Geology 229Engineering and Environmental

GeologyLecture 25

Mass Movement and Landslides (Ch. 14)

Mass Movement1. Introduction2. Mass movement definitions3. Mass movement agents4. Mass movement types

IntroductionLandsides, subsidence, rock falls, etc. are all related to a process of gradation (making rough shapes to be flat), a massive portion of earth mass is moved from one place to another place. Thus, we need discuss mass movement first.

What drives mass moving:Mass movement Agents

• Gravity• Running water• Glacial ice• Wind• Wave action (shorelines)• Strong ground shaking

(earthquakes, tsunamis)

Mass movement Classification

• Sharpe’s classification is a typical one for mass movement

• This is the Table 14.1 of the textbook

• It classifies the movement by direction of movement, and type of mass involving movement

Two concepts in Sharpe’s Classification

• Flow:Flow involves failure of the mass along

countless internal planes of slippage, along which small amounts of relative movement have occurred.

• Slip: Slip involves the movement of a solid mass

along a single failure plane or a well-defined failure zone. The center of gravity of the mass is translated downward under gravity.

TRB Classification

• TRB stands for the Transportation Research Board, a research advisory body under the umbrella of the National Research Council (NRC).

• This classification is shown as Table 14.2 of the textbook

Two concepts in TRB Classification

• topples:Blocks of rock or soil topple over – top heavy through the reduction in block size owing to the development of closely spaced fractures, also known as over turning, and is upside falling down.

• Lateral spreading: Lose shear strength by excessive pore pressure

during earthquake liquefaction.

Falls

• In Sharpes’ classification, falls was included in slips but it is separated out in the TRB Classification;

• For falls, the rock/soil mass travels most of the distance in the air.

Some concepts for slope failure

• Mass movement:The use of the term ‘mass movement’

has more geological emphasis, it is long-term, and occurred naturally.

• Slope movement: Use of ‘slope movement’ has more

engineering emphasis. It usually refers process that is short-term, and means to prevent its occurrence.

Some concepts for slope failure (cont.)

• Slope instability:The use of ‘slope instability’ has more engineering emphasis. The concept involves both materials and mechanisms of failure.

• landslide: The term ‘landslide’ is a general, collective term. It includes all (or at least most) slope failures.

• Complexa combination of 2 or more mechanisms defined above.

(a) Topple (b) Earth slump

(c) Rock slide (d) Soil creep

(e) Solifluction (f) earth flow(mud flow)

(g) Debris (h) lateral spreadavalanche

Lateral Spreading caused by the 1906 San Francisco Earthquake at Moss Landing, CA

(USGS Professional Paper 993)

10The Damage of Embankment StructuresThe Damage of Embankment Structures

ToyokoroToyokoro

Collapsed EmbankmentCollapsed Embankment

A massive landslide on the coast

A massive mud flow caused by landslide in winter time

Land subsidence and sinkholes

Sinkholes found in people’s backyard! (Cheshire, CT)

January 10, 2005: A 25-foot boulder blocks Topanga Canyon Blvd. near Malibu, southern California, after a massive mudslide killed 3 and had up to 21 missing (AP).

Highly fractured rocks for potential rock falls and toppling

Equilibrium Analysis

• For a piece of mass to move, particularly, before a slip to occur, the resisting force on the slipping plane must be greater than the driving force.

• At the point of slip starting, the driving force must be equal and start to prevail the resisting force.

Translational slip

For impermeably rocks, water may accumulate on interfaces of rock/soil or rock/debris blocks (the plane of weakness). If the driving force is greater than the resisting force on the weakness plane, translational slip may occur.

For the resisting force on the interface originated from the friction between the two blocks. The coefficient of friction for most solid earth materials is between 0.1 -1.5 as the extreme end values.

µ = tanφµ is a material property so is φ. Corresponding to 0.1 and 1.5, φ is between 5.7 and 56.3 degrees.

Rotational mass movement

Rock blocks can also more rotationally - toppling.

Toppling

When

b/h<tanθ b<htanθ

Toppling happens. where

b - the thickness of the block;

h - the height; and

θ- the dipping angle.

Question: If a dense limestone layer with height of 6.5 m dips into a road cut at 20 degrees, what joint spacing perpendicular to the bedding will cause toppling to occur?

b = htanθ = 6.5 x tan(20) = 6.5 x 0.364 = 2.37 m