BE Civil Engineering (IOE, TU) Engineering Geology I (IOE, CE 552) Question Paper 2079 Nepal

Definition of a mineral (2 marks). A mineral is a naturally occurring, inorganic, homogeneous solid with a definite (but not fixed) chemical composition and an ordered internal atomic (crystalline) structure.

The five criteria, and why each is required:

Physical properties for identification (4 marks).

• Lustre: appearance of reflected light — metallic (galena, pyrite) vs non-metallic (vitreous, pearly, resinous, earthy).
• Hardness: resistance to scratching, measured on Mohs scale 1–10.
• Cleavage: tendency to break along planes of weak bonding, giving smooth surfaces; described by number and angle of planes (e.g. mica = 1 perfect; feldspar = 2 at ~90°; calcite = 3 rhombohedral).
• Fracture: breakage NOT along cleavage planes — conchoidal (quartz), uneven, hackly.
• Colour: often diagnostic but unreliable for many silicates (e.g. quartz varies widely).
• Streak: colour of the powdered mineral on an unglazed porcelain plate — more reliable than colour (haematite = red-brown streak even when grey).
• Specific gravity: ratio of mineral mass to mass of equal volume of water; e.g. galena ≈ 7.5, quartz ≈ 2.65.

Mohs scale and the unknown (2 marks). Mohs scale is an ordinal (relative) scale of ten reference minerals:

1 Talc   2 Gypsum  3 Calcite  4 Fluorite  5 Apatite
6 Orthoclase  7 Quartz  8 Topaz  9 Corundum  10 Diamond

A mineral of higher number scratches all lower ones. Relative hardness is found by testing which reference minerals the unknown scratches and which scratch it.

The unknown scratches fluorite (4) → unknown H > 4, and is scratched by orthoclase (6) → unknown H < 6. Therefore $4 < H < 6$, i.e. H = 5 → apatite.

Quartz vs mica silicate structure (2 marks).

• Quartz is a framework (tectosilicate): every $SiO_4^{4-}$ tetrahedron shares all four oxygen corners, giving a strong three-dimensional network with bonds equally strong in all directions. Consequently quartz has no cleavage and breaks by conchoidal fracture.
• Mica (e.g. muscovite, biotite) is a sheet (phyllosilicate): tetrahedra share three of four oxygens to form continuous 2-D sheets, weakly bonded to adjacent sheets by interlayer cations (K⁺). The weak inter-sheet bonds give mica one perfect basal cleavage, so it splits into thin elastic flakes.

Thus crystal structure directly controls the mechanical breakage behaviour observed in hand specimen.

Physical vs chemical weathering (3 marks). Physical (mechanical) weathering breaks rock into smaller fragments without changing its chemical composition. Processes:

1. Frost wedging (freeze–thaw of water in joints).
2. Thermal expansion/exfoliation (insolation, sheeting due to unloading).
3. Salt crystallisation and biological/root wedging.

Chemical weathering decomposes minerals by altering their composition. Processes:

1. Hydrolysis (feldspar → clay).
2. Oxidation (Fe-minerals → iron oxides/hydroxides).
3. Carbonation/solution (calcite dissolved by carbonic acid) and hydration.

Feldspar → clay reaction (2 marks). Hydrolysis of potassium feldspar (orthoclase) by carbonic acid:

$$2KAlSi_3O_8 + 2H_2CO_3 + 9H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 4H_4SiO_4 + 2K^+ + 2HCO_3^-$$

Products: kaolinite (clay), dissolved silica, and potassium + bicarbonate ions carried away in solution.

Climatic control (2 marks).

• Hot, humid climates → chemical weathering dominates (abundant water + high temperature accelerate reactions → deep lateritic/residual soils).
• Cold or arid climates → physical weathering dominates (frost action in alpine zones; thermal/insolation in deserts) with little chemical alteration.
• In Nepal: high Himalayan zone → frost-dominated mechanical weathering; mid-hills with monsoon → intense chemical weathering and deep residual soils.

Engineering significance of deep weathering profile (1 mark). Deep residual soils in the mid-hills mean foundations may rest on weak, variable, partly-weathered material; reduced bearing capacity, high susceptibility to landslides on monsoon-saturated slopes, difficulty locating sound bedrock, and the need for deeper foundations / slope stabilisation.

Surface-area calculation (2 marks). Original cube, side $L_1 = 0.50\ \text{m}$:

$$A_1 = 6 L_1^2 = 6 \times (0.50)^2 = 6 \times 0.25 = 1.5\ \text{m}^2$$

Number of small cubes (side $0.05\ \text{m}$) from the big cube:

$$N = \left(\frac{0.50}{0.05}\right)^3 = 10^3 = 1000$$

Surface area of one small cube:

$$a = 6 \times (0.05)^2 = 6 \times 0.0025 = 0.015\ \text{m}^2$$

Total new surface area:

$$A_2 = N \times a = 1000 \times 0.015 = 15\ \text{m}^2$$

Increase:

$$\Delta A = A_2 - A_1 = 15 - 1.5 = \mathbf{13.5\ m^2}\ \text{(a 10-fold increase).}$$

Significance: mechanical fragmentation hugely increases the surface area exposed to air and water, dramatically accelerating subsequent chemical weathering. Physical and chemical weathering therefore act synergistically.

Strike and dip (2 marks).

• Strike: the compass direction (bearing) of the horizontal line formed by the intersection of an inclined bed with a horizontal plane.
• Dip: the angle of maximum inclination of the bed measured from horizontal, in a vertical plane perpendicular to strike (true dip). Strike and the true-dip direction are always at 90°.
• Rule of V's: where an inclined bed crosses a valley, its outcrop trace forms a V; the V points (closes) in the direction of dip for beds dipping upstream, and the relationship of the V to topographic contours indicates dip direction and approximate amount (a vertical bed shows no V; a bed dipping downstream more steeply than the valley floor V's downstream).

Classification of folds (3 marks). (i) By attitude of the axial plane:

• Symmetrical (upright): axial plane vertical, limbs dip equally in opposite directions.
• Asymmetrical: axial plane inclined, limbs dip unequally.
• Overturned: axial plane strongly inclined, both limbs dip in the same direction (one limb inverted).
• Recumbent: axial plane horizontal.

(ii) By limb dip / form:

• Anticline: up-arched, oldest beds in core.
• Syncline: down-warped, youngest beds in core.
• Isoclinal: limbs parallel (equal dip same direction).
• Monocline: local step-like flexure in otherwise flat beds.

Sketch (text): Anticline ∩ (limbs diverge downward), Syncline ∪ (limbs converge downward); axial plane bisects the fold.

Classification of faults (2 marks).

• Normal (gravity) fault: hanging wall moves DOWN relative to footwall; due to tension/extension.
• Reverse fault: hanging wall moves UP relative to footwall; due to compression (a low-angle reverse fault, <45°, is a thrust).
• Strike-slip (transcurrent) fault: horizontal movement parallel to strike (dextral/sinistral).
• Oblique-slip fault: combination of dip-slip and strike-slip.

Apparent dip calculation (2 marks). Formula relating apparent dip $\delta$, true dip $\theta$, and angle $\beta$ between the section line and the strike:

$$\tan\delta = \tan\theta \cdot \sin\beta$$

Given $\theta = 35^{\circ}$, $\beta = 40^{\circ}$:

$$\tan\delta = \tan 35^{\circ} \times \sin 40^{\circ} = 0.7002 \times 0.6428 = 0.4501$$ $$\delta = \tan^{-1}(0.4501) = \mathbf{24.2^{\circ}}$$

The apparent dip (24.2°) is less than the true dip (35°), as expected for any section not perpendicular to strike.

Engineering consequences of unfavourable dip (1 mark).

1. Daylighting beds dipping out of (towards) the cut face promote planar sliding/rock falls along bedding.
2. Adverse dip increases excavation support cost and requires flatter cut slopes, rock bolting, or benching for stability.

Plate tectonics theory (2 marks). The Earth's rigid outer shell (the lithosphere) is broken into about a dozen major plates that move over the weaker, ductile asthenosphere. Plate motion is driven by mantle convection, ridge-push (at spreading centres) and slab-pull (at subduction zones). New lithosphere is created at mid-ocean ridges and consumed at subduction zones, conserving surface area. It unifies sea-floor spreading, continental drift and palaeomagnetic evidence.

Three plate boundaries (3 marks).

Origin of the Himalaya & Nepal seismicity (2 marks). The Himalaya is a continent–continent collision orogen. The Indian Plate, after closure of the Tethys Ocean, collided with the Eurasian Plate (~50 Ma). Continental crust is buoyant and resists subduction, so the collision produced enormous crustal shortening, thickening and uplift, forming the Himalaya. The major thrusts — MCT (Main Central Thrust), MBT (Main Boundary Thrust) and MFT (Main Frontal Thrust) — sole into the Main Himalayan Thrust (MHT) at depth. Ongoing convergence locks the MHT, accumulating strain that is periodically released as great earthquakes (e.g. 1934 Bihar–Nepal, 2015 Gorkha), making Nepal one of the most seismically active regions on Earth.

Slip-deficit estimate (1 mark). Shortening rate accommodated on the MHT (the slip that must eventually be released) $= 20\ \text{mm/year}$. Over a seismic gap of $t = 250\ \text{years}$:

$$D = 20\ \text{mm/yr} \times 250\ \text{yr} = 5000\ \text{mm} = \mathbf{5.0\ m}$$

Hazard implication: a locked segment storing ~5 m of slip deficit could rupture in a single great (M ≈ 8+) earthquake, since coseismic slip of several metres corresponds to such magnitudes. This quantifies the well-documented Himalayan seismic-gap hazard and underlines the need for earthquake-resistant design in Nepal.

Geomorphology (1 mark). Geomorphology is the scientific study of landforms, their origin, evolution, form and the surface processes (weathering, erosion, transport, deposition) and tectonics that shape the Earth's surface.

Geomorphic cycle of erosion (3 marks). Proposed by W. M. Davis: an uplifted landmass is progressively worn down through stages toward a low, flat peneplain.

• Youth: steep gradients, rapid downcutting (vertical erosion dominant), V-shaped valleys, waterfalls and rapids, few tributaries, narrow valley floors.
• Maturity: gradient reduced, lateral erosion increases, widest variety of landforms; well-integrated drainage, meanders begin, valley widens, relief at maximum then declines.
• Old age: very low gradient, broad flat floodplains, large meanders, ox-bow lakes, levees; deposition dominant, landscape reduced toward a peneplain with residual monadnocks.

Physiographic divisions of Nepal (south → north) (3 marks).

1. Terai (Gangetic Plain): flat alluvial plain, <300 m; recent sediments, fertile.
2. Siwaliks (Churia Hills): youngest molasse sediments (sandstone, mudstone, conglomerate); fragile, landslide-prone; bounded by MFT (south) and MBT (north).
3. Lesser Himalaya (Mahabharat Range): low-grade metamorphic and sedimentary rocks; between MBT and MCT.
4. Higher (Greater) Himalaya: high-grade gneiss, schist, migmatite above the MCT; highest peaks incl. Everest.
5. Tibetan-Tethys (Inner) Himalaya: fossiliferous Tethyan sedimentary sequence north of the Greater Himalaya (e.g. Thakkhola, Manang).

Gradient calculation (1 mark). Vertical drop $h = 1500\ \text{m}$; channel length $L = 60\ \text{km} = 60000\ \text{m}$. Gradient as a ratio:

$$S = \frac{h}{L} = \frac{1500}{60000} = \frac{1}{40} = \mathbf{1:40}$$

Gradient in m/km:

$$S = \frac{1500\ \text{m}}{60\ \text{km}} = \mathbf{25\ m/km}$$

Interpretation: 25 m/km (1:40) is a steep gradient, characteristic of a youthful, high-energy mountain river with strong vertical erosion (downcutting), high transport capacity and V-shaped valleys — typical of Himalayan headwater streams in Nepal.

Rock cycle (3 marks). The rock cycle describes the continuous transformation among the three rock families driven by internal (tectonic, magmatic) and external (weathering, erosion) processes.

Text flow:

   Magma --(crystallisation/cooling)--> IGNEOUS ROCK
      ^                                      |
      | (melting)                            | (weathering, erosion, transport)
      |                                      v
  METAMORPHIC ROCK <--(heat & pressure)-- SEDIMENT
      ^                                      |
      |                                      | (deposition, compaction, cementation)
      +--(heat & pressure)-- SEDIMENTARY ROCK

• Magma cools → igneous rock.
• Any rock weathers/erodes → sediment → compaction & cementation → sedimentary rock.
• Any rock subjected to heat & pressure → metamorphic rock.
• Deep burial/heat → melting → back to magma. Any rock can be converted to any other.

Classification of igneous rocks (2 marks). By mode of occurrence:

• Extrusive (volcanic): cooled at surface, fine-grained/glassy (rapid cooling) — e.g. basalt, rhyolite.
• Intrusive (hypabyssal/shallow): medium-grained, in dykes/sills — e.g. dolerite.
• Plutonic (deep intrusive): slow cooling, coarse-grained — e.g. granite.

By silica (SiO₂) content:

• Acidic (>66% SiO₂, light) — granite, rhyolite.
• Intermediate (52–66%) — diorite, andesite.
• Basic (45–52%, dark) — gabbro, basalt.
• Ultrabasic (<45%) — peridotite, dunite.

Examples (1 mark): Extrusive = basalt; Plutonic = granite.

Definition and joint vs fault (2 marks). A joint is a fracture (crack) in rock along which there has been no appreciable displacement parallel to the fracture surface; joints usually occur in parallel sets.

Engineering importance of joints (2 marks).

• Control strength and deformability of the rock mass (intact rock is far stronger than jointed mass).
• Provide pathways for groundwater seepage (permeability) — important for dams, tunnels.
• Govern slope and tunnel stability: control wedge/planar failures and overbreak.
• Determine block size and ease of excavation/rippability and rock support requirements.
• Influence weathering depth (water ingress along joints).

Volumetric joint count (2 marks). $J_v$ is the total number of joints per cubic metre, summed over all sets:

$$J_v = \frac{1}{S_1} + \frac{1}{S_2} + \frac{1}{S_3}$$

With $S_1 = 0.40\ \text{m}$, $S_2 = 0.50\ \text{m}$, $S_3 = 1.0\ \text{m}$:

$$J_v = \frac{1}{0.40} + \frac{1}{0.50} + \frac{1}{1.0} = 2.5 + 2.0 + 1.0 = \mathbf{5.5\ joints/m^3}$$

Comment: On the standard ISRM block-size scale, $J_v$ between 3 and 10 indicates large blocks (low joint density). The rock mass is moderately blocky, generally favourable for stability, requiring relatively light support.

Metamorphism and agents (1.5 marks). Metamorphism is the solid-state transformation of a pre-existing rock (protolith) into a new rock with different texture and/or mineralogy in response to changed physical/chemical conditions, without complete melting. Principal agents: heat (temperature), pressure (lithostatic and directed/stress), and chemically active fluids.

Contact vs regional metamorphism (1.5 marks).

Foliation and lineation (1 mark).

• Foliation: planar, parallel arrangement of platy minerals (e.g. mica) or compositional banding produced by directed pressure — slaty cleavage, schistosity, gneissic banding.
• Lineation: a linear (1-D) fabric — parallel alignment of elongate/needle minerals or fold-axis-related lines within the rock.

Metamorphic equivalents (1 mark).

Karst topography (2 marks). Karst is a distinctive terrain produced by the dissolution (carbonation/solution) of soluble rocks — mainly limestone (and dolomite, gypsum) — by slightly acidic groundwater:

$$CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2\ \text{(soluble)}$$

Conditions required:

1. Soluble rock (thick, pure limestone) at or near the surface.
2. Well-developed joints/bedding for water to penetrate.
3. Humid climate with adequate rainfall and vegetation (source of $CO_2$).
4. Water table below the surface allowing vertical and lateral flow / a hydraulic gradient.

Three karst landforms (2 marks).

• Sinkholes (dolines): closed surface depressions where rock has dissolved or collapsed; surface drainage disappears underground.
• Caves and caverns: underground solution channels and chambers, often with stalactites/stalagmites (dripstone).
• Disappearing (sinking) streams / springs: surface streams sink underground and re-emerge as large springs; also limestone pavements and underground drainage networks.

Dam-site problem on karstic limestone (1 mark). Karstic limestone is riddled with solution cavities and conduits, giving very high secondary permeability. A reservoir would suffer severe water leakage through the foundation/abutments, potential piping, sinkhole collapse and loss of storage — requiring extensive (and costly) grouting/cut-off treatment or site rejection.

Formation of sedimentary rocks (1.5 marks). Sediments are produced by weathering and erosion of pre-existing rocks, transported by water/wind/ice, deposited in basins, and converted to rock by lithification = compaction (removal of pore water under burial) + cementation (precipitation of silica, calcite, iron oxide between grains). Chemical/biological precipitation also produces sediment.

Classification by origin (2 marks).

Superposition and unconformity (1.5 marks).

• Law of superposition: in an undisturbed sequence of sedimentary strata, each bed is younger than the one beneath it and older than the one above — gives relative age.
• Unconformity: a buried erosion/non-deposition surface representing a gap (hiatus) in the geological record. Three main types:

1. Angular unconformity — tilted/folded older beds overlain by horizontal younger beds.
2. Disconformity — erosion surface between parallel strata.
3. Nonconformity — sedimentary rocks overlying eroded igneous/metamorphic basement.

Siwalik Group of Nepal (1 mark). The Siwaliks (Churia) are Mio-Pliocene to Pleistocene molasse sediments — alternating sandstone, mudstone and conglomerate — deposited by braided/meandering rivers in the foreland basin as the rising Himalaya shed its erosional debris southward. They demonstrate the clastic sedimentary cycle (uplift → weathering → fluvial transport → deposition → lithification) and are notoriously weak and landslide-prone.

Tectonic/geological zones of Nepal and boundary thrusts (3 marks). From south to north, separated by major north-dipping thrusts:

  Indo-Gangetic Plain (Terai)
  ---- MFT (Main Frontal Thrust) ----
  Sub-Himalaya (Siwalik / Churia)
  ---- MBT (Main Boundary Thrust) ----
  Lesser Himalaya (Mahabharat)
  ---- MCT (Main Central Thrust) ----
  Higher (Greater) Himalaya
  ---- STDS (South Tibetan Detachment System) ----
  Tibetan-Tethys Himalaya

• Terai/Indo-Gangetic plain: Quaternary alluvium.
• Sub-Himalaya (Siwaliks): young, weak molasse; bounded by MFT and MBT.
• Lesser Himalaya: low-grade metasediments; MBT to MCT.
• Higher Himalaya: high-grade crystallines (gneiss, schist) above the MCT.
• Tethys Himalaya: fossiliferous sediments above the STDS.

Significance of the MBT zone (1.5 marks). The MBT is a major active thrust separating weak Siwaliks from Lesser Himalayan rocks. The zone is characterised by intense fracturing, shearing, crushed/sheared rock, fault gouge, high seismicity and abundant landslides. Engineering significance: poor and highly variable rock quality, instability of cut slopes and tunnel faces, high water inflow along the fracture zone, and elevated seismic hazard — all demanding special design and support.

Why geological investigation is essential (1.5 marks). Nepal's terrain is young, tectonically active, seismically hazardous, with fragile lithologies and steep, landslide-prone slopes. Prior geological investigation is essential to:

• identify active faults, weak/sheared zones and groundwater conditions;
• assess slope stability, bearing capacity and excavability;
• choose safe alignments/sites and appropriate support, reducing the risk of failure, cost overruns and loss of life. Skipping investigation in such terrain greatly increases the chance of dam leakage, tunnel collapse and highway landslides.