Slope stabilization: What is and how is it carried out? (2022)

10/22/2022

Slope stabilization: What is and how is it carried out? (2022)

Slopes are masses of land or soil with an inclination on its external surface. Generally, they are unstable structures susceptible to external factors such as human activities, weather phenomena, seismic activity, among others. Because of that they need to be stabilized.

A common and effective way to improve the stability of slopes is through the use of precast concrete elements. Other techniques include slope stabilization with wire mesh, with anchors, and with shotcrete.

The construction and stabilization of slopes is one of the most common activities today, both in civil works, infrastructure construction, communication routes, agricultural and mining activities, among others.

What are slopes?

Slopes are all those permanent earth structures, of natural or artificial origin, that have a slope or inclination. The slopes can be:

Natural: they are called slopes and are generated by geological or hydrogeological processes, such as the formation of mountains or the erosive action of rivers, the displacement of land masses due to climatological factors, the occurrence of earthquakes, the movement of glaciers, etc.
Artificial: they are made by man with a specific purpose, generally, in the construction of buildings, communication routes (roads, highways, railways, etc.), infrastructure works (ports, airports, etc.) and in activities such as agriculture or mining.

What is slope stabilization?

Stabilizing a slope consists of applying geotechnical engineering methods to consolidate its constituent elements, increase its internal resistance, reduce destabilizing forces, as well as guarantee its safety and permanence over time.

Currently there are a variety of methods to stabilize slopes, some of the most used are:

Reinforced Soil Slopes (RSS): These are a form of reinforced soils that incorporate flat stabilizing elements on slopes with a maximum inclination of 70 degrees.
Mechanically Stabilized Earth Walls (MSEW): These are more complex structures that include reinforcing elements such as steel bars or plates, anchors, piles, steel or polymer mesh, geotextiles, among others.

One of the most used methods in the construction of mechanically stabilized slopes is the use of prefabricated concrete elements , which are reinforced with other materials, to obtain a structure of high resistance and quality.

The precast concrete elements are mass-produced in specialized industrial facilities and then transported to the site where the slope will be built for final installation.

Other techniques include the use of mixed and cast–in-place concrete, the use of shotcrete (called Shotcrete or gunite), or the use of geotextiles and certain plant species (grasses, plants, trees, shrubs, etc.)

How are slopes stabilized?

Slopes are stabilized through the application of various engineering techniques that generally include one of the following methods:

Surface protection measures: Their purpose is to avoid the degradation of the slope surface due to the action of external factors (for example, rainfall) and also the detachment of rocks or displacement of the soil. They include the cleaning of the land at the base, head and lateral surface of the slope, the use of double or triple torsion metal mesh for the containment of rocks, use of geosynthetic materials (including geotextiles) and reforestation.

Geometry modification: It consists of changing the shape of an unstable slope by removing material from its head, adding material to its base, or a combination of both techniques, to provide greater stability to the terrain. Another common way of changing the geometry of the slope is to reduce its slope, by removing material along its entire surface, or by creating stepped terraces.

Drainage measures: These are techniques that counteract the negative effects of water on the stability of slopes. If the water is not properly channeled, it permeates the ground and increases its weight, which increases the loads on surface, natural or artificial, of the slope. Water can also cause erosion and entrainment of material, increase the water table below the slope, undermine the foundations of the structure, increase the pore pressures of the constituent materials, which decreases the general stability of the slope.

Use of resistant structural elements: Such as anchors, piles or micro-piles, which increase the slope’s resistance to failure. Anchors generally employ tensioned steel members, coated with a protective material, that are inserted into a concrete retaining wall. Piles are high-strength, prefabricated structural elements that are inserted into holes drilled in the ground and completed using foundation techniques. Micro-piles are rigid structural elements of smaller diameter, which are inserted in large quantities into the ground like needles, that is why this technique is known in English as soil-nailing.

Construction of walls and retaining elements: They are intended to provide a physical barrier that provides mechanical resistance to the slope, increasing its stability and containing possible failures, rockfall or material displacement. There is a variety of retaining walls and elements , including anchored walls, retaining walls, shotcrete walls (Shotcrete or gunite), masonry walls, gabion walls, precast element walls, etc.

What is soil slope stability?

The stability of slopes in soils refers to the ability of the slope to maintain its shape and functionality over time in a certain environment.

Movements due to slope instability

Whether it is natural or artificial slopes, you want the slope to maintain its shape and fulfill the function for which it was built. However, there are instabilities in the slopes that can cause some of the following movements:

Landslides: This happens when a mass of material, generally from the head of the slope, separates from the main structure and precipitates in free fall. They are usually large rocks and can cause catastrophic damage and put people’s lives at risk. In certain cases, they can result in rotational slips or translational slips. Rotational slips occur along an internal slip surface that is roughly circular and concave in shape. Translational slips occur when a mass of material moves parallel to an approximately flat or slightly undulating external ground surface.
Overturning: Is caused when blocks of material detach from the main structure of the slope through a rotatory movement, due to the forces of gravity or the action of hydraulic forces.
Flows: They are displacements in which the material behaves in a similar way to viscous fluids. Generally, they are produced due to the action of hydraulic forces, such as river floods, heavy rainfall, glacial melting, etc.
Complex movements: When combinations of the previous types of displacement occur.

Less slopes results in greater stability, thus there is less probability of occurrence of any of the aforementioned movements.

How is slope stability determined?

To determine the stability of a slope, it is necessary to carry out a series of geotechnical studies (soil characterization), mechanical (determination of shear forces and shear resistance) and hydrogeological studies (phreatic level, piezometric level, porosity, flow networks), among others.

Soil types

According to their resistant behavior these can be:

Cohesive or coherent soils: They are those in which, due to their physical and chemical characteristics, the application of external forces is necessary to separate their constituent materials. They are usually waterproof.
Non-cohesive or incoherent soils: They are those in which their constituent materials only have internal cohesion when they are wet and water fills the interstitial spaces. They are permeable.

Shear stresses and shear strength

When it comes to determining the stability of a slope, it is necessary to carry out studies to evaluate the internal stress state of the ground and the resistance to shear stresses that occur within the slope.

Shear stresses are internal loads that tend to produce relative tangential displacements of some layers of the material in comparison to others. Shear strength is the ability of the material to resist the action of shear forces.

When the shear forces exceed the shear strength of the material, the probability of slope instability and movement rises.

Hydrogeological aspects

Water is one of the elements that influences slope stability the most. When the slope material absorbs water, it increases its weight and, therefore, the load on the resistant elements. If the material is porous, water can seep into the cracks and decrease the stability of the floor.

The water can run off the external surfaces of the slope, causing erosion and weakening the structure. It can also seep, generating sub-surface flows, raising the water table and lowering the slope’s resistance to internal and external unbalancing forces.

For these reasons, it is extremely important to carry out the following hydrogeological studies:

Analysis of the water table: Amount of water accumulated below the base of the slope.
Analysis of the piezometric level: Indicative of the potential of the hydraulic forces within the slope.
Study of the porosity of the material: Related to the ability of the soil to absorb water.
Study of the flow networks: They allow to assess -approximately- the water current lines within the slope and their respective hydraulic pressures.

In general, for a slope to be stable, the balancing forces must be greater than the unbalancing forces.

What factors influence the stability of a slope?

The factors that influence the stability of slopes are classified as:

Conditioning factors: These determine the overall stability of a slope. Among them, the most important are the geological structure and lithological characteristics of the soil, as well as the hydrogeological conditions and the morphology of the terrain.

Triggering factors: These act as generators of instabilities or initiators of landslides, overturning, flows and movements that modify the shape and functionality of the slopes.

The factors that influence slope stability can be natural or result from human activities.

Natural factors

The natural factors that influence slope stability are mainly geological, climatological, hydrogeological and biological.

Geological factors: They include the type of geological formations, the displacement of tectonic plates, seismic and volcanic activity.
Climatological factors: Precipitation (rain, snow, hail) are triggering factors that can modify the stability of slopes.
Hydrogeological factors: Groundwater, the action of rivers, hydraulic erosion, the action of ocean waves against cliffs, melting and displacement of glaciers, among others. These phenomena have a great influence on the stability of slopes.
Biological factors: The vegetation cover serves as natural protection on the surface of the slopes. The roots contribute to increasing the stability and resistance of the soil, as well as to the absorption of water and mineral salts. The stems mitigate the dragging of material, preventing soil erosion. The leaves of the plants avoid the direct impact of precipitation on the ground.

Factors due to human activity

The factors caused by the action of man are generally linked to the construction of infrastructures, civil works or mining and/or agricultural activities.

Mining activities: They require the construction of slopes by excavation (open pit mining) or tunnels and galleries (underground mining), as well as the use of explosives whose expansive waves can affect the stability of the land.
Agricultural activities: They require the construction of hydraulic works such as canals, dams, irrigation and drainage systems, which affect the water table and piezometric level of the soil.
Construction of communication routes: They often require the construction of slopes and embankments for stabilization of the land and layout of roads or railways. Once built, the traffic of vehicles and trains imposes dynamic loads and vibrations on the floors.
Civil works, buildings and infrastructures: They generally need some type of slope stabilization, retaining walls, walls with anchors, etc. In the case of bridges, the use of buttresses and pillars is required, as well as the stabilization of the ground at the end of the work.

How are geotextiles used in slope reinforcement?

Geotextiles are used in conjunction with other construction materials to increase strength and improve slope stability.

Geotextiles, like geonets and geogrids, are a type of geosynthetic elements. That is, polymeric materials, used for geotechnical applications that include the stabilization of the internal structure of the slopes, and in some cases, serve as the basis for the plant species that are used to reinforce and stabilize the slopes.

Geotextiles are permeable fabrics, woven or non-woven, made up of polyester or polyolefin fibers, which allow the passage of water, but not the entrainment of solid material, which makes them ideal for drainage systems and protection against erosion.

Polyester geotextiles (PET): They are recommended in soils with a pH of 9, including organic, saline, ferruginous soils or with the presence of other metals such as copper, chromium, cobalt or manganese.
Polyolefin geotextiles: This group includes propylene (PP) geotextiles and high-density polyethylene (HDPE) geotextiles. They are useful for soils with a pH of 9 but can also be used in alkaline, organic, calcareous or clay soils.

Although geotextiles are not affected by corrosion, they can be degraded by other physical-chemical processes such as hydrolysis, oxidation or exposure to ultraviolet (UV) light. They are also susceptible to high temperatures, so the recommended operating temperature range is between 12°C and 30°C.

For this reason, geotextiles used for slope reinforcement must receive special chemical treatments and not be exposed to direct solar radiation. With proper treatment and protection, geotextiles have a useful life of 75 to 100 years.

What is the objective of determining the stability of a slope?

The final objective of determining the stability of a slope is to identify possible imbalances in its structure in order to proceed to eliminate or neutralize them. This is done through the application of different analysis and calculation methods, according to the nature and particular characteristics of each slope.

The methods of slopes analysis and calculation depend, firstly, on the type of instability or imbalances that they present and, secondly, on the type of corrective measures that are applied.

According to the instabilities and internal imbalances, the types of faults that occur on the slopes can be classified into the following groups:

Infinite slope failures: For practical purposes, a slope is considered to be infinite when the thickness of unstable material is small compared to the height of the slope, therefore, the slip surface is parallel to that of the slope.
Finite slope failures: In this case, the amount of unstable material is considerable and the failure occurs through an approximately cylindrical internal surface, defined by a radius of gyration (r) and a center of rotation (o).

There are two main types of slope stability corrective measures:

Homogeneous measures: They consist of improving the stability of a slope without modifying its original constituent materials. For example, lowering its water table or modifying its geometry, removing material from the head and adding heels or berms of the same material at the base of the slope.
Heterogeneous measures: These are measures that improve the stability of a slope by introducing elements other than the original materials of the structure, such as retaining walls, anchors, piles, sprayed concrete, geotextiles or plant species specially selected for this purpose.

In this way, the slope analysis methods take these characteristics into consideration to develop calculation procedures that allow the determination of safety factor. This expresses the confidence margin so that the slope does not fail.

Some of the most used slope stability analysis methods are:

Exact Methods: Planar Rupture Method and Wedge Rupture Method.
Approximate voussoir methods: Jambu method, Fellenius method and Simplified Bishop method.
Precise Voussoir Methods: Morgenstern-Price Method, Spencer Method and Rigorous Bishop Method.
Calculation Methods in Deformations: They are a particular type of numerical methods that lead to approximate solutions.

Determining the stability of a slope, using the appropriate calculation method and obtaining an appropriate factor of safety, allows the design and construction of safer and more reliable slopes.

What is necessary for slope stabilization?

To achieve slope stabilization, it is necessary to carry out geotechnical studies, analyzes and stability calculations, economic, environmental and safety considerations. This can be summarized in three steps:

First step: Specify the location and functionality of the slope. Based on this information, all the necessary geological, lithological, stratigraphic, edaphological, hydrogeological and climatological studies are carried out to identify possible instabilities, conditioning and failure-triggering factors.

Second step: Select the appropriate analysis and calculation methods. Then, decide the most appropriate engineering solutions for slope stabilization from a geotechnical point of view.

In this phase, it is decided whether homogeneous or heterogeneous corrective measures will be used. For example, the use of prefabricated concrete elements, geotextiles, walls with anchors, plant species for reforestation, modification of the geometry of the slope or lowering its water table.

Third step: Carry out the pertinent economic, environmental and social impact studies, always keeping in mind that the best solution will be the one that presents the lowest cost and highest safety margins to guarantee the stability of the slope over time.

The construction and stabilization of slopes is a specialized and very complex activity. It requires not only numerous studies and geotechnical capabilities, but also the participation of multidisciplinary work teams and, in some cases, significant financial investments.

For this reason, it is important to have the best professionals, with extensive and proven experience in the area, when carrying out a construction or slope stabilization project.

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