BE Civil Engineering (IOE, TU) Engineering Geology I (IOE, CE 552) Question Paper 2078 Nepal

Definition of a mineral

A mineral is a naturally occurring, inorganic, homogeneous solid with a definite (but not always fixed) chemical composition and an ordered internal atomic (crystalline) arrangement. The five essential criteria are: naturally occurring, inorganic, solid, definite chemical composition, and ordered atomic structure.

Principal physical properties and their structural basis

Mohs scale of hardness (1 = softest to 10 = hardest)

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

Hardness is reliable because it is a direct expression of bond strength in the crystal lattice, is essentially independent of impurities and weathering, and is easily and reproducibly tested in the field (fingernail $\approx 2.5$, copper coin $\approx 3.5$, knife/steel $\approx 5.5$, glass $\approx 5.5$).

Silicate classification (basis = polymerisation of the $\text{SiO}_4$ tetrahedron)

The fundamental building block is the silica tetrahedron, one $\text{Si}^{4+}$ surrounded by four $\text{O}^{2-}$ (the $[\text{SiO}_4]^{4-}$ anion). Subclasses are defined by how many corner oxygens are shared between adjacent tetrahedra:

Isolated (nesosilicate)   : 0 oxygens shared   -> Olivine
Single chain (inosilicate): 2 oxygens shared   -> Pyroxene (augite)
Double chain (inosilicate): alternating 2 & 3  -> Amphibole (hornblende)
Sheet (phyllosilicate)    : 3 oxygens shared   -> Mica (biotite)
Framework (tectosilicate) : 4 oxygens shared   -> Quartz / Feldspar

Representative rock-forming silicates: Isolated -> Olivine; Single-chain -> Augite (pyroxene); Framework -> Quartz (or orthoclase feldspar).

Definition

A fold is a wave-like bend or flexure produced in originally planar rock layers by compressive (ductile) deformation of the crust.

Geometric elements

• Limbs: the two sides (flanks) of a fold sloping away from the hinge.
• Axial plane: the imaginary plane that bisects the interlimb angle, passing through successive hinge lines.
• Hinge line: the line of maximum curvature on a folded surface.
• Plunge: the angle the hinge line makes with the horizontal.
• Interlimb angle: the angle measured between the two limbs (tightness of the fold).

Classification

(i) By attitude of axial plane: upright, inclined, overturned, recumbent, vertical.

(ii) By interlimb angle:

Numerical determination of interlimb angle

With an E–W hinge, both limb dips are measured in the same N–S vertical cross-section, so the dips are coplanar and additive about the crest.

Each limb makes an angle with the horizontal equal to its dip. The interlimb angle $\theta$ measured through the fold (across the top, an anticline) is:

$$\theta = 180^\circ - (\text{dip}_N + \text{dip}_S) = 180^\circ - (40^\circ + 30^\circ) = 180^\circ - 70^\circ = \mathbf{110^\circ}$$

Since $120^\circ > 110^\circ > 70^\circ$, the fold is an open fold. Because one limb (40 deg) is steeper than the other (30 deg), the axial plane is slightly inclined (asymmetric fold).

        Axial plane (slightly inclined)
            |
    N  40\   |   /30  S
        \    |  /
         \   | /
          \  |/   interlimb angle = 110 deg
  ---------- crest (hinge, E-W) ----------

Engineering significance

• Tunnels: In an anticline the crown is in compression and self-supporting, but the limbs may shed wedges along bedding; a syncline concentrates groundwater inflow at the trough. Tunnel alignment perpendicular to the fold axis is generally more stable than parallel.
• Dam foundations: Steeply dipping limbs can create leakage paths and differential settlement; the axial-plane zone is intensely fractured and weak. Beds dipping upstream are favourable (resist sliding) whereas beds dipping downstream promote foundation sliding and reservoir leakage.

Plate tectonics

Plate tectonics is the unifying theory that the Earth's rigid outer shell (the lithosphere) is broken into several large and small plates that move (a few cm/yr) over the weak, ductile asthenosphere, driven mainly by mantle convection, ridge-push and slab-pull. Plate interactions at boundaries produce earthquakes, volcanoes, mountain belts and ocean basins. It synthesises continental drift (Wegener) and sea-floor spreading (Hess).

Three types of plate boundary

Origin of the Himalaya

During the Mesozoic the Indian plate separated from Gondwanaland and drifted northward, consuming the intervening Tethys Ocean by subduction beneath the Eurasian plate. By about 50–55 Ma the Tethys was closed and the two continental plates collided. Because continental crust is too buoyant to subduct, the collision caused crustal shortening, stacking and uplift, raising the Himalayan range. India continues to underthrust Eurasia (~ 2 cm/yr), keeping the belt seismically active.

Longitudinal tectonic divisions of Nepal (south to north)

The convergence is accommodated by a series of north-dipping thrusts that separate these belts:

South (younger, lower)                          North (older, higher)
  Terai/Gangetic plain
   --- MFT (Main Frontal Thrust) ---
  Siwaliks (Churia) - Neogene molasse
   --- MBT (Main Boundary Thrust) ---
  Lesser Himalaya - low-grade metasediments
   --- MCT (Main Central Thrust) ---
  Higher Himalaya - high-grade gneiss, leucogranite
   --- STDS (South Tibetan Detachment System, normal fault) ---
  Tethys (Tibetan) Himalaya - fossiliferous sediments

• MFT – southernmost active thrust, Siwaliks over Terai alluvium.
• MBT – Lesser Himalaya thrust over Siwaliks.
• MCT – Higher Himalayan crystallines thrust over Lesser Himalaya; a major ductile shear zone.
• STDS – a north-dipping normal (extensional) fault separating the Higher from the Tethys Himalaya.

Thus the Himalaya is the type example of a continent–continent collisional orogen, and Nepal's geology is essentially a south-vergent thrust stack expressing that ongoing collision.

Physical vs chemical weathering

Principal processes

Physical: frost wedging (freeze–thaw), thermal expansion/insolation, exfoliation (pressure release/unloading), salt crystallisation, biological (root) wedging, abrasion.

Chemical: hydrolysis (most important for silicates), oxidation (e.g. of $\text{Fe}$), carbonation, hydration, dissolution.

Hydrolysis of feldspar to clay (kaolinite)

Orthoclase (K-feldspar) reacting with carbonic acid charged water:

$$2\,\text{KAlSi}_3\text{O}_8 + 2\,\text{H}_2\text{CO}_3 + 9\,\text{H}_2\text{O} \rightarrow \text{Al}_2\text{Si}_2\text{O}_5(\text{OH})_4 + 4\,\text{H}_4\text{SiO}_4 + 2\,\text{K}^+ + 2\,\text{HCO}_3^-$$

The insoluble residue $\text{Al}_2\text{Si}_2\text{O}_5(\text{OH})_4$ is kaolinite (clay); potassium and bicarbonate are carried away in solution and silica is released as $\text{H}_4\text{SiO}_4$.

Factors controlling rate and depth of weathering

1. Climate – temperature and rainfall (chemical weathering fastest in warm, wet conditions).
2. Rock type / mineralogy – Bowen's reaction series: olivine/Ca-feldspar weather fastest, quartz most resistant.
3. Rock structure – joints, fractures and bedding give access to water.
4. Topography and drainage – steep slopes shed water and debris; gentle slopes retain moisture and deepen profiles.
5. Time and vegetation/biological activity.

Engineering significance in the Middle Hills of Nepal

The warm-temperate, monsoon climate and steep relief of the Middle Hills produce thick residual soils and deeply weathered, friable rock (saprolite) over fresh bedrock. Engineering consequences:

• Slope instability: the weathered mantle is the main source of shallow landslides and debris flows during the monsoon; the soil–rock interface is a common slip surface.
• Foundations: low and variable bearing capacity; deep, uneven rockhead requires deeper foundations or careful exploration.
• Excavation: easy to excavate but collapses readily; cut slopes need flatter angles and drainage/protection.
• Construction materials: highly weathered rock is unsuitable as aggregate or rockfill.

Hence the depth and degree of weathering must be mapped (weathering-grade classification) before siting roads, dams and buildings in this terrain.

Definition

A fault is a fracture or fracture zone in rock along which there has been appreciable relative displacement of the two sides parallel to the fracture.

Elements of a fault

• Fault plane: the surface along which movement occurs (its orientation given by dip and strike).
• Hanging wall: the block resting above an inclined fault plane.
• Footwall: the block lying below the inclined fault plane.
• Throw: the vertical component of the displacement.
• Heave: the horizontal component of the displacement.
• Slip: the actual relative displacement measured on the fault plane (net slip / dip slip / strike slip).

Classification by relative movement

• Normal fault: hanging wall moves down relative to footwall; caused by tension (extension); fault plane usually steep.

   footwall |\
            | \  <- hanging wall drops down
            |  \______
  

• Reverse / thrust fault: hanging wall moves up relative to footwall; caused by compression. A low-angle (< 45 deg) reverse fault is a thrust (e.g. MCT, Nepal).
• Strike-slip (transcurrent) fault: blocks move horizontally past each other; dextral (right-lateral) or sinistral (left-lateral); e.g. San Andreas Fault.

Numerical: throw and heave of the normal fault

Given: dip of fault plane $\delta = 60^\circ$, net (dip) slip $s = 25\,\text{m}$ measured along the dip of the plane.

For dip slip, the slip vector lies in the fault plane along the dip direction, so:

Throw (vertical component):

$$T = s \sin\delta = 25 \times \sin 60^\circ = 25 \times 0.8660 = \mathbf{21.65\ m}$$

Heave (horizontal component):

$$H = s \cos\delta = 25 \times \cos 60^\circ = 25 \times 0.5000 = \mathbf{12.50\ m}$$

Check: $\sqrt{T^2 + H^2} = \sqrt{21.65^2 + 12.50^2} = \sqrt{468.7 + 156.25} = \sqrt{624.9} \approx 25.0\,\text{m}$ = slip. Correct.

        |\  s = 25 m (along dip)
     T  | \
 21.65m |  \  60 deg
        |___\
          H = 12.50 m

Two field criteria to recognise faults

1. Abrupt truncation or repetition/omission of strata across a line; offset of marker beds.
2. Presence of slickensides, fault breccia/gouge, drag folds, or a topographic fault scarp / aligned springs.

The rock cycle

The rock cycle describes the continuous transformation of rock material among the three rock classes through geological processes.

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

Any rock may be uplifted and weathered to sediment, buried and lithified, metamorphosed by heat and pressure, or melted to magma and re-crystallised, so the cycle has many shortcuts.

Distinguishing characteristics

Nepalese examples

• Igneous: Granite (e.g. Higher Himalayan leucogranite).
• Sedimentary: Sandstone (Siwaliks/Churia).
• Metamorphic: Gneiss (Higher Himalayan crystallines) or slate/phyllite (Lesser Himalaya).

Dip and strike

• Strike is the compass direction (bearing) of the horizontal line formed by the intersection of an inclined bed with a horizontal plane.
• True dip is the angle of maximum inclination of the bed measured in a vertical plane perpendicular to the strike.
• Dip direction is always at $90^\circ$ to the strike, and the true dip is the steepest dip on the surface (any other direction gives a smaller apparent dip).

Calculation of true dip

The line from the $200\,\text{m}$ point to the $160\,\text{m}$ point is stated to be along the line of steepest descent, i.e. the dip direction. Along this line:

• Vertical drop $= 200 - 160 = 40\,\text{m}$
• Horizontal distance $= 100\,\text{m}$

The true dip angle $\delta$ is:

$$\tan\delta = \frac{\text{vertical drop}}{\text{horizontal distance}} = \frac{40}{100} = 0.40$$ $$\delta = \tan^{-1}(0.40) = \mathbf{21.8^\circ}$$

The $180\,\text{m}$ borehole lies on the structure contour midway in elevation, confirming a uniform planar dip. The seam therefore dips at about 21.8 deg (gradient 1 in 2.5) towards the $160\,\text{m}$ borehole, and the strike runs at right angles to that direction.

Geomorphology

Geomorphology is the scientific study of landforms and the surface processes (weathering, erosion, transport, deposition by water, wind, ice and gravity) that create and modify them, together with their evolution through time.

Fluvial (river) cycle of erosion (Davisian normal cycle)

Youth:    /\  /\   V-shaped, steep
Maturity: \__~~__/   open V, meanders
Old age:  ~~~~~~~~   wide flat plain, ox-bows

Relevance to Nepal

• Bridges: youthful Himalayan rivers cut deep gorges with bedrock at depth and high erosive/scour power; abutment siting must avoid bank erosion and account for monsoon flood scour.
• Hydropower: steep youthful reaches give large head suitable for run-of-river plants, but high sediment load causes turbine abrasion and reservoir sedimentation; understanding the erosion stage guides intake and desander design.

Joint vs fault

A joint is a fracture or crack in rock along which there has been no appreciable displacement parallel to the fracture surface. A fault differs in that it shows appreciable relative displacement of the two sides. Joints usually occur in sets; faults are individual displacement surfaces or zones.

Rock Quality Designation (RQD)

RQD (Deere, 1964) is a modified core recovery index that estimates rock-mass quality from drill core. It is the percentage ratio of the sum of lengths of intact, sound core pieces longer than 10 cm to the total length of the core run:

$$RQD = \frac{\sum (\text{length of core pieces} \ge 10\,\text{cm})}{\text{total core run length}} \times 100\%$$

Computation

Sum of pieces $\ge 10\,\text{cm}$:

$$22 + 15 + 30 + 11 + 18 + 24 = 120\,\text{cm}$$

Total core run $= 1.5\,\text{m} = 150\,\text{cm}$.

$$RQD = \frac{120}{150} \times 100\% = \mathbf{80\%}$$

Since $75\% \le 80\% < 90\%$, the rock-mass quality is GOOD.

Five major geological zones of Nepal (south to north)

(These are separated respectively by the MFT, MBT, MCT and STDS.)

Why the Siwaliks are landslide-prone

• The rocks are young, soft, poorly cemented and weakly consolidated (sandstone–mudstone–conglomerate), so they weather and disintegrate rapidly.
• They form the steep, frontal Churia hills with high relief and active tectonic uplift along the MFT/MBT (frequent fracturing and seismicity).
• Mudstone interbeds soften when wet, creating weak slip surfaces, and the intense monsoon rainfall saturates the slopes.
• Deforestation and road cutting further destabilise the fragile slopes.

The combination of weak young lithology, steep tectonically active slopes and heavy monsoon makes the Siwaliks the most landslide- and debris-flow-prone belt in Nepal.

(Answer any four; each ~2.5 marks.)

(a) Bowen's reaction series and weathering

Bowen's reaction series shows the order in which silicate minerals crystallise from cooling magma: a discontinuous branch (olivine -> pyroxene -> amphibole -> biotite) and a continuous branch (Ca-plagioclase -> Na-plagioclase), converging to K-feldspar -> muscovite -> quartz. Minerals that crystallise first/at high temperature (olivine, Ca-plagioclase) are farthest from surface conditions and are least stable / weather fastest; those crystallising last/at low temperature (quartz) are most resistant to weathering. This explains why beach sands are quartz-rich.

(b) Unconformity and its types

An unconformity is a buried erosion or non-deposition surface representing a gap (hiatus) in the geological record. Types:

• Angular unconformity: tilted/folded older beds overlain by younger flat-lying beds.
• Disconformity: erosion surface between parallel strata.
• Nonconformity: sedimentary rocks resting on eroded igneous/metamorphic basement.
• Paraconformity: obscure, near-parallel break with little visible erosion.

(c) Outlier and inlier

• Outlier: an isolated outcrop of younger rocks completely surrounded by older rocks (e.g. an erosional remnant capping a hill).
• Inlier: an outcrop of older rocks completely surrounded by younger rocks (e.g. older rocks exposed in a valley/eroded anticline core).

(d) Index minerals and metamorphic grade

Index minerals are minerals that are stable only over a particular range of temperature/pressure and so indicate the grade (intensity) of metamorphism. In pelitic (clay-rich) rocks the classic sequence of increasing grade is: chlorite -> biotite -> garnet -> staurolite -> kyanite -> sillimanite. Mapping the first appearance of each defines isograds; their presence lets the geologist read the metamorphic conditions a rock experienced.

(e) Apparent dip and true dip

The true dip is the maximum inclination of a bed (perpendicular to strike). An apparent dip is the inclination seen in any vertical section not perpendicular to strike, and is always less than the true dip. If $\alpha$ = true dip, $\beta$ = apparent dip, and $\theta$ = the horizontal angle between the strike-normal (true-dip direction) and the section line, then:

$$\tan\beta = \tan\alpha \cdot \cos\theta$$

When $\theta = 0$ (section perpendicular to strike) $\beta = \alpha$; when $\theta = 90^\circ$ (section along strike) $\beta = 0$.