The highway would also have 12 bridges, most ranging between 40 and 50 metres in length, but the one at Sokorong stretching more than 118 metres.
Officials said the estimated cost of the highway, which is funded by the government of India, was 1677.589 million ngultrum (Bt872.9 million).
Project director Jangchub Gyeltshen said the rocky mountainous terrain made drilling difficult and slow, with one difficult, 2km stretch taking eight months to cut.
Once opened, the road is expected to boost business in the previously isolated east of the Himalayan country.
“It will facilitate establishment of infrastructure and boost trade activities in the six eastern dzongkhag (district) and others,” Jangchub Gyeltshen said.