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Moderate quake can bring Dhaka down

Experts blame it on unplanned urbanisation, violation of building code

Helemul Alam
Earthquake poses a serious threat to living in Dhaka city where a tremor measuring seven on the Richter scale may destroy about 35 percent buildings and kill around 25,000 people, say experts.
Mehdi Ahmed Ansary, vice-president of Bangladesh Earthquake Society and professor of civil engineering at Bangladesh University of Engineering and Technology (Buet), blames the risk on unplanned urbanisation and construction of buildings defying the Bangladesh
National BuildingCode (BNBC).
He said depending on what time of day earthquake strikes, 22,000 to 28,000 people might perish in structural collapses. The num- . ber of seriously Injured may range between 86,000 and 1,07,000.
Such an incident might place a very severe strain on emergency relief and healthcare infrastructure.
Ansary, who conducted a survey on Dhaka city with a group of his students during 1999-2007, said the direct economic loss from an
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earthquake of eight magnitude is estimated at SL76 NWlforDhaka^ reasonable’com pared to other recent devastating earthquakes.
According to a UN study conducted in 1999, Dhaka is the world's most earthquake vulnerable city followed by Tehran, said ASM Maksud Kamal, an expert on earthquake and tsunami preparedness.
He said about 90 percent buildings in the capital are non-engineered, which is very risky even for a moderate earthquake.
According to geologists, the topsoil condition of Dhaka city, divided into two types, is not structurally fit to bear the load of the ongoing jumbled growth of concrete structures. If a major earthquake shook the capital and its periphery, most parts of the city might experience a trail of destruction, theyfear.
Maksud Kamal said as Dhaka is surrounded by four rivers-Buriganga, Turag, Shitalakhya and Balu--35 percent of its area is covered with hard soil or red soil (Pleistocene Terrace) but the rest is soft soil, mostly marshy land, some flood plain and abandoned channels and some wide belly.
He said the seismic waves propagate with high velocity through red soil and last for a short period whereas the waves stand for a longer period in soft soil. So, the damage on soft soil is greater than red soil.
The topsoil condition of
the belt covering Rampura, the old town, Buriganga, Mohammadpur and Shyamoli is Jess capable of taking the load of heavy construction compared to the condition of the Mirpur-Uttara-Tongi-Gazipur belt, he said.
The eastern fringe of the city will suffer the highest damage and casualty as the major components of the soil in this area are sand and water, he added.
Each building based on its height swings in both directions in a certain predominant mode. When the frequency of the soil media and that of the building in a certain place correspond, there is a high probability of having potential damage, Maksud said.
Earthquakes like the 1999 earthquake in Chi-Chi, Taiwan, and the Mexico earthquake of 1986 proved this phenomenon known as site effect, he added.
"Investigation in the different types of soil in Dhaka conducted in 2004 reveals that nine to 14-storey buildings constructed on soft soil will experience higher ground acceleration and durations during an earthquake due to site effect. Out of 300 square kilometres of the area of Dhaka city surrounded by the rurag, Buriganga and Balu rivers, one-third area will experience this phenomena,” Kamal said. He said the situation has not changed so far.
Ansary said in many cases even the Real Estate and Housing Association of Bangladesh (REHAB), which has 361 members, also does
not follow the BNBC while constructing multi-storey buildings.
Many developers follow the earthquake rules while designing buildings, but they do not implement the rules during construction, he said.
Ansary emphasised the need for a greater role on part of Dhaka City Corporation, Rajdhani Unnayan Kartripakkha and the Public Works Department for monitoring implementation of the BNBC.
REHAB President Tanvirul Haque Probal, however, claimed that most of their members follow the BNBC properly. 'As the members of REHAB are in competition to survive in the market, it is very necessary to maintain quality during construction and following BNBC,' he reasoned.
Probal said the REHAB members have constructed 5-7 percent buildings of Dhaka city.
Prof Ansary said buildings of different housing projects on low-lying areas fiUed without following Private Housing Project Development Rules 2004 are under the highest risk.
During filling the wetland, housing projects do not maintain quality, which affects the condition of the land of the project areas, he said, adding that many developers spend only Tk 50,000 against a requirement of at least Tk 10,00,000 to fill up a piece of land.