Preventing large, natural landslides is difficult
Preventing large, natural landslides is difficult, but common sense and good engineering practices can help to minimize the hazard. For example, loading the top of slopes, cutting into sensitive slopes, placing fills on slopes, or changing water conditions on slopes should be avoided or done with caution. Common engineering techniques for landslide prevention include provisions for surface and subsurface drainage, removal of unstable slope materials, construction of retaining walls or other supporting structures, or some combination of these.
Surface and subsurface drainage control are usually effective in stabilizing a slope. The objective is to divert water to keep it from running across or infiltrating into the slope. Surface water may be diverted around the slope by a series of surface drains. This practice is common for roadcuts (Figure 1a). The amount of water infiltrating a slope may also be controlled by covering the slope with an impermeable layer such as soil-cement, asphalt, or even plastic (Figure 1b). Groundwater may be inhibited from entering a slope by constructing subsurface drains.
A drainpipe with holes along its length is surrounded with permeable gravel or crushed rock and is positioned underground so as to intercept and divert groundwater away from a potentially unstable slope.
Although grading of slopes for development has increased the landslide hazard in many areas, carefully planned grading can be used to increase slope stability. In a single cut-and-fill operation, material from the upper part of a slope is removed and placed near the base. The overall gradient is thus reduced, and material is removed from an area where it contributes to the driving forces and is placed at the toe of the slope, where it increases the resisting forces. However, this method is not practical on very steep, high slopes. As an alternative, the slope may be cut into a series of benches or steps. The benches are designed with surface drains to divert runoff. The benches reduce the overall slope of the land and are good collection sites for falling rock and small slides (Figure 2).
Retaining walls constructed from concrete, stone-filled wire baskets, or piles (long concrete, steel, or wooden beams driven into the ground) are designed to provide support at the base of a slope (Figure 3). They should be anchored well below the base of the slope, backfilled with permeable gravel or crushed rock (Figure 4), and provided with drain holes to reduce the chances of water pressure building up in the slope (Figure 4).
The evolution of a retaining wall is shown on Figure 5. A shallow landslide along a road causes a problem (Figure 5a). The wall is shown during construction in 1999 in Figure 5b. The finished wall in 2001 now stabilizes the slope (Figure 5c). Preventing landslides can be expensive, but the rewards can be well worth the effort. It has been estimated that the benefit-to-cost ratio for landslide prevention ranges from approximately 10 to 2000.
That is, for every dollar spent on landslide prevention, the savings will vary from $10 to $2000. The cost of not preventing a slide is illustrated by the massive landslide in Utah, known as the Thistle slide. In April 1983, this slide moved across a canyon, creating a natural dam about 60 m (197 ft) high and flooding the community of Thistle, the Denver Rio Grande Railroad and its switchyard, and a major U.S. highway (Figure 6). The landslide and resultant flooding caused approximately $200 million in damages.
The Thistle slide involved a reactivation of an older slide, which had been known for many years to be occasionally active in response to high precipitation. Therefore, it could have been recognized that the extremely high amounts of precipitation in 1983 would cause a problem. In fact, a review of the landslide history suggests that the Thistle landslide was recognizable, predictable, and preventable! Analysis of the pertinent data suggests that emplacement of subsurface drains and control of surface runoff would have lowered the water table in the slide mass enough to have prevented failure. The cost of preventing the landslide was estimated to be between $300,000 and $500,000, a small amount compared with the damages caused by the slide. Because the benefit-to-cost ratio in landslide prevention is so favorable, it seems prudent to evaluate active and potentially active landslides in areas where considerable damage may be expected and possibly prevented.
Warning of Impending Landslides
Landslide warning systems do not prevent landslides, but they can provide time to evacuate people and their possessions and to stop trains or reroute traffic. Surveillance provides the simplest type of warning. Hazardous areas can be visually inspected for apparent changes, and small rockfalls on roads and other areas can be noted for quick removal. Human monitoring of the hazard has the advantages of reliability and flexibility but becomes disadvantageous during adverse weather and in hazardous locations.
Other warning methods in-clude electrical systems, tilt meters, and geophones that pick up vibrations from moving rocks. Shallow wells can be monitored to signal when slopes contain a dangerous amount of water. These methods are part of real-time monitoring (Figure 7).
After a slide has begun, the best way to stop it is to attack the process that started the slide. In most cases, the cause of the slide is an increase in water pressure, and in such cases, an effective drainage program must be initiated.
This may include surface drains at the head of the slide to keep additional surface water from infiltrating and subsurface drainpipes or wells to remove water and lower the water pressure. Draining tends to increase the resisting force of the slope material, thereby stabilizing the slope.
Edward A. Keller ( https://www.es.ucsb.edu/ )