After the April 25, 2015 earthquake, a lot of people have realized a very scary fact, a house can’t be made strong enough if the soil it rests is not strong. One of the signs of weak soil is liquefaction during earthquake. In the recent earthquake, we could observe liquefaction in different places in Kathmandu, causing a number of structure to settle and fail.
What is liquefaction?
Liquefaction is a process that occurs during an earthquake in which, soil particles are rearranged such that the water and air between the particles of soil is squeezed out. The pressure built in the water in the soil reaches such that the soil particles ‘float’ in the water and the soil behaves more like liquid than a solid. The water under pressure is squeezed out through cracks in the ground. The water usually brings out the silt and sand particles on the ground surface.
As a result of liquefaction, the ground surface can tilt or settle. The structure on the liquefied ground are usually damaged by tilting or sinking. Following illustration gives the overview of the liquefaction process.
1. Illustration of the process of liquefaction.
After April 25 earthquake and after shocks that followed, water came out of the ground in some places, sand and gravel also came out in some places. These are the signs of liquefaction. This problem was most pronounced in Gongabu area, Balaju, New Bus Park, Machapokhari, and Goldhunga areas. In those areas, some buildings have tilted and others have sunken in the ground. Water and sand had also come out of the ground in Khumaltar area in Lalitpur and Kharipati area in Bhaktapur. Other places of high liquefaction potential are the areas near rivers like Balkhu, Manohara corridor, Bishnumati corridor, and Bagmati corridor.
Similar liquefaction was observed in the 1934 earthquake. The following text is taken from the only official document written about the earthquake by Brahma Shumsher J B Rana.
2. A copy of 1934 Earthquake report explaining liquefaction in Kathmandu by Brahma Shumsher JB Rana (in Nepali language).
When compared to the 1934 earthquake, less amount of liquefaction was observed in the 2015 earthquake. Liquefaction is a directly correlated to the duration of the shaking during earthquake.
In a report by Kathmandu Municipality and JAICA in 1998 to 2000, high probability of liquefaction was predicted.
Photos showing signs of liquefaction in Earthquake 2015
3. Damaged road surface, April 25. 2015 Earthquake, Nepal
4. Water and sand coming out of ground, April 25. 2015 Earthquake, Nepal
6. Sand and water ooze out of water, April 25. 2015 Earthquake, Nepal
7. Liquefaction in Hetuda (photo credit: @razzulovely in Twitter)