The criss-crossing canals that once earned Bangkok the moniker "Venice of the East" have long gone, but the city still draws one comparison with its famed Italian cousin: it too is sinking.

"Bangkok is sinking at varying rates throughout the city… The settlement rate can be more than 12 centimetres per year in the worst areas of Bangkok," says structural engineer Geoffrey Warnes.

Venice has sunk around 23 centimetres (9 inches) over the last century, studies estimate.

Built on the swampish banks of the Chao Phraya River, Bangkok’s top layer of soft clay soil is an engineer’s budgetary nightmare.

"Bangkok has some of the worst clay in the world," says Vithaya Punmongkol, a civil engineer working with the Mass Rapid Transit Authority of Thailand, which is currently constructing a subway system for Bangkok and her 10 million people.

The first 15 metres "is of the most concern to us because it cannot bear much of a load from above", he says.

The subway and its stations, therefore, are being built 20 metres underground, he explains, compared to subways built on more stable ground only going down ten to 15 metres.

"It costs more, of course," he says. "The deeper you go, the more expensive it is."

The challenges can be just as great above ground.

The problem set in around 20 years ago: As Bangkok’s population exploded, buildings shot up and pumps went to work searching for groundwater in lieu of a piped water supply.

"The geotechnical conditions upon which Bangkok is founded are the main reason for settlement. However, artesian water withdrawal exacerbates the situation," structural engineer Warnes adds.

Somkid Buapeng, chief of the groundwater technical and planning section of the Department of Mineral Resources says authorities soon recognised the problem of subsidence could be traced mostly to groundwater pumping.

"After we knew it was due to overpumping we started the mitigation of land subsidence by controlling the amount of groundwater pumping," Somkid says.

Today, Somkid says, the department does not allow pumping where piped water supply is distributed. In the areas where the problem has been brought under control, subsidence occurs at just under one centimetre per year.

However the pumping persists. According to the Ministry of Industry, some 2.2 million cubic metres of water is pumped from the depths of Bangkok each day, allowing the soil above to gradually depress into the earth below.

Flooding that brings Bangkok’s concreted canals back to life and the chaotic city to a standstill is the result.

Teeradej Tangpraprutgul, deputy director of the Bangkok Metropolitan Administration’s drainage and sewerage department, explains that Bangkok’s average land level is about one metre above sea level.

"But given high tides and the rainy season, the water levels of the Chao Phraya can be about 1.7 to two metres (above sea level). During a very high tide, the level can be 2.1 metres," Teeradej says.

The city is fighting back.

"We have many flood protection facilities according to a masterplan. So if rain is in normal range, 60 millimetres per hour, we can protect the city. But sometimes it happens to be 100 millimetres or more than that, which isn’t normal … but we are pretty sure we can drain the water within two hours," Teeradej says, adding that in 1983 some areas remained flooded for two months.

Long-term Bangkok resident Aaron Frankel says the city’s fight has bred some success.

"I remember when I was in high school, having to ride a bicycle down the street to get there as there were no other vehicles that could," he says.

"Flooding has gotten way better … In a strong storm now it will go up for four, five, maybe six inches, which is a pain — it splashes you and you have to take off your shoes off to walk through it — but it’s not as bad as it was," he says.

The protection comes at a cost. The Technical Service Centre of Chulalongkorn University estimates that in 1998 flood control cost the BMA a stunning 20 billion baht (476 million dollars), although Teeradej says this seems on the high side and may have included capital works.

The centre estimates that the annual cost for maintenance and repair of structural damage to buildings due to subsidence was more than two billion baht (47 million dollars) for 1998, while the cost of filling land before construction was about 13 billion baht (305 million dollars) per year.

Further evidence of subsidence can be seen in the extra steps — or the lack of them — leading up to skyscrapers and pedestrian bridges where the ground has literally slipped away as foundations have safely held firm.

Jim Bhandhumkomol, deputy director general of the BMA’s Public Works Department, describes the situation as "quite serious".

"It is quite a serious problem because it (the unstable soil) can create very large differential settlement in structures, as you can notice from approaches to bridges," he says.

Re-laying the approaches to some 500 bridges in the Bangkok road system every two to three years is one of the maintenance jobs subsidence creates, Jim says.

And he doesn’t expect the maintenance to ease up anytime soon.

"It will take years before the subsidence can be significantly reduced. It’s quite a big problem to solve."

/ Travel