Almost everybody in the developed world suggests that ‘Drop Cover Hold’ (or Duck, Cover and Hold) is the best policy during an earthquake. A lot of NGOs and INGOs had suggested the techniques and taught it to the Nepali school children. The April 25 earthquake showed that the ‘Drop Cover Hold’ notion was wrong in Nepali contest, in a very disastrous way. A lot of corpses positioned in ‘Drop Cover Hold’ were found in the rubble after the earthquake. Many believe, a lot of those who died might have survived if they had run out instead of taking cover in the crumbling buildings.
What is Drop, Cover and Hold ?
In the event of an earthquake, people are instructed to ‘Drop’ down so that they don’t fall and take ‘Cover’ under something strong. People are asked to get under sturdy bed, desk or table to avoid falling objects. They are instructed to ‘Hold’ on something to keep themselves steady until the earthquake shaking stops. The person can get out to the open space after the shaking stops.
This method holds good if the building is constructed to deal with earthquake. Most of the structures in earthquake prone zones in the world are made earthquake resistant and they can hold on for a while, until the people escape after earthquake before falling down. That is why the method is widely publicized. Following are some of the official documents of different countries.
Screenshot from – http://www.eqc.govt.nz/be-prepared/earthquake
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.
The Manakamana Temple in Gorkha district was shaken by the earthquake of April 25, 2015. In a close-circuit video, the temple is seen shaking violently during the earthquake. Some of the walls of the temple were damaged although the temple didn’t fell down.
Watch the live video of earthquake in Manakamana temple:
The rebuilding process after a devastating earthquake is a tricky business. People are scared and impatient, many engineers are clueless. There are also some people who want to benefit from the situation. I am dealing only with the engineering aspect of the rebuilding process. This podcast is prepared after reading an article by a professor in a popular online media, setopati.com.
Prof. Padma Khadka has written an article titled ‘Don’t worry if you have cracks on beams and columns’. I believe, people should worry and hence this podcast.
Prof. Khadka is a geotechnical engineer. But, his suggestions are nowhere near geotechnical in nature. Please listen to the podcast and comment. Continue reading
In the second podcast of NepaliEngineer we discuss the possibility of seismic microzonation. The question was to prepare a map so that a person can know the seismicity of the place while purchasing or building a structure. We discuss the possibility of having such a map by the government. In the course of the preparation for the podcast, I had collected various hazard maps and concluded:
- although the hazard maps are good for research purpose, they don’t serve as a guideline when making land purchases for residential purpose.
- there is no consistency between different hazard maps prepared by different authorities (see the papers below).
- these maps can not be reliable during earthquake.
In the podcast, I discuss about the difficulty in producing a reliable earthquake hazard map useful for land use management.
Some other documentations I found in a quick search include:
- Preliminary Study For Evaluation Of Earthquake Risk To The Historical Structures In Kathmandu Valley (Nepal); by Sudhir R. Shrestha, Madan B. Karkee, Carlos H. Cuadra, Juan C. Tokeshi, and S. N. Miller; at World Conference on Earthquake Engineering, Vancouver, B.C., Canada, August 1-6, 2004 (download link, pdf)
In association with kguff.com, we are starting a new podcast of NepaliEngineer.com. The technical aspects of the podcast is being managed by kguff and the technical aspects of the content by NepaliEngineer.com
The first podcast is about the Code of Conduct of Nepal Engineering Council (NEC). All the related documents of NEC can be downloaded here.