Mitigation of Damages from Downslope Flows

once a landslide has occurred it is difficult to recover property losses. some areas in California were once very expensive ocean-view real estate, but once large slumps started moving whole neighborhoods downslope along systems of curved faults, the properties became worthless. The best way to avoid financial and property loss in places like this is to not try to rebuild, as once the land slips in these regions, it takes huge engineering efforts to prevent further movements. sometimes the only way to prevent additional landslides is to completely reengineer the slopes, changing steep slopes into terraced low-angle hills. Even this type of engineering may not be enough to prevent additional slides, so the best protection is to avoid building on land that has a history of sliding.

Despite these cautions, many new developments in landslide mitigation techniques make living in mountainous or hilly terrain safer. Most early landslide repair and mitigation techniques involved the building and emplacement of buttresses along the toes of landslides to stop their forward advance and emplacement of pipes and other features to promote water drainage from within the slides. Later, in the middle part of the 20th century, as equipment for moving regolith became larger, it became more common to remove entire slide masses from the sides of hills, and to recompact the material to stabilize the slopes. since the 1990s, new products such as geomembranes and geotextiles that can hold rego-lith in place and allow water to escape, have greatly increased the ability to stabilize slopes to make them safer from sliding.

The beginning of slope reengineering to prevent or mitigate landslides is thought to have started in the early 1830s, along railroad lines in England and France. With the industrial revolution in the late 1800s, engineers used steam engines to excavate slopes to 1:1 (horizontal to vertical), or a 45-degree slope. Steeper slopes were covered with masonry retaining walls, holding the slopes back with gravity. When slopes failed in downslope movements, they were typically repaired by cutting the slope back to more gentle slopes, or, if space in urban areas did not permit this, the slopes were reinforced with concrete or masonry walls.

After World War II, large earthwork projects were employed in the United States, particularly associated with construction started as a result of the Interstate Highway Act of 1955. At this time, a new style of landslide mitigation became common, that of excavating the entire slipped area, installing subdrainages, then refilling and compacting the slopes with the excavated material. These so-called buttress fills are still the most common form of landslide repair in the United States, and are moderately effective in most cases. Slopes can be modified and slip surfaces removed, and the subdrainages keep pore water pressures to a minimum.

Since the 1990s, new materials have been developed that help engineers mitigate the effects of landslides and reduce the risks of additional slope failures on repaired slopes. These materials are known as geo-synthetics and geomembrane materials, and include many individual types of construction and materials. Pavement cloths are tack-coated membranes that are overlain on existing pavement, then paved over. They serve to hold the pavement together but allow water to escape through the membrane. Filter cloths are used beneath roads and railroad ballast and on hillsides to prevent settlement of gravels into the underlying soils. They help to stabilize slopes and prevent hillside drains and other embankments from settling. Liner membranes are impervious to water and can be used to isolate areas of contaminated or clean groundwater from regional ground-water systems. Drainage membranes are constructed as composites of the above materials, and can be used in the construction of retaining walls, combining different effects of not allowing water in some places, and forcing water to drain in others that are less hazardous. Other materials, known as geogrids, can be stretched across slopes, and these materials add strength and support to toes of slopes that might otherwise collapse.

Since the 1960s, soils on slopes have been engineered by mixing materials into the soil so that they have additional strength, much like the natural effect of having abundant roots in a soil. These reinforced earth walls have become common along highways and above retaining walls. In other cases, strong materials are partly buried along the toes of slides or bases of slopes that could potentially fail. These reinforcements are typically designed as grids, increasing the strength of the toe of the slopes so that they are less susceptible to failure. These grids are typically extended into the slope for a distance of about 1.5 times the slope height. In addition, the surfaces of faces of the slopes are wrapped with the reinforcement grids, and then the surface between the grids is planted, further promoting slope stability.

See also earthquakes; geological hazards; soils; volcano; weathering.

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