The work, spanning some 51 kilometers in Binh Thuan Province and 17 km in Lam Dong, is expected to cost VND1.4 trillion (US$54.6 million).
The roadbed will be widened from 5.5 meters to 9-12 m.
Nearly 400 workers and 150 machines are currently working at the construction site.
But only 45% of the land required for the project has been cleared in Binh Thuan as much of it is under forests and have to have their purpose converted. The contractor is working on sections where lands have already been cleared.
The Dai Ninh mountain pass near the Lam Dong border has high mountains and deep ravines, and the complex terrain makes construction challenging.
A 103-meter-long, 11-meter-wide and 23-meter-high reinforced concrete bridge is being built to reduce hazards.
Steel cages suspended from cranes are used by workers to access the top of bridge piers that are over 20 meters above the ground.
Bui Duy Khoa, an engineer, said the piling phase has been completed and the construction of the piers are underway.
At a sharp curve over a deep ravine, widening or reinforcing the roadbed using conventional methods is challenging. So this section has been designed with semi-viaducts, allowing part of the road to rest on natural ground and widening of the roadbed to 12 meters.
In flatter areas, concrete retaining walls are being built to serve as strong embankments. Soil is then added to widen the roadbed.
The builders are installing the steel mesh for retaining walls along the Dai Ninh mountain pass in the Song Luy protective forest area before pouring concrete.
The pass's complicated terrain presents significant a major challenge to widening the roadbed. Excavators dig into the side of a hill to widen the road, reducing the curvature.
Warning signs and ropes are installed at both ends of the construction area for safety.
The soil and rocks excavators dig from the mountain along the pass are transported to a nearby storage site.
Excavators work at the top of the pass near Duc Trong District in Lam Dong. The excavated soil is being used to fill the roadbed.
In areas where land has been cleared, teams are rapidly moving in construction equipment.
At a recent meeting, Cao Son Dung, director of the Binh Thuan Department of Agriculture and Environment, urged that the Bac Binh District People's Committee to ensure the lands required are handed over to the contractor by April 30.
