Question 7 of 7: Short Answer and Fill in the Blank
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams May 2015 — 04-BS-14, Geology. Closed-book, 3 hours. Five questions constitute a complete paper: Questions 1-4 are mandatory and one of Questions 5-7 must be chosen; every question (5, 6, and 7) is answered here as a complete study resource. Each question is worth 20 marks.
Reference texts: Marshak, Earth: Portrait of a Planet (structural geology, relative dating, weathering, glacial and fluvial landforms); Goodman, engineering-geology mapping methods (strike and dip, three-point problem); Freeze & Cherry, Groundwater (Darcy flow, piezometers).
Question 7: Short Answer and Fill in the Blank (20 marks)
Mass wasting is the downslope movement of rock, regolith and soil under gravity; whether and how it occurs is controlled by the balance between the forces driving movement (chiefly gravity, resolved as shear stress on a potential failure surface) and the material's resistance to movement (shear strength). The main controlling factors are:
Slope angle (gradient). Steeper slopes increase the gravity-driven shear-stress component acting parallel to the slope while reducing the normal-stress component that generates frictional resistance; every material has a characteristic angle of repose/stability above which it fails.
Water content. Water adds weight (driving stress), and in fine-grained material raises pore-water pressure, which directly reduces the effective normal stress and hence the frictional shear strength (Mohr-Coulomb / effective-stress principle); saturation is the single most common trigger of rapid mass-wasting events (debris flows, rapid earthflows).
Vegetation cover. Root systems mechanically reinforce soil (added cohesion) and vegetation intercepts/transpires rainfall, reducing infiltration and pore pressure; removal of vegetation (fire, logging, land clearing) is a well-documented trigger for increased slope failure.
Material type and structure. Unconsolidated, poorly-sorted or weathered/clay-rich material has low shear strength and fails readily; the orientation of bedding planes, joints, foliation or fractures relative to the slope face is critical — a discontinuity that "daylights" (dips out of the slope face at an angle less than the slope angle) is a ready-made failure surface, while one dipping into the slope is comparatively stable.
Overloading / undercutting. Added weight at the slope crest (construction fill, stockpiles) increases driving stress, while removal of support at the toe (stream/wave erosion, excavation, road cuts) removes resisting force; both push the slope toward failure without changing the material itself.
Seismic shaking. Earthquake ground motion applies a transient additional (dynamic) shear stress and, in saturated granular material, can trigger liquefaction (a sudden, large loss of shear strength), both of which can initiate mass wasting on slopes that were otherwise stable under static conditions.
Climate. Freeze-thaw cycling wedges open joints and reduces rock-mass cohesion over time, and climate governs the frequency/intensity of the rainfall events that saturate slopes, so climate acts as a long-term background control on how often the water-content trigger above is reached.
2) Drainage patterns
[Figure not reproduced: Drainage pattern block diagrams a through e. See the official exam paper or the cited reference text.]
Five block diagrams (a-e) of drainage networks to be labelled by pattern type.
Diagram
Drainage pattern
Why
a.
Dendritic
Irregular, tree-like branching of tributaries at varied angles into the main channel — the signature of uniform, flat-lying rock with no structural control on flow direction.
b.
Rectangular
Streams and tributaries follow a grid of near-right-angle bends, controlled by a perpendicular joint/fracture set in the bedrock that channels are forced to follow.
c.
Trellis
Long parallel main streams (following the strike of weak rock) joined by short tributaries entering at right angles down the flanks of the labelled "ridges of resistant rock" — the diagnostic pattern of tilted/folded alternating strong and weak strata.
d.
Radial
Streams radiate outward in all directions from the central summit of the conical (volcano-like) landform, the classic pattern on an isolated dome or cone.
e.
Deranged
An irregular, disorganized network with numerous small lakes/ponds and no coherent overall trunk-and-tributary organization, typical of young, low-relief, recently-glaciated terrain where the drainage network has not yet had time to integrate.
