L3-Landslides_Triggers - The Unstable Earth: Landslides...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: The Unstable Earth: Landslides Part 3 P t 3: Causes & Triggers 1980 Mt. St. Helens landslide & eruption, USA 1 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Landslide Learning Goals III By the end of this lecture, you will be able to: 1) Compare and contrast landslide causes and how they differ from landslide tri ers triggers. 2) Compare and contrast several of the key triggers of landslides and how they affect the force balance equation (i.e. Factor of Safety). 3) Differentiate the mechanism by which liquefaction landslides develop in loose sands and sensitive clays. 4) List the different human activities that contribute to increased landslide hazards. 2 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 1 Landslide Processes When assessing if and/or when a landslide might occur, it’s important to understand that the processes involved comprise a continuous series of events from cause to effect. effect Cause geological mechanical hydrological geomorphological biological bi l i l human Effect Trigger rock slide topple mud flow rock fall f debris flow Cause: makes slope susceptible to movement without actually initiating failure. 3 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Landslides are Complex Systems!! Causes geological mechanical hydrological geomorphological biological human Cause: makes slope susceptible to movement without necessarily initiating failure. 4 of 31 Landslides: Erik Eberhardt Trigger: initiates failure. EOSC 114 – 2009/10 2 Landslide Processes Cause Effect geological mechanical hydrological geomorphological biological human rock slide topple mud flow rock fall debris flow Trigger Cause: makes slope susceptible to movement without actually i iti ti t ll initiating failure. major rainfall inf ll events Trigger: initiates failure. 5 of 31 FS=1.0 Landslides: Erik Eberhardt gradual weakening of the slope Catastrophic Failure EOSC 114 – 2009/10 Landslide Causes and Triggers Cause Landslides may have several causes, including geological, morphological, morphological physical and human… geological mechanical hydrological h d l i l geomorphological biological anthropogenic … but only have one trigger. 6 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 3 Landslide Triggers FS = resisting forces driving forces = water pressures shear strength (F) friction cohesion [n – ] shear stress () FS > 1.0 represents a stable situation FS < 1.0 denotes failure Triggers intense rainfall rapid snowmelt p m water-level change volcanic eruption earthquake shaking rapid erosion 7 of 31 By definition, a trigger is an external stimulus causing a near-immediate response in the form of a landslide by… … rapidly increasing stresses … reducing the strength Landslides: Erik Eberhardt EOSC 114 – 2009/10 Natural vs. Engineered Slopes October 21, 1966, 144 people, 116 of them children, were killed when a tip of coal waste slid onto the village of Aberfan in South Wales. 8 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 4 Earthquake Triggers Strong ground shaking during seismic activity is a very common triggering mechanisms for landslides in many different topographic and geologic settings. Area of Landslides (km 2) 1000000 100000 10000 Rock Avalanche 1000 Rock Slumps 100 Soil Flow 10 Falls LIQUEFACTION 1 0.1 0 2 4 6 8 Earthquake Magnitude 9 of 31 Landslides: Erik Eberhardt Earthquakes as small as M 4.0 may trigger landslides in l d lid i susceptible slopes, and larger earthquakes can 10 generate thousands of landslides. EOSC 114 – 2009/10 Earthquake Triggers – Rock Fall Rockfalls: Earthquake shaking often dislodges loose rock resting on steep slopes. 10 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 5 Earthquake Triggers – Rockslides Earthquakes can be doubly destructive as they both induce shear stresses and weaken the internal structure of the slope material. 56% of the total cost of damage of the 1964 M9.2 Alaska earthquake was caused by earthquake-triggered landslides. 11 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Huascaran Rock & Ice Avalanche In 1970, a M7.5 earthquake 30 km off the coast of Peru triggered a 60 million m3 avalanche of ice and rock from the summit of Nevado Huascaran, the highest peak in Peru. Peru The slide debris travelled some 20 km, reaching speeds of over 100 m/s. Part of the landslide jumped a 200m ridge, wiping out the town of Yungay and killing all but about 100 of its 20,000 inhabitants. 12 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 6 Huascaran (Before and After) 13 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Earthquake Triggers: Spreading-Type Landslides SpreadingIn areas where loose soils are saturated with water, seismic shaking (i.e. from an earthquake) can cause water pressures to dramatically increase. This results increase in liquefaction, the sudden loss of strength in the soil transforming its behaviour into that of a liquid. zero [n – ] The most significant cause of damage resulting from the Alaska earthquake was due to liquefaction landslides (spreads). The ground dropped an average of 11 m with houses sliding about 180 m. 14 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 7 Liquefaction - The Fraser Delta The False Creek area is at risk because it is fill. A C B E D 10 The Fraser Delta is underlain by deep water-saturated, loose, silty-sand deposits. This makes it highly vulnerable to earthquakes and damage due to liquefaction landslides (i.e. lateral spreads). 15 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Sensitive Clays & Spreads Sensitive clays are young marine clays that are deposited with a “cardhouse” like structure. During an earthquake, this cardhouse structure may collapse and liquefy. 16 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 8 Canada’s Quick Clays The St. Lawrence river valleys are lined with thick beds of sensitive marine clays. Upon disturbance (e.g. earthquake), these soils may undergo a significant loss of shear strength, liquefying and flowing as a viscous mud, rapidly devastating large areas of relatively flat terrain. 17 of 31 1993 Lemieux, Ontario Landslides: Erik Eberhardt EOSC 114 – 2009/10 Triggers – Volcanoes Deposition of loose volcanic ash on hillsides commonly is followed by accelerated erosion and frequent mud or debris flows triggered by intense rainfalls. Volcanic eruptions have directly gg g triggered some of the largest historic landslides. Lahar - debris/ mudflow originating on the slopes of a volcano. They occur when pyroclastic flows rapidly melt snow/ice, saturating loose volcanic deposits. 1985 Nevado del Ruiz, Colombia - Within 4 hours of erupting, lahars travelled 100 km and left behind a wake of destruction: 23,000+ deaths!! 18 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 9 Volcano Triggers - Mount St. Helens The 1980 Mount St. Helens eruption is an interesting case of a volcanic eruption triggered by a massive 2.8 km3 rockslide, rockslide which was itself triggered by ongoing volcanic activity. 2 1 http://www.youtube.com/ watch?v=bgRnVhbfIKQ 19 of 31 3 Landslides: Erik Eberhardt 4 EOSC 114 – 2009/10 Volcano Triggers - Mount St. Helens 20 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 10 Triggers – Human Activities (Anthropogenic) Triggers intense rainfall rapid snowmelt water-level change volcanic eruption seismic loading rapid erosion anthropogenic 21 of 31 In addition to the natural occurrence of many landslide triggers, these mechanisms may also be induced through human activities. g … … … … … … excavation of slope near its toe loading of slope at its crest drawdown of water reservoirs irrigation blasting (mining, construction, etc.) jumping up and down on edge of cliff Landslides: Erik Eberhardt Case Study - Frank Slide EOSC 114 – 2009/10 Turtle mountain before the slide. The collapse of Turtle Mountain on April 29th, 1903 destroyed part of the town y p f of Frank, Alberta, and claimed the lives of 76 people (Canada’s worst natural disaster in terms of fatalities). 22 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 11 Frank Slide - Anthropogenic Trigger? pre-1903 W-SW E-NE 23 of 31 The cost of mining: Exposed seams of coal under Turtle Mountain were quickly mined after their discovery to feed the recently completed railway. M l t d il Men working ki underground had reported strange movements as much as seven months prior to the landslide. Large support timbers had cracked, and coal - under pressure - had begun to "mined itself". Landslides: Erik Eberhardt EOSC 114 – 2009/10 Engineered Slopes – Panama Canal 24 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 12 Engineered Slopes - Mining 25 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 Landslide Cause & Effect Triggers Cause geological mechanical hydrological geomorphological biological human intense rainfall rapid snowmelt water-level change volcanic eruption l earthquake shaking rapid erosion anthropogenic Trigger Effect rock slide topple mud flow rock fall debris flow Having identified a landslide hazard, this then leads to mitigation (trying to fix the problem). In some cases, it may be more economical to repair the effects of a landslide than to remove the cause altogether. 26 of 31 Landslides: Erik Eberhardt EOSC 114 – 2009/10 13 ...
View Full Document

This note was uploaded on 04/01/2010 for the course EOSC 114 EOSC 114 taught by Professor Stull during the Spring '10 term at The University of British Columbia.

Ask a homework question - tutors are online