Manali, Oct 3 (IANS): After a decade of sheer hard work of the Border Roads Organisation (BRO), Prime Minister Narendra Modi on Saturday inaugurated the strategic Atal highway tunnel, one of the world's most challenging and a marvel of engineering motorways, in the Himalayas that will also bring the infantry combat vehicles closer to the Line of Actual Control.

Donning a BRO cap, Modi interacted with BRO Director General Harpal Singh, who explained to him through a photo exhibition, about the hurdles the agency faced during the tunnel excavation. Singh explained in detail the challenges encountered during the heavy water influx of Seri rivulet.

The 9.02 km-long horseshoe-shaped single-tube, two-lane tunnel -- the world's longest motorable tunnel at over 3,000 metre above the sea level, came up under the 3,978 metre Rohtang Pass in the Pir Panjal range some 30 km from here in Himachal Pradesh.

Earlier, Chief Minister Jai Ram Thakur welcomed him along with Defence Minister Rajnath Singh and Union Minister of State for Finance Anurag Thakur upon his arrival at the SASE Helipad near here.

Both Rajnath Singh and Thakur reached here a day earlier.

"The all-weather tunnel can take any military traffic, even armoured vehicles," a BRO official aware of the development told IANS.

However, the all-weather road to forward areas of Ladakh requires more tunnels, either at Shikunla or at the high passes located on the 475-km Manali-Leh road for round-the-year connectivity.

Considered a wonder of human perseverance, the Rohtang tunnel, a dream of former Prime Minister Atal Bihari Vajpayee and named after him posthumously, was completed after 10 years of sheer hard work by the BRO with an outlay of Rs 3,200 crore.

Talking exclusively to IANS, project Director Colonel Parikshit Mehra said the tunnel has manoeuvred one of the largest shear zones in the history of highway tunneling.

"A length of 587 m across the Seri-nullah or rivulet zone took us four years and the balance 8.4 km took almost the same time," he said.

Mehra, who did a twin master's degree in tunneling engineering, including the one from Austria, said during excavation the temperature inside the tunnel rose to 55 degrees Celsius before breakthrough and it hardly crossed 20 degrees after that.

On its vulnerability, he said, "The deep tunnels in general are not vulnerable to the tectonic effects since they move as a rigid body with shock waves.

"However portal buildings are vulnerable to earthquakes and in our case earthquake loads have been considered in design."

On the decision to keep the main and escape tunnel within the same opening was with a viewpoint not to disturb a large extent of rock mass and restrict excavation to a specific area only, Colonel Mehra said.

Globally, the escape tunnel is built separately along the main tunnel.

And no wonder the working conditions were relatively short and tough too owing to climatic conditions. "The north portal of the tunnel was accessible only for five-six months in a year,' he said.

The tunnel construction, a blessing for the people of landlocked Lahaul-Spiti district who mark a new beginning in their lives this winter, was abandoned on numerous occasions, primarily due to the complex geology that included fracture zones and fault lines.

Colonel Mehra said all major works have been completed. Now cleaning and final touches are going on inside the tunnel.

He said for at least two years the BRO would regulate the movement of the vehicles through the tunnel. Later, it would be handed over to the local civil authorities.

The tunnel will shorten the distance between this Himachal Pradesh tourist resort and Keylong, the headquarters of Lahaul-Spiti, by 46 km, shortening the travel time by nearly three hours.

With the maximum speed limit of 80 km per hour, the tunnel is expected to see traffic of 3,000 cars and 1,500 trucks a day.

The tunnel has consumed 12,252 metric tonne steel, 1,69,426 metric tonne cement and 1,01,336 metric tonnes of concrete, and excavated out 5,05,264 metric tonnes of soil and rocks by adopting the latest Austrian tunnelling method for construction.

The construction contract of the tunnel has been awarded to Strabag-Afcons, a joint venture between India-based Afcons Infrastructure and Austria's Strabag.

Chandigarh-based Snow and Avalanche Study Establishment (SASE) has designed mechanical structures to ensure the safety of motorists by countering avalanches on both ends of the tunnel that remain under snow even during peak summer.

Engineers of SASE, a Defence Research and Development Organisation (DRDO) laboratory, said self-escape snow galleries have been erected for the safety of motorists after studying the local dynamics of avalanches like force and velocity.

The tunnel's foundation stone was laid by United Progressive Alliance (UPA) Chairperson Sonia Gandhi on June 28, 2010, in the Solang Valley near Manali, some 570 km from the national capital.

The completion of the Atal tunnel is a key element in the Defence Ministry's attempts to make the entire 475 km-long Manali-Keylong-Leh highway, used by the armed forces to reach forward areas in Ladakh bordering China and Pakistan, motorable almost round the year.