iGeo 2016, Писмен тест — Задача 1. Section A: Landslides

• 1. B
• 2. January to April

1. Answer: point marking
B (1m).

2. Answer: point marking
January to April (1m).

3. Answer: point marking
Any reasonable list of different triggers (not initial conditions like geology, topography (relief) – steepness of slopes or gravity etc.), each correct answer 0.5m.
Landslide is “Movement of a mass of rock, debris or earth down a slope” (slides, falls, flows, topple and (lateral) spread).

• Weathering.
• Heavy precipitation (rainfall, storms and tropical cyclones etc.).
• Hydrogeology (aquifers saturation, seepage, change in groundwater level, liquefaction etc.).
• Snowmelt (seasonal).
• Retreat of glaciers.
• Flooding.
• Fluvial erosion (undercutting of cliffs and banks by rivers etc.).
• Coastal erosion (wave action, tidal currents, wave currents; high winds – aeolian erosion).
• Slopes dried out and cleared by fire (excluding human induced fires).
• Earthquakes and its vibrations.
• Tectonic movements.
• Volcanic activity and eruptions.
• Tsunami.
• Previous landslides.
• Fall of a meteorite.
• The effect of global warming (perhaps not totally natural factor) was considered only in case of its direct influence explained (i.e. the retreat of glaciers, the melting of permafrost, increase of sea water, heavier and longer rainfalls etc.).

4. Answer: point marking
Any reasonable list of different triggers, each correct answer 0.5m.

• Removal of vegetation.
• Interference with or changes to natural drainage.
• Damming of water.
• Irrigation.
• Modification (cutting and filling) of slopes by development by construction of buildings, urbanisation etc.
• Modification (cutting and filling) of slopes by development by construction of roads, railways, tunnels etc.
• Vibrations from heavy traffic.
• Leaking pipes (water, sewer).
• Mining activities, blasting and excavation or displacement of rocks.
• In residential development, natural river beds are often filled with soil to make a flat ground. Such filing will make an unstable slope susceptible to earthquake-induced landslides even when the slope is very gentle.
• Inadequate agricultural practices (crop planting and animals grazing counted separately).
• Recreational activities (pressure on surface caused by uncontrolled trekking etc.).
• Climate changes caused by human activities (CO$_2$ emission).
• Nuclear weapons testing blasts.
• War (bombing).

5. Answer: point marking
Any reasonable list of different damages no matter the category.

• Fatalities.

  • Killing people and animals.
• Damage.

  • Engulfing people, animals, crops and buildings causing injuries and damage to property.
  • Cutting off all infrastructures: roads, power lines, pipes, and communication lines, leaving people stranded.
  • Ruining economic viability including food gardens and cash cropping areas.
• Secondary effect.

  • Alteration of the landscape. Landslides (debris avalanche) can create huge gaps in the vista and dam rivers, flooding surrounding areas and creating lakes.
  • Landslide dam outburst floods. Failure of a landslide dam, which has trapped large volumes of water behind it, can cause catastrophic flooding downstream.
  • Tsunamis. Large landslides underwater or into the sea may create tsunamis.
  • Reef damage. Landslides cause severe soil erosion and deposit this sediment in rivers, which then carry it into the sea causing considerable damage to the surrounding coral reefs.

6. Answer: level marking

• No other remedy will yield better results than avoiding landslide-prone areas altogether by consulting engineering geologist or a geotechnical engineer to evaluate the potential for landslides and other geology-related problems.

Landslide may be prevented by stabilisation of slope. There are three basic approaches:

• Geometric methods, in which the geometry of the hillside slope is changed. Rock slope and soil slope geometric modifications will require different techniques.
• Hydrogeological methods, in which an attempt is made to lower the groundwater level or to reduce the water content of the material.
• Mechanical methods, in which attempts are made to increase the shear strength of the unstable mass or to introduce active external forces (e.g. anchors, rock or ground nailing) or passive external forces (e.g. structural wells, piles or reinforced ground) to oppose the destabilising forces.

Once constructed, remedial measures must be inspected and maintained. Lack of maintenance can cause renewed landslide movement.

• Preserving vegetation (reforestation):

  • Trees, grasses, and vegetation can minimise the amount of water infiltrating into the soil, slow the erosion caused by surface-water flow, and remove water from the soil.
  • Although vegetation alone cannot prevent or stop a landslide, removal of vegetation from a landslide-prone slope may initiate a landslide.
• Improving surface and subsurface drainage:

  • Surface water should be diverted away from the landslide-prone region by channelling water in a lined drainage ditch or sewer pipe to the base of the slope but not to be allowed to pond.
  • Ground water can be drained from the soil using trenches filled with gravel and perforated pipes or pumped water wells.
  • Swimming pools, water lines, and sewers should be maintained to prevent leakage, and the watering of lawns and vegetation should be kept to a minimum.
  • Clayey soils and shales have low hydraulic conductivity and can be difficult to drain.
• Constructing piles and retaining walls (terraces, benches):

  • (Friction) piles (metal beams, as wooden beams and telephone poles are not recommended for use as piles because they lack strength and can rot) are fixed to extend into a competent rock layer below the landslide.
  • Retaining walls can be constructed by adding lagging (metal, concrete, or wooden beams) horizontally between the piles.
  • Such walls can be further strengthened by adding tiebacks and buttressing beams.
  • Retaining walls also are constructed of concrete, cinder blocks, rock, railroad ties, or logs, but these may not be strong enough to resist landslide movement and could topple.
  • Sheet piling.
• Rock fall protection:

  • Loose blocks of rock are attached to bedrock with rock bolts, long metal rods that are anchored in competent bedrock and are threaded on the outside for large nuts.
  • Geo Grid, shotcrete.
• Removal and replacement (altering the slope gradient):

  • Landslide-prone soil and rock can be removed and replaced with stronger materials, such as silty or sandy soils.
  • Must include measures to prevent continued weathering of the remaining rock.
• Legal approach:

  • Land use regulation, abstaining from building houses / practicing agriculture etc. in landslide-prone places.