BE Civil Engineering (IOE, TU) Engineering Geology I (IOE, CE 552) Question Paper 2076 Nepal

Definition of a mineral

A mineral is a naturally occurring, inorganic, homogeneous solid with a definite (but not necessarily fixed) chemical composition and an ordered internal atomic (crystalline) structure. Five criteria: (i) naturally occurring, (ii) inorganic, (iii) solid, (iv) definite chemical composition, (v) ordered crystalline structure.

Principal physical properties for identification

1. Crystal form / habit – external shape (e.g. cubic, prismatic, fibrous).
2. Cleavage & fracture – tendency to break along planes of weakness (number and angle of cleavage directions) vs. irregular breakage (conchoidal fracture).
3. Hardness – resistance to scratching, measured on Mohs scale (1 talc → 10 diamond).
4. Lustre – appearance of reflected light (metallic, vitreous, pearly, resinous, dull).
5. Colour & streak – body colour and colour of powder on a streak plate.
6. Specific gravity – relative density.
7. Diaphaneity – transparent, translucent, opaque.
8. Special properties – reaction with dilute HCl, magnetism, taste, double refraction.

Distinguishing the three minerals

Key diagnostic tests: calcite effervesces with cold dilute HCl and is soft (H=3); quartz is hard (H=7) with no cleavage and conchoidal fracture; orthoclase has two cleavages at ~90° and intermediate hardness (H=6).

Importance of silicates

Silicates make up about 90–95% of the Earth's crust and are the dominant rock-forming minerals. Their stability ranges (Bowen's reaction series) control igneous crystallisation, weathering behaviour and the engineering properties of rocks.

Structural classification of silicates (based on linkage of the SiO₄ tetrahedron):

• Nesosilicates (isolated tetrahedra) – e.g. olivine, garnet.
• Sorosilicates (paired tetrahedra) – e.g. epidote.
• Cyclosilicates (rings) – e.g. beryl, tourmaline.
• Inosilicates (chains) – single chain: pyroxenes; double chain: amphiboles.
• Phyllosilicates (sheets) – e.g. micas, clays.
• Tectosilicates (3-D frameworks) – e.g. quartz, feldspars.

As sharing of oxygen atoms increases from nesosilicates to tectosilicates, the Si:O ratio rises and resistance to weathering generally increases.

Definitions

• Strike: the trend (compass bearing) of the horizontal line formed by the intersection of an inclined plane with a horizontal plane.
• True dip: the maximum angle of inclination of the plane measured in a vertical plane perpendicular to the strike.
• Dip direction: the horizontal direction of the steepest descent, at right angles to strike.
• Apparent dip: the inclination of the plane measured in any vertical section that is not perpendicular to the strike. It is always less than or equal to the true dip.

Governing relation

$$\tan(\delta_a) = \tan(\delta)\,\sin(\beta)$$

where $\delta$ = true dip, $\delta_a$ = apparent dip, and $\beta$ = angle between the section line (trend) and the strike.

Given: $\delta = 40°$, $\beta = 45°$.

$$\tan(\delta_a) = \tan(40°)\times\sin(45°)$$ $$= 0.83910 \times 0.70711 = 0.59333$$ $$\delta_a = \arctan(0.59333) = 30.68°$$

Apparent dip along the $45°$ section = 30.7° (approximately 30°41′).

(a) Section along the strike ($\beta = 0°$):

$$\tan(\delta_a) = \tan(40°)\times\sin(0°) = 0 \Rightarrow \delta_a = 0°$$

The bed appears horizontal in a strike-parallel section.

(b) Section along the true-dip direction ($\beta = 90°$):

$$\tan(\delta_a) = \tan(40°)\times\sin(90°) = \tan(40°) \Rightarrow \delta_a = 40°$$

The apparent dip equals the true dip = 40°, as expected.

This confirms apparent dip ranges from $0°$ (along strike) up to the true dip $40°$ (perpendicular to strike).

Plate tectonics

The theory states that the Earth's rigid lithosphere is broken into about a dozen major plates that move (a few cm/yr) over the weaker, ductile asthenosphere. The driving mechanism is mantle convection, aided by ridge-push and slab-pull. Plate interactions occur at boundaries and produce earthquakes, volcanism and mountain building. The theory unifies continental drift (Wegener) and sea-floor spreading (Hess).

Three types of plate boundaries

1. Divergent (constructive) – plates move apart; new lithosphere forms. Example: Mid-Atlantic Ridge; East African Rift.
2. Convergent (destructive) – plates move together. Sub-types: ocean–ocean (island arcs, e.g. Japan), ocean–continent (Andes-type subduction), continent–continent (collision, e.g. Himalaya).
3. Transform (conservative) – plates slide past each other; lithosphere neither created nor destroyed. Example: San Andreas Fault.

Tectonic evolution of the Himalaya

• About 180 Ma ago India separated from Gondwanaland and drifted northward across the Tethys Ocean.
• Tethyan oceanic lithosphere was subducted beneath the Eurasian plate; the ocean progressively closed.
• Around 55–50 Ma, India collided with Eurasia (continent–continent collision). Because continental crust is buoyant, instead of subducting it caused crustal shortening, stacking and uplift.
• Continued northward push (~4–5 cm/yr) produced the Himalaya through a series of south-verging thrust faults, dividing Nepal into tectonic zones bounded by:
  • MFT (Main Frontal Thrust) – southernmost, active boundary with the Terai.
  • MBT (Main Boundary Thrust) – separates Siwaliks from Lesser Himalaya.
  • MCT (Main Central Thrust) – separates Lesser from Higher Himalaya.
  • STDS/SHFZ – South Tibetan Detachment in the north.
• The Indian plate underthrusts Nepal along the gently dipping Main Himalayan Thrust (MHT).

Relation to seismic hazard of Nepal

Nepal lies on this active collision zone where strain accumulates on the locked MHT and is released as great earthquakes. The 1934 Bihar–Nepal (M~8.0) and the 2015 Gorkha (Mw 7.8) earthquakes both ruptured the MHT. Implications:

• Nepal is in a high seismic-hazard zone; recurrence of M≥8 events is a real threat.
• Engineering structures must follow seismic-resistant design codes (NBC).
• Site selection must consider faults, ground amplification in valley sediments (e.g. Kathmandu basin) and slope stability under seismic loading.

Physical (mechanical) weathering – disintegration of rock into smaller fragments without change in chemical composition. Processes:

1. Frost wedging / freeze–thaw – water freezes in cracks, expands ~9%, prying rock apart.
2. Thermal expansion–contraction (insolation) – diurnal heating/cooling causes differential expansion and exfoliation.
3. Unloading / pressure release – removal of overburden causes sheeting joints.
4. Biological / root wedging and salt crystallisation.

Chemical weathering – decomposition involving chemical change of minerals to new, more stable products. Processes:

1. Hydrolysis – feldspars react with water/CO₂ to form clay minerals (e.g. kaolinite).
2. Oxidation – Fe-bearing minerals react with oxygen (rust-coloured limonite/hematite).
3. Carbonation / solution – CO₂ + water forms carbonic acid that dissolves limestone (karst).
4. Hydration – water absorbed into mineral structure (anhydrite → gypsum).

Numerical solution

Core recovery:

$$\text{Core recovery} = \frac{\text{total recovered core}}{\text{total core run}}\times 100 = \frac{170}{200}\times 100 = \mathbf{85\%}$$

RQD uses only intact pieces ≥ 10 cm long.

Pieces: 25, 8, 14, 11, 6, 18, 22, 9, 12, 35 (cm). Pieces ≥ 10 cm: 25, 14, 11, 18, 22, 12, 35 (the 8, 6, 9 cm pieces are excluded).

Sum of pieces ≥ 10 cm:

$$25+14+11+18+22+12+35 = 137\ \text{cm}$$ $$\text{RQD} = \frac{\sum(\text{intact pieces}\ge 10\text{cm})}{\text{total core run length}}\times 100 = \frac{137}{200}\times 100 = \mathbf{68.5\%}$$

Classification (Deere's RQD chart):

RQD = 68.5% falls in the 50–75% band, so the rock-mass quality is FAIR.

Answers: Core recovery = 85%, RQD = 68.5% → Fair rock mass.

Geomorphology is the scientific study of landforms, their origin, evolution, form and the processes (endogenic and exogenic) that shape the Earth's surface.

Fluvial (river) cycle of erosion (Davisian concept) – an uplifted land surface is progressively worn down by a river toward base level, passing through three stages:

1. Youth stage – steep gradient, rapid vertical (downcutting) erosion. Narrow V-shaped valleys, waterfalls, rapids, gorges, potholes; few tributaries.
2. Mature stage – gradient lower, lateral erosion dominates. Valley widens, meanders develop, floodplains begin to form; well-integrated drainage.
3. Old stage – very gentle gradient near base level; deposition dominates. Broad floodplains, large meanders, ox-bow lakes, natural levees, deltas.

Characteristic landforms

Erosional: V-shaped valley, gorge/canyon, waterfall, rapids, potholes, river terraces, incised meanders.

Depositional: alluvial fan (mountain front, common in Nepal's Bhabar zone), point bar, natural levee, floodplain, ox-bow lake, braided channel deposits, delta.

Engineering significance for siting in Nepal

• Bridges: Site at stable, narrow, straight reaches with firm rock; avoid meander bends (bank erosion) and zones of active scour. Foundation depth must exceed maximum scour depth.
• Dams: Need a narrow gorge (youthful reach) for an economical wall and a watertight, competent rock foundation; assess sediment load (reservoir siltation is severe on Himalayan rivers) and reservoir-rim stability.
• Highways: Avoid actively eroding outer banks and unstable terrace edges; provide adequate cross-drainage, culverts and bank protection (gabions, spurs) in the Terai and valley reaches; beware of debris-flow alluvial fans.
• General: understanding aggradation/degradation, flooding and channel migration is essential because Himalayan rivers carry high sediment and have flashy, monsoon-driven discharge.

Fold: a wave-like bend or flexure in originally planar rock layers produced by compressive (ductile) deformation.

Parts of a fold

   limb        AXIAL PLANE        limb
      \           |              /
       \        crest           /
   _____\______ /\ ____________/______  <- a folded bed
              /    \
         hinge      hinge line (axis)

• Limbs: the two sides (flanks) of the fold.
• Hinge / axis: line of maximum curvature.
• Axial plane: surface containing the hinge lines, dividing the fold symmetrically.
• Crest & trough: highest and lowest points of the folded surface.
• Plunge: inclination of the fold axis from horizontal.

Classification

• Anticline: up-arched fold; oldest beds in the core; limbs dip away from the axis.
• Syncline: down-warped (trough) fold; youngest beds in the core; limbs dip toward the axis.
• Symmetrical: axial plane vertical; both limbs dip equally in opposite directions.
• Asymmetrical: axial plane inclined; limbs dip at different angles.
• Overturned: axial plane strongly inclined; both limbs dip in the same direction (one limb is inverted).
• Recumbent: axial plane nearly horizontal; fold is lying on its side.

Fault: a fracture or zone of fractures in rock along which there has been appreciable displacement (relative movement) of the two blocks parallel to the fracture surface.

Terms

• Fault plane: the surface along which displacement occurs.
• Hanging wall: the block lying above the inclined fault plane.
• Footwall: the block lying below the fault plane.
• Throw: the vertical component of the displacement (dip-slip).
• Heave: the horizontal component of the displacement measured perpendicular to strike.

   hanging wall
        \  fault plane
   ______\
   |throw \______
   |......|  footwall
    heave

Types of fault and stress regime

The Himalayan thrusts (MFT, MBT, MCT) are reverse/thrust faults formed by compression from the India–Eurasia collision.

Rock cycle

The rock cycle describes the continuous transformation of rock among the three major types through geological processes (melting, cooling, weathering, erosion, deposition, lithification, metamorphism, uplift).

            melting
   MAGMA  <----------  (any rock)
     |                      ^
 cooling/                   | melting
 crystallisation           |
     v          heat &      |
  IGNEOUS  --pressure--> METAMORPHIC
     |                      ^
 weathering,                | heat & pressure
 erosion,                   |
 transport,            SEDIMENTARY
 deposition  ----compaction & ----
            cementation (lithification)

Classification

1. Igneous rocks – formed by cooling and solidification of magma/lava.

   • Intrusive (plutonic): slow cooling, coarse-grained — e.g. granite.
   • Extrusive (volcanic): rapid cooling, fine-grained — e.g. basalt.

2. Sedimentary rocks – formed by weathering, transport, deposition and lithification of sediments (or chemical/organic precipitation).

   • Clastic: sandstone; Chemical/organic: limestone.

3. Metamorphic rocks – formed by transformation of pre-existing rocks under heat and/or pressure in the solid state.

   • From limestone → marble; from shale → slate (or gneiss from granite).

These transformations are reversible and continuous, illustrating that no rock is permanent.

Nepal is divided, from south to north, into five major tectonic/geological zones bounded by north-dipping thrust faults:

Major boundary thrusts (south → north):

• MFT – Main Frontal Thrust (Terai / Siwalik boundary; presently active).
• MBT – Main Boundary Thrust (Siwalik / Lesser Himalaya).
• MCT – Main Central Thrust (Lesser / Higher Himalaya).
• STDS – South Tibetan Detachment System (Higher Himalaya / Tethys; a normal-sense detachment).

This zonation reflects the southward propagation of thrusting during the ongoing India–Eurasia collision.

Joint: a fracture or crack in rock along which there has been no appreciable displacement (negligible movement) of the two sides. (Contrast: a fault shows appreciable relative displacement along the fracture.)

Classification by origin

• Tension joints – formed by tensile stress (e.g. due to folding, cooling).
• Shear joints – formed by shear stress; often conjugate sets.
• Cooling joints – contraction during cooling of lava (columnar joints in basalt).
• Sheeting (unloading) joints – pressure release / exfoliation.

Classification by geometry (relative to bedding/strike)

• Strike joints – parallel to the strike of beds.
• Dip joints – parallel to the dip direction.
• Oblique (diagonal) joints – at an angle to strike and dip.
• Bedding joints – parallel to bedding planes.

Numerical solution

Mean joint spacing for the set, measured along a scanline perpendicular to the set:

$$s = \frac{\text{scanline length}}{\text{number of joints}} = \frac{12.0\ \text{m}}{18} = 0.667\ \text{m}$$

Mean joint spacing ≈ 0.67 m (667 mm).

The joint frequency (number per metre) for this set, which is its contribution to the volumetric joint count $J_v$:

$$\lambda = \frac{1}{s} = \frac{18}{12.0} = 1.5\ \text{joints/m}$$

Contribution to $J_v$ = 1.5 joints/m.

Comment: A spacing of ~0.67 m falls in the 'moderately wide / widely spaced' range (ISRM: 0.2–0.6 m = moderate; 0.6–2 m = wide). A frequency of only 1.5 joints/m from this set indicates relatively low fracturing, favouring good rock-mass quality (higher RQD, higher strength and lower permeability). If additional sets were present, the total $J_v$ would rise and rock-mass quality would decrease.

(Answer any two; all three given here.)

(a) Glacial landforms

Glaciers erode and deposit material as they flow.

• Erosional landforms: cirque (armchair hollow), arête (sharp ridge), horn (pyramidal peak, e.g. Matterhorn-type), U-shaped valley, hanging valley, roche moutonnée, striations.
• Depositional landforms: moraines (lateral, medial, terminal, ground), drumlins (streamlined hills), eskers (sinuous ridges), outwash plain, erratics. In Nepal, glacial moraines dam glacial lakes that pose a GLOF (Glacial Lake Outburst Flood) hazard.

(b) Mass wasting / landslides

Mass wasting is the down-slope movement of rock, soil and debris under gravity. Types: falls, topples, slides (rotational/translational), flows (debris/earth flow), creep.

Causes:

• Increased shear stress: steepening of slopes, undercutting by rivers, loading, earthquakes.
• Reduced shear strength: water saturation (high pore pressure), weathering, weak bedding/joint planes dipping out of slope, deforestation.
• Triggers in Nepal: intense monsoon rainfall and seismic shaking (e.g. 2015 earthquake-induced landslides).

(c) Karst topography

Landscape developed by chemical solution (carbonation) of soluble rocks, mainly limestone/dolomite, by slightly acidic water. Features:

• Surface: sinkholes (dolines), karren/lapies, disappearing streams, dry valleys, poljes.
• Subsurface: caves, caverns with stalactites (ceiling) and stalagmites (floor), underground drainage.
• Engineering concern: ground subsidence, collapse, leakage from reservoirs founded on karstic limestone, and difficult foundation conditions.