Engineering Geology in Himalayas

A discontinuity is a plane of break or weakness in a rock mass that has little or no tensile strength, such as bedding, joint, foliation, fault, thrust or shear zone.

In the Nepal Himalaya, the major discontinuities are large-scale tectonic thrusts that separate the five tectonic zones from south to north.

Major discontinuity systems

Other local discontinuities: bedding planes, foliation, joints and faults within each zone.

Engineering significance

• Thrust zones contain crushed, sheared and weak rock with low strength; they cause tunnel collapse and squeezing.
• They are conduits for groundwater, giving large inflows in tunnels and seepage at dams.
• They control landslides and slope failures; many big slides lie along MBT and MCT.
• Active thrusts (MFT, MBT) are sources of earthquakes; structures must be kept away or designed for fault movement.
• Discontinuity orientation relative to a slope or tunnel axis decides wedge and planar failures, overbreak and support requirement.
• Detailed mapping of discontinuities is required in site selection of dams, tunnels and roads.

The Lesser Himalaya lies between the MBT in the south and the MCT in the north. It is made of phyllite, slate, schist, quartzite, limestone, dolomite and some granite, and is intensely folded and faulted.

Major problems and mitigation

General mitigation: detailed geological mapping, good surface and sub-surface drainage, avoiding deep cuttings, slope monitoring and early warning.

Lesser Himalaya

The zone has weak, folded and sheared rocks (phyllite, schist, slate) and steep terrain.

• Problems: landslides, rock fall, weak foundation rock, tunnel squeezing, karst in limestone, erosion, MBT/MCT earthquakes.
• Mitigation: gentle slopes and retaining structures, surface drainage, rock bolts and shotcrete, bio-engineering, careful alignment, grouting in karst areas.

Terai (Indo-Gangetic plain)

The Terai consists of thick, unconsolidated Quaternary alluvium (gravel, sand, silt, clay) with a gentle slope.

• Problems:
  1. Frequent floods and river bank cutting; shifting of river channels.
  2. Sediment deposition and aggradation of river beds.
  3. High water table and waterlogging.
  4. Low bearing capacity and large settlement of soft clay and silt.
  5. Liquefaction of loose saturated sand during earthquakes.
  6. Scarcity of rock aggregate (only boulders and gravel from rivers).
  7. Arsenic contamination of shallow groundwater.
• Mitigation: embankments and spurs, river training, raised foundations or pile/raft foundations, soil compaction and ground improvement, drainage, deep tube wells for safe water, use of river gravel with proper testing.

A discontinuity is any plane or surface of separation in a rock mass across which the rock has negligible tensile strength and which interrupts the physical continuity of the rock. It is a general term covering bedding planes, joints, foliation, cleavage, faults, shear zones and thrusts.

• Discontinuities make a rock mass weaker, more permeable and more deformable than intact rock.
• Their orientation, spacing, persistence, roughness, aperture and filling control slope stability, tunnel behaviour, seepage and foundation safety.
• In Nepal Himalaya, the major discontinuities are MFT, MBT, MCT and STDS.

The Siwalik (Churia) zone lies between the MFT and the MBT. It is made of young, poorly consolidated Neogene-Quaternary molasse deposits: sandstone, siltstone, mudstone, clay and conglomerate.

Engineering geological problems

1. Landslides and slope failure: soft, weakly cemented rocks with heavy rain, steep slopes and tectonic dip.
2. Gully erosion and badland topography: easy erosion of friable sandstone and mudstone.
3. Debris flow and flash floods: steep torrents carry large sediment loads to the Terai.
4. Weak and swelling mudstone: low strength, slakes on wetting; difficult for tunnel and foundation.
5. Active tectonics: the MFT and MBT are active, giving earthquakes and fault scarps.
6. Seepage and groundwater: coarse sandstone and conglomerate are permeable; low water retention gives dry springs.
7. Poor construction material: rock is weak for aggregate, and soft boulders.
8. Roads: frequent cut-slope failures and road washouts.

Mitigation

Gentle slopes, retaining walls, drainage, afforestation in the Churia range, check dams, river training and proper alignment away from active faults.

Siwalik zone

Composed of weakly cemented sandstone, mudstone and conglomerate.

• Landslides, gully erosion and debris flow due to weak rock and steep slope.
• Swelling and slaking of mudstone; poor aggregate.
• Active tectonics along MFT and MBT; earthquakes.
• Heavy sediment load carried to the plains and floods.

Higher Himalaya zone

Composed of high-grade metamorphic rocks: gneiss, schist, migmatite, quartzite and granite, with very high relief and glaciers.

• Rock fall, rock avalanche and large rockslides on steep, jointed slopes.
• Snow avalanche, glacier hazards, GLOF and permafrost thaw.
• Moraine and glacial deposits unstable; debris flow.
• Tunnel problems: high overburden gives rock burst and squeezing in shear zones; high in-situ stress.
• Earthquakes along the MCT and active faults.
• Frost action and very cold climate affect construction.
• Access and construction are difficult at high altitude.

Higher Himalaya

Made of high-grade metamorphic rocks (gneiss, schist, migmatite, quartzite) and granite, with very steep, glaciated terrain.

• Rock fall, rock avalanches and landslides on steep jointed slopes.
• Snow avalanches, glacier retreat and GLOF; unstable moraines.
• Rock burst, squeezing and high stress in deep tunnels.
• Seismic hazard along the MCT.

Tethys Himalaya

Made of fossiliferous Palaeozoic-Mesozoic sedimentary rocks (limestone, shale, sandstone) in a cold, arid, high-altitude setting.

• Landslides in weak shale and slate; rock fall in limestone.
• Karst and cavities in limestone.
• Frost shattering, permafrost and freeze-thaw damage.
• Glacier, GLOF and snow avalanche hazards.
• Intense folding and faulting; STDS zone is weak.
• Wind erosion and debris deposition; poor accessibility.

The Higher Himalaya contains the highest peaks and is made of gneiss, schist, quartzite and granite with high relief, glaciers and snow. Its major geological hazards are:

1. Glacier hazards and GLOF: melting glaciers fill moraine-dammed lakes that burst and cause floods.
2. Snow avalanches on steep slopes, which destroy roads and settlements.
3. Rock fall, rock avalanche and landslides from steep, jointed, frost-shattered rock slopes.
4. Debris flows from moraines and glacial deposits during heavy rain or melting.
5. Earthquakes due to active thrusts such as the MCT and related faults.
6. Frost action and permafrost thaw which weaken ground.
7. Rock burst and squeezing in deep tunnels because of high stress.

The Himalaya is vulnerable to geological hazards because:

• It is a young, tectonically active mountain formed by the continuing collision of the Indian and Eurasian plates; India moves north by about 4-5 cm per year.
• Rocks are highly folded, faulted and fractured, with active thrusts such as MFT, MBT and MCT.
• Very steep slopes and high relief give high gravitational energy.
• Intense monsoon rainfall saturates slopes and increases erosion.
• Many rocks are weak (phyllite, schist, Siwalik mudstone).
• Frequent earthquakes, glacier melting and deforestation add to the instability.
• Rapid, unplanned road construction and human settlement increase the risk.

Geological problems of the Himalaya arise from its young tectonics, weak and fractured rocks, steep slopes and monsoon rains. Mitigation measures are:

1. Detailed geological investigation before construction: mapping, geophysical survey, borehole and hazard zoning; avoiding active faults, unstable slopes and landslide-prone ground.
2. Slope stabilisation: reduce slope angle, benching, remove loose material, retaining and breast walls, gabions, rock bolts, anchors, wire mesh and shotcrete.
3. Drainage control: surface drains, catch drains, sub-surface drains and horizontal drains to lower pore pressure.
4. Bio-engineering: vegetation, grass and tree plantation to bind soil and reduce erosion.
5. River and flood control: embankments, spurs, check dams, river training, and monitoring of glacial lakes (lowering lake level, early warning systems).
6. Earthquake resistant design following the Nepal Building Code and keeping structures away from active faults.
7. Tunnel support: systematic classification and flexible support in squeezing and weak zones.
8. Monitoring and early warning, land-use planning and community awareness.
9. Afforestation and control of unplanned road cutting.