Landslides

Yes, in a way you can. People who work in mountain landslide areas start a landslide on purpose so that they can control when one will start. People who work in rock slide areas also do things like putting up netting on the side of a hill to stop a future slide and plant plants to hold the dirt down.

Soil Slopes

There are a number of potential solutions which are used to try and prevent a landslide occurring in soil slopes which commonly involve either increasing the strength of the soil mass or reducing the stress acting upon it.

Soil nailing is a process whereby steel bars are grouted into holes drilled into the unstable soil slope face. These are placed in a regular array and act to reinforce the soil by increasing it's tensile and shear strength.

Mini piling is similar in concept to soil nailing however the piles are commonly of higher strength and larger dimensions than a soil nail and so would be used in situations where a greater restraining force is required to support the soil mass but also require more extensive and expensive works to place.

Other solutions include lime stabilisation which involves boring columnar holes into the soil and mixing a lime or other grouting material into the fill material which acts to increase the soil strength, inhibiting the formation of shear planes and halting soil movement.

Another solution may be de-watering which is a process where the groundwater table is lowered. This acts to reduce the pore water pressure in the soil mass in turn increasing the in-situ stress and hence the shear strength of the soil which can halt movement.

Another potential solution is the planting of vegetation on the soil slope whereby the roots will act to bind the soil mass increasing it's strength and acting to inhibit the formation of shear surfaces which are necessary for slope failures / movements to occur in soils. Vegetation will also act to reduce the infiltration rate of water into the soil which can reduce the fluctuations in pore water pressure which will also act to increase the stability of the soil slope.

Rock Slopes

Areas that may be prone to rock falls or rock slides are commonly assessed by an engineering geologist. They will attempt to identify regions where the rock mass forming the slope or cliff face is at risk of failure.

These regions are likely to be those where there is weak material (for example a band of shale in other higher strength sedimentary rocks) or areas where the structure of the rock mass makes a failure more likely to occur. An example might be where the bedding planes or joints within the rock mass dip steeply towards the slope face.

When they identify areas where failure is a potential risk, they will then suggest a solution depending on the exact nature of the potential problem. Solutions may include rock bolts which are used where discrete blocks in a rock mass might fall from the face of a cliff and the bolt or anchor essentially acts to secure the block to the stable rock mass behind. Rock anchors perform a similar role to rock bolts but are used where greater restraining forces are required.

Other solutions include the scaling of the face. This involves removing loose material and small loose blocks that have the potential to fall. The engineer may also recommend that retaining structures be built in front of certain portions of the weak rock mass to act to support weakened material. Rock catch netting may also be attached to the face of a potentially unstable slope to catch small blocks which may fall from the face. Larger collapse may be halted by rock catch fences which are high strength steel posts secured firmly into the ground and which are connected by rows of high tensile strength steel cable. These are placed at the base of slopes or cliffs to halt any larger blocks which may fall from the cliff face.

Another solution which is commonly used before the application of rock bolts is "shotcreteing". This is when a reinforcing steel mesh is bolted to the rock face and then high viscosity concrete is sprayed onto the face. This sets and acts to support the rock face. An extreme solution may be to actually trim the rock face back to an angle that is stable however this is commonly not possible due to both practical and economic considerations.

Essentially, a landslide results from the failure of soil and rock on hillsides to remain securely in place, as for one reason or another, they lose cohesion. Another major force is gravity. Gravity causes stress for slope materials such as rocks and soil, because it is constantly acting to try to pull the materials downslope.

Any of the following factors can contribute to landslides:

• Tree clearing and deforestation: with fewer trees and tree roots to secure the soil, landslides can more easily occur.
• Earthquakes: earth tremors can destabilise hillsides.
• Excessive rainfall: super-saturated soils can not hold up against gravity.
• Use of landfill to build up hillsides and roadways: landfill is not a stable as solid ground or rock.
• Mining activities, quarrying and excavation: blasting causes vibrations which can have the same effect as eartquakes and tremors, while undermining a slope causes instability.
• Drought: drier soil is less cohesive than damp soil.

We know gravity is the ultimate force behind any landslide and that weathering plays a part. But what pulls the trigger to set a slide in motion?

Land surfaces are held together by multiple forces. The most important of these is friction. Some soil particles, like clay, cling to each other tightly, while others, like sand, are only loosely joined. All landscapes are held together by friction between the sediment cover and the underlying bedrock, some more tightly than others. If something is introduced to disrupt the friction on an incline, a landslide slips into action. Landslides occur when gravity overcomes the force of friction.

Several common causes of landslides are:

• Water: Perhaps the most common trigger of a landslide, water reduces the friction between the bedrock and the overlying sediment, and gravity sends the debris sliding downhill. In sand and clay soils, a small amount of water may increase stability. You've likely seen this when building a sand castle or working with clay. However, too much water causes the sediment to flow, which is why many landslides occur after rainstorms.

• Earthquakes: If the Earth's crust vibrates enough to disrupt the force of friction holding sediments in place on an incline, a landslide can strike.
• Wildfires: Plants help to stabilize the soil by holding it together like glue with their roots. When this glue is removed, the soil loosens, and gravity acts upon it much more easily. The loss of vegetation after a fire makes the razed land susceptible to slides.
• Volcanoes: Several characteristics of volcanoes make them a fertile starting point for especially destructive landslides. On the next page, you'll learn just how powerful these volcanic landslides can be.

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