October 13, 2021

Case study: How we made it possible to plan tunnels in the Himalayas with the help of a revolutionary geoscanning method

For the first time ever, the rock conditions of the challenging Himalayas were mapped with the use of helicopter geoscanning and exploratory drilling follow-up. This pioneering work, done in 2013, made it possible to analyse the ground conditions in practically inaccessible areas.

Eight years ago, the Bhutanese government started planning a new tunnel to replace a dangerous mountain pass close to the capital Thimpu. The project required a comprehensive mapping of the rock quality along the tunnel alignment. This is easier said than done in the highly challenging conditions oft he Himalayas – an area where normal methods of manual drillings are close to impossible to perform.

Hence, the Bhutanese Department of Geology and Mines reached out to experts at the Norwegian Geotechical Institute (NGI), experts that later founded EMerald Geomodelling.

Location of the 70 km long old road over Dochu La pass (3100 m) and the two new road tunnel options (red doted lines) from Yesipang to Nabesa. (From Bhasin et al. 2016)

Through a unique and revolutionary solution, we used helicopter geoscanning – or more precisely speaking, airborne electromagnetic (AEM)resistivity imaging – to map the bedrock properties in 3D. This was the first time this type of technology had been used in the Himalayas, but the Bhutanese saw the potential and seized the opportunity.

How it was done

With the help of a Nepalese helicopter and Danish AEM system provider, we were able to obtain the data we needed.

Cooperating with a Danish AEM system provider and Nepalese helicopter company, we were able fly over the mountain chain with the equipment and scan most of the ground conditions at the planned tunnel area.

With only one profile of ground based seismic analysis afterwards, we were able to confirm exactly what the scanning data had indicated.

This was an essential breakthrough in the planning process of the tunnel. The ground data provided us the necessary insight to plan the tunnel portal’s optimal location on the slope of the mountain.

What we discovered

The helicopter scanning data also revealed important information about the rock conditions, such as a weakness zone in the southern part of the planned tunnel. This was an important discovery, as is resulted in adjustments on the planned line.

In the northern part, we were able to locate areas with solid bedrock and shallow weathering that were confirmed by a refraction seismic profile. This led to the conclusion of placing the tunnel entry at this location.

All of the analyses were done in cooperation with local geologists and geotechnical engineers, so that we could be confident we interpreted the data correctly.  

The green to blue colours (high resistivity) indicate resistive material typical for intact bedrock whereas red to yellow (low resistivity) indicate conductive material that later was confirmed as deep weathered rock with refraction seismic.

What did the client say?

"The results from the survey were useful in pin-pointing areas where potential weak zones may be encountered along the tunnel alignment. Geophysical surveys performed on the ground of the portal of the tunnel matched the output results from the AEM surveys. We believe AEM surveys are useful for planning of such infrastructures in mountainous terrain." - Dr. Dowchu Drukpa, Chief Geophysicist at the Department of Geology and Mines, Royal Government of Bhutan.

What happened next?

As mentioned, this work began in 2013, with the intention of starting the tunnel building soon afterwards. Unfortunately, funding opportunities for these projects are limited. Thus, the construction of the tunnel has yet to begin.  

However, the results of the geoscannings were not wasted, as it proved that the airborne scanning method can provide necessary ground data to plan construction projects in the Himalayas and in similar conditions – something that had never been done before.  

This has enabled EMerald Geomodelling to continue developing the technology and applying it on other projects in the area, such as construction projects in the Indian Himalayas and hydropower pre-investigations in the high Andes.

The whole team for the Bhutan Project. Andi A. Pfaffhuber, CEO of EMerald Geomodelling, in the middle.

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