Question

Explain the difference between a stable and unstable slope. What factors determine the angle of repose of a material? What features are likely to serve as failure surfaces?

Short answer

A stable slope is in equilibrium with balanced forces, while an unstable slope experiences movement or failure. The angle of repose, which is the steepest angle that a material can form before becoming unstable, depends on factors like particle size, shape, moisture content, and internal friction. Failure surfaces, which are areas where structural integrity is compromised, can include features like geologic discontinuities, changes in material properties, groundwater, and human-induced disturbances.

Step-by-step solution

Define stable and unstable slopes

A stable slope is a slope that remains in equilibrium, where the forces acting upon it are balanced, and the slope does not experience any movement or failure. On the other hand, an unstable slope occurs when the forces acting upon it are no longer balanced, causing the slope to move or fail. This can lead to a landslide, a rockslide, or other types of slope failures.

Determine the angle of repose of a material

The angle of repose is the steepest angle that a granular material can form before the slope becomes unstable. Several factors determine the angle of repose of a material: 1. Particle size: Smaller particles have a larger surface area for contact and friction, leading to a higher angle of repose. 2. Particle shape: Irregularly shaped particles interlock and provide more resistance to sliding, resulting in a higher angle of repose. 3. Material moisture content: Moisture can increase the cohesion between particles, leading to a higher angle of repose. However, too much moisture can decrease friction, resulting in a lower angle of repose. 4. Material internal friction: Materials with higher internal friction are more resistant to sliding and have a higher angle of repose.

Identify features likely to serve as failure surfaces

Failure surfaces are planes or zones where the structural integrity of a material is compromised, leading to a slope failure. Some features likely to serve as failure surfaces include: 1. Geologic discontinuities: Joints, bedding planes, and faults are natural planes of weakness in rocks that may act as failure surfaces. 2. Changes in material properties: A sudden change in material properties, such as varying grain sizes or internal friction, can create weak zones that may serve as failure surfaces. 3. Groundwater: The presence of groundwater can reduce the shear strength of a slope, making a failure more likely. 4. Human-induced disturbances: Activities such as excavation, loading, or artificial drainage can create or activate potential failure surfaces. In summary, stable slopes have balanced forces, while unstable slopes experience movement or failure. The angle of repose is determined by factors such as particle size, shape, moisture content, and internal friction. Features such as geologic discontinuities, material property changes, groundwater, and human-induced disturbances can serve as failure surfaces in slopes.

Key concepts

Angle of Repose

Understanding the angle of repose is crucial when analyzing the stability of slopes made from granular materials, such as sand or gravel. It represents the steepest angle at which a pile of unconsolidated grains remains stable and is a visual guide to the slope's inherent stability.

The factors that determine this critical angle are complex and interconnected. Particle size can increase stability; smaller grains create a higher angle due to their increased surface contact and friction. The shape is also a key component, as non-spherical particles can interlock and resist movement better than smooth, round ones. Moisture content plays a dual role: a certain amount can bind particles together and increase cohesiveness; too much water, however, acts as a lubricant, significantly reducing the angle.

Material internal friction, often measured as a 'friction angle,' is another factor. Materials with higher intrinsic frictional resistance will exhibit greater resistance to slippage, thereby sustaining a steeper angle of repose. Yet, it's not just about a single grain's properties — the overall pile's behavior under gravity's pull and the frictional forces between grains dictate this defining angle. Therefore, both natural occurrences and human activities affecting these factors can alter the stability of slopes.

Slope Failure

The term 'slope failure' describes the process where a slope becomes so unstable that it cannot sustain its own weight or resist external forces, ultimately leading to its collapse. This can manifest as landslides, rockslides, or even gradual soil movement.

Critical to understanding slope failure is the balance, or rather imbalance, of forces. Shear strength, which helps to maintain the slope's integrity, is pit against the shear stress, the force that works to destabilize the slope. When the stress becomes too great or the strength too reduced, failure occurs.

Contributory factors include gravitational forces, saturation of the slope material (causing a reduction in shear strength), overloading (from natural accumulation or human activities), and removal or erosion of support from the slope's base. Human activity should not be underestimated — construction, deforestation, and water management can all upset the precarious equilibrium of a slope. Understanding these triggers can help in predicting and preventing failures, mitigating risks for local communities, and preserving the landscape.

Failure Surfaces

A 'failure surface' is typically a plane within the slope along which cohesion is lost, leading to the separation and movement of a mass of soil or rock downward. Identification of potential failure surfaces is a vital part of geotechnical investigations.

Naturally occurring planes of weakness such as joints, faults, and bedding planes contribute to creating these surfaces. They are preemptive lines along which a slope is more likely to fail, as they represent a drastic change in material resistance or cohesive properties of the slope.

However, not all failure surfaces are natural. Human activities can induce weaknesses too — excavation, loading of landmasses at the crest, or even poorly managed water can create an artificial disintegration point. Additionally, a change in material properties like a transition from high to low friction materials can spur the creation of failure surfaces.

Managing water content is especially pivotal; it affects the hydrostatic pressure within slopes and can rapidly diminish the effective stress — the component of soil strength keeping it cohesive. By recognizing these areas before wide-scale construction or engineering projects, steps can be taken to reinforce prone areas, thereby reducing risk and enhancing long-term stability.