|
|
|
Earthquakes: Quake engineering saves lives
It is often said that amongst all natural disasters, earthquake is the most gentle. Earthquake does not kill anyone. It is the collapse of man-made structures that kills.
This apparently paradoxical and simple remark summarizes the significance of earthquake engineering. In some countries such as Japan and America tolls from quakes of 7.0 to 8.0 magnitudes are rather small. But earthquakes of similar magnitudes in Asian countries like Pakistan, India, Turkey, Indonesia and Iran have been claiming a lot of lives. It is generally felt that if relevant seismic codes are strictly followed, it is possible to mitigate the disaster.
The discipline of earthquake engineering was initiated in 1934 almost simultaneously in Japan and America. Progress in the area was rather slow until the mid-sixties. In 1964, the Niigata earthquake in Japan and the Alaska earthquake in the US caused huge economic losses and took heavy death tolls.
Consequently, the two countries started making accelerated progress in earthquake engineering. They specially made a lot of headway towards making industrial establishments safe. Incidentally, in 1935 a remarkable contribution was made by a city which is now part of Pakistan. The 7.5 magnitude Quetta earthquake of May 30, 1935, had caused great losses. Back then the subcontinent was ruled by the British.
The Army Corps of Engineers introduced a small but very effective measure to protect the houses from seismic forces. In a brick or stone masonry wall, holes of about 10mm diameter were provided at vertical distances of 30 to 40cms. In these holes reinforcement was inserted and fixed with mortar. Such measures strengthened the walls and prevented their collapse. These are known as “Quetta Bonds”.
Seismo-sediments pose a big threat to the safety and capacity of dams. Mangla Dam is located in a seismically active area
At present, Pakistan and India have totally forgotten the “Quetta Bond”. However, the British Code for reinforced concrete masonry does mention the bond. Not only is the practice in vogue, but it is known as inserting “Headers”.
It needs to be reiterated that all new constructions in the seismically prone areas must follow the relevant codes. Existing houses could be strengthened by appropriate measures. The provision of seismic belts at the lintel- or roof-level is very effective. This consists of a wire mesh of about 50cms width to be wound on the house.
This mesh has to be fixed at regular intervals with nails or nuts and bolts. Similarly, introducing diagonal bracings on the walls is quite effective.
Another important point to consider is the “remote damage due to surface waves”. The area in the vicinity of the epicentre (usually within 30 to 50km radius) suffers heavily following a quake. Beyond this distance the degree of damage goes on reducing with distance.
At a distance of about 70 to 90km the damage is minimal. However, tall buildings located at a distance beyond 100km but less than 300km can suffer heavily. This damage is due to surface waves. The collapse of a portion of the Margalla Towers is a case in point. Such surface wave damages were seen during the 1986 Mexican earthquake and also during the 2001 Bhuj earthquake.
It has been seen that a number of people do not consult any structural engineer during construction. They are mostly dependent on the mason, who is generally not too familiar with the codes. The mason advocates simple, cheap and quick methods.
In order to avoid such practices, masons may be imparted suitable training in earthquake-resistant constructions. Such a training programme would definitely help in the design and construction of earthquake-resistant structures.
Hydrology provides for another very important point. When a moderate to major earthquake (with 6.5 or more on the Richter scale) hits the catchment area of a river, huge amounts of loosened soil, fractured rocks and other geological material roll down under gravity and wind to be deposited in the river. These are known as seismo-sediments and the process is known as seismo-sedimentation.
The sediments formed in this manner are very high and the river is charged to its full sediment carrying capacity. If there is a dam downstream then the entire sedimentation is deposited in the dam. As such the generation of seismo-sediments is a big threat to the safety and capacity of dams. The flow of seismo-sediments continues almost for one year or so after the earthquake. Incidentally, the Mangla dam is located in a seismically active region.
The rate of sedimentation in this and other reservoirs may be checked and monitored. Bhakra dam in India had suffered to some extent due to seismo-sedimentation caused by a 6.5 magnitude earthquake at a distance of about 200km upstream. Similar cases have been cited in Mexico and Indonesia. However, one of the most illustrative examples is from New Zealand. A dam was to be constructed on river Inanaguha. All relevant aspects were studied during the planning stage. The site was near a seismic zone. It was estimated that an earthquake of magnitude 8.0 or so would be able to fill up the entire reservoir by seismo-sediment. The site was abandoned due to this very reason. It is, therefore, pertinent to examine all the dams that could be affected by seismo-sediments.
The other major areas that need to be examined from an engineering point of view include roads, bridges, railways, airports, telephone lines, electric lines and powerhouses. In order to inculcate the spirit of earthquake disaster mitigation in people, it is always useful to impart training to students and common people.
Japan observes Sept 1 as its National Disaster Mitigation Day. On this day the entire nation undergoes an earthquake drill. If Pakistan currently doesn’t observe any such day, it would be appropriate if it decides to observe Oct 8 as its Disaster Mitigation Day.
In the end, let me discuss earthquake damage survey. The survey teams should comprise geologists, civil engineers, structural engineers, architects and economists. As the recent earthquake has affected Pakistan and India both, it may be useful to have joint surveys. Surveys must be supplemented by suitable satellite photos taken during the pre-seismic, co-seismic and post-seismic periods. Though the event was a national calamity, this could be used to further our knowledge about earthquake engineering, hydrology and geology.
The author is a senior seismologist based in Pune, India. E-Mail: arun_bapat@vsnl.com
|
|
|
|
