Chapter 7

Mass wasting and

Subsidence

Objectives

• Basic processes, types of flow• Driving and resisting forces (safety factor) and

how it is related to slope stability• Slope processes and the influence of slope

angle, topography, vegetation, time etc• Influence of human use• Methods of identification, prevention., warning

etc• Processes related to land subsidence

Introduction

• Landslides and related phenomena cause damage and loss of life

• General term:– Mass wasting (gravity driven)

• Downslope movement of rock or soil as more or less coherent mass under the action of gravity

• Other terms: earth flows, landslides, mudflows, rockfalls, debris flows, creep

• Also to include: subsidence

Hazard Fact Sheet (USGS) • The term landslide includes a wide range of ground movement, such

as rock falls, deep failure of slopes, and shallow debris flows. Although gravity acting on an over steepened slope is the primary reason for a landslide, there are other contributing factors:

– erosion by rivers, glaciers, or ocean waves create oversteepened slopes – rock and soil slopes are weakened through saturation by snowmelt or heavy

rains – earthquakes create stresses that make weak slopes fail – earthquakes of magnitude 4.0 and greater have been known to trigger landslides – volcanic eruptions produce loose ash deposits, heavy rain, and debris flows – excess weight from accumulation of rain or snow, stockpiling of rock or ore, from

waste piles, or from man-made structures may stress weak slopes to failure and other structures

– Slope material that become saturated with water may develop a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.

Bases for classification

• Type of material involved

• How the material moves

• The moisture content

• How fast the material moves

Types of flow (remolded)• Creep

– Mm per year

• Debris flows– High velocities – (in the ocean, 100 km/h)

Landslides that move as a unit

• Slumps– Rotational slides

• Glide blocks– Coherent masses

• Falls– talus

Types of landslides

• Example on rock slide– Norwegian

valley

• Video clip

Slope types

• Hard rock/talus• Typical for Norwegian

valleys/fjords

Soft rock/sediments

Causes of debris flows

• Rain induced– Antecedent rainfall, saturated soil– Access rain– Depend on various parameters such as

vegetation

• Earthquake

• Man made effects

Forces on the slopeMechanics of slides

• Stability of a slope– Relationship between

• Driving forces• Resisting forces

• Slope stability is evaluated by computing a safety factor –SF-, defined as– The ratio of resisting forces to driving forces

• SF > 1, stable, SF< 1 unstable

CCy = C sin

Cx = C cos

30o

30o

W= mg

mg sin

mg cos

h=d sin

d = h/sin

Role of Gravity and Slope Angle

• Gravitational force acts to hold objects in place by pulling on them in a direction perpendicular to the surface.

• The tangential component of gravity acts down a slope: it causes objects to move downhill.

Role of Gravity and Slope Angle

• Shear stress is the downslope component of the total stress involved.– Steepening a slope by erosion, jonting it by

earthquake, or shaking it by blasting, can cause an increase in shear stress.

• Normal stress is the perpendicular component.

Figure B 13.1

Figure B 13.1

Friction angle

cohesion

Effective stress

Effective Stress

• Total stress consists of two parts:– One portion of stress is carried by water – equal intensity in

all direction– The other portion of stress is carried by the soil solids at

their points of contact– Total stress =effective stress ’+pore water pressure u

= ’ + u• Effective stress – the sum of the vertical components of the

forces developed at the points of contact of the solid particles per unit cross-section area of the soil mass. ’ = - u applicable to fully saturated soil

Principle of effective stress (Terzaghi, 1925 &1936)

Infinite slope analyses

• http://www.tagasoft.com/Calculators/slope/eqn_is1_html#input

Driving and resisting forces depend on:

• Type of materials

• Slope and and topography

• Climate

• Vegetation

• Water

• Time

Role of material

• Two basic patterns of movement– Rotational –Translational

Rotational slides

• Occur along curved slip surfaces

• Movements follows a curve

• Comment in slope soils

Translational slides

• They are planar

• Occur along inclined slip planes with a slope– E.g bedding plane– Weak clay layer

• Soil slips – Very shallow slides in soil above

Other types of materials and flow

• Shale slopes/ slopes on weak volcanic pyroclastic materials– Creep

• Earthflows; mudflows– Downslope flow of saturated materials

Role of slope and topography

• Slope, relative importance, note – The difference in height h is the important

one– More frequent slide on steep slope

• Note in marine conditions,– Steep slope, minor slides, more frequent

Slope angles and landslidesThe slope angle for each of the 145 recent landslides was determined from the

U.S. Geological Survey topographic maps. The slope angle is the angle between the horizontal and the ground surface. Figure 13 relates the frequency of landslides to the slope angle. The average slope angle for landslides is 22.2

degrees with 75% of the landslides on slopes greater than 15 degrees. One landslide is on a slope of only 5.7 degrees.

Fig. 13. The frequency of recent landslides for a given slope angle. The frequencies were determined by counting the number of landslides for 5-degree intervals of slope angle. The average slope angle for recent landslides is 22.2 degrees.

Marine slides

1 2 3 4 5 6 7 8 9 10 12 14 14

slope

The role of climate

• Climate influences – the amount and timing of water– the form, rain or snow

• Note different slides types in various climatic regions– Dry/cold/humid

The role of vegetation

• Veg. provides a cover– Cushions the impact of rain

• Veg/root systems – An apparent cohesion

• Veg adds weight to the slope

• Note the impact of deforesting

The role of water

• Slopes becomes saturated

• Infiltration of water deep into the slope, late response

• Water can erode, decreasing stability

• Quick clay

Quick clay

• Marine clay – flocculated (card house)

• Subaerially exposed (isostatic uplift)

• Fresh water drainage and leaching

• Reduced content of e.g. K+

• Reduced stability and collapse of clay structures

The role of time

• Classical case – Vaiont dam– Slide into the dam

Other factors

• Timber harvesting

• Urbanization– E.g. Rio de Janeiro

• Combination of steep slopes and fractured rocks covered with thin soil

• Slopes were logged • Urban development on slopes• Vegetation cover has been removed • Etc

Identifying Potential Landslides

• Aerial photographs

• Information on past landslides

• Soil properties etc

• Risk/probability of occurrence

Preventing landslides

• Difficult to prevent!

• Possible trigger mechanisms:– Loading the top of slopes– Cutting into sensitive slopes– Placing fills on slope– Changing water conditions on slope

Landslide Hazard Zonation

• Inventory maps

• Land risk map

• Control and Stabilization

Techniques for landslides prevention

• Provisions for surface and subsurface drainage

• Removal of unusable slope materials

• Construction of retaining walls

• Supporting structures

Drainage control

• Surface and subsurface drainage– Note the importance of effective stress

• Grading, material from upper part of a slope is removed and placed near the base– Increased resisting forces

• Cut into a series of benches

Slope support

• Retaining walls

• Other suggestions?

Warning methods

• Electrical systems

• Tilt meters

• Geophones

• In most cases, the cause of the slides is an increase in water pressure – note effective stress

• Note snow avalanches

Withdrawal of fluids-Subsidence

• Withdrawal of fluids– Oil/gas

• Ekkofisk oil field in the North Sea

– Groundwater

• Consequence– Subsidence

• E.g. Ekkofisk oil field• Central California, pumping of ground water

SinkholesSalt domes

• Voids, large open spaces such as caves formed by chemical weathering within soluble rocks, limestone, dolomites etc

• Serious subsidence associated with salt mining– Water is injected, salt dissolves and

supersaturated water is pumped out

Summary (1)

• Slope failure involve – Flowage, slumping, sliding, falling of earth

materials– Complex combination of sliding and flowage

• Forces – interaction of several variables– Type of material, topography, slope angle,

climate, vegetation, water and time

Summary (II)

• Common driving forces:– Weight of slope materials

• Note safety factor:– Ratio of resting versus driving forces, – SF > 1, stable slope

Summary (III)

• The importance of water– Water can erode the base of slopes– Excess water increases the weight– Reducing effective stress

– Reduction of resting forces

• Importance of human impact– Change of groundwater, logging, change of

vegetation etc.

Summary (IV)

• Minimizing landslides hazard– Mapping and monitoring– Prevention is difficult

• Possible techniques– Drainage control, grading of slopes,

construction of retaining walls

• Subsidence– Due to withdrawal of fluids, water, oil and gas

Vaiont Dam, Italy

• Vaiont Dam, 262m high, on the Vaiont River, a tributary of the Piave River, in Venetia, NE Italy in the south-eastern part of the Dolomite Region of the Italian Alps, near Belluno, about 100 km north of Venice.

• The dam, one of the highest in the world, was completed in 1961 • After heavy rains in 1963, landslides into the Vaiont reservoir

caused the stored water to spill over the dam, sweeping away the village of Longarone and flooding nearby hamlets.

• The Vaiont reservoir disaster is a classic example of the consequences of the failure of engineers and geologists to understand the nature of the problem that they were trying to deal with.

• During the filling of the reservoir a block of approximately 270 million m3 detached from one wall and slid into the lake at velocities of up to 30 m sec-1 (approx. 110 km h-1). As a result a wave over topped the dam by 250 m and swept onto the valley below, with the loss of about 2500 lives. Remarkably the dam remained unbroken.

•The slip surface of the slide was along weak bedding planes in the dipping limestone valley.

•Inadequate geological surveying of the mountain surrounding the projected lake contributed greatly to the disaster.

•The measures taken by the authorities (lowering the water level in the lake after prolonged rainfall), while intended to stem movement of the hillside, actually made the situation worse. 

•As the valley filled with water after completion of the dam in 1960, an ancient landslide on its upper southern side, adjacent to the dam, began to move again, in episodes of slow creeping movement.

•It was later established that this was due to groundwater, unable to escape into the floor of the now - flooded valley, saturating a layer of clay within the rocks beneath it.

Vaiont Dam

This accumulation of water high in the slope was most marked during periods of heavy rain, although the association of heavy rainfall and creep went unnoticed.

The presence of the impermeable clay layer in the bedrock was not recognized at the time and it was assumed that the movement was due to local saturation of the rocks below the level of water in the reservoir, at the toe of the creeping landslide, rather than accumulation of water pressure in the entire mountainside.

It was therefore proposed to regulate the movement of the landslide, and thus allow it to settle to a new equilibrium, by lowering the level of water in the lake when an episode of creep was in progress, until the toe of the landslide was no longer saturated and the creeping stopped.

The reservoir was then allowed to refill and the drainage cycle repeated whenever creeping movement occurred again.

Events that led to the disaster

• The engineers concluded that if there were any shearing, it would take place such that it would cause a chair like deformation. This would then result in a braking reaction. After analysing the seismic information gathered, it was determined that the walls had a very high modulus of elasticity and that even though small slides were likely to occur, the outcome of these slides could be managed

Events that led to the disaster• Around Feb 1960, the dam was filled and only one month later a tiny detachment

slide occurred. • Significant amounts of mass material continued to slide and eventually on Nov 4th, 70

000 m3 of material slid down into the lake. • The team of engineers decided to solve the problem by varying the water level in the

reservoir by constructing drainage and bypass tunnels. • From Oct 1961, to Sep 1963, the engineers who increased and decreased height of

the water controlled the reservoir levels. This was done with the intent to control the landslides.

• On Oct 9th 1963, a mass of earth and rock slid down the hill and blocked the gorge completely.

• The material eventually travelled 140m up the opposite bank and finally terminated its movement after 45 seconds. At the time the reservoir contained 115 million m3 of water.

• A wave of water was pushed up the opposite bank and destroyed the village of Casso, 260 m above lake level before over-topping the dam by up to 245 m. The water, estimated to have had a volume of about 30 million m3, then fell more than 500 m onto the villages of Longarone, Pirago, Villanova, Rivalta and Fae, totally decimating them. However the dam was not destroyed and is still standing today.

Vaiont dam• It is likely that increasing the level of the reservoir drove up pore

pressures in the clay layers, reducing the effective normal strength and hence the shear resistance.

• Resistance to movement was created by the chair-like form of the shear surface.

• Dropping the level of the reservoir induced hydraulic pressures that increased the stresses as water in the jointed limestone tried to drain.

• It has been estimated that the total thrust from this effect was 2 - 4 million tonnes.

• Failure occurred in a brittle manner, inducing catastrophic loss of strength. The speed of movement is probably the result of frictional heating of the pore water in the clay layers.