SLOPES are in a state of equilibrium when the angles of rest or repose of the material of which they are composed do not exceed a critical value. Thus, slopes may be seen to be in a state of dynamic equilibrium where input balances output but where one exceeds the other the slope will reach a state of entropy and ‘crashes out’ by adjusting its angle of rest to return to a new state of equilibrium.
Slopes occur everywhere in our daily lives from road and rail embankments, road cuttings into hillsides to earthworks in new road schemes, and on building excavation sites. Most are at their angle of repose but can easily be disturbed by vibrations of heavy traffic, earthquake tremors, and very heavy rainfall.
Remember the Ranau magnitude 5.9 earthquake of June 5, 2016 that shook the rock sides of Mount Kinabalu and killed 19 people? Many more lives have been unnecessarily lost through indiscriminate construction of buildings on slopes prone to slope failures. This has been no better illustrated by the collapse of shanty towns through landslides on the outskirts of Mexico City and Sao Paulo in Brazil, occasioning the huge loss of lives of workers and their families, swept away and buried under landslides.
The construction of new roads needs careful review of the landscapes through which they pass and especially if solid rock has been blasted through to create gorge-like thoroughfares or to widen an existing road. I can think of two potential hazard spots on the M5 motorway in South West England, one on the outskirts of Bristol and the other bypassing Exeter in Devon. In both places the steep bare rock faces, created by blasting through hillsides, are now covered with thick wire mesh in an attempt to prevent rockfalls onto the roads beneath and even rockfall warning signs are placed by the roadsides.
Heavy vehicles certainly produce vibrations as seen on remote narrow mountain roads in the Himalayas where frequent rockfalls and landslides occur let alone avalanches from overladen snow slopes. How many people have lost their lives in the recent massive Turkey-Syria earthquake through landslides and debris avalanches, by-products of the initial quake and its aftershocks? Alas, we will never know.
Factors affecting a slope’s stability
The stability of a slope is essentially determined by two factors: the angle of the slope and the strength of the materials of which the slope is composed. The steeper the slope, as in the case of bare rock faces, the greater the chance that rockfalls are likely. This is especially true where disused quarries are found or where a sea cliff is undercut by wave action. The greater the tidal range, the larger the wave cut notch at the base of the cliff. Rock falls usually occur in horizontally bedded sedimentary rocks and cliff slides in unconsolidated material such as glacial drift deposits.
Granite, basalt, and gneiss are very strong rocks whilst some metamorphic rocks such as schist are moderately strong. Sedimentary rocks vary in strength from limestones such as dolomite and most sandstones (like Millstone grit) to mudstones which are very weak. The mode of folding of the rocks during uplift has significance in slope failures particularly in sedimentary and metamorphic rocks. Where the dip of the strata is into the slope or parallel with the slope, greater resistance is had. When the dip of the strata is towards the slope or parallel with the slope, greater erosion occurs.
The composition and structure of the rocks also affects their strength as some minerals are more susceptible to weathering than others. Feldspars in rocks break down easily into clays thus producing an easily removed material when the regolith (rotted rock) is drenched with rain, resulting in mudslides.
Unconsolidated sediments are weaker in general than sedimentary rocks because the sediments are not cemented or compressed together. The amount of water that the rotted rock contains is the most important factor in controlling its strength. In unconsolidated material there are air spaces or pores which, when filled with water, allow the grains to be pushed apart, reducing the amount of friction between the grains.
With persistent, heavy rainfall, such as the 17mm input of rain in less than five hours that Kuching experienced on the evening of Feb 2, 2023, it was not surprising that mudflows and slope failures occurred. The rain significantly increased the mass of the material on slopes by increasing the gravitational force pushing it down as well as lubricating the material thereby lessening the cohesion of the particles making up the slope. The recent December 2022 disastrous loss of 31 lives in Batang Kali, Selangor was the result of such.
‘Triggers’ for mass wasting
The most common trigger is that of an increase in water content caused by heavy rain, the rapid melting of snow or by volcanic eruptions. Volcanoes often create convective rainfall by blasting material high into the atmosphere which acts as condensation nuclei for water droplets which when falling on ash laden slopes quickly produce mudflows. Earthquakes often lead to deadly mass wasting events including snow avalanches such as those caused by a 7.8 magnitude quake in Nepal in 2015.
Freezing and thawing of rocks are significant in rock falls where the rock is prized apart by frost wedges leading to the development of scree or felsenmeer slopes on the side of mountains. The largest scree slopes occur in previously glaciated valleys where valley glaciers carved out steep sided, flat floored valleys. When the glacier melted, the valley sides lacked the support of the glacier and thus collapsed. Clearly the undercutting of coastal cliff faces leads to massive rock falls often onto beaches below and the collapse of houses and hotels built initially on cliff tops for their sea views.
Man-made structures to mitigate slope failures
Groynes built at right angles to sea cliffs and wave directions trap sand and arrest its movement along a beach thereby allowing the sand to be a massive wave energy soak by protecting the bases of cliffs from erosion. Boulder piles dumped at the base of cliffs give similar protection from erosion. Rock falls are reduced by bolting wire mesh netting into overstep slopes and concrete cascades or steps built into the slope take surplus water away to rivers below.
Limitations have been imposed on the height and steepness of mining waste tip heaps since a massive (former National Coal Board) slag heap, composed of coal shale, collapsed on a primary school at Aberfan in South Wales in 1966 resulting in the tragic deaths of 144 people including 116 children. The slope failed through progressive spring sapping by a stream which emerged at the foot of the slope after a period of very heavy rainfall. Today, in former mining areas, slag heaps have been lowered, grassed over or forested to create walking trails and recreational parks.
Avalanches and rock falls have been reduced in Austria and Switzerland by the creation of road tunnels through Alpine areas and by the construction of reinforced concrete roofs over some mountain roads. Snow avalanches are still caused by inconsiderate off piste skiers in avalanche risk areas.
Despite our increased understanding of how stable hill slopes work, we still risk slope instabilities by deforestation and bad land management practices in farming and house building. We cannot, however, control nature and under stress a slope will change its form.
With ever-increasing frequency and intensity of rainfall and rising sea levels, we can expect a greater number of landslides, mudslides, and rock falls annually owing to climate change but we can be more prudent as to where we choose to build houses, settlements, and transport links to avoid looking potential disasters in the face.