Landslides have always posed a hazard to humans and property. Appropriately, the technical term for landslides and related phenomena is “mass wasting:’ To a large degree the cause of mass wasting is gravity, the attraction of smaller particles to a larger, denser, lower mass. Landslides are possible when the particles that make up the land lose the cohesiveness that has held them in place.
A Java slide in 1006 A.D. impounded a lake, flooded a densely populated area, and destroyed the Hindu cultural center of Java. China’s 1920 disaster in the Kansu district — where a massive landslide of thick loess deposits of the type of windblown topsoil that com-prises eastern Nebraska’s rich farm-land — killed 200,000 people.
Factors causing landslides
There are many clues to landslide danger. Any area with hills may be subject to some form of landslides. In dry rock areas, cracks and fissures in the rock show a weakness that may lead to rock falls or other forms of wasting. Any sign of previous landslides —whether new and clearly visible or old and partially covered by plants —means that conditions are right.
Soil types are one important condition that can lead to slides. Clays are notorious for sliding when wet. But any loose, non-compacted soil that overlies a harder, or slippery material could slide with enough precipitation. Rain, snow, and water — runoff, irrigation, river or stream — can cause an increased risk of landslides.
Other natural factors that can increase the risk of soil movement include seismic activity from earth-quakes and volcanoes. Areas bordering on the Pacific in the “Ring of Fire” are sites of much mass wasting.
Human activities also cause landslides:
- clearing the land of trees, shrubs and deep-rooted native grasses;
- paving areas and covering over soil;
- cutting off the toe of a slope;
- even moving heavy equipment across unstabilized soil.
All the above human actions can trigger landslides.
Although what we think of as “the land” may appear to be a solid mass, it is actually a conglomeration of separate particles. At a very simple level, we can consider the earth to be comprised of rocks and soil, which is made up of eroded rocks.
Rocks are classified as igneous, metamorphic, or sedimentary. Igneous rocks formed by the cooling of molten lava and magma are associated with volcanic activity. Although volcanoes may deposit pyroclastic materials around their vents in a cone, the tendency for molten lava is to form a flat flow of rock.
Sedimentary rocks develop when materials, sediments, precipitate out of a solution, and then dry as flat strata. Metamorphic rocks are sediments trans-formed by heat, pressure, and chemical action. In brief, rocks are sediments transformed by heat, pressure and chemical action. In brief, rocks tend to lie in horizontal planes. Hills and valleys are developed either by movements that tilt the flatbeds of rocks into slopes or by erosion wearing through layers of rocks, as when a river cuts its course across the land.
If the rocks existed in an atmosphere where they were not affected by wind, rain or temperature, they would retain the hard, cohesive texture that makes them rocks rather than soil. However, these forces do erode rocks, break them apart and make their cohesion fail on certain slopes.
Picture the land as a three-dimensional puzzle fit together with minute grains of dirt. As long as the grains are held together (compacted) the soil is stable. But if something loosens the structure, the pieces fall apart.
Volcanic activity and landslides
The most dramatic disrupters of soil cohesion are also the most obvious. Volcanic activity is violent and often results in the emission of materials that themselves are destructive. But they are also responsible for one of the most cataclysmic of landslides, the “Iahar,” the debris flow which rapidly denudes the volcanic slope down which it slides and which may be far worse when volcanic heat melts ancient glacial ice.
Much of the worst destruction associated with the 1980 eruption of Mt. St. Helens in southwest Washington was caused by the lahar, which scoured the Toutle River valley and destroyed 123 houses in Toutle itself. But the absence of glaciers is no protection from volcano caused mudflows. Most volcanic eruptions are accompanied by heavy rainfall.
The flowing mass of ash and pyroclastic debris that followed the eruption of Italy’s Mt. Vesuvius in 79 A.D. buried the town of Herculaneum so completely that it was lost to the world. for centuries.
Earthquakes can also cause mass wasting. In most cases, the physical jolt is sufficient to loosen the cohesion that has held rocks and soil in place. In 1959, the Hebgen Lake earthquake in the Madison Canyon of Montana, near Yellowstone, caused a massive rockslide of more than 21 million cubic yards of rock. Within a minute the mass of rocks slid across the valley and up the canyon’s opposite wall.
A popular tourist destination, the area had attracted a number of visitors when the earth quaked on August 17, 1959. Fortunately, only 23 were killed in the disaster.
But the causes of most landslides are far less dramatic. And, more importantly, they are more predictable, which means that it’s possible to limit the losses caused by landslides if one plans carefully.
The Water Factor
Perhaps the most important factor in landslides is water. A little water can improve soil cohesion by “lubricating” the soil particles so they compact together more tightly wet than they do dry. If you’ve ever built a sandcastle on the beach, you’ve undoubtedly experimented to find the proper level of dampness to keep your castle standing.
If you add too much water to the sand or any soil, the water lubricates the particles so thoroughly that the cohesion is destroyed, and the castle’s walls collapse. Pour on enough water, and the particles will demonstrate liquefaction, a process that turns the water-logged soil into a mush that acts more like a milkshake than solid ground.
Landslides and rockslides are, overall, the most destructive and catastrophic forms of mass wasting. Millions of tons of rock and soil can plunge down slopes at speeds of more than 100 miles per hour.
In Peru’s 1970 earthquake the western part of the summit of Mount Nievados Huascaran collapsed and a rockslide 3,000 feet wide and one mile long raced down the mountain at more than 100 miles per hour to bury ,the city of Yungay and several nearby villages, killing 18,000 people in the process.
Landslides are generally rapid movements common on stream banks, sea cliffs, and mountainsides. Landslides frequently begin with the slumping of material high on the slope and then move down and out along a definite curved plane. Although landslides are usually less dramatic than other types of mass wasting, the direct costs are widespread and increasing as our population increases.
A 1973 study by the Federal Highway Administration estimated annual repair costs for landslide damage to federal highways at $50 million. A decade later, during the winter and spring of 1982-1983, landslide damage to the California state road system came to $166 million.
The costs of landslides to the environment are also high. Much of the land that slides is, by definition, the nutrient-rich topsoil. When this layer is lost, crops grown on the same slopes will grow less vigorously or not at all. Slopes that have suffered landslides are also subject to recurring erosion, which can lead to deterioration of stream quality as the result of sedimentation.
Even without man’s intervention, landslides occur in nature. But in undisturbed areas they are rare because of the plants that grow on the slopes. Most landslides are caused by water or erosional factors such as wind, heat, and cold that disturb or alter the slope’s angle of repose (naturally stable angle).
The bare soil, as we said earlier, consists of rains fitted together much like the pieces of a jigsaw puzzle. When something disturbs the fit, the soil is loosened, and gravity leads it to slide downhill.
Water is one of the most common disrupters of the soil’s cohesion, but when plants cover the slopes, then their roots help to hold the soil physically in place and, more importantly, drink up the excess moisture that threatens the cohesion.
The water is then transported through the plant along with mineral nutrients derived from the soil to feed the plant, and the excess moisture transpires through the plant’s leaves into the atmosphere. When a fire ravages a forest to lay bare the land, the roots help to maintain the soil in place for five or six years.
As new plants grow in the mineral-enriched soil that remains after the fire, their roots grow to replace those of the burned trees and shrubs. Because the plants destroyed by the wildfires are replaced by other plants, most likely the offspring of the originals, the stability of the slope is maintained. This article will teach you how to protect your home from wildfires and what fire-resistant plants to plant around your house.
The importance of plants
When timber is harvested, or mines are opened, however, the stability of the slopes is often affected because heavy equipment disturbs the site (most dramatically when roads are cut into the mountains), soil nutrients are not replaced, and new plant species are introduced.
During the time between clear-cutting and reforestation, the slopes are denuded. More rain hits the earth harder and more directly than when trees sheltered the earth. The water saturates the ground because the tree roots that drank up the excess are no longer there, and in the winter the snow cover is more direct, leading to a greater volume and speed of snow-melt in the spring.
Although the slides that are frequent in timberlands may seem an extreme example, they are actually fairly representative of the process that leads to landslides: plants that provided the important function of evapotranspiration are removed, and the soil’s angle of repose is disturbed by road cuts and the movement of heavy machinery.
The same disruptive activities also characterize road building and housing development, activities which are even more destructive to the stability of the slope because the vegetation is replaced by buildings and pavement, neither of which absorb water and both of which increase run-off and soil saturation.
The fact that the best level sites in urban areas are already developed means that builders are now working on steeper, less stable slopes that are more likely to slide. In Southern California, 25 to 30 percent of major landslides result from construction.
Before building on any slope, you need to determine the slope’s stability and what factors maintain its angle of repose. Clay soils, for example, are particularly likely to slide.
Before building or settling on a building site, you should consult with local zoning officials or engineers who have access to environmental geology maps that indicate areas of potentially unstable slopes. In rural areas, it’s best to consult with an engineering geologist who can advise you on the soil type on the surface as well as the underlying rock strata.
If the danger of landslides does exist, you may need to install a drainage system in the slope to carry excess water away from the slip surface. Such systems are difficult and expensive to install. Alternatively, you might want to build a retaining wall to prevent a slope’s descending on to a cleared area. To build in some areas, you may need to drive pilings to bedrock in order to provide a solid footing for your building.
At best, leave trees and brush in place if you’re building on a hillside. If the slopes have already been cleared, you must carefully consider the nature of the slope before deciding on the best way of stabilizing it.
Planting for soil stabilization
The first thing you must determine is whether the slide surface is shallow or deep. If it is shallow, then you can begin by planting native grasses, which will protect the soil from the infiltration of water from rain, grow roots to wick excess water from the slope and begin building the network of roots that helps tie the particles of soil together.
Once the grasses are established, plant herbs and shrubs. If your slope is very steep — more than 45 degrees — do not plant trees until the shrubs and bushes are well established. Choose plants that do not need to be irrigated since adding moisture to your hillside will only increase the probability of a landslide.
Instead, choose deciduous trees such as oaks, which consume more water than evergreens and grow more slowly so that they do not overburden the fragile hillside. One tree to avoid is the spruce, which is known for its rapid growth and shallow root system.
In areas frequently threatened by wildfires, such as Southern California, you’ll also want to choose drought and fire-resistant species. In the West some good choices are Pacific red elder, snowbrush and huckleberry. The slopes will achieve their greatest stability when they are forested, but you must balance the advantages of stability against the danger of forest fires.
When growing a forest is not appropriate, you may choose to stabilize your slopes mechanically. One of your first steps would be to provide drainage so that excess water can be removed. If clay overlies sand, gravel, or another permeable surface, you can drive perforated pipes horizontally through the clay into the sand or gravel.
The drains will be most effective if they slope down from the horizontal. Eventually, however, drains do become blocked and must be bored out to restore their effectiveness.
Retaining walls are another means of controlling an unstable slope and controlling existing landslides. At their simplest, these require heavy manual labor and rip-rap to stop the slide. More formally, low walls of concrete, block, or, in cases where a supply of inexpensive timber is plentiful, wood may be built to stop an active slide.
Control is best with drainage behind and under the wall, and when the wall is massive: the rip-rap wall of massive boulders which takes up a lot of space is usually more effective than a concrete wall. In any case, low, thick walls that lean back into the slope are more effective than tall, thin vertical ones.
In rocky slopes, rock bolts and rock anchors can be driven into the deteriorating rock face to hold it in position. Bolts and anchors work by compressing the stone so that tension holds it together.
Another alternative in impermeable soil is hardening. The first method is electro-osmosis, a method of draining the soil by creating an electrical field by placing two electrodes in the soil, setting up a potential difference and causing the water in the soil to migrate to the cathode, a perforated pipe which is then drained by pumping out the water.
In shallow landslides in stiff materials characterized by surface cracks, grouting the cracks by filling them with portland cement can be effective. The grouting displaces the water in the cracks, and then, as it hardens, provides a solid skeleton that stabilizes the soil.
A related method is to bore into the slope with an auger to the slippery soil beneath and mixes lime into the auger hole. The lime mixes with the clay and stabilizes the soil in the boreholes, creating solid columns in the slope. The slope must, however, rest for sixty days before it is stable enough to withstand loading.
While many of these techniques require heavy equipment that may not be available or affordable, the old practice of terracing a slope can be accomplished with hand took and family labor.
Another advantage of building terraces is that the flat surfaces provide areas suitable for gardening. An appropriate choice of plants (those not requiring irrigation) can increase stability while beautifying the slope and providing food for the family. If you are terracing slopes above your home site, you will probably also want to provide drainage away from the house.
A last word
Although our increasing knowledge about the earth means that we can better plan the ways we disturb the landscape, our understanding is still incomplete. And anyone who considers how man’s activities are changing the natural stability of slopes will have reason to consider carefully before building on or near sloping land.