Things are going downhill fast: understanding massive landslides. Prof. Dave Petley. 7th Oct 2019

Dave told us that a land­slide occurs when a slope col­lapses.  He showed us a pic­ture of a huge land­slide in New Zealand which left two cows and a calf marooned on an untouched meadow that was sur­roun­ded by land­slide debris.  Sometimes there is no obvi­ous reason that one sec­tion of a slope col­lapses but other parts are stable.

There are dif­fer­ent types of slide and dif­fer­ent scales of slide.

In Alaska a moun­tain slide slid 15 km across a gla­cier, but in a cliff fall in Staithes a block about the size of a coffee cup hit a girl on the beach and killed her.

Probably the land­slide that most people know about was the dis­aster at Aberfan in 1966.

The spoil tip, loc­ated above the town, col­lapsed and slid down into the school and killed 118 people in there.  He said that this was a shame­ful event, because the spoil could have been taken away but the tip was situ­ated where it was because that was the cheapest place to put it.  The Coal Board did remove it later, but it took money raised from the public appeal for the town to pay for the removal.

Sometimes land­slides have reached the sea and caused tsuna­mis and three massive slides in Brazil in 2019 killed 400 people as the slide front became chaotic.

There are five key pro­cesses involved in land­slides.

1/         The role of water.

Friction is a key con­trol.  Water cre­ates buoy­ancy and reduces fric­tion.

2/         Liquefaction.

As earth mater­i­als deform they go from solid to fluid and the block picks up          mater­ial and speeds up.

(Dave showed a pic­ture of a car half-buried in a road in Alaska, 2018.  He said      that the embank­ment found­a­tions lique­fied as the slope slipped.)

3/         Rocks with defects are much weaker that those without defects.

Multiple hori­zontal defects or cracks are more likely to fail.

4/         Earthquakes in moun­tains gen­er­ate many slope fail­ures.  Nepal has mul­tiple            slides.

5/         We don’t under­stand why the biggest slides travel so far and so fast.

The slides are fre­quent in spring­time when snow and ice melts.

They are very chaotic, they slide far and then con­tinue to creep.  We don’t know   why they become chaotic and break up in to much smal­ler particles and ’liquefy’.

These con­di­tions cannot be sim­u­lated in the labor­at­ory.

Big rock falls are not under­stood.

Landslides are often part of a highly com­plex chain of events.

A land­slide in Sulawesi in 2018 was prob­ably caused by an earth­quake and pos­sibly irrig­a­tion con­trib­uted.

In the north of India, as the con­tin­ent travels north­wards on the tec­tonic plate push­ing up the Himalayas, earth­quakes are caused.  Dave showed satel­lite photos on which the fault line and fault rup­tures were shown.  The are stretched for over 200 km.  These faults cause earth­quake waves.

He showed photos of an area along this 200km fault line where he said 40 — 50% of the land­scape had slipped.  In one place the moun­tain had slipped and flowed down the valley.  More that a cubic kilo­metre (i.e. 2.5 bil­lion tons) of mater­ial blocked the valley, 500metres deep.

In 2007, in a panda con­ser­va­tion area in China, an earth­quake and land­slide caused great damage to Beichuan town.  Dave showed a pic­ture of a crane that had the arm that holds the coun­ter­weight bent upwards.  The crane had been jolted as it dropped over 6.5 metres in the earth­quake. The land­slide and buried Beichuan Middle School, killing 700 people.  The school was a primary and sec­ond­ary school so all the school-age chil­dren in the town were in it at the time.  As Chinese par­ents were allowed to have only one child this was a major cata­strophe. 10 minutes later a whole hill slid down, bull­doz­ing build­ings and caus­ing a massive loss of life

In 2005 in Kashmir a land­slide killed 600 people.  Two women, who were cut­ting grass at the top of the slope, were on an unbroken mass of mater­ial which slid down.  The women ‘surfed’ over 2.5 km on the land­slide and sur­vived.

The chaotic mass blocked the valley and a lake built up behind the dam.  As water loosens the mater­ial the dam could col­lapse at any time, so the Army evac­u­ated all the 1 mil­lion people who lived there until they could drain the dam.

A ques­tion was asked about ava­lanches, which are much easier to pre­dict and there­fore could be stud­ied in more detail than land­slides.  Dave said that they were sim­ilar but dif­fer­ent because the mater­i­als in an ava­lanche were snow, ice and water so much less fric­tion that in a land­slide, and as the solid mater­i­als slid they melted and the water made them even more fric­tion­less.

This was a very inter­est­ing talk that was well illus­trated with slides and a video.

If you want to know more about land­slides Dave has a blog: