Transcript
Geologically Active – Williams et al. (eds)
© 2010 Taylor & Francis Group, London, ISBN 978-0-415-60034-7
Low-cost road for the development of Nepal and its engineering
geological consequences
R.K. Dahal
Department of Geology, Tribhuvan University, Tri-Chandra Campus, Ghantaghar, Kathmandu, Nepal
S. Hasegawa
Department of Safety System Construction Engineering, Kagawa University, Takamatsu, Japan
N.P. Bhandary & R. Yatabe
Department of Civil & Environmental Engineering, Ehime University, Matsuyama, Japan
ABSTRACT: Construction of roads in the mountains of Nepal is quite complicated
because of steep slopes, thick soil profiles, weak rockmass and the extreme rainfall of the
monsoon season. In the name of “low cost”, many roads of Nepal do not have any standard
engineering structures. As a result, low cost road construction and maintenance programs
are widely affected by landslide and debris flow triggered by monsoon rainfall. Generally,
shallow failure occurred along the roadside, both in uphill as well as downhill slopes, are
major geological problems of roads of Nepal. Although, cost effective techniques are very
important for a developing country like Nepal, experience of Nepal reveals that low cost
roads are not the best solution for sustainable development of underdeveloped countries.
1 Introduction
Nepal is a mountainous country situated in the heart of the Himalaya. This young mountain
chain is also famous worldwide for its very active tectonics. Furthermore, it lies in an area of
strong monsoon precipitation, with the rainfall distribution being such that over 90 percent
occurs within a short three months. Extreme rainfall events are very common which can
bring over 400 mm of rain within 24 hours. Thus, these geological and climatic factors make
the mountain slopes of Nepal highly vulnerable for to rainfall triggered landslides.
Construction of roads in the mountains of Nepal is complicated because of steep slopes,
thick soil profiles, weak rock mass and torrential rainfall due to the monsoon. This paper is
intended to describe the engineering geological issues of low cost roads in Nepal with some
illustrations. Later in the paper, some mitigation measures used to stabilize the slopes are also
evaluated with some illustrations.
2 Road construction practice in Nepal
A significant factor in the development of a country is the provision of improved access to
physical facilities and social services. Nepal’s total road network and density are low and only
43% of the population has access to all-weather roads. In the last ten years, road construction projects in Nepal have been considered a major part of infrastructure development. The
growth in roads shows an exponential increase from 376 km in 1950 to 20,600 km (including
usable rural roads) by 2007 (a period of 58 years). Even now the road density is low, about
6 km/100 sq km. Most of the roads (either rural roads or national highways) in Nepal are low
cost roads, not built to international standard. Nowadays, there are many projects of district
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or rural road construction. These roads are being constructed with the participation of the
people but very littletechnical supervision. Many international donor agencies are providing
funds for local governments to construct roads. Some of the major donor agencies for low
cost road projects in Nepal are the Asian Development Bank (ADB), World Bank (WB), UK
Department of International Development (DFID), the Swiss Agency for Development and
Cooperation (SDC), Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ, an international cooperation enterprise from Germany), Japan International Cooperation Agency
(JICA), and the United Nations World Food Program (WFP).
High-paid short term consultants from developed countries work on these road construction projects and provide engineering as well as socioeconomic consulting service for low cost
road construction. But, most of the time, international consultants only suggest economical
road construction practices rather than sustainable road construction methods suited to the
highly dynamic Himalayan environment. It is not unusual for the total cost of a rural low
cost road project to be lower than one day’s travel and daily allowance for an international
consultant. For example, for 2009/10, the District Development Committee of Dolakha district has approved 100 low cost rural road projects and most of the rural roads are 10 to
15 km long whereas the budget sanctioned for each road project is only about US$270!! (District Technical Officer, Dolakha district, pers. comm.). In fact, road construction practices in
Nepal are mostly guided by the desire of the donor agencies and the interests of local political
leaders. This is partly attributable to the lack of uniform and mandatory national standards
and guidelines. Many stretches of road do not have any standard engineering structures and
the roads are usually constructed on side slopes requiring cut and fill. Locally available materials are used to protect the cut slope, usually dry stone retaining walls or bioengineering are
used for protecting slopes.
In recent years, district or rural road construction projects have already grown by 11%
annually. A related fact is that more than 50% of the Village Development Committees
(VDCs) from all over Nepal have a User Committee for rural road construction. Nowadays,
many slopes in the mid-hills are dissected by earthen roads and, due to the lack of engineering standards; these roads are almost unusable for vehicle movement. As a result, low cost
roads are prone to shallow and deep seated landslides during the monsoon. The poor engineering conditions of the roads hampers the delivery of social services in the remote hill and
mountainous districts and directly or indirectly affects the country’s economic development.
Lack of connectivity in presence of low cost road projects are major impediments to the
development of Nepal.
In Nepal, about one third of the total road length is earthen roads (33%), with the remainder of the network comprising gravel roads (22%) and bitumen sealed roads (45%). The
Nepal Road Standards classifies roads as follows:
• National Highways connecting the length and width of the country (Figure 1)
• Feeder Roads providing access to important trade centres and district headquarters from
the national highways
• District Roads providing access between a district’s headquarters and its trade centres.
• Urban Roads within the city area
• Rural Roads to provide access to rural settlements and agricultural centres
National highways and feeder roads are included in the Strategic Road Network (SRN)
under the jurisdiction of the Department of Roads (DoR); the SRN comprises nearly one
third of the total road length. The remaining two thirds are considered as district roads, rural
roads, urban roads, agricultural roads and capillary roads which are under the jurisdiction
of Department of Local Infrastructure Development and Agricultural Roads (DoLIDAR)
and local bodies (District Development Committees, Village Development Committees and
Municipalities).
Major highways like Prithivi Highway, Tribhuvan Highway, Siddhartha Highway,
and East-West Highway are continuously in the maintenance phase. New highways like
Dhulikhel-Sindhuli Road, Chhinchu-Jajarkot Road, Surkhet-Jumla Road, Katari-Okhaldhunga Road, Mid-Hills Roads and Beni-Jomsom are in the construction phase. Likewise,
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Figure 1. Strategic road network of Nepal.
there are many more road projects running at a local level with the financial aid of many
international donor agencies.
Low cost road construction and maintenance programmes are widely affected by landslide
and debris flows triggered by monsoon rainfall. The failures also vary in severity and losses.
Roadside slope failures have not commonly resulted in major loss of life, because most catastrophic failures have occurred in less populated areas. Economic and financial losses are,
however, substantial. Generally shallow failures occurring along the roadside, both on the
uphill as well as the downhill slopes, are major problems for Nepalese roads. These problems
are excessive in both major highways and rural roads. Through enforcement of the Public
Works Directives (PWD) in 2001some degree of uniformity has been achieved in the project
implementation practices. Table 1 lists existing road construction practices adopted for the
construction of roads in Nepal.
3 Geological issues OF roads IN Nepal
3.1 Geology and geomorphology of Nepal in brief
Having emerged as a result of tectonic uplift of sedimentary deposits, the rock-mass in
the Himalaya has a high degree of fragility and a greater tendency to undergo accelerated
decomposition under the influence of climatic factors. With 83% low to high mountainous
areas, Nepal covers approximately one third of the Himalayan mountain ranges in the central
Himalaya. The Nepal Himalaya has eight well-defined regional geomorphologic zones in a
north–south direction and each of these zones has a unique altitudinal variation, slope and
relief characteristics, and climatic pattern. Figure 2 provides the structural framework and
geological map of Nepal.
Controlled by the monsoonal winds and regional geomorphology, the climate of Nepal
is extremely varied. It ranges from seasonally humid subtropics to semiarid alpine, but in a
more global sense, the climate of Nepal is tropical monsoon, except for parts of the northern
area, which lie in the rain shadow of the Himalaya and have a cold semi-desert climate. It
is often said that the wet monsoon over the Himalaya provides almost 90% of South Asian
water resources. Orographic effects are the main cause of extreme monsoonal rainfall in
Nepal, which usually begins in June and ends in September.
3.2 Landslides as major geological issues of roadside slopes
Factors such as excessive rainfall and human intervention are the main triggers of landslides
along the roadside slopes of Nepal. Factors such as groundwater conditions, river under
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Table 1. Existing road construction practices in Nepal (modified after Dahal et al. 2006).
Landslide
occurrence
Methods
Generic guidelines
Labour intensive
method
Applied for construction of early roads
in Nepal, labour groups employed as
labour contract, no work contracts, no
heavy equipment used except
work tools, mostly full cut roads,
structures and embankments
minimized, side casting of surplus
material permitted,
blasting for rock breaking,
Embankment and fills compacted
with light equipment, equipment
used for pavement construction
Applied in highways, feeder roads
and urban roads, earthwork equipment
used for cut, slope trimming and
embankment construction, mechanized
compaction of backfill and
embankments, labourers
used for minor works—drainage, slope
finishing, subgrade preparation etc.,
full range of equipment used for pavement
construction, blasting for rock
breaking permitted, large contractors
employed for construction
Mostly used for district roads and feeder
roads, only light equipment used, no
heavy equipment used, small local
contractors used for civil work contracts,
maximum use of local labourers for works,
limited blasting permitted for rock
breaking, large scale and long distance
haulage not practical
Stage construction of road formation
(1.5 m, 2.5 m, 4.0 m) in combination with
bioengineering, only local labourers used
through community based organizations,
wages equally distributed, no profit
retained by community based
organizations, no contractors may
participate, balanced cut and fill
principle—no haulage of surplus material,
bioengineering measures as an integral
part in each stage, natural compaction
principle—no artificial compaction, only
local material used except some gabion
wires, use of cement discouraged,
blasting for rock breaking not permitted,
road formation out cambered for
sheet flow—no side drains, equipment
may be used only for gravelling and
pavement, prevailing method for rural
and agricultural roads, also applicable
for construction of highways and
feeder roads, method is inherently
poverty focused and uses poorest people
Conventional
Mechanized Road
Construction
Practice
Labour-Based
Road Construction
Method
Low-cost
Environment-friendly
and Participatory
(LEP) Method
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High Risk of
Landslides
Need extensive
costly mitiga
tion measure
High Risk of
Landslides
Need extensive
costly mitiga
tion measure
Medium Risk of
Landslides
Need extensive
mitigation
measure
Generally
destroyed
by monsoon
rainfall so
seasonal road
Shallow failures
are prominent
Needs routine
care by
community
Figure 2. Simplified Geology and physiogeography of Nepal (modified after Dahal & Hasegawa
2008).
cutting and deforestation of slopes are also facilitating landslides on roads. In this context, it
is easy to accept that landslides are the major engineering geological issues in low cost roads
of Nepal. Brief scenarios for landslide occurrences in the main physiographic provinces of
Nepal are given in Table 2.
3.3 Roadside slope failures
For the prediction of rainfall triggered landslides, the concept of hydrological landslidetriggering thresholds has been already developed to some extent. Similarly, a correlation
between rainfall intensity, rainfall duration and landslide events are also already established
(Dahal & Hasegawa, 2008). Moreover, the influence of rainfall on landslides usually depends
on landslide dimensions, kinematics, material involved, etc. Experiences show that shallow
failures are usually triggered by comparatively short intense storms (Campbell, 1975; Wieczorek et al., 2000; Kim et al., 2004) whereas most of the deep-seated landslides are affected by
long-term variation of annual rainfall as well as daily rainfall which has to last at least some
couple of years. In the Himalayan context, shallow landslides are associated with torrential
rainfall brought by the monsoon (Dahal & Hasegawa, 2008).
Not only shallow landslides, but also deep-seated landslides are usually associated with the
monsoon rainfall. Many roadside slopes along the highways consist of considerable numbers of deep-seated landslides which undergo prolonged creep during torrential rainfall. For
example, a large and well known landslide of the Prithivi Highway, named the Krishnabhir
landslide (Figure 3), occurred after some days of intense precipitation, in the Trishuli River
valley (Lesser Himalaya) at the beginning of mid of August 2000 and blocked the Prithivi
Highway, the main entry route of Kathmandu, for 11 days. Because of low cost mitigation
measures and “wait and see” principles, the highway was blocked many times since 2000 and
it was stabilized in 2005.
Shallow landslides that generally flow downslope at a very high velocity are found to be
the most devastating. In various parts of Nepal extreme rainfall can reach up to 200 mm
in 24 hours and studies shows that this value is enough for triggering landslides on steep
slope (>30°) in the Lesser Himalaya (Dahal & Hasegawa 2008). The intense rainfall that
occurred in Chitawan (central Nepal) on July 2003 resulted in a series of landslides affecting
steep roadside slopes of the Mugling-Narayangadh Road, all having similar characteristics.
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Table 2. Landslide problems in various physiographic provinces of Nepal.
Physiographic
provinces
Siwaliks (Churia)
Range
Mahabharat Range
Midlands
Fore Himalaya
Higher Himalaya
Trans Himalaya
Major issues
Made up of geologically very young unconsolidated
and easily disintegrated sedimentary rocks such as mudstones,
shale, sandstones, siltstones and conglomerates. Rainfall is
normally in the range of 2000 to 2500 mm per year. The combination
of geology and rainfall makes the Siwaliks highly susceptible to
landslide processes.
The Upper Siwaliks contain thick beds of loose and fragile
conglomerates. The Lower and Middle Siwaliks have problems caused
by alternating beds of mudstones and sandstone. Mudstones can
flow when saturated with water, which results in overhanging
sandstone beds; when such sandstone beds are well jointed they
easily disintegrate into blocks.
The Mahabharat Range is the most important barrier to the monsoon
clouds and therefore it greatly influences the rainfall distribution
pattern in Nepal.
The southern face of the range receives extensive rainfall in comparison
to the Midlands with a very high frequency of high intensity rainfall.
Those areas made up of rocks such as limestone, dolomite, marble
and granites have the more stable slopes whereas those underlain
by rocks such as phyllite, slate, intercalations of phyllite and quartzite,
depict the terrain most prone to landslides.
A gentle topography compared to the Siwaliks and Mahabharat ranges.
Thick soil formations are found on slopes because of deeply
weathered rocks.
Usually considered as a rain shadow of the Mahabharat with
1000–2000 mm annual precipitation but some areas also record
incidences of high rainfall.
Slopes are very prone to landslides after intense rainfall.
Most of the population of Nepal lives in the Midlands which is
intensively cultivated. Irrigation systems can be found in every terrace
on the slopes as well as on old landslide debris; mismanagement
of irrigation canals can be noticed everywhere. These improper
agriculture practices generally create landslides or reactivate old
landslides and usually damage whole villages on the slopes.
The frontal portion of the Higher Himalaya.
Main rock types being phyllite, schist, marble, quartzite, and gneiss.
Tectonically very active and uplifting at a high rate. The topography
is steep and rugged.
High rainfall; 2000 to 3500 mm, so another vulnerable area for landslide
occurrence.
Some landslide dams are also noticed in narrow river valleys of this
province.
The highest area of Himalaya, including all elevated peaks and their
slopes above 5000 m.
Main rock types are gneiss, schist, marble and quartzite.
Vertical or steep rocky slopes are very common.
There is little or no soil cover on slopes; rock related failure
phenomena are very common, Because of very low population
and nil infrastructure development, the degradations of the Higher
Himalaya do not draw the attention of planners and researchers.
Situated in the rain shadow zone of the greater Himalayan Range,
this zone has very low average annual rainfall, so soil related
landslides are less frequent but debris flows in snow fed streams are
quite common. Riverbanks composed of alluvial and glacial
moraine possesses bank failure problem.
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Figure 3. Krishnabhir landslide along the Prithivi highway, a major route connecting the capital city
with the rest of the country.
These gravitational mass movements occurred suddenly, generally extending for tens or even
hundreds of metres and damaged almost 80% of a total 36 km highway stretch.
Based on classifications proposed by Varnes in 1978 and Cruden et al. (1996), most of the
landslides on roadside slopes can be classified as translational landslides, rotational landslides, followed in many cases by debris flow. Nevertheless, some highway landslides are very
large landslides of a complex nature. The failure surface of translational landslides on slopes
is generally at a depth of 2–4 m and appears to affect the whole hill slope. In fact, such small
scale landslides are noticed in large scale landslide mass as seen in the Mugling-Narayangadh
road (Hasegawa et al. 2009). Rotational slides are generally seen on thick colluvial deposit as
well as residual soil where water was not properly managed on cut slopes.
Debris flows are even observed in the Jomsom area (Jomsom-Kagbeni-Muktinath road)
where annual rainfall is only 250 mm. Many parts of the Mugling-Narayangadh road were
damaged due to debris flows and debris slides. These landslides are generally initiated at
small areas at the tops of hills or heads of gullies and they flow with extremely high velocity and erode a path damaging everything in the way, finally leaving a deposit rich with high
debris load. These debris flows generally have very a shallow depth and most of the debris
flows triggered by rainfall appear on roadside slopes with slope angles mainly between 30°
and 40°. Failures are rarely recorded on slopes of <20°. The materials involved in all flows are
very similar, mainly gravel silt of colluvial origin. Regarding rainfall infiltration behaviour
and soil moisture characteristics during the rainy season on roadside slopes, it can be suggested that the primary failure mechanism is related to increasing water content as a result of
downward movement of a wetting front (Dahal et al. 2009). Natural piping in colluvial soil is
another factor responsible for landsliding noticed during field visit.
4 Mitigation MEASURES
4.1 Drainage measures
There are some accepted practices of drainage measures in roads of Nepal. Mainly DhulikhelSindhuli Road, Dharan-Dhankuta Road, Pokhara-Baglung Road, Arniko Highway, and
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Figure 4. An economic approach of water management at Arniko Highway (modified after Dahal et al.
2006).
East-West Highway have relatively good management of surface runoff at cut slopes and a
reduced landslide frequency in comparison with low cost rural roads. In these roads, surface
runoff of up to 1 m3/s has been drained out with lined catch drains and riprap channels
designed to cope with storms of a minimum 25 years return period. Drainage of subsurface/
surface water has been managed using french drains (depth 1.5 m to 2.5 m). The generally
effective depth of a network of French drains is to the lower saturation level from ground surface. Excellent performance of French drains was observed at both landslides and cut slopes
of Arniko Highway (Figure 4), whereas some unlined catch drains around the cut slopes
along the rural roads were possessing adverse effects on the slope. Similarly, many retaining
walls and toe walls constructed without managing subsurface water have been damaging low
cost roads extensively.
4.2 Structural support measures
In roadside slopes of Nepal, structural support measures are also applied in combination
with drainage and bioengineering measures. Various types of wall, such as gabions, stone
masonry and composite masonry, have been using to mitigate failure. A few support structures, such as rock bolts, earth anchors, anchored walls, are also found to be used in some
highways.
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Table 3. Bioengineering measures practiced in Nepalese roadside slope.
Type of plants
Bioengineering system
Hard wood plant
Palisades, tree planting,
bamboo planting,
fascines, live check dam
Shrub planting,
Vegetated riprap,
Brush layering,
Live staking
Grass seeding or broadcasting,
grass planting in diagonal lines,
horizontal lines, downward lines,
chevron lines, herring bone lines
or random pattern.
Shrub
Grass
4.3 Bioengineering measures
In Nepal, comprehensive bioengineering work has been carried out during the last 30 years
and large experience has been collected in slope stabilization using vegetation (Howell 1999;
Dahal 2001; Florineth et al. 2002). Bioengineering systems, which are generally found to
be applied in roadside slopes of Nepal as low cost mitigation measures, are presented in
Table 3.
5 Conclusions
Landslides and debris flows triggered by torrential rainfall events are common in Nepal
Himalaya. Such events are highly localized along both old and recently constructed low cost
roads and losses and damage are extremely high. Nowadays, there are many roads to remote
villages of Nepal, but none of them are functioning; they are just in progress report of donor
agencies and past working experiences of local contractors.
Road planners, consultants and executors in Nepal have now realized the need of an integrated approach to site investigation, analysis, design and construction of mitigation structures on landslides. But still the Department of Roads, Department of Local Infrastructure
Development and Agricultural Roads and local bodies (District Development Committees,
Village Development Committees and Municipalities) do not have a single geologist as an
employee. As a result, proper site investigation for engineering geology is always lacking.
International donor agencies are also supporting various Nepalese road construction
authorities to butchering young mountains without proper geological investigations. As an
immediate action, international donor agencies need to give due attention to these issues.
What is the use of low budget roads which are not functional after construction? Thus, allocation of low budgets for many road projects should be stopped and should due attention
should be given to develop one or two sustainable road projects per year from the limited
budgets. It will help to construct modern roads in mountains of Nepal with full attention
to the mitigation of roadside slope failures. Giving more attention to the quality than the
quantity of roads, donor agencies and the Government of Nepal can change road construction practices in Nepal. All authorities engaging in road construction should think primarily
about the geological risk of roadside slopes rather than the cost effective techniques of road
construction.
From this study, it is understood that concepts, approaches and techniques for this unique
Himalayan terrain at an affordable cost are still challenging. Although cost effective techniques are very important for a developing country like Nepal, the Nepalese experiences
reveal that low cost roads are not the best solution for sustainable development of underdeveloped countries.
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