Slope Failures and Soil Mechanics Principles

Flashcards of key vocabulary and concepts from the lecture notes.

Landslide

A general term used to describe the downslope movement of soil, rock, and organic materials under the effects of gravity and also the landform that results from such movement.

Landslide

Downslope movement of rock or soil, or both, occurring on the surface of rupture-either curved (rotational slide) or planar (translational slide)-in which much of the material often moves as a coherent or semicoherent mass with little deformation.

Fall

Begins with the detachment of soil or rock, or both, from a steep slope along a surface on which little or no shear displacement has occurred. The material subsequently descends mainly by falling, bouncing, or rolling.

Rockfall

Falls are abrupt, downward movements of rock or earth, or both, that detach from steep slopes or cliffs. The falling material usually strikes the lower slope at angles less than the angle of fall, causing bouncing.

Topple

Recognized as the forward rotation out of a slope of a mass of soil or rock around a point or axis below the center of gravity of the displaced mass.

Slide

A downslope movement of a soil or rock mass occurring on surfaces of rupture or on relatively thin zones of intense shear strain.

Rotational Landslide

A landslide on which the surface of rupture is curved upward (spoon-shaped) and the slide movement is more or less rotational about an axis that is parallel to the contour of the slope. The displaced mass may, under certain circumstances, move as a relatively coherent mass along the rupture surface with little internal deformation.

Translational Landslide

The mass in a translational landslide moves out, or down and outward, along a relatively planar surface with little rotational movement or backward tilting.

Spread

An extension of a cohesive soil or rock mass combined with the general subsidence of the fractured mass of cohesive material into softer underlying material.

Lateral Spreads

Lateral spreads usually occur on very gentle slopes or essentially flat terrain, especially where a stronger upper layer of rock or soil undergoes extension and moves above an underlying softer, weaker layer.

Flow

A spatially continuous movement in which the surfaces of shear are short-lived, closely spaced, and usually not preserved.

Debris Flows

A form of rapid mass movement in which loose soil, rock and sometimes organic matter combine with water to form a slurry that flows downslope.

Lahars (Volcanic Debris Flows)

Also known as volcanic mudflows. These are flows that originate on the slopes of volcanoes and are a type of debris flow. A lahar mobilizes the loose accumulations of tephra (the airborne solids erupted from the volcano) and related debris.

Debris Avalanche

Are essentially large, extremely rapid, often open-slope flows formed when an unstable slope collapses and the resulting fragmented debris is rapidly transported away from the slope.

Earthflow

Can occur on gentle to moderate slopes, generally in fine-grained soil, commonly clay or silt, but also in very weathered, clay-bearing bedrock. The mass in an earthflow moves as a plastic or viscous flow with strong internal deformation.

Slow Earthflow (Creep)

The informal name for a slow earthflow and consists of the imperceptibly slow, steady downward movement of slope-forming soil or rock. Movement is caused by internal shear stress sufficient to cause deformation but insufficient to cause failure.

Translational Failure

A translational slide occurs when a slope fails along a weak zone of soil. Before coming to a stop, the sliding mass can travel a significant distance. Translational slides are common in coarse-grained soils.

Rotational Failure

A common type of failure in homogeneous fine-grained soils is a rotational slide. It has a point of rotation on an imaginary axis parallel to the slope. When rotational failure occurs, the failed surface will begin to move outwards and downwards. This occurs when rotation by a slip surface causes the slope surface to curve.

Wedge Failure

Occurs when forces from neighboring soils shatter a soil mass along joints, cracks, fissures, and weak zones, block or wedge slides occur. This failure can happen when the soil contains weak layers or joints that are created as the slope is constructed from two different, non-compatible materials.

Compound Failure

A combination of rotational and translational sliding failure.

Flow Slide

When internal and external factors drive a soil to behave like a viscous fluid and flow down even modest slopes, spreading out in multiple directions, a flow slide develops. In flow slides, the failure surface is poorly defined. Multiple failure surfaces are common, and they alter as the flow progresses.

Erosion

Natural and man-made slopes are constantly eroded by water and wind. Erosion alters the slope's shape, leading to slope failure or, more accurately, a landslide.

External Loading

Loads placed at the crest of a slope (the top of the slope) increase the gravitational load and can lead to slope failure. A berm, or load placed at the toe, will boost the slope's stability. Problem slopes are frequently remedied with berms.

Excavations

Total stresses are reduced during excavation, and negative porewater pressures are created in the soil. Negative porewater pressures dissipate with time, resulting in a reduction in effective stresses and, as a result, a decrease in soil shear strength.

Dutch Cone Test

A 60° cone with a base area of 1.55 inch (10 cm2) is pushed into the ground at a constant rate of approximately 0.8 inch (20 mm/s). The cone's resistance to penetration is determined, as is the sleeve's frictional resistance. The test is also known as the cone penetration test (CPT) and does not require borehole drilling.

Piezocone Penetration Test

Enables simultaneous measurements of cone resistance, qc, sleeve friction, fs, and pore pressure, ut, opening up new avenues for soil identification and classification, as well as the interpretation of soil parameters.

Vane Shear Test

This test entails embedding a four-blade vane in undisturbed soil and rotating it from the surface to determine the torsional force required to shear a cylindrical surface. This force is then converted to the shear strength of the undrained material.

Drained Conditions

Drained conditions are ones in which load fluctuations are slow enough, or last long enough, for the soil to establish equilibrium and there are no extra pore pressures induced by the loads. Pore pressures are governed by hydraulic boundary conditions in drained situations.

Undrained Conditions

Changes in load occur more quickly than water can flow into or out of the soil under undrained settings. The behavior of the soil in response to changes in external loads controls pore pressures.

Rapid (Sudden) Drawdown

The decreasing of the water level near to a slope at such a fast rate that the soil does not have enough time to drain appreciably causes rapid or sudden drawdown.

Earthquake

During an earthquake, the acceleration caused by the seismic ground motion causes the soil to be subjected to cyclically fluctuating stresses.

Vibrating Wire Piezometer

Are pressure-sensitive, underwater measurement sensors used in a range of geotechnical applications to monitor pore water pressure and ground water levels.

In-Place Inclinometers

Can detect new movement, an acceleration of movement, and the direction of movement. “In-place” inclinometers are installed in a borehole cased with inclinometer casing. The wiring for the inclinometer can be buried and the boring covered with a locking cap to vandal-proof the installation.

Tiltmeter

Are used to track and measure changes in the inclination and vertical rotation of a structure.

Vibrating Wire Extensometers

Are used in a number of geotechnical applications to measure the displacement or general deformation of rock, soil, and concrete structures.

Settlement Plates

Is the most popular type of settlement monitoring system.

Crack Gage

Are designed to measure movement across surface cracks and joints.