Lecture 20 - Landslide Analysis and Mitigation

what is the typical output from a limit equilibrium analysis?

factor of safety = resisting forces/driving forces

factor of safety = shear strength/shear stress

FS > 1 = stable situation

FS < 1 = denotes failure

how does water effect soil grains?

- soil grains separate, effective stress lowers

- lowers factor of safety

how do high pore pressures affect the stability of a slope?

- increase in pore pressure along any potential failure plane acts against effective normal stress

- decreases the frictional strength component along the sliding surface

describe thr two landslide processes

cause: geological, mechanical, hydrological, geomorphological, biological, human

effect: rock slide, topple, mud flow, rock fall, debris flow

define cause and trigger in terms of landslides

cause: makes slope susceptible to movement without initiating failure

trigger: initiation of failure by an external stiluli causing a near-immediate response in the form of a landslide by rapidly increasing stresses and reducing the strength

name different types of landslide triggers

- intense rainfall

- rapid snowmelt

- water-level change

- volcanic eruption

- earthquake shaking

- rapid erosion

why does heavy rainfall trigger landslides?

- rapid infiltration of rainfall causes temporary rise in pore-water pressures

- mechanism that generates most shallow landslides during storms

describe how earthquakes can trigger landslides

- strong ground shaking during seismic activity

- can induce shear stresses and weaken the internal structure of the slop material

- factor of safety decreases (cohesion and effective force decrease)

describe how humans can trigger landslides

- excavation of slopes near its toe

- loading of slope at its crest

- drawdown of dam reservoirs

- irrigation

- artificial vibration (pile driving)

- blasting (mining, construction)

landslide avoidance

- landslide hazard maps may be used to plan the "best" route for a new highway, pipeline, or rail line

- route minimizes the number of major landslide hazard areas encountered

visual indicators of past landslides

- lack of trees

- accumulation of material at the bottom of the slope (may be overgrown if landslide happened a long time ago)

- head scarp at top of slope

- condensed tree rings (sign that tree was stressed)

- misaligned fences

- fresh material over older material

landslide prevention - remove source

remove mass of soil/rock within the slope or all unstable material

landslide prevention - reinforce source

increase resisting forces by applying a resisting force at the toe of the landslide or by adding anchors

landslide prevention - reinforce drainage

- control of surface and subfurace water will reduce effects of seepage (which increases the driving force on a landslide and) and pore pressures that reduce the effective strength

- drill drainage holes from a tunnel to drain the slope from beneath

landslide protection - rockfall protection

- let falls occur and control their distance and direction travelled

- dynamic barrier systems -> minimize costs by making yielding elements replaceable following a rockfall

landslide protection - debris flow protection

- remove debris from the flow mass

- prevent more debris from being entrained

- decrease flow's velocity and erosive capabilities

factor of safety

- ratio of the resisting force over the driving force

- shear strength over shear stress in landslide mechanics

how does groundwater impact the effective stress of soil?

- reduces the shear strength of soil

- increases liklihood of a landslide

examples of key triggers for landslides

- intense rainfall

- earthquake shaking

- human activity

- rapid snowmelt

- water-level change

- volcanic eruption

- rapid erosion

- all impact force balance equation

name the three different strategies for dealing with landslide hazards

- avoidance

- prevention

- protection