Dam blasting exonerated of damage to dzongs blame

After the installation of a vibration-monitoring device, complaints have even dried up

MHPA: Almost two years after the installation of a vibration-monitoring device, no recent damage has been reported on the Choetse dzong and Ta dzong from blasting at the Mangdechu hydropower project dam site.

Earlier, the site located 1.6km from the town had allegedly caused damage to the historical monuments and residential building structures.

“Neither the dzongkhag officials nor residents complained of damages on the structures since the vibration-monitoring device was installed at the Choetse dzong premises on June 15, 2013,” the project’s executive engineer, Ugyen Tenzey, said.

The ground vibration-measuring device, known as Minimat Plus, was installed at the dzong to record vibrations from blasting, or tremors caused by human or animals.  It can record a tremor of at least peak particle velocity (PPV) 0.5 millimetres per second (mm/sec).

The device in 2015, recorded 154 readings from January 10 till May 7, while in 2014, it detected 17 readings.

Save for nine occasions, the blasting monitoring report for May 2015 and 2014 revealed that the blasting limit was within the permissible threshold.

“The permissible limits correspond to the standards of the Directorate General of Mines Safety (DGMS), India, which is also adopted by the Department of Geology and Mines,” the report stated.

The permissible threshold prescribed by DGMS is 2PPVmm/sec for historical and sensitive structures, such as Choetse and Ta dzongs.

10PPVmm/sec is the permissible limit for industrial buildings, while domestic houses made of cement and bricks are 5PPVmm/sec.

“Except for readings on April 17, 25 and 26 in 2015, most of the reading are below the threshold values, indicating that the vibrations from the blasting wouldn’t have caused any damage to the structures,” the report stated.

While foot stamping of the project staff, who went to download the data from the device caused 8.18PPVmm/sec reading on April 17, others were attributed to the earthquakes in Nepal.

As per the event report data, the vibration of 8.18PPVmm/sec was recorded at 11:29am, while the device was turned off at 11:30am to download data.

“There is no correlation with the blasting because no explosions are carried out after 7am,” Ugyen Tenzey said.

Similarly, 4.04PPVmm/sec, which occurred at 12.16 noon on April 25, is attributed to the quake in Nepal.  The similar higher record of 2.03PPVmm/sec on the following day at 1:13 pm was also inferred was caused by the aftershocks of the same earthquake.

In 2014, six readings on March 18, 19, 22, April 3 and 10, had exceeded the DGMS threshold of 2PPVmm/sec. But the blasting, according to the project’s report, was not responsible for pushing the reading above the threshold.

The report, however, stated that no reasonable correlation could be drawn that the blasting caused the readings to accelerate because of unreasonable time difference.  All six readings were recorded between 48 minutes to four hours after the closest time of blasting.

15-30 minutes is the closet time from blasting, which could be associated with the upward surge of the readings. “Either human or monkeys around the devices could have caused the reading to shoot up,” Ugyen Tenzin said.

Meanwhile, the thromde thuemi, Karma Latho, also said that no recent reports of complaints were lodged on damage of structures. “If there was one, I’d be the first to be informed,” Karma Latho said.

Officiating curator from Ta dzong, Dorji Wangmo, said no fresh cracks have been detected in Ta dzong since 2012 end, when the blasting was allegedly causing damage to structures.

Ugyen Tenzey said the glass attached to the wall of the dzong was also intact.  Glass is usually used to monitor the air overpressure from the blasting.

“If the blasting at any point exceeds the limit, the glass breaks and if it doesn’t, then it’s within the limit,” he said.

Internationally accepted blasting factsheet states that human bodies can detect PPV of 0.2mm/sec, while the 2PPVmm/sec is the amount of vibration that can cause damage to the structures.

“So, any vibrations felt by humans couldn’t necessarily induce damage to the structures,” Ugyen Tenzey said.

By Tempa Wangdi

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