Collapsed Lao Dam ‘Was Built On A Sinkhole’

Source: AsiaTimes

That is the claim of a Stanford University engineering expert who has scientifically analyzed the site of the disaster that likely killed hundreds of Lao villagers

The site of a tragic dam collapse in southern Laos three months ago was geologically unsuitable for such a project, an American engineer has claimed.

Richard Meehan is an MIT-trained engineer who teaches at Stanford University in the United States, who has experience in designing and building dams in northeastern Thailand. Meehan recently published a diagnosis on the Xe-Pian Xe-Namnoy dam failure.

The disaster in Attapeu province in southern Laos is believed to have killed many hundreds of people, although the official death toll is just 43 with 28 others missing.

On the evening of July 23, half a dozen villages below a western ‘saddle dam’ of the Xe-Pian Xe-Namnoy project were swamped in a tidal wave of mud and water. Environmentalists in Thailand monitoring the spate of dam building in Laos suspect at least 800 people were killed.

They allege the communist government in Vientiane has kept the true scale of the disaster under wraps because it would jeopardize dozens of other dams envisaged under its plan to be the “Battery of Southeast Asia.”

Meehan said he used space technologies and information about local soils to complete an assessment of the dam failure, which is available online at Stanford’s Blume Earthquake Engineering Center.

‘Western arm of reservoir built on sinkhole’

He said satellite ground elevation data and imagery, readily available information about mineral explorations, plus experience on dam projects in similar terrain in northeast Thailand and other comparable areas of tropical soils, “show that the extended western arm of the [saddle dam] reservoir was built on a sinkhole” that probably did not retain any of the heavy summer rain that drenched the Bolaven Plateau.

“Rising water pressures, still within the normal operating specifications for the project but unprecedented in geological time, enlarged underground free-draining conduits within the naturally fragile basalt ridge line that supported the new saddle dam,” he said. “With a rising water level and loss of support, the brittle earth fill dam began to sink into the void and crack extensively.

“The rising reservoir then cascaded over or through the fragmented [dam] crest, washing away the remnants of the dam and 15 meters of erodible foundation and spilling, late at night, half a billion cubic meters of water and debris… onto small villages hundreds of meters below.”

Porous terrain, ‘known risk factors’

He described “known risk factors” at the site as geological formations with high subsurface water mobility, including “volcanic lavas with high leakage potential”, plus cracks and voids, and tropical soil “long known by dam builders for their instability.”

“The dam embankment consisted almost entirely of fill composed of local residual soils susceptible to rapid internal erosion and disintegration when cracked and subjected to high water pressures.

“These high-risk factors raise the question of whether this type of tropical geography is suitable for the aggressive and loosely managed ‘hydropower rush’ that is now underway in remote regions of Laos. Whether the predictable foundation and water retention problems advanced were ever investigated or considered during construction of the saddle dam is not known.”

The disaster raised questions not only about the care with which Laos’ dam projects are being built, he said, but also “the suitability of the country’s tropical geography for engineering concepts largely imported from western countries.”

Asked about the future of the Xe-Pian Xe-Namnoy project, Meehan wrote in an email: “When I first looked at this saddle dam site after the failure I couldn’t understand why they built this important saddle dam in such a geologically unsuitable place when a far more reliable site could be found nearby – even if that would involve trimming the total storage capacity of the project a bit.

“If they ever plan to revive the Xe-Pian Xe-Namnoy project after this bad experience a relocation of the saddle dam to a safer site would be a good start. But the entire project unquestionably needs a review.”

Silence in Vientiane raising concern

The Lao government set up committees in August to determine the root cause of the disaster and review the safety of other existing dams, including those still under construction. New hydropower projects were suspended pending inspections.

However, three months after the tragedy the government has said little about the disaster. “With no information released on its progress, there is rising concern over a lack of transparency and a failure to call those responsible to account,” The Nation newspaper in Bangkok said in an editorial last week.

“The outside world needs to know about the progress of the investigation since the Xe-Pian Xe-Namnoy dam project is jointly run by firms from South Korea and Thailand,” it said.

Most of the electricity generated from the 410MW project was due to be exported to Thailand. The world needed to know that such operations “meet international standards,” the editorial said. The dam collapse flooded 19 villages, including areas as far downstream as Stung Treng in northern Cambodia.

Homes and land for victims

District officials in Attapeu announced last week that the construction company building the dam had ordered about 500 modular homes for people left without shelter after the disaster.

District governor Bounhom Phommsane was reported to have told the Vientiane Times that authorities were also preparing to clear 1,750 hectares of land and would allocate one hectare to each displaced family to support themselves.

Meanwhile, politicians in South Korea have asked questions about the role of the Korean construction company that built the dam, SK Engineering and Construction, and whether alleged cost-cutting  – including lowering the height of auxiliary dams by an average of 6.5 meters – played a significant role in the dam’s collapse.

Currently, at least 5,000 people have been left homeless and are living in tents in Sanamxay district.

Ratchaburi Electricity Generating Holding Plc, a Thai power company, has provided five million baht (US$152,000), Korea Western Power firm has given more than $1 million and SK has donated more than $2.8 million, according to a report in the state daily.

Xe-Pian Xe-Namnoy Power Company (PNPC) has reportedly spent $800,000 on temporary housing and has provided over $10 million to the Lao government for financial relief and recovery work.

Meanwhile, speculation is mounting that the dam investigation is accelerating and that criminal charges could be laid this month against at least one of the four companies involved.