Kathmandu-Terai fast track's longest tunnel breaks through its first tube

What happened

Mahadevtar is in Makwanpur and connects the Dhedre and Mahadevtar sections of the expressway. Meroauto reported that excavation of its 3.486 km left tube is complete. The 3.422 km right tube had more than 3 km dug. Concrete lining was finished on more than half of the left tube and more than a third of the right tube. The contractor is China State Construction Engineering Corporation, under a contract worth Rs 21.61 billion.

The fast track has seven twin-tube tunnels, about 11.21 km in total if each tunnel is counted once. Two are already through. At Lendanda, both tubes broke through in May and July 2024, and secondary lining is complete. At Dhedre, both tubes broke through in May and August 2024, and finishing work is in its final stage. Chandram Bhir, Devichaur, Sisautar and Mauri Bhir were still being excavated.

The expressway is 70.977 km long and has been managed by the Nepali Army since 2017. Its estimated cost is Rs 211.93 billion. Meroauto reported physical progress of 48.72 percent in mid-July 2026. On 30 September, a six-month progress report from the Prime Minister's secretariat put it at 50.02 percent and listed the Mahadevtar breakthrough as an achievement. Officials warned that the mid-April 2027 target will be hard to meet.

The engineering behind it

A 2024 paper in the project's own KTFT Journal, by engineers working on the tunnel, describes how Mahadevtar is built. The tubes are excavated with the sequential excavation method, also called the New Austrian Tunnelling Method (NATM). Each excavated section is about 14.6 metres high and 10 metres wide. The tunnel has 13 connecting passages between the two tubes, nine for people and four for vehicles, so traffic and rescue teams can cross if one tube is blocked.

In NATM, as general knowledge, engineers dig the face in stages and support the rock quickly with sprayed concrete, called shotcrete, rock bolts and steel ribs. They measure how the rock moves and adjust the support as they go. Mahadevtar's engineers graded the rock with the Rock Mass Rating (RMR) system, which scores rock strength, cracks, joints and water. Each score sets a standard support type, from light support in good rock to heavy support in poor rock.

The rock did not match the design. The paper says the tunnel passes through Lesser Himalayan rocks near the Mahabharat Thrust, a major fault zone. Engineers expected about 810 metres of marble but found only about 250 metres, with phyllite in other places. Weak rock and joints caused collapses in both tubes. The team fixed them with extra ribs, rock bolts, grouting, steel pipes driven ahead of the face, and by replacing some sections. Where the tunnel crossed the thrust itself, the paper describes a zone about 20 metres thick of dark, crushed rock that broke easily by hand.

Collapse zones need special care. The paper says the team refilled collapsed areas with excavated rock to stop further falls, and that explosives were not allowed in such sections. There, the rock was broken with a mechanical breaker instead. This shows the main idea of NATM: design follows what the rock actually does, not only what the drawings predicted. To see what lies ahead, the team used ground-penetrating radar, which the paper says can detect cavities, water and rock changes up to about 30 metres beyond the tunnel face.

What it means in Nepal

Long twin-tube road tunnels are new for Nepal. The fast track shows Nepali engineers the full sequence of hard-rock tunnelling: geological mapping, support design, excavation, collapse treatment, lining and safety passages. The KTFT Journal itself is a sign of this, because it lets project engineers record lessons from the work so later teams can learn from them. The Mahadevtar paper, for example, records where the rock differed from the design report and which repairs worked, which is useful for any future tunnel in similar Himalayan rock.

The main delay is at the start of the road, not in the hills. The Kathmandu Post reported in April 2026 that construction in the Khokana area of Lalitpur was stalled by land acquisition disputes and local opposition. One option was to move the toll plaza about 3.3 km to Pharsidol and build a bridge linking Dukuchhap and Pharsidol. The Cabinet must approve a change to the project report before work can start on a new alignment.

The lesson is that a road is only useful when its whole length is open. A tunnel can break through on time while one disputed section holds back the corridor. Engineers who work on such projects need to understand land, consultation and approvals as well as rock and concrete, because these decide the opening date as much as the excavation does. The six-month government report shows how such projects are tracked: by a single percentage that combines many separate work fronts.