What is slope retention: definition, types and importance in civil engineering
Slope retention is the techniques and structures to stabilize and protect sloped terrains from soil erosion, structural failure, and aesthetic degradation. These systems are also known as anchored mesh systems. Slope retention improves aesthetics and promotes environmental sustainability using retaining structures, vegetation, and other methods. This prevents the slope from sliding or collapsing. These systems are designed on a site-to- site basis and crucial in civil engineering as unstable slopes damage roads, buildings, and other infrastructure.
Causes of slope instability
Understanding the cause of slope instability helps good slope management and prevention. They are:
Soil properties
When the shear strength of the soil is higher than the shear stress, like poorly compacted or loose soils, it leads to slope failure. Cohesive soils such as clays are more prone to plastic deformation making them fail under loading. Sandy soils, on the other hand, are more unstable under saturated conditions as capillary forces become less effective when water content increases.
Water infiltration
Water infiltrating the soil increases the pore pressure, reducing the effective stress and shear strength of the soil. Surface water runoff, especially when the soil has a low permeability, leads to landslides. In some cases, it also leads to hydraulic fracturing.
Vegetation
Vegetation removal through natural events like fire or human activities loses soil support causing excess water accumulation. It will also exacerbate erosion and instability.
Seismic activity
The dynamic forces generated during seismic activity, especially an earthquake, cause both vertical and horizontal stresses on slopes, which reduce the soil’s stiffness. This process is detrimental under saturated, undrained conditions, making slopes prone to failure.
Human activities
As urban populations expand, there is a growing demand for land for housing, agriculture and recreational activities. Poorly designed infrastructures lead to soil compaction and increased runoff, causing flooding. It also increases land degradation and instability.
Slope angle
The angle of a slope directly impacts the gravitational forces acting on the soil or rock mass. The steeper the slope, the greater the force of gravity acting on the material, hence more prone to sliding. Higher slope angles reduce the frictional resistance between the soil or rock layers.
Rapid drawdown
Rapid drawdown occurs when the water level in a reservoir or waterbody near the slope is quickly lowered. This leads to slope instability as reduction in water pressure loses the support of the soil or rock.
Types of slope retention methods
Different methods are there for the different challenges posed by soil types, slope geometries, and environmental conditions.
1. Geometric Techniques
Geometric methods modify the slope’s shape to reduce shear stress. Techniques include flattening the slope to decrease gravitational forces, excavating unstable soil or rock, constructing pressure berms for added support, re-compacting slip debris, and replacing materials with free-draining ones like gravel, crushed rock, and geotextiles to reduce pore water pressure.
2. Hydrological Techniques
Hydrological methods manage water within and around slopes, using drainage systems and inverted filters to lower groundwater levels and reduce pore pressure, preventing soil weakening.
3. Chemical and Mechanical Techniques
- Grouting: Injecting cementitious materials into soil or rock increases cohesion and shear strength.
- Retaining Walls: Structures such as gravity walls, cantilever walls, and anchored walls provide lateral support to slopes.
- Soil Nailing: Inserting steel bars into pre-drilled holes within the soil aids reinforce it against movement.
- Geosynthetics: Utilizing geotextiles and geogrids improves drainage and stability by reinforcing soil layers.
4. Vegetative techniques
Bioengineering: Techniques such as live stakes, where cuttings or branches are planted into the slope to grow and stabilize the soil, and erosion control blankets made from natural fibers (like coir or jute) ,establish vegetation on the slope.
Live Fascia and Brush Mattresses: These involve laying down brush or plant materials over slopes, facilitating immediate erosion control until vegetation takes root.
5. Structural Defense Systems
Active systems prevent material displacement on slopes using anchors to secure unstable materials and metal meshes to provide surface protection against falling debris. Passive systems contain displaced materials without limiting movement. It uses gabion walls to absorb energy from falling rocks and dynamic meshes to flexibly restrain displaced materials.
Importance of geosynthetics for slope retention
Geogrids are the means of soil reinforcement, interlocking with soil particles, to dissipate loads and enhance shear strength. Geotextiles are used as a filter and separator, which allows infiltration through limiting soil movement, a key element for slope stability. Geocells provide three-dimensional confinement of soil, improving stability on steep slopes by reducing erosion and strengthening load-bearing capacity. Geomembranes are used for impermeability and blocking infiltration of water that could cause slope instability. Finally, geonets allow drainage, helping to manage excess water and to alleviate pore pressure in the slope.
Design considerations
While designing slope retention systems, standard designs are utilised based on soil mechanics and other factors ensuring their effectiveness in numerous conditions.
Soil type
Cohesive soils such as clay behave differently under stress than granular soils such as sand, thus influencing the wall performance over time. Evaluating mechanical properties of the soil allows the engineers to estimate its stability with respect to the weight of the retaining wall.
Slope geometry
The shape of a slope i.e, angle, height, directly affects its stability. Steeper slopes are prone to failure because of increased gravitational forces on the soil mass.
Load and external forces
Loads are categorized into dead loads (weight of the wall and backfill), live loads (transient loads such as vehicles), and lateral earth pressures. These factors must be considered to prevent utilization exceeding capacity. Moreover, wind loads also affect design, predominantly in areas with large exposed surfaces.