Mass Movement

Mass Movement

Heave

Heave is the slowest of all mass movements and refers to the slow lifting of soil particles perpendicular to the slope due to wetting/drying (soil expands and lifts when wet and contracts and falls back vertically when dry) or freezing/thawing (ice crystals expand forcing soil particles closer to surface of slope, when thawed the displaced material falls back vertically, gravity, moving down slope).

Soil creep

The slow downhill movement of dry unconsolidated material or soft rocks results in soil creep. Movement is less than 1cm/year. It occurs on vegetated slopes of more than 5 degrees.

Solifluction

Solifluction is the slow downhill flow of saturated soil. It is a common process in periglacial environments, where the seasonal thawing of the uppermost layer (the active layer) provides sufficient water to enable flow to occur. The water reduces the effects of cohesion and friction, thus promoting movement. The term gelifluction refers to solifluction that takes place on top of frozen ground.

Earth/debris flows

When regolith on slopes of 5-15 degrees becomes saturated with water, it begins to flow.

Mudflows

These involve the rapid movement of rock and weathered debris mixed with water down valleys - a turbulent, structureless mixture of sediment and water. They occur on steeper slopes and are much more rapid movements, exceeding 1km/hour.

Causes of mudflows

• Steep slopes

• Narrow valleys

• Deforestation/removal of vegetation

• A thick regolith

• Slope vibration (traffic, construction etc)

• Addition of water to slope

Landslide

Landslides are single dramatic events when a section of a hillside becomes unstable, shears away and moves downhill. The shear stresses in the slope exceed the shear strength of the soil or rock.

Causes of landslides

• Slope angle (steeper)

• Geological structure (fractures)

• Rock type

• Amount of water present (increase weight of slope)

• Removal of the toe of the landslip, basal undercutting

• Tectonic processes

• Alignment of the strata (especially if it is parallel to the slope surface)

• Loading of the head of the slope

Planar landslides

These are caused when weathered rock becomes detached from the parent rock usually along a bedding plane or joint. As a result, the break is often clean and straight. Gravity pulls the material as one down the slope along the surface of the rupture. The exposed upper cliff is known as the scarp.

Rotational slides

“Slumping”

As movement occurs along a curved failed surface causing the upper surface to tilt back. This often occurs when saturated, weaker rocks (clay) slide over stronger or impermeable rocks (granite). This can often be seen in coastal cliffs.

Rock fall

Sudden movements of debris on slopes of more than 40 degrees. Once material has broken from the rock surface, it will bounce or fall vertically and collect at the bottom of the slope as scree or talus.

Causes of rock falls

• Intense physical or chemical weathering

• Tectonic processes

• Slope angle (steeper)

• Geological structure (fractures etc)

• Alignment of strata - especially if vertical

• Basal undercutting (sea/river or human activity)

Classification

Carson and Kirby - 1972

Po Shan Road - facts

• 18th June 1972

• 16th-18th 650mm

• May-June 1400mm

• 67 deaths

• Upper slopes up to 30 degrees

• Granite and volcanic rocks (hydrolysis)

Po Shan Road - causes

• Rainfall

• Lack of vegetation

• Basal undercutting

• Construction site - loading of slope

• High population density

Po Shan Road - management

• Geotechnical Engineering Office of the Civil Engineering Development

• Hazard maps

• Piezometers

• Groundwater drainage (90m long, 20 y/o)

• Gabions

• Retaining wall

• Slope reinforcement

• Afforestation

• Slope regrading

• Land use zoning