GEOL 3400 Geologic Hazards Natural Hazards Geologic Hazards

GEOL 3400: Geologic Hazards Natural Hazards: Geologic Hazards Slope and Surface Failures Slope Failures: Mass Wasting

Geologic Hazards: Slope Failures An Introduction GEOL 3400: Geologic Hazards • material on a slope is unstable and wants to move downslope – most of the time it moves very slowly • sometime large volumes move rapid and with catastrophic results • mass movements or mass wasting are large scale transfer of material downslope under the influence of gravity – slow version is creep – fast variety is landslides • in the United States, mass movement causes $1. 5 billion in damages and 25 deaths yearly • the speed of a mass movement determines its potential for hazardous behavior – slow-moving masses cause property damage but rarely fatalities – in contrast, fast moving masses can produce deaths 21 -Sep-21 2

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards An Introduction 21 -Sep-21 • gravity is the agent that causes downward and outward movement of landslides • force that gravity exerts on a particle or object on a slope is related to the slope of the surface – is the weight of object times sin of angle of slope from horizontal 3

Geologic Hazards: Mass Wasting Types of Mass Movements GEOL 3400: Geologic Hazards • there are three basic types of mass movements: falls: movement of material from a topographic high by free-fall or – falls bouncing slides: movement of a semi-coherent mass along a failure surface – slides flows: any mass that moves like a viscous fluid by flowing down and over a – flows surface • the types of movement exhibited by these types of movements varies: – falls move downward – slides and flows move downward as well as outward 21 -Sep-21 4

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Falls • occurs when rock separates from main slope, e. g. cliff, mountain • mass falls through the air via free fall – when it hits the ground it bounds and rolls • some triggers of falls include: – heavy rains – frost wedging – earthquakes • rock falls and small volume avalanches flow horizontally only twice the vertical distance they fall • rock falls with volume of 1 million m 3 often travel long distances – sometimes 25 times vertical fall – long run-out flows of this type are called sturzstorms 21 -Sep-21 5

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Falls 21 -Sep-21 • farmers quarried slate from base of mountain until cracks opened up in hillside above • mass began to disintegrate as it fell • hit floor of quarry and disintegrated completely • shot out from mountainside ledge into valley flowed 2, 230 m • processes: fall, jump, surge 6

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides slides, also known as landslides, occur above a failure surface • slides • basal failure surfaces are: – curved concave upward; or – nearly planar • two types of slides are important – rotational slides – translational slides 21 -Sep-21 7

Geologic Hazards: Mass Wasting Types of Mass Movements: Slides – Rotational Slides GEOL 3400: Geologic Hazards • rotational slides move down and out on curved slip surface – movement is rotational along an axis parallel slope – Swedish circle is arc of circle drawn through the basal surface • used to calculate driving versus resisting forces – doing movement, head of slide moves down and rotates backward • toe moves upward and onto a surface not disturbed by slide – top of slump creates depression in which water can collect • can seep down and lubricate basal surface • increases likelihood of new movement – head of scarp is also unstable and could produce new movement – this type of slide moves only short distance • rotating motion decreases driving mass and increases resisting mass naturally 21 -Sep-21 8

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Rotational Slides 21 -Sep-21 9

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Rotational Slides 21 -Sep-21 10

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Rotational Slides 21 -Sep-21 11

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Rotational Slides 21 -Sep-21 12

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Translational Slides • when mass slides down and outward along surface of weakness it is a translational slide – surface can be joints, faults, clay layer – this type of movement can move as long as it is on the sliding surface and has driving mass • can travel long distances – sliding mass may: • move coherently as a block slide • deform and disintegrate to form debris flow • lateral spreading occurs when underlying material fails and flows causes coherent material to break apart 21 -Sep-21 13

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Translational Slides 21 -Sep-21 14

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Translational Slides 21 -Sep-21 15

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Translational Slides 21 -Sep-21 16

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Translational Slides 21 -Sep-21 17

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Slides – Translational Slides 21 -Sep-21 18

Geologic Hazards: Mass Wasting Types of Mass Movements: Flows GEOL 3400: Geologic Hazards • flows are mass movements that behave as viscous fluids – flow is driven by gravity • flows consist of a mixture of sediment, water and air – sediment ranges from large blocks to fine material like snow – water contents vary from dry to saturated • the type of flow varies depending upon the: – proportions of sediment, water and air – physical and chemical properties of the material moving • massive falls often change to highly fluidized flows – rapid flows can kill large numbers of people • two types of flows: – a slurry flow is water saturated – a granular flow is a flow that is not water saturated 21 -Sep-21 19

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows 21 -Sep-21 20

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows: Slurry 21 -Sep-21 • a slurry flow is a moving mass of watersaturated sediment – sediment particles are separated and supported by water between them – these flows are dense enough to move boulders in suspension • they can also roll larger blocks – sediment deposit produced when a slurry flow stops is unsorted • that is, the sediment in the deposit is not separated by size 21

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows: Slurry 21 -Sep-21 • a slurry flow traveling at the slowest velocities, < 1 m/yr are soilfluction • this is the downslope movement of water-saturated soil and regolith • occurs where soil is saturated for long periods of time • recognized on slopes by presence of distinctive lobes, sheets and ridges 22

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows: Slurry 21 -Sep-21 23

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows: Slurry 21 -Sep-21 24

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows: Slurry 21 -Sep-21 • the fastest moving slurry flows are mudflows • the density of mudflows varies from that of a muddy stream to wet concrete – depends on water content • on volcanoes, mudflows are called lahars 25

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows 21 -Sep-21 26

Geologic Hazards: Mass Wasting Types of Mass Movements: Flows GEOL 3400: Geologic Hazards • granular flows are mixtures of sediment, water and air that are not water saturated 21 -Sep-21 – movement results from contact and collision between grains • they are mostly dry with air not water filling the voids between the sediment grains • the slowest granular flow is creep which moves about 1 cm/yr – most widespread of the mass movements – generally affects soil and uppermost part of the bedrock 27

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows • creep results from expansion and contraction of soil – expands and moves up perpendicular to the surface • swelling is caused by: – freezing of water in voids – change from water to ice involves a 9 % increase in volume – absorption of water by soil minerals – particularly important when soils include a lot of the clay minerals – volume expansion by solar heating • contracts and moves downslope – shrinkage caused by: • thawing • drying • cooling 21 -Sep-21 28

Geologic Hazards: Mass Wasting Types of Mass Movements: Flows GEOL 3400: Geologic Hazards • creep is controlled by: – presence/absence of plants – animal activity – heating and cooling – wetting and drying • creep is too slow to actually observed – generally recognized by the effects it has on objects resting on the slope – displaced or tilted features, e. g. tilting of telephone poles, displacement of fences • creep is faster, the steeper the slope 21 -Sep-21 29

Geologic Hazards: Mass Wasting Types of Mass Movements: Flows GEOL 3400: Geologic Hazards • graphic 21 -Sep-21 30

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows 21 -Sep-21 31

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows 21 -Sep-21 32

Geologic Hazards: Mass Wasting Types of Mass Movements: Flows GEOL 3400: Geologic Hazards • earthflows move with velocities in the range of 1 m/d to about 1 m/hr so they are faster than creep 21 -Sep-21 – commonly occur where the ground is intermittently saturated • often triggered by excessive rainfall – once started, earthflows can continue for days to months to years – earthflows typically head in scarps 33

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Types of Mass Movements: Flows 21 -Sep-21 34

Geologic Hazards: Mass Wasting Types of Mass Movements: Flows GEOL 3400: Geologic Hazards • debris avalanche is faster than earthflows 21 -Sep-21 – movement of dry or nearly dry granular flow – air fills almost all pores – familiar example is the slip of grains on the face of a dune • the fastest granular flows (< 1 km/hr) are debris flows – these flows are very fast – highly destructive and dangerous events – they are rare and therefore poorly studied 35

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Causes of Slope Failure • landslide is mass of material whose center of gravity has moved downward and outward • divided into two parts – tear-away zone: zone at top of slope where material has pulled away – pile-up zone: place at base of slope where material that has moved piles up • two types of causes for slides – external – internal 21 -Sep-21 36

Geologic Hazards: Mass Wasting Causes of Slope Failure - External GEOL 3400: Geologic Hazards • three processes that cause slope failure – steepening of slope – removing support from base of slope – adding mass to top of slope • landslides often move above a curved surface – mass that moves can be divided into resisting and driving masses – slope is stable if resisting mass outweighs the driving mass • decrease resisting or increasing driving will cause the slope to fail 21 -Sep-21 37

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Causes of Slope Failure - Internal • long term processes are attacking the materials that make up a slope – weaken material and can result in failure • these are internal causes of slope failure and include: – inherently weak materials – water in different roles – decreasing cohesion – adverse geologic structures • clay minerals are commonly a cause of slope failure – most common sediment – very small minerals – formed by chemical weathering at the Earth’s surface – clays have layered structure • expand shrink as they absorb and lose water – strength of rock determined by smallest minerals in rock • absorbing water causes clays to weaken • water on outside of grains moves them apart • water absorbed internally expands clay structure and weakens the grips between the layers 21 -Sep-21 38

Geologic Hazards: Mass Wasting GEOL 3400: Geologic Hazards Causes of Slope Failure - Internal • the presence of water weakens Earth materials in several different ways – weight of water filling voids can make slope fail – absorbed internally and externally to weaken surface materials with clays – dissolves material the cements rocks together – groundwater flow can erode material • piping (subsurface erosion) can create extensive cave systems thereby weakening a hill – pore pressure moves grains away from each thereby reducing friction • high pore pressure can cause mass movement or slope failure • quicksand is produced when sand is supersaturated with pressurized water 21 -Sep-21 39

Geologic Hazards: Mass Wasting Causes of Slope Failure - Internal GEOL 3400: Geologic Hazards • slopes may be weak due to pre-existing geologic conditions – these include: • old slide surfaces • rock layering dipping less than slopes • geologic structures within the rock – ancient surfaces are zones of weakness that are reused again and again • often areas of ground-up rock – the orientation of rock layers relative to slope is important in determining where the slope is strong or weak • when units dip less than slope, they will intersect the slope – produces potential surfaces for slipping and mass movement – these are known as daylight bedding – if the layers dip into the slope, they form barriers to movement • rock characteristics that promote mass movement include: – – – 21 -Sep-21 rocks not cemented strongly clay layers provided surface for sliding soft layers sliding off stronger layers joints create weak rock old fault surfaces are potential slide surfaces 40

Geologic Hazards: Mass Wasting Causes of Slope Failure - Internal GEOL 3400: Geologic Hazards • failure generally happens due to a variety of causes – lot of reasons bring slope near to failure – something then triggers the event 21 -Sep-21 41

Natural Hazards: Geologic Dealing with Mass Wasting GEOL 3400: Geologic Hazards • Prediction & Hazard Assessment – reconstruction geologic past to determines where mass-wasting events have occurred in the past – use this to identify areas susceptible – limit the use or types of use of these threatened areas • Prevention & Mitigation – engineering solutions include: • • 21 -Sep-21 retaining walls drainage pipes grading diversion walls 42

Natural Hazards: Geologic GEOL 3400: Geologic Hazards Summary 21 -Sep-21 43