An aerial view of the Oso landslide's bare source scar, debris path and deposits across the valley.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

Selected circular slip. 1 scene unit = 10 m. no vertical exaggeration.

Initial-slope factor of safety 1.48 · maximum pore pressure 16.0 kPa. Resistance exceeds driving demand here: increase pore pressure to compare.

How to read these results

The Bishop calculation uses cohesion of 2 kPa, a friction angle of 15° and unit weight of 20 kN/m³ for the selected initial slope.

Mass movement

Identify the main parts of a rotational slump and explain how rainfall can reduce the stability of a slope.

Drag to rotate. Hold Ctrl and scroll to zoom, or pinch on a touch screen.

Why it happens

Gravity drives a slope downhill while friction and cohesion resist movement. Rising pore-water pressure can reduce effective stress and resistance, making a chosen slip surface less stable.

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Oso, Washington, 2014: an aerial photograph of the landslide source, travel path and deposit. The displaced material crossed the valley floor.

Try a comparison

Increase Pore pressure and relate falling resistance to effective stress. At the highest setting, play the prescribed rotational movement.

Reading the result

The factor of safety describes the initial slope and selected surface; the later rotation is an illustration of movement, not a calculated runout.

A common misunderstanding

Rain can destabilise a slope only by making the material heavier.

Added water can increase weight, but rising pore-water pressure can also reduce effective stress and frictional resistance along a potential slip surface. Drainage, cohesion, slope geometry and material properties influence stability, so the effect is not explained by added mass alone.

Cases

Exam language

Rotational slump

A landslide in which a mass of soil or rock rotates along a curved slip surface. The upper part commonly drops and tilts back, while material moves outward near the toe. Water pressure can reduce the slope’s resistance to failure.

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Slumping above Redcliff Point, Dorset. Rotational movement displaces blocks along a curved shear surface; the surface scars and tilted ground provide evidence.

Shear surface

A surface along which adjacent masses move relative to each other. In a landslide it separates moving material from the underlying slope and may be curved or nearly planar. It is distinct from the exposed cliff or landslide scar.

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Oso, Washington, 2014: an aerial photograph of the landslide source, travel path and deposit. The displaced material crossed the valley floor.

Crown

The upper edge of a landslide, where displaced material separates from stable ground. Cracks near the crown can outline the developing failure. It is distinct from the toe, where displaced material accumulates farther down the slope.

Toe

The downslope end of a landslide or other mass-movement deposit. Displaced material may thicken or bulge there as it slows. It differs from the crown, where material separates from the stable slope.

Pore pressure

The pressure of water or gas within pores in soil or rock. Increasing water pressure can reduce effective stress and resistance to slope failure.

Till

Unsorted sediment deposited directly by glacier ice, often containing a mixture from clay to boulders. Its poor sorting distinguishes it from many meltwater deposits reworked by flowing water. Till can form several types of glacial landform.

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A natural exposure of glacial till. Ice deposits a mixture of grain sizes directly, often without the sorting produced by flowing water.

Mass movement

The downslope movement of rock or soil under gravity, including falls, slides and flows. Water can increase weight or pore pressure and may be mixed with the moving material. Gravity provides the driving force rather than a transporting river or wind.

Process0.40Pore pressure0.15