ESYS150LECTURE 10

MASS MOVEMENTSIMPORTANT CONCEPTS

Gravity

Creep/Landslides

External and Internal causes of slope failure

MASS MOVEMENTS

Falls

Slides

Flows

Subsides

IMPORTANT CONCEPTSROLE OF GRAVITY

Gravity causes the downward and outward movement of landslides and the collapse of subsiding ground.

Eventually it will flatten all slopes

The force of gravity is the mass of a body x the sine of the slope.

If can remove the initial resistance to motion the body will move.

Earthquake, heavy rain could give initial energy

IMPORTANT CONCEPTSCREEPMovement down slope of soil and uppermost bedrock

Block diagram showing the effects of creep.

Most commonly seen by its effects on telegraph poles, fences and trees.

The soil zone slips in ultra slow

movement as particles shift in response to gravity

IMPORTANT CONCEPTSCREEPHow creep works

Surface materials expand perpendicular to slope (1 to 2) as a result of freezing, wetting or heat of sun.

Upon thawing,drying or cooling they contract (2 to 3) under pull of gravity vertically.

Do not go back to old position. Thus have a slow movement down slope i.e. creep

IMPORTANT CONCEPTSLANDSLIDEFast moving mass-movement

Causes most fatalities.

Landslide is a mass whose center has moved downwards and outwards.

Has a tear-away zone upslope where material has pulled away and a pile-up zone where material had accumulated.

IMPORTANT CONCEPTSLANDSLIDEMajor topographic features

Features include crown, head scarp, basal surface of rupture, transverse cracks, transverse ridges and radial cracks. All created in the downward and outward movement

IMPORTANT CONCEPTSEXTERNAL PROCESSES CAUSING FAILUREThree major ones

On arcuate failure surfaces have balance between the driving mass and the resisting mass. Changing either can create a landslide

Processes include: 1) steepen slope, 2) remove support from bottom of slope, and 3) add mass high up on slope.

IMPORTANT CONCEPTSINTERNAL CAUSES OF SLOPE FAILUREClays

Clays form during chemical weathering due to acidic fluids such as water, CO2 charged water and organic acids decomposing minerals created at high pressures and temperatures.

Creates totally different internal structure. Clay minerals are built like books and have many unfilled atomic positions in the crystal structure.

Typically can have their strength dramatically reduced by adding water which also causes expansion.

IMPORTANT CONCEPTSINTERNAL CAUSES OF SLOPE FAILUREQuick Clay: Ontario, Canada 1993

Fine grained rock flour left behind during the retreat of the glaciers and deposited in a nearby sea. The clay and silt particles are loosely packed and held together as a rock by sea salts.

When the sea retreats, the sediments are uplifted and the glue removed by fresh water. Anything can cause the house of cards to collapse

IMPORTANT CONCEPTSINTERNAL CAUSES OF SLOPE FAILUREThe five roles of water

1) Sediments have high porosities. When these void spaces are filled with water the sediment is much heavier and the driving mass increased.

2) Water is easily absorbed and attached externally to clay minerals with a major decrease in strength.

3) Water flowing through rocks can dissolve the minerals that bind the rocks together. The removal of the cement makes the rock easier to move or a slope easier to collapse.

4) Water can physically erode loose material creating caverns.

5) Pressure builds up in water trapped in the pores of sediments being buried deeper and deeper. Sediments can compress but water does not compress. Get abnormally high pore-water pressures which “jacks up” the sediment and makes it very easy to move.

IMPORTANT CONCEPTSINTERNAL CAUSES OF SLOPE FAILUREThe role of flowing water

Schematic cross section of ground water flowing through poorly consolidated rock. The water will carry sediments to the stream creating a series of caverns that seriously weaken a hill.

IMPORTANT CONCEPTSINTERNAL CAUSES OF SLOPE FAILURESlope stability

Addressed by use of Coulomb/Terazaghi equation where

s = c + (p - hw) tan øWhere s = resistance due to shear, c = the cohesion of the sediment layer p = load of sediment and water above a slide surface

hw = weight of water above the potential surface. = internal angle of friction.

Strength comes from cohesion + the weight of the sediment.Weakness from the pore water pressure and the internaL angle of friction.

Clays have high cohesion but a very low failure angle. Sands have poor cohesion.Granites have very high failure angle.

IMPORTANT CONCEPTSINTERNAL CAUSES OF SLOPE FAILUREQuick sand

Example of the Coulomb-Terazaghi equation.

The pore water pressure hw equals the weight of the sands p.

Leaves cohesionless sand with no shear stress.

With no shear stress you will sink into the sand when you walk on it.

IMPORTANT CONCEPTS

ADVERSE STRUCTURES

1) Ancient slip surfaces are weaknesses that tend to be reused over time. These surfaces are especially slippery when wet.

2) The orientation of the sediment layers can create strong or weak conditions. Sediment layers dipping into the hill are very stable, dipping in the same direction but shallower than the slope have daylight bedding. Potentially dangerous condition.

3) Rocks have inherent weakness that set-up slope failure. Lack of cement, clay layers, soft rocks, splitting joints, faulting surfaces.

TRIGGERS

Basic causes bring slopes close to failure.Rain, earthquakes or humans create trigger.

MASS MOVEMENTSCLASSIFICATIONSpeed of movement and water flow.

On left have mass movement speed versus moisture content.

On right have rates of travel for mass movements

MASS MOVEMENTSCLASSIFICATIONFalls, Flows, Slides and Subsides.

Falls and subsides involve vertical drops. Slides and flows involve downward and outward motion.

Sliding involves a coherent mass.

Flowing involves the moving mass behaving like a viscous fluid.

MASS MOVEMENTSFALL10 July 1996 Yosemite

MASS MOVEMENTSSLIDESRotational

Downward and outward movement on a curved surface.

Note the rotation of head and the up movement of the toe.

Swedish circle analysis of slope stability has a compass set at the center of rotation. Use this to compute driving and resisting forces.

Note backward tilted head and bulged toe. Toe helps stability, tilted head catches water.

MASS MOVEMENTSSLIDESRotational: 1976 Ensenada slide

The slide was associated with a pronounced head scarp shown here.

A rotation of coherent beds downwards and outwards and the formation of a pronounced bulge.

Not all rotational slides are as simple as this because of discontinuities on the surface.

MASS MOVEMENTSSLIDESRotational slide: 1976 Ensenada

Top) View northward along Highway 1 showing seaward shift of the highway. This was created by the downward and outward movement of the center of the slide.

Bottom) Toe of the Ensenada slide. Note that the ocean floor was lifted above sea level stabilizing the slide.

MASS MOVEMENTSTRANSLATIONAL SLIDESThree types

1) Move as coherent blocks. Pt Fermin 1929

2) May deform and break-up as a debris slide. 1963 Viaont, Italy

3) Involve lateral spreading where the underlying material fails and flows, Anchorage, Alaska 1964

MASS MOVEMENTSSLIDESTranslational slide: Pt Fermin, Ca 1929

Cross section showing block on top of a inclined slippery layer which day lighted under the ocean.

Block moves towards the unsupported offshore.

Though triggered by watering from yard irrigation seeping down to layers of weak clays which expand and lose strength and sliding begins.

MASS MOVEMENTSSLIDESTranslational: Head of Point Fermin 1929 slide

MASS MOVEMENTSSLIDESTranslational Debris Slide: Vaiont Italy 1963

Reservoir built on a) sedimentary rock layers with beds dipping towards the reservoir, b) fractures formed in the rocks due to expansion after retreat of glaciers and river cutting Canyon, 3) weak clay layers and numerous Limestone caverns.

After very heavy rains the slide slumped down the fracture surfaces into the reservoir.

MASS MOVEMENTSSLIDESTranslational slide: Debris Slide, 1963 Vaiont Italy

Huge area of slope slid into the reservoir. Debris at 150 M above the water level. A huge wall of water climbed over the dam and swept down the river bed as a 70m high wall of water causing extensive fatalities in the towns down stream. The Dam stood. Has been called world’s worst dam disaster

MASS MOVEMENTSFLOWSMovements that behave like fluids with internal reorganization

Loess flows - the flow of loose silt, Gansu Province China, 1920. The mountains walked.

Earth flows - wet flows moving slowly on slick surface, Portuguese Bend Ca, 1958

Debris flows (Sturtzstroms) - massive rock falls that convert into highly fluidized rapidly moving Debris flows. Elm, Switzerland, 1881

MASS MOVEMENTSFLOWSEarth flow: Portuguese Bend 1958 and following.

Cross section through Portuguese Bend showing the seaward dipping reflectors, bentonitic clay layers, a preexisting slide surface and waves eroding the base of the slope.

An ancient earth flow site which was unstable but not moving. 1950’s put development on area by beach. No sewers. Within a few years lower slope started to move.

Watering and sewage disposal seeped down to bentonite layers. They expanded, lost strength and started to move.

MASS MOVEMENTSFLOWSEarth flow: Portuguese Bend 1958

View from the sea. Note the pier and Crenshaw Boulevard.

MASS MOVEMENTSFLOWSEarth flow: Portuguese Bend

View of the toe of the bulged up earth flow in 1959. Note the remains of the houses and the roads and the damaged pier.

MASS MOVEMENTSFLOWSSlumps and Debris flows, La Conchita

Heavy rains led to failure at depth triggering a slump and earthflow in 1995 which moved very slowly during day. No loss of life.

Heavy rains led to a faster moving debris flow in 2005 which killed 10 people.These flows typical of coastal flows near coast in southern California.

MASS MOVEMENTSFLOWSDebris flow: Elm, Switzerland 1881

Three stages, rock fall, a jump and fluidization of the debris, and then a flow of the disintegrated rock mass down the valley floor using air and dust as the internal fluid.

Many fatalities in Elm 2.25 km away.

MASS MOVEMENTSFLOWSSubmarine flows: Hawaii

Left) Slump and debris-avalanche deposits cover more than five times the area of the islands.

Right) Created by the collapse of flanks of the Islands. The zone of normal faults associated with the injection of magma at Kilauea appear to be head scarps for giant mass movements.

MASS MOVEMENTSSUBSIDENCE

SLOW

Delta compaction, New Orleans

Oil withdrawal, Houston

Groundwater withdrawal, Mexico City

CATASTROPHIC

Limestone sinkholes

MASS MOVEMENTSSUBSIDENCESlow: Mississippi Delta, New Orleans

Left) Loosely packed sand and clay have high porosities near the surface. When buried they expel the water and contract over a period of time.

Right) As much as 100 ft of subsidence of the Mississippi Delta around New Orleans over the past 20,000 years. Has been faster recently up to 3m in the past 50 years making city very prone to damage from hurricanes.

MASS MOVEMENTSSUBSIDENCECatastrophic: Sink Holes

View of sinkhole formed by the collapse of a limestone cavern on May 31 1981 in winter park Florida. Sinkhole is 100 m across by 30 m deep.

Most of southern and eastern US lies on limestone rocks created from the deposition under seawater of soft shells made of calcium carbonate.

Uplifted above seawater they are prone to dissolution from circulating acidic fresh ground water. Creates unstable caverns underground.