6: Slopes and Discontinuities

Hazard

Any potential source of harm, such as a landslide, earthquake, or flood.

Exposure

The situation of people, infrastructure, and assets located in a hazard-prone area.

Vulnerability

The susceptibility of a community or system to harm from a hazard.

Risk

The probability of harm occurring, calculated as the product of Hazard, Exposure, Vulnerability, and sometimes Cost.

Orogeny

The process of mountain building through tectonic forces, such as continental collision, involving folding, faulting, and uplift.

Percent Slope

A measure of steepness calculated as (Rise / Run) * 100.

Slope Degree

The angle of a slope measured in degrees, calculated as the arctan (Rise / Run).

Mass Wasting

The downslope movement of soil and rock under the direct influence of gravity; synonymous with slope failure.

Landslide

A type of mass wasting involving relatively rapid downslope movement of earth materials.

Shear Force

The component of gravitational force acting parallel to a slope, which drives downslope movement.

Normal Force

The component of gravitational force acting perpendicular to a slope, which increases frictional resistance.

Shear Strength

The internal resistance of a material to sliding, primarily composed of friction and cohesion.

Angle of Repose

The steepest angle at which a pile of unconsolidated granular material remains stable.

Cohesion

The binding force between particles in a material, such as clay, that contributes to shear strength.

Discontinuity

A planar weakness in a rock mass, such as a joint, fracture, fault, bedding plane, or foliation, which reduces its overall strength.

Foliation

The parallel alignment of platy minerals in metamorphic rocks, creating planes of weakness.

Bedding Plane

A surface separating successive layers of sedimentary rock, representing a change in depositional conditions.

Consolidated Slope

A slope composed of cemented or compacted solid rock, where strength comes from chemical bonds.

Unconsolidated Slope

A slope composed of loose sediments, where stability relies on friction and cohesion between particles.

Sackung

The very slow, deep-seated gravitational deformation of a rock slope, often measured in millimeters per year.

Rock Fall

A type of mass wasting where rock fragments detach and fall vertically or nearly vertically from a steep face.

Talus Slope

A pile of rock debris (scree) accumulated at the base of a cliff from rockfalls.

Rock Slide

The movement of a large, coherent body of rock downslope along a planar discontinuity like a fracture or bedding plane.

Slump

A rotational slide of unconsolidated material along a curved failure surface, often triggered by excess water.

Head Scarp

The steep, often curved, upslope boundary of a slump or landslide where the material has pulled away.

Mudflow

A flow of saturated fine-grained sediment (silt and clay) that moves as a viscous fluid, often on gentle slopes.

Debris Flow

A rapid flow of saturated, coarse-grained material (sand, gravel, boulders) that often originates in steep stream channels.

Cut Slope

A slope created by cutting into and removing material from a natural hillside, typically for construction.

Fill Slope

A slope created by placing and compacting excess excavated material onto a hillside to form a stable embankment.

Balanced Cut and Fill

A construction method where the volume of material excavated (cut) equals the volume used to build embankments (fill).

Full Bench Cut

A construction method where the road is built entirely on a shelf cut into a steep hillside, with no fill material used.

Through Cut

A construction method where material is cut out to form a roadway through a ridge or hill, creating steep slopes on both sides.

Inclinometer

A geotechnical instrument used to monitor subsurface lateral movement and deformation within a slope or embankment.

Debuttressing

The removal of support at the toe of a slope, often by natural erosion or human excavation, which increases shear stress and instability.

Pore Water Pressure

The pressure of water within the pores of a soil or rock; increased pressure reduces effective stress and shear strength.

Surface Tension

The property of moist sediment where water films between grains create a binding force, increasing strength compared to dry or saturated states.

Progressive Failure

A slope failure mechanism where deformation and weakening occur incrementally over a long period, leading to eventual catastrophic collapse.

Key Block

In rock slope stability, a critically located block whose removal can trigger widespread failure of the slope.

Ground Shaking Trigger

The initiation of slope failure due to seismic waves from an earthquake, which can reduce shear strength and increase shear stress.

Revegetation

The process of replanting and rebuilding soil on disturbed slopes to stabilize them and prevent erosion.

Retaining Wall

A structure designed to resist the lateral pressure of soil on a cut or fill slope, providing stabilization.

Rock Bolt

A long anchor bolt used in rock engineering to stabilize rock slopes or tunnels by transferring load from unstable exterior to confined interior rock.

Avalanche Shelter

A defensive structure built to protect roads or railways from falling rock, snow, or debris.

Drainage Control

The management of surface and subsurface water through systems like ditches, drains, or pipes to reduce pore pressure and increase slope stability.

Slope Monitoring

The continuous or periodic measurement of slope movements using instruments like GPS, inclinometers, or crack meters to provide early warning of failure.