Perhaps the most famous and visible part of the long, shared history of Britain and Nepal is the fearsome Brigade of Gurkhas, which has operated as part of the British Army for over 200 years.

The UK is also a popular destination for many among the Nepalese diaspora. And as some of these men and women are engineers, they have naturally set up their own representative group. The Society of Nepalese Engineers UK (SONEUK) helps promote their interests, facilitate networking, and share knowledge.

However, one member in particular has increasingly been thinking about the infrastructure needs of the Nepalese capital, Kathmandu.

Binod Lal Amatya, a geotechnical lead for Arcadis based in London, has been working on an ambitious proposal for a Kathmandu Valley metro system that could revolutionise transport in the region. At a joint meeting of the ICE and SONEUK in Kent, UK, he set out his proposal. Although he hastened to add that this study is still very much in the advocacy phase, and that development may be five to 10 years away.

Walking through the Kathmandu city centre, the need for a metro is all around you. The city currently has no public transport system. Private syndicates operate fleets of microbuses, which are designed for 15 people but usually carry up to 25. More than a million motorbikes (and 34% of all Nepalese vehicles) are owned by the four to six million people living within the Kathmandu Valley, and the average speed during rush hour traffic is a demoralising 7km/h.

Although the city was already sprawling, one SONEUK engineer tells Tunnels and Tunnelling that the recent civil war also caused many to flee to the capital, increasing its population. Once in the metropolitan city, many would be reluctant to return to the rural areas.

Situation in the Valley

Kathmandu itself is located in a valley surrounded by hills – with the impressive Himalayas dominating the horizon. A particular feature of the place is the sheer number of world heritage sites – Kathmandu Valley has been a spiritually significant place for a very long time and boasts seven of these sites, giving it a rank of ‘Outstanding Universal Value’. The seven include the Durbar Squares of Hanuman Dhoka (Kathmandu), Patan and Bhaktapur, the Buddhist stupas of Swayambhu and Bauddhanath and the Hindu temples of Pashupati and Changu Narayan.

As mentioned above, rush hour is far from the serene experience one imagines when one thinks of the Himalayan nation. The government is moving to resolve some of these issues with surface infrastructure works and a programme to bring in bigger buses, but this is problematic. Expanding the roads necessarily requires heritage destruction, and the backlash is so strong that the police and even the military are sometimes required to protect these poorly thought-out projects.

Nepal is the latest in a long series of countries new to the idea of urban underground works, and officials will need to be shown the immense benefits that can be earned when engineers help them to unlock their underground space for the benefit of the people and the economy.

In the words of Amatya, “surface infrastructure squeezes our temples”, and the solution as far as he sees it, is an underground metro. Monorail has also been discussed, but the feeling is that has an unsustainably limited capacity, and also there is the fact that it is something of a closed shop – that you are dependent on one, or a limited number of suppliers and engineers to solve all of your future needs.

This all makes the globally-applied, and far less-disruptive metro solution by far the more palatable.

The first step is to pick an alignment, which in the case of Kathmandu could follow the two major rivers of the valley – the Bagmati and the Bishnumati – or existing major road alignments. The current advocacy study opted primarily for the latter.

As shown in the map, the proposed tunnels will largely follow existing roads to connect the west with the east and the north with the south of the city, with another line acting as the city’s orbital route. A further line, the Bishnumati, follows the River Bishnumati.

The linear routes will be underground in the city centre and either at-grade or elevated elsewhere. The total underground length is expected to be approximately 65km, with a further 120km in the wider valley. This includes a second orbital line further outside the city. Amatya believes that it is essential to link this metro system into a wider Nepalese network. The nearby city of Bhaktapur, for example, is far smaller than Kathmandu, but is densely populated and could also benefit from underground infrastructure.

Back inside the city, nine major stations will be spaced at locations critical for interchange and local economic development. These are expected to be three- to four-storey structures spaced at intervals of 2 to 6km along the network. Cost and technical reasons mean that smaller, local stations will be built at intervals of 0.5 to 1.5km between these major hubs. The smaller stations could be constructed by cut-and-cover or by enlarging the running tunnels.

Geological Conditions

Kathmandu Valley was once a lake, but it has drained over the last 120,000 years. This makes Mexico City a useful case study for planners, as it is also drained and experiences seismic problems. Other useful case studies are the Jakarta Metro in India and Los Angeles Metro in the United States.

The region is characterised by silty sand in the north of the valley (fluvial deposits), with silty clay and clayey silt in the south (lacustrine deposits). At the predicted tunnelling depths the undrained strength is typically 50-75kPa. The east is also more clay-dominated than the west. Boreholes, while limited in number and almost entirely restricted to a depth of 25m, indicate that the water table is incredibly close to the surface close to the rivers, and 4 to 5m below ground level in other places. However, groundwater data has been varied.

The Kathmandu ‘Kalimati’ black clay is a popular clay for pottery and is less stiff than London Clay. London clay can be scratched; Kalimati can be moulded. Although the clay does get firmer as you get deeper.

The ground conditions and the need to minimise the risk to surface structures means that a TBMs will be needed. For the east-west and north-south lines, which may come before the circular, two TBMs per line is preferred. But as this study is about education and advocacy, it is too early to be certain what decisions will be made.

The ground conditions suggest that an EPBM for the south and a slurry machine for the north would be optimal but the likely economic restrictions on the project (Nepal’s GDP is only USD 30bn) mean that EPBM might be the preferred option. Further studies have shown that going with the cheaper EPB option would indeed be feasible, if combined with ground treatment in the sandier areas.

Soft ground tunnelling is less wellknown in Nepal, but TBMs themselves have recently been introduced to the country, for the 12.3km hard rock drive for the Bheri Babai Multipurpose Diversion Project in the far west of the country, which made use of a 5.06m Robbins double shield machine (see feature, Tunnels and Tunnelling International April 2019, pp.41-45). Aside from this, work has primarily been drill and- blast, so paying the price for a TBM, and cost in general, will be a hurdle if this project moves to the development phase.

The 6.3 to 6.7m-diameter TBMs will hopefully achieve an advance rate of 130 to 150m per week operating 12-hour shifts, allowing for unplanned maintenance. Maintenance breaks would be in the station boxes, which would be excavated prior to machine arrival. TBMs will excavate with a lag time of 30 days to allow the ground around the leading tunnel to stabilise.

Structures

The running tunnels will be excavated to an approximately 5.4 to 5.8m internal diameter with 11 to 12m between centres. The lining will be a steel-reinforced precast concrete lining consisting of 5 to 7 segments plus a key. The segments will be 275-300mm thick, and more segments may be preferable as they offer lower strains in a seismic event, it also improves the constructability as the segments are easier to handle and less likely to be damaged. The rings will be bolted radially and circumferentially and fitted with hybrid hydrophilic/EPDM gaskets for water-tightness.

Cross-passages will be spaced at every 500m and will have an internal diameter of 3m. Due to the clayey silt or silty sand at the tunnel level, excavation of these structures does pose a risk of ground collapse, so ground treatment will be needed. Cementitious jet-grouting is envisaged, 4.5m above and 4.5m below the cross-passage axis. The cross passage lining primary lining will be SCL and the permanent will be cast in-situ, with thicknesses varying depending on location, but at least 150mm for both layers.

As for local station design, variations will be needed depending on whether they are built in the north, or the more central and southern areas. Studies considered a multi-storey underground structure measuring 100 to 150m in length, 28 to 35m wide and 25 to 30m deep. Although locations have been identified, it was difficult owing to the lack of open spaces that did not have historical or religious value.

As an example of a northern structure, ‘Thamel Station’ is located in some existing gardens adjacent to a Nepalese government building. Ground conditions are predominantly silty sand with layers of sand down to a depth in excess of 30m. The station and the tunnels sit well within the sandy layer, but the groundwater level is believed to be around 5 to 8m below the surface. A three-storey box 120m long by 35m wide by 25m deep will be built top-down and will need to be fully waterproofed against the high water table and porous sandy conditions, both for temporary and permanent works.

Initially this will be achieved by 2m-diameter, jet-grouted columns installed down to the clay level. This will also allow for easier bored piling works and reduce ground movement during excavation. Permanent grouting to protect nearby vulnerable buildings would also be done at this stage.

Longer-term waterproofing will be achieved by a continuous retaining wall. Diaphragm walls are too costly because of the large amount of bentonite that would be needed to support the sandy soil, so hard-soft secant piled walls are the more likely solution. These piles would be 1m in diameter and 35m deep. A grout curtain will also be required when the TBM passes through the station, and permeation grouting for settlement control may be required at this time.

The ‘New Baneshwor Station’ will be built within the International Convention Centre in ground conditions comprising silty clay and clayey silt at 5 to 10m below ground, overlain by interbedded sandy and clayey/silty strata. Groundwater is expected as high as 3m below ground level. The station will be 144m long by 28m wide by 33m deep. Jet grouting of the ground around the station box will be required for temporary waterproofing during construction. For the actual structure, again built top-down, the study assumes a reinforced concrete box structure with diaphragm walls consisting of 5m-wide panels that are 1.5m thick and 45m deep. Two water stops of full station wall height will be at each panel joint for post-construction waterproofing.

Feasibility

The cost of the project for a developing country such as Nepal is high. Tunnel costs are expected to be USD 80 to 120M per kilometre, so the middle of this range, USD 100M is being assumed. The local stations could cost USD 40 to 50M. In terms of works package sizes, a 5km section of line plus a station is possible, with 2.5 years for civil works and 1.5 years for fit-out, with all stations being under construction at about the same time. Amatya suggests that to build the 22km east-west and north-south lines in a five-year period, a robust programme would be needed. The east-west could be implemented as a phase one, or pilot scheme, but as said before everything is still very preliminary Technically the project is very feasible, and the logistics of Nepal are workable, as has been shown in many other densely populated and remote environments.

The cost to the nation is high, but as Amatya points out, many of the benefits are intangible, with metro stations providing an opportunity to build communities and further drive economic development. A metro system has the ability to completely remake a capital and a nation.

Kathmandu needs a transport masterplan, but not one focused purely on the railway itself. It should be a holistic approach, with a special focus on development.

Expensive or not, the act of failing to invest in the underground has a cost of its own, one that hits the daily routine and prosperity of the people of Kathmandu. Similarly, the government needs to be persuaded that elevated or surface works are very damaging and disruptive.

Amatya has proposed a Kathmandu Metro Rail Authority to the Nepalese government and is drawing on the resource of young engineers working on their MSc courses at Warwick University in the UK, and students at Tribhuvan University in Nepal.

Perhaps with this new cooperation between Britain and Nepal, and along with other international experience, the dream of a metro to solve the problems of Kathmandu will be realised.