A landslide is the movement of rock, debris, or earth down a slope under the influence of gravity. This occurs when the forces pushing material downslope exceed the slope material’s internal shear strength or resisting forces. While gravity is the primary driving force, common triggers include water saturation, which adds weight and reduces soil strength, and the presence of weak geological material. Prevention requires engineering and biological methods designed to decrease driving forces or increase resisting forces.
Managing Subsurface Water
Water saturation causes slope instability by increasing soil mass weight and reducing friction and cohesion between particles. The most effective way to address this is by lowering the groundwater table, which reduces pore water pressure. Reducing this pressure increases the soil’s resistance to movement.
Engineers manage surface runoff using ditches, berms, and catch basins installed above unstable areas to divert water away from the slope. These surface drains must be properly lined and graded to ensure rapid flow and prevent water from seeping into the ground.
For deep-seated instability, subsurface drainage is required to pull water out of the soil mass. This is often achieved using horizontal drains, which are perforated pipes drilled into the slope. These drains allow groundwater to flow out by gravity, increasing effective stress and boosting shear strength.
Alternative subsurface techniques include trench drains and vertical relief wells. Trench drains are gravel-filled trenches that intercept shallow groundwater flow and redirect it away from the potential failure plane. Vertical relief wells lower the deeper groundwater table by pumping or gravity flow, useful where water is trapped beneath less permeable layers.
Structural Support Systems
When drainage and geometry modification are insufficient, structural support systems physically restrain the unstable slope. These systems increase resisting forces by introducing artificial elements that buttress, anchor, or reinforce the soil mass. System selection depends on the depth and mechanism of the potential failure surface.
Retaining structures, such as cantilever or gravity walls, hold back the soil mass and prevent downslope movement. For complex slides, deep foundation elements like micropiles or steel piles transfer the unstable soil load to deeper, stable layers. These piles act as shear pins, physically stabilizing the sliding mass.
Internal slope reinforcement increases the shear strength of the slope material. Soil nailing involves drilling steel bars into the slope face and grouting them in place to create a cohesive reinforced zone. This increases the internal strength, often followed by a facing of shotcrete to prevent surface erosion.
Ground anchors or rock bolts are tensioned cables installed into the slope and secured deep within stable rock formations. These anchors exert a compressive force on the unstable mass, which increases the normal stress across the failure plane and boosts resistance to sliding.
Modifying Slope Geometry
Modifying slope geometry involves physically reshaping the hill to reduce gravitational driving forces. This approach alters the mechanical balance of the slope by reducing the slope angle or redistributing the soil mass.
Slope reduction, or flattening, decreases the overall steepness of the slope to a stable angle of repose. Cutting back the slope face significantly reduces the driving force component of gravity. This method requires excavation but provides a long-term solution.
The creation of benches or terraces divides a steep slope into shorter, less steep segments. These steps interrupt the slope’s continuity, reducing the distance material can travel and allowing for controlled diversion of surface runoff.
Instability concentrated at the top of the slope can be addressed by removing unstable material, known as unloading the head. Removing weight decreases driving forces. Conversely, adding a buttress fill to the toe of the slope increases resisting forces by providing a counterweight against movement.
Bioengineering and Vegetation
Bioengineering uses live plants and plant materials, often combined with minor civil structures, to stabilize slopes. This approach is suited for preventing shallow slides and surface erosion where the failure plane is close to the ground surface. Plants stabilize slopes through mechanical reinforcement and hydrological regulation.
Deep root systems of grasses, shrubs, and trees act as natural tensile reinforcement, binding shallow soil particles and increasing shear strength. This root matrix anchors topsoil to deeper, stable layers, preventing surface slumps. Fast-growing species with dense, fibrous root systems are commonly selected.
Vegetation manages water through evapotranspiration, where plants draw water from the soil and release it as vapor. This process removes excess moisture, helping maintain lower pore water pressures during heavy rainfall. The plant canopy also intercepts rainfall, minimizing surface erosion.
Specific bioengineering techniques include live fascines, which are bundles of viable branch cuttings placed in trenches perpendicular to the slope contour. These bundles provide immediate physical support and trap sediment. As the cuttings root and sprout, they form a continuous, living barrier that reinforces the slope.