Classification of Landslides

Ngatapa, Tairawhiti view large image HERE

Figure 2.
Falls

Falls are the simplest kind of landslide movement and are generated on the steepest hill slopes. They occur where rock discontinuities such as joints, bedding surfaces and foliation are relatively closely spaced. Weathering contributes to the breaking of the rocks along these discontinuities, or clefts, further weakening them. Water occupying the discontinuities increases the "cleft-water pressure" and further weakens the clefts and is responsible for many incidences of rockfall. Once the overall rock strength is overcome along the cleft the rock shears along the discontinuity triggering a rockfall. In Norway 60 % of rockfalls occur in April-May and October-November, which correspond to the seasons of snowmelt and rainfall maximum. The debris from rockslides accumulates beneath the cliff as talus or scree or is carried away by streams (Carson and Kirby, 1972, p. 125-128; Costa and Baker, 1981, p. 248-249).

An example of a Topple, from Pingchau Island, Hong Kong. The failure occurred within sedimentary rocks. The orientation of the rock slabs on the coastal platform indicates failure of a tall wedge of rock (originating in the rock face behind the geologists) along a joint plane. View large image HERE

Topples

Topples (slab failures) occur when a slab of rock is pushed outwards along a joint away from the rock face either by the adjacent rock units or water or fluid in the cracks between the units of rock. The unit being pushed rotates forward about a pivot point below or very low in the unit and creates a wedge of rock that will rotate forward until it eventually topples over. As it moves forward tension cracks are generated in the main rock unit and water enters these which then creates joints and starts the process over again. Although the creeping rotation of the wedge may be slow once it has moved far enough to overcome resistance to failure the collapse of the wedge is sudden (Carson and Kirby, 1972, p. 112-120; Costa and Baker, 1981, p. 251).

An example of a topple occurred when Threatening Rock (New Mexico) collapsed destroying a portion of the village of Pueblo Bonito. The village was occupied about 1000 AD. and the hazard that Threatening Rock posed was recognised then. Then the villagers tried to shore up the creeping slab of rock. It is calculated that the slab was then moving about 0.5 mm per year. Before it toppled this rate had increased to 50 mm per year. Movement accelerated in winter as a result of frost action and freeze-thaw of water at the base of the slab (Costa & Baker 1981; Carson and Kirby, 1972, p. 133-134).

Slab and wedge failures are characteristic of well-jointed rocks. Slab failures occur when sliding takes place along a weakened joint on an inclined bedding plane. The jointing structure of the rock may be such that two inclined bedding planes intersect, effectively isolating a block of rock. If the intersection of these bedding planes dips towards the face of the cliff face then failure is possible.

Slides

There are two main types of slide, (1) translational slides and (2) rotational slides (Costa and Baker, p. 251-255)

(1) Translational slides have a relatively flat, planar surface of movement along one or several surfaces. In block slides the slide mass moves relatively quickly as a single or few rock units along steeply dipping bedding or planar joints. In a rock slide the slide mass is broken into many units. Debris slides often occur in surficial deposits, i.e. the loose material which blankets the slope, along a surface of rupture approximately parallel with the underlying bedrock. The material may slide as a coherent mass or may break into smaller parts. With increasing movement downslope debris slides break apart more rapidly causing the mass to tumble downslope like a snow avalanche. The momentum of the debris mixture allows it to run out beyond the foot of the slope. These events, which can be the most hazardous of all mass movements are called debris avalanches.

(2) Rotational slides have a curved surface of rupture and produce slumps by their backward rotational movement. Many landslides that begin with slumping at their main scarp will often transform to sliding or flowage further downslope. Rotational slides are typically deep-seated and occur in relatively homogeneous material and the failure plane often assumes a semi-circular shape.

The 1963 Vaiont Dam disaster, Italy , was caused by a huge 2 km long, 1.6 km wide, 150 m thick and 240 million m³ rockslide that displaced water in the reservoir impounded by the Vaiont Dam. This landslide filled the reservoir for 2 km upstream of the dam and caused a huge surge wave that travelled across the reservoir, overtopped the dam and created a 70 m flood wave in the valley below that killed 3,000 people.

Spreads

Spreading failures occur when sediment (rock or soil) suddenly loses strength and starts to flow. This process is termed liquefaction. If the sediment lies beneath firmer rock or soil then the overlying material is broken up and spreads too. Typically this phenomenon occurs in very fine-grained sediments and in particular in glacial quick clays. These clays are very sensitive and have considerable strength when in an undisturbed condition. However, if disturbed, e.g. by earthquakes or explosions, they become "quick" and can flow on slopes as shallow as 1^o (Costa and Baker, 1981, p. 255-258)

Soil comprises grains which are in contact with one another. Below the water table the spaces (voids) between these grains are filled with water. As long as the grains stay in contact the soil will retain its strength. However, if pressure exceeding the weight of the soil is put upon the water in the voids the soil dilates, and the soil turns into a slurry which behaves like a fluid and has virtually no bearing strength. This process can happen very quickly and structures built on top of soil which behaves in this way will sink into the slurry.

Liquefaction can be triggered by earthquakes (see study guide 4). Earthquake waves disrupt and reorganise the soil particles which decreases void space and thus increases pore pressure with pressure increasing with each passing shock. The size of the grains in the soil is an important factor. Fine-grained soils, with a large proportion of silt and clay, are less likely to liquefy because the capillary water tension between gains is high. Fine- to medium-grained sands do not allow rapid draining of pore water but have void spaces large enough to make capillary cohesion irrelevant. These sand sizes are common in recently deposited marine and river sediments, and so liquefaction commonly occurs in these materials during earthquakes.

Although liquefaction generally occurs in water-saturated soils, it can also involve air. During the 16 December 1920 Kansu earthquake, 200,000 people were buried by landslides triggered by liquefaction of fine-grained loess. The shear strength of the loess was exceeded by the earthquake effects but low permeability of the fine-grained soil prevented air from escaping from the loess, which then liquefied.

Quickclays in the St. Lawrence lowland of Canada, the Anchorage area in Alaska and Scandinavian countries accumulated in marine coastal areas during glaciations when large quantities of clay was generated by glaciers grinding down rock inland which was then washed downstream and into the sea. The salt water caused the clay particles to bind together in a house of cards fashion with chemicals in the water providing a glue which created a very strong bond between the clay particles. Water filled the spaces between the clay particles. These clays accumulated on the seafloor. As the glaciers melted receded and their weight stopped pressing down on the land the land rebounded and raised the muds above sea level. Freshwater then washed the chemicals (the glue) out of the clays leaving them in unstable state. When they are shaken, usually during earthquakes, the "house of cards" structure collapses, the water is released and the clay flows.

Quick clays also occur in freshwater glacial clays. Here small electrical forces hold the clays together, rather than a chemical glue. Shaking of the clay, e.g. during earthquakes, overcomes these forces and the sediment liquefies.

Debris flows

Most slope failures involve only small movements along the shear plane before stability is regained and sliding ceases. Thus movement is generally confined to the limits of the unstable slope. However landslide debris can become very mobile after slipping to the extent that debris and contained water is mixed together and then flows downslope as a fluid, and this is termed a debris flow. Debris flows can also be triggered by melting snow or rainfall saturated loose material on hillslopes which then pours downslope. Debris flows move downslope at a few kilometres an hour, leaving low ridges of material, or levees, at their margins (Stratham, 1977, p. 93). They contain 20 to 80 % particles coarser than sand and generally flow very quickly and are destructive (Costa and Baker, 1981, p. 258)

Debris flows tend to flow down preexisting channels and their movement resembles wet concrete, and the front of the flow is usually armoured with boulders which are pushed along in front of the flow. Debris flows usually contain fine-grained soil material, and this gives the flow cohesion and allows it to transport very coarse material. The velocity of the debris flow depends on the thickness of the flow; once this falls below a critical value flow ceases completely and the material is rapidly. Plug flow can also occur in debris flows. In this situation only material at the margins of the flow is moving and material in the centre of the flow, which may include cobbles and boulders, is rafted along as a coherent plug (Costa and Baker, 1981, p. 258-260; Fig. 2).

Many of the most destructive debris flows occur after heavy rainfall on hillslopes denuded by deforestation (e.g. through forest clearance or fire). They are generally associated with debris slides and debris avalanches. A spectacular occurred along the Serra das Araras escarpment, Brazil, in 1967 when heavy rain triggered hundreds of debris slides along partings between crystalline bedrock and the overlying soil. Soil and rock slid into the valley floors, cutting the main highway between Rio de Janeiro to Sao Paulo, and badly damaging a hydroelectric complex that supplied power to most of Rio de Janeiro (Costa and Baker, 1981, p. 261-262).

Creep

In contrast to the processes described so far some mass movements proceed at rates that are barely detectable. Creep is the imperceptibly slow down slope movement of regolith (soil and weathered rock). On much of the New Zealand hill country evidence of creep can be seen in the formation of small terraces spaced at vertical intervals of less than a metre which follow the contours of the hillsides Factors which contribute to creep include frost heaving, wetting and drying, animal activity and the growth and decay of plants (Carson and Kirby, 1972, p. 173-176).