The incident in Mae Taeng district occurred as Pratyawat Wasu-anant, 24, and Pathomporn Siriwat, 23, went into tunnel No 6 for a geological inspection yesterday morning (Mar 2). The men worked for Italian-Thai Development Plc.
The tunnel near Ban Pasak Ngam in tambon Mae Hoe Phra was being built to divert water into the district’s Mae Kuang Dam reservoir.
The tunnel is one of several being drilled and blasted under a Royal Irrigation Department project to divert water from Mae Taeng Dam into the Mae Ngat Dam reservoir, and then into the Mae Kuang Dam reservoir.
Wongpan Wongsamut, chief engineer of the bureau, said standard practice demands that shortly after the drilling and blast work, geologists have to go inside the tunnel to check the interior and make a detailed report on the characteristics of the rock and soil in the tunnel.
The report helps engineers design the strongest method of cementing the tunnel interior, he said.
Mr Wongpan said it was possible a fragile rock layer may have crumbled, causing a large rock to fall onto the geologists. A number of other people were injured.
Noppadol Pianvech, a construction expert with the project’s consulting firm, said the tunnel interior was made up of sandstone and mudstone.
However, he denied an earlier news report that water seepage had contributed to the soil absorbing too much water, causing the collapse.
He explained that the water found inside the tunnel did not come from the soil but was tap water released during construction, and waiting to be drained. He said the rock that killed the men measured one metre by three metres.
Mr Noppadol said a group of workers had entered the tunnel before the geologists and reported nothing unusual. The area sits on the Kanang fault line.
Safety measures will be added to tunnel drilling and blasting procedures, he said, adding the incident did not result from a flaw in the construction blueprint or from the collapse of the scaffolding.
Mr Wongpan, meanwhile, said a more thorough safety check would be put in place and the builder would need to work to reinforce the strength of the tunnel wall.
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