Techniques of prevention
Prevention of slides, as for landslides in general, comprises several successive stages. First, investigations must be carried out in order to identify the sectors prone to slide. These investigations lead to slide hazard assessment. Then, elements at risk are identified and their vulnerability is assessed in order to include slide hazard in town planning. The result is a zoning map which defines terrains which can be built or not, according to the degree of hazard and vulnerability (go to 12.1.More information on landslide mapping). Therefore, the town is obliged to control all water discharge (drainage waters, waste waters, rain waters) and to avoid digging being liable to destabilize the slope.
Techniques of protection
These techniques are usually used for reducing small-scale slide hazards. In large-scale and very deep slides, in combination with protection techniques such as underground drainage, the best way is to continuously monitor the site and take urban planning measures.
Techniques of protection can be classified in two distinct categories:
– “Active techniques”: they avoid the triggering of the slide. They mainly consist in increasing the strength of the slope-forming material, by drainage or internal soil reinforcement systems.
– “Passive techniques”: they seek to control the consequences of the slide. They consist in external reinforcement systems which retain the material on the slope.
A. Active techniques
– Drainage techniques
Drainage of water is the most important operation for the correction of existing or potential slides. Drainage will both reduce the weight of the mass tending to slide, and increase the strength of the slope-forming material. Surface, subsurface and underground drainage techniques exist. Figure 1 show different drainage techniques used to stabilize a slope (from TRB, 1978).
Surface drainage is strongly recommended as part of the treatment for any slide or potential slide. Interceptor drains and drainage ditches collect and carry surface waters away from the instable slope. These systems provide a rapid surface drainage. A ditch is between 0,5 and 1 metre deep, with triangular, rectangular or trapezoidal section.
These methods to control surface runoff are effective when used in conjunction with subsurface and underground drainage techniques. It is agreed that groundwater constitutes the most important contributory cause for the majority of landslides (See 3.1.1. More details on the role of water). Thus, the most successful methods used for both prevention and correction of slides consist of entirely or partially groundwater control.
Draining trench is usually employed for subsurface drainage. The trench is 5-6 metres deep. A drain pipe perforated in its upper part is installed at the bottom of the trench. A membrane can be put, to prevent contact between backfill and surrounding material or to avoid clogging of the drain. Then, the trench is filled with pervious rock mixture, generally with clean granular material or pebbles. At the surface, the trench must include inspection holes to control functioning of the drainage system.
Finally, drainage waters must be collected and carry away from the unstable slope. Figure 4 shows an example of impervious collecting system.
Draining trench is not efficient in case of active landslide because it can break up and is insufficient for deep landslides. Underground drainage is necessary.
Underground drainage implies boreholes drilling. It consists of subhorizontal drains, when water table is too deep to be lowered with surface or subsurface drains. When the instable mass is deep and when the movement is slow (1cm/year), vertical drains and pumped wells are used.
Underground drainage is more expensive and more difficult to accomplish than surface and subsurface drainage. Drilling operations must be carried out carefully, during dry seasons when the displacements are the slowest. Then, water table level and displacements must be monitored during one year at least.
Moreover, problems exist concerning subdrains: clogging by materials such as dispersive clays, cohesionless silts, or root system of dense vegetation. Rodents can also make homes of subdrain outlet pipes, or these pipes can become overgrown near their discharge point, causing the system to back up. Hard groundwater can deposit calcium carbonate around the percolation silts in subdrain pipes or well casings.
Finally, it is necessary to do geotechnical and hydrogeological studies before beginning the treatment, because the effectiveness and the frequency of use of the various types of drainage techniques vary according to the geology and the climatic conditions. That is why many practitioners prefer to use drainage in combination with other measures, such as retaining structures.
– Slope remodelling
Another method to stabilize a slope is remodelling. The principle is to remove the upper part of the slide by excavation and to buttress the toe of the slide by filling and compaction. This technique can be used during construction or stabilization of existing roadways slopes. But in case of natural terrains, this method is not efficient because the quantity of material necessary to remove is generally too important. However, shallow remodelling can be done.
– Role of vegetation
Vegetation has positive and negative effects on slope stability (3.1.2. More information on landslide causes>>). In many cases, planting is considered as a long-term strategy for natural hazards prevention, more particularly for slide risk management. Figure 5 shows a good example of positive effect of the vegetation on slope stability. The root network of a banyan tree stabilizes a nearly vertical cut in colluvium on a trail at Diamond Head, Hawaï. Water percolating through the colluvium serves to propagate the expanding root system. The concept of reinforced earth (see below) emanates from this kind of natural process.
– Soil reinforcement using Geogrid
Geogrid is usually used during the construction of embankments near highways, or to repair existing landslides. It corresponds to the concept of reinforced earth. The principle is to provide tensile reinforcement through frictional contact with the surrounding soil. Soil-reinforcing grids serve to increase the unit shear strength of any soil which they are placed, thereby offering much higher long-term factors of safety than are possible trough simple compaction.
The grids are an-open-mesh construction, usually composed of polypropylene or high-density polyethylene impregnated with carbon-black ultraviolet radiation inhibitors. The size of Geogrid depends on the level of intended loading once buried in the ground. Reinforcing grids are generally placed at lift separations of 0,6-1,2 metres. Embedment lengths generally vary from 1 to 1,5 times the slope height (figure 6). Soil is then spread over the mesh and compacted (figure 7). The area immediately adjacent to the free face usually requires local compaction with hand-operated vibratory equipment.
Wrapping the slope face with the grids is an option (figure 8 and 9). Face wrapping generally provides a mulch surface to resist erosion and promote planting. Vegetation serves to protect the grids from ultraviolet degradation and vandalism. It provides an aesthetic surface and also reduces erosion. Face wrapping also helps retard rodent burrowing into the slope.
Figures 8 and 9: Two views of a Geogrid-reinforced landslide repair accomplished in 1986 in Crockett, California. The lower view shows the finished slope six weeks after completion. The slope has be hydroseeded and Geogrid face acts as a good mulch mat (from Slosson and al., 1992)
B. Passive techniques
A slope can be stabilized increasing the forces resisting the mass movement with an external applied force system. These systems usually apply a resisting force at the toe of the sliding mass. These are basically the same systems than employed for fall risk prevention: buttresses and bulkheads, retaining walls and box gabion catch walls (go to 2.1.2.2. more information on fall).
The principle of the buttress is to provide sufficient dead weight near the toe of the unstable mass to prevent moment. A buttress design provides an additional resistance component near the toe of the slope to ensure an adequate safety factor against failure. Several important highway sections have been constructed with or treated remedially by a buttress.
Care must be taken in the construction phases to ensure adequate depth for founding the buttress and prescribed quality for the layer on which the buttress is placed.
Reinforced earth wall is a construction material that involves the use of backfill soil and thin metal strips to form a mass that is capable of restraining large imposed loads. The concept is about the same than Geogrid, but the final structure is an external wall. The face of a reinforced earth wall is usually vertical, and the backfill material is confined behind either metal of unreinforced concrete plates.
As a buttress, reinforced earth wall acts as a gravity structure placed on a stable foundation, and it must be designed to resist the slope driving forces (overturning, shearing internally, sliding at or below the base).
Gabion wall is also a reinforcement structure to stabilize slopes go to 2.1.2.2. more information on fall).
For all these reinforcement systems, it is important to create a draining backfill with pervious material upstream the structure, in order to prevent water accumulation behind the wall and negative effects on the slope.
Another retaining structure developed in 1985 by French engineers of the “Laboratoire Central des Ponts et Chaussées” is the “Arma Pneusol”, a combination of tyres, backfill and grids layers, basically to stabilize highway slopes. The structure is planted to provide an aesthetic surface. With this reinforcement system, worn tyres are also recycled.
Anchor walls are also be used. The principle is the same than employed for fall risk protection techniques (go to 2.1.2.2. more information on fall). Shotcrete or retaining wall can be used in combination with passive or active anchored cables (figure 16). This protection technique is rather an “active” system, which increases the internal strength of the slope-forming material.
Finally, in most applications, a combination of the various methods outlined above is used. For example, soil-reinforcing grids can be mixed with any number of facing elements such as gabions, wire mesh or masonry blocks. Figures 17 and 18 show an example of rock-filled, gabion-faced embankment with Geogrid.
References
BESSON L., 2005. Les risques naturels: de la connaissance pratique à la gestion administrative. Editions Techni. Cités, Voiron, 60 p.
FLAGEOLLET J.C., 1988. Les mouvements de terrain et leur prévention, Masson, Paris, 224 p.
GUEFFON M., 2005. Les travaux de prévention actifs contre les glissements de terrain : stabilisation et drainage des zones instables. Risques Infos n°16, p. 13-16
SLOSSON E., KEENE A.G., JOHNSON J.A., 1992. Landslides/Landslide mitigation. In: Reviews of Engineering Geology, Volume IX, Colorado
TRANSPORTATION RESEARCH BOARD, 1978. Landslides: Analysis and Control. National Academy of Sciences, Special Report 176, Washington DC