Water seepage into layers of sand beneath Suvarnabhumi Airport is partly responsible for cracks and ruts in areas where heavily-laden planes manoeuvre, according to an investigation.

The Tortrakul Yomnak fact-finding committee said yesterday seepage was responsible for at least 80 per cent of taxiway damage.

But the runway cracks stemmed from a different root cause, the committee findings said. Surfaces of both runways were supposed to be coated with polymer to increase strength. The probe discovered three-thirds only of the 4,000 metre-long runways were coated.

"It's a design problem. The designer probably wanted to save costs but they forgot we have to use the other end of the runways for take offs when the prevailing wind changes in the cool season," Tortrakul said. "We can probably do away with replacing surfaces because we found no structural damage."

According to the committee, there is no need to close the entire airport. But, the committee was unable to recommend how water could be drained to minimise the potential spread of cracks on taxi lanes and taxiways in the near term, or how to improve drainage to eliminate the problem.

"Technically, there are many ways to drain the water. Other experts should determine the best way with the least shortcomings and lowest costs," committee foundation engineer Suebsak Prom-boon said. "I assume our committee's mandate expires today."

It remains unclear if other factors - such as subsidence - may have contributed to the cracks or represent a host of additional problems.

Committee soil expert Prof Dr Surachat Samphantarak said airfield settlement will not cease after draining because it is built on a layer of soft clay.

He said although clay immediately below the airport structure had been strengthened a decade ago by prefabricated vertical drainage (PVD) - draining water from the layer - untreated, deeper clay would continue to settle.

It may subside faster than the natural rate because it now has to bear the weight of the entire airport.

"Slowly but surely the clay is settling," Surachat said. "If the pavement is not strong enough we may have differential settlement in different areas. Swampy soil subsides at different rates," he said. That can result in more ruts and cracks.

"I also doubt if the PVD technique was done right. How do we know the contractors completed the drainage process. There is no data supporting the [contractors'] claim settlement in the treated soil layer was stopped [before paving commenced].

As a result of uncertainty, Airports of Thailand (AOT) president Saprang Kalayanamitr announced yesterday he will suggest the government retain international experts to inspect the airport and advise on solutions for both the airfield and passenger terminal.

"Nobody can guarantee anything today," Saprang said. "This [Tortrakul] committee has had only two weeks. The [pavement] problem will certainly continue into the future and we need help from foreign experts. But the government will have to decide if this is the way it wants to proceed because hiring experts may involve a lot of money. I was told there are few [companies] in the world that have the adequate expertise." 

The affected taxi areas appear to share a common trait, Suebsak said. Water from heavy rain during construction and additional flooding after completion entered the drainage system and seeped into the 1.5-metre-deep sand layer below the pavement. Trapped water mixed with sand destabilises the sand layer, causing cracks on the surface.

While it appears ruts and cracks are confined to about 80,000 square metres of airfield now, experts said it was likely damage could spread to most of its two million square metres.

That is owing to increasing traffic and because the layer is already saturated.

A drainage solution is needed before the next wet season. "I say most of the taxi lanes and taxiways have been trashed," Suebsak said.

Experts said the key issue was that water should never have seeped into the sand layer and suggested drainage systems were "clearly not designed properly".

"The designer probably forgot the two-year period of pavement construction was long enough for a lot of rainwater and other water to get into the sand layer," Suebsak said

The committee could not provide the names of the designers.

Lack of information and cooperation from AOT staff and board members was cited as a "major stumbling block" affecting problem solving and operations.

"We were frustrated we didn't get the information we asked for," Saprang revealed. "But now I have removed people who were part of the problem. I hope the situation will get better with the new people I have put in charge."

Nantiya Tangwisutijit

The Nation