Slope protection techniques and risk estimation [+Examples]
Slope protection encompasses all those actions and measures to prevent erosion on a slope and thus contain the detachment or sliding of rocks from the slopes. Its main purpose is preserving the integrity of the structure, preventing accidents and avoiding material damage or human life losses.
Slopes are permanent earth structures that have an inclination compared to a horizontal surface. They can be natural (originated by the action of geological, hydrological, climatic forces, etc.) or artificial (created by human activity) to fulfill a specific purpose.
Some of the most common uses of slopes are the stabilization of land in the construction of buildings, civil works, communication routes (such as roads, highways, highways, etc.), agricultural activities and mining operations.
Why is slope protection necessary?
In the case of artificial slopes, created for the stabilization of land in the construction of infrastructures and in agricultural or mining activities, it is necessary to protect the slopes to guarantee their physical integrity.
In the case of natural slopes, when they are close to towns or areas of human activity, such as crops, national parks, recreational areas, and especially on communication routes, it is necessary to protect the slopes to avoid accidents that endanger human lives or that can generate economic and material damage.
This way, it is very important to take measures for slope protection in order of preventing the detachment or sliding of materials from the slope, or containing said materials before they reach the areas of human activity.
Why do slopes fail?
Slopes fail when the set of unbalancing forces acting on the structure exceeds in magnitude the balancing forces of the slope, which causes a total or partial collapse of the structure, with detachment or displacement of material.
The unbalancing forces can be internal or external:
- Internal: Those related to the geological and lithological characteristics of the slope, type of soil and materials that make it up, water table, etc. For example, the nature of the rocks, if the slope is in a seismic zone, among others.
- External: Those related to meteorological, hydrological, climatic, biological factors or even human activity. For example, erosion by the action of the wind or by the action of water, the use of land for agricultural, livestock or mining activities, among others.
What are the main types of slope failure?
Different types of faults can occur on slopes, which affect their stability and physical integrity. The main types of slope failure are:
- Landslides: They are displacements of material in which rocks and other solid components separate from the slope and fall freely at high speeds. Depending on the type of terrain and material characteristics, this type of movement can be the trigger for other slope failures.
- Overturning: Occur when constituent elements of the slope, generally blocks of material, rotate about and break away from the main body of the slope. Normally, this type of failure occurs due to the weight of the material itself or the action of external unbalancing forces.
- Flows: Are faults in which the slope materials move as a fluid or are dragged by the action of hydraulic forces.
- Complex movements: They are types of faults in which two or more of the previous types of displacements are combined.
Techniques for slope protection must take into account these types of faults, as well as the particular characteristics of each terrain, weather conditions, use of the slope, among other factors, in order to guarantee the effectiveness of the protection measures used.
How to protect slopes?
The execution of slope protection works requires previous studies of the land and the area to be protected, in order to determine the instabilities present in the slope and estimate the risks of failures occurring.
Currently, there are various methodologies for estimating the risk of detachment and fall of material from slopes. One of the most used is the Rockfall Hazard Rating System (RHRS), developed in Oregon and applied in several states of the United States.
How is the RHRS method of slope risk estimation applied?
The Rockfall Hazard Rating System (RHRS) is mainly applied for slope protection in road and highway construction, and has the following components:
- A uniform method for inventorying slopes.
- A classification of the slopes in three categories (A, B and C), according to their level of risk.
- A prioritization and detailed study of the most dangerous slopes (type A).
- The design of slope protection and erosion control systems.
- Execution of slope protection works.
- Periodic review and updating of the database of slope failures.
According to the RHRS method, slopes are preliminarily classified, according to their estimated level of risk, in the following categories:
- Type A slopes: These are structures with a level of risk that varies between high and moderate. They deserve a more detailed study to determine their instabilities and failure probabilities. They are the most critical from a security point of view.
- Type B slopes: These are structures that present a level of risk that varies between moderate and low. From a safety point of view, they deserve less rigorous and detailed studies than type A slopes.
- Type C slopes: These are structures in which it is very unlikely that the slope will fail and, if it does occur, the probability that the dislodged or displaced material will reach inhabited or transited areas is very low.
Some of the factors that the RHRS method takes into account to estimate the risk of falling material on slopes include the following:
- Position of the detachments.
- Estimated annual frequency of material detachments.
- Time of the year with the highest occurrence of failure events.
- Approximate amount and size of each material detachment.
- Physical description of the detached or displaced material.
- Extent of loosened or displaced material.
- History of failure events and registered accidents.
- Expert opinion on the causes of landslides.
- Frequency of slope maintenance tasks.
- Estimated cost of slope maintenance work.
In this way, the RHRS method establishes criteria, as well as technical and economic parameters for the prioritization of slope protection measures according to their estimated level of risk.
Another way to estimate the risk of detachment or sliding of material from a slope is through the use of software and computer tools, specially designed for this purpose.
Which software exists for slope failure estimation?
Currently there are numerous software for estimating slope failures, some of which include:
- ROXIM: It is a software for the simulation in two dimensions (2D) of slope failures developed by the University of Durham, in the United Kingdom.
- RocFall: It is a slope failure estimation and analysis software (2D) created and marketed by the company Rocscience.
- CRSP: Acronym for Colorado Rockfall Simulation Program, is a software for 2D simulation of slope failures.
- EUROBLOC: It is a non-commercial software that allows the simulation in three dimensions (3D) of faults in slopes.
- STONE: It is another non-commercial software for 3D simulation of slope failures.
Once the risks of occurrence of slope failures have been evaluated, classified and estimated, either through an empirical methodology (such as the RHRS), or through the use of a computational tool (such as the aforementioned software), the most appropriate slope protection methods can be applied.
What are the types of slope protection?
The types of slope protection are classified into active and pasive structural defense systems, according to their form of action.
Active structural defense systems:
Active structural defense systems, also called surface solutions, are all those slope protection techniques that fix the slope material in its original location to prevent its detachment or displacement.
These techniques include the use of anchors, retaining walls, slope protection with metal mesh, geotextiles or geomembranes, among others. They are called surface solutions because they generally cover an area of the external surface of the slope.
- Anchors: These are metal structures that are housed in holes drilled in the slope and then filled with cement. The anchors work in tension and are used to stabilize both rock slopes and loose soil slopes.
The anchors can be temporary, if they are used as a temporary stabilization measure (while other protection measures are applied), or they can be permanent, if they are applied as a definitive stabilization measure for the slope.
Anchors can be passive , if no stress is applied to them after they are installed; they can be active , if they are prestressed up to their allowable working load after installation; or they can be mixed, if a stress below their allowable load is applied to them after installation.
- Walls: These are structures that are built at the base of slopes to increase their stability. Some types of walls function as active structural defense measures and others act as passive structural defense measures.
Some of the types of walls that function as active structural defense measures include cast-in-place concrete, riprap, and shortcrete walls.
The retaining wall is a structure that supports some material -usually earth or water- and its main function is to prevent the overflow of the material it contains. For this reason, it is designed to withstand the temporary or permanent thrust of the floor and prevent it from overturning or sliding due to the horizontal or inclined loads exerted on it.
The gravity wall is characterized by the usage of its own weight to prevent slipping or overturning. They are walls that lack armor, usually being built of prefabricated concrete or other materials. They rarely have a toe or heel and their recommended height ranges from 2.0 to 12.0 meters with the use of geotextile mesh as an anchor and support for the loads of the fill to be used.
The cast-in-place concrete walls are built with the help of formwork and have the advantage that they can adapt to the geometry of the slope along its surface, thus allowing the use of structural reinforcements to be optimized.
Breakwater walls are a type of stone slope protection. They are built with large and heavy stone blocks. They have the advantage of lower cost, less environmental impact and better drainage than other types of walls.
Shotcrete walls, also called gunite walls, are built by spraying a mixture of concrete and additives through a pneumatic system. The concrete is fixed to the slope surfaces due to the speed of the impact and the physical-chemical action of the additives.
- Metal meshes: They are special networks made of high-resistance galvanized steel that are fixed to the external surface of the slope. The most common are triple twist meshes and reinforced triple twist meshes.
These meshes are fixed to the slopes in order to prevent the movement or detachment of material in unstable areas. For this reason, they are widely used as rock slope protection measures.
- Geotextiles: These are permeable synthetic materials, generally polyethylene or polyester, used for drainage systems and for the protection of the external surfaces of slopes. They can be woven or non-woven depending on the type of application.
Geotextiles are widely used in combination with certain plant species to protect slopes against erosion. Plants not only reduce soil erosion and improve slope drainage, but also help protect geotextile materials from ultraviolet (UV) light, preventing their degradation and increasing their useful life.
Passive structural defense systems:
Passive structural defense systems, also called linear solutions, are all those techniques for the protection of slopes that do not prevent the detachment or displacement of the slope material, but contain it before it affects or endangers an area of human activity.
These techniques are divided into rigid or semi-rigid systems (with little deformation capacity), such as prefabricated concrete walls, gabion walls, among others, and dynamic systems (with greater deformation capacity), including dynamic meshes, steel etc
- Precast concrete walls: They are built with elements of industrially manufactured concrete, then transported and installed in the required place.
Due to the fact that the shape of its elements is determined by a mold, precast concrete walls cannot adapt to the geometry of the slope and, therefore, are used not to fix the slope material but to contain its displacement.
However, the advantage of precast concrete walls is that they can be designed in numerous configurations. They can have structural reinforcements, such as steel bars, buttresses, among others.
- Gabion walls are built by overlapping galvanized wire mesh prismatic cells that are filled with small stone blocks. Generally, they help protect slopes from erosion.
- Dynamic systems: such as meshes, steel cables, etc., are protection systems whose purpose is to transform the kinetic energy (due to speed) of the materials that are released or displaced, into deformation energy.
In this way, dynamic systems have great flexibility and deformation capacity, which allows them to retain the material that comes off or moves from the slope, which is why they are widely used as a protection measure for slopes on roads and railways.
Conclusion
Slope protection is essential to guarantee the stability of the slope and avoid structural failures, which can cause accidents, material damage or put human lives at risk.
There are currently numerous slope protection techniques, which are classified into active structural defense systems, or surface solutions, and passive structural defense systems, or linear solutions.
Active structural defense systems act by fixing the material to the surface of the slope in order to prevent its detachment or displacement. Passive structural defense systems do not prevent the detachment or displacement of the slope material, but contain or retain it, preventing it from dangerously affecting nearby human activity areas.
Some of the most common slope protection methods include the use of anchors, walls, triple-twist metal mesh, geotextiles, and plant species. Some of the most frequent applications of slope protection techniques involve highway slope protection and slope erosion control systems.
In this link you can see some slope protection works carried out by COMOPA CA.
