay, as if it was perfectly designed to make tsunamis more intense and more deadly. This complex configuration, perhaps, makes it very difficult to model potential tsunamis. It is hard to issue timely and accurate warning ahead of time to people who may be affected.
But it is not the first time earthquakes and tsunamis have caused mass destruction and death in Indonesia. A combination of plate tectonic activity in the region, shape of the coastline, vulnerable and dense communities, together with a less than efficient early warning system, have all combined to make the 7.5 magnitude on 29 September 2018 extraordinary, to say the least.
Why do people crowd to places of tectonic activity?
Asking this question, is like asking how long is a piece of string?
In December 2004 and in March 2005 there was a pair of “subduction zone” earthquakes along the Sunda Trench off the west coast of Sumatra. The magnitude of this tremor and quake was as much as 9.1 in December 2004.
We all know in Sri Lanka what this earthquake did. It generated a devastating tsunami that killed a quarter of a million people all around the islands of the Indian Ocean.
Liquefaction
Several strong earthquakes around the world have resulted in a phenomenon called “earthquake liquefaction”.
What seems to have happened according to experts is “the seismic generation of pore water pressures and the softening of ground soils, gravel and silt, with the ground soaked by previous heavy rain, turning the ground soil to behave in a manner similar to liquid, resulting in mudslides”. This made the ground unsafe to support the foundations of buildings and other infrastructure.
I know I am not an engineer, nor a soil specialist, but I am trying to the best of my understanding to give the impression to my readers that “nature itself prepares the ground that man occupies” ready for devastation by natural forces, which man cannot control.
Looking at the pictures, the roofs of the houses, particularly aluminium or “tin sheets” were all blown and scattered away by the tsunami, which made rescue work more perilous.
Ground Improvement methods
Given the above conditions for liquefaction, researchers are now developing ground improvement methods to increase the stability of homes and other structures built on top of soils prone to liquefaction driving a strong earthquake.
Researchers from University of Texas at Austin Cockrell School of Engineering are developing ground improvement methods prior to any construction work.
This knowledge is needed to develop foundation design solutions.
“Four ground improvement methods have initially been selected for testing: rapid impact compaction (RIC); rammed aggregate piers (RAP) which consist of gravel columns; low mobility grouting (LMG) and construction of a single row of horizontal beams (SRB) or a double row of horizontal beams(DRB) beneath existing residential structures via soil cement mixing.” This additional study and materials have been provided by research at Purdue University, Indiana, USA.
What can we in Sri Lanka do to combat liquefaction?
It is all well and good for rich communities to play with modern research, but we have to be practical in our approach.
Effective engineering infrastructure to protect life and to mitigate the economic, environmental and social impact of tsunami and effects of earthquake liquefaction requires the ability to accurately assess the likelihood of liquefaction and its consequences.
Besides, it is never a wasted effort or expense, to evaluate at regular intervals our early warning systems and bring them up to date even with foreign assistance, which we hope is carried out by our Disaster Management Department.
A very rigourous risk assessment and scrutiny of planning permissions issued for house building and road and bridge construction, adjoining our coasts and river valleys, should be commissioned at the earliest, if it is not already in place.
It may even be necessary to pass legislation for flood prevention barriers at known sites prone to flooding, as well as commissioning the installation of ground foundation improvement methods.
Climate change is now part of our livelihood.
Awaiting for disasters to happen to take concerted action is too little, too late. It is an island wide duty of government to prepare for such eventuality.
