How human activity can result in mass movement on slopes
excavations: most common way human activity can result in mass movement is
where ground is removed. (road/rail cuttings, to make level ground for a building).
in areas prone to mass movements it can create a slope which is too steep to be
stable and therefore liable to failure. where an excavation removes the toe of an
old landslip this can reactivate the feature and lead to further movement. new
slopes are being created and must be done in a way that ensures new slopes are
stable and not liable to mass movements.
waste heaps: from quarrying and mining. steep slopes and made of material which
is unconsolidated or highly porous. newly created steep slopes may be unstable
and liable to slope failures. e.g. Aberfan, wales; slope failure of a coal mine spoil
top caused by a build-up of water in the accumulated rock waste which started to
move downhill as a mudflow. killed 116 children and 28 adults.
loading by building: building on top of a slope liable to landslip can add sufficient
mass to the ground that it will trigger a landslide
loading by water: when rock cuttings/building projects are carried out, drainage
may be disturbed, diverting water into these areas. water has a lubricating effect
on unconsolidated material and saturated clays are unstable, all of which can lead
to landslips. extra water weight in the rock is a factor. if saturated sands/clays are
shaken by an EQ then liquefaction may happen. before an EQ, water pressure in
sand is low but shaking increases it, allows sand particles to move relative to each
other, acting like a liquid.
removal of vegetation: deforestation, construction projects or even leaving land
bare after cultivation can increase surface run off leading to mudflows in
susceptible areas.
traffic vibrations: movement of heavy vehicles is not a sole cause of mass
movement but can be a trigger for movements.
How human activity can result in erosion on slopes
removal of vegetation: overgrazing, soil exposure during cultivation, cultivating in
areas of low rainfall, construction projects. all these activities can lead to bare surfaces liable to rain splash erosion, sheet erosion, rill erosion and gullying. after
very heavy rainfall it could lead to mudflows.
ploughing up and down slopes: creates pathways for surface run off which can lead
to the development of rills.
destroying soil structure: poor agricultural practices (over cropping/allowing the
organic content of the soil to deteriorate) leads to destruction of the crumb
structure that binds soil together. leaves the soil loose and prone to erosion both
by wind and running water.
Strategies to reduce mass movement and impact on slopes
methods to prevent slides and falls, often on artificially created slopes
pinning (rock bolts, dowel bars and ground anchors): drill a long hole through loose
blocks into stable rock beyond. metal rod inserted and fixed in place with a
resin/expansion bolts. metal plate then bolted into road outside. rock bolts and
dowel bars a short but ground anchors may be long cables used to stabilise whole
landslides areas.
Netting: metal. fastened to road cuttings to prevent loose blocks falling onto road.
Gabions: boxes of metal mesh. fold flat for transport, assembled on site and filled
with rocks. stabilise toe of a landslip.
Drainage: excess water on slopes add mass, lubricates and is a factor in formation
of flows and slides. moving water from vulnerable slopes prevents mass
movements. dig a trench and fill with a highly permeable aggregate.
Grading: making slopes gentler. slope angle is a feature in mass movements.
steeper are more unstable. artificial need to be gentle if a movement risk. requires
more excavation and produces more waste rock to be transported and disposed,
increased costs. slope angle of natural decreased to reduce risk.
Afforestation: planting trees/vegetation reduces soil erosion and reduce mass
movement. increased interception and evaporation losses are greater. roots absorb
water and increase transpiration. less surface run off and infiltration to add mass to
the rocks. roots bind soil and loose rock.
Grouting: injecting permeable rocks with cement to reduce pore water and
increase strength
Shotcrete: loose rock surfaces sprayed with concrete which can prevent loose
blocks falling from the slope
mapping hazards: landslides occur when old ones are re-activated as a result of
heavy rainfall, excavations or earthquakes. detailed mapping helps planners to
decide which areas should be avoided by buildings or decide what precautionary
measures need to be taken. can make use of historical accounts but often looks for
topographic features.
Case study: the Merriespruit tailings dam disaster
gold is extracted from ground at very low purities and processing produces large
quantities of fine-grained waste mixed with water known as tailings (slime).
waste is deposited in slime dams which are prominent landscape features, appears
as rectangular step fast topped hills.
made by constructing a ‘day wall’ perimeter which was allowed to settle and dry
out.
activity often done during the day under supervision.
after this and often at night, the slurry was pumped into the night pan, between
the perimeter walls.
drainage system was installed in the dam to drain away the water plus any rain
water.
250 houses in Merriespruit, suburb of the goldfield’s town of Virginia when the dam
was constructed in 1978.
22 February 1994 in the pm there was a thunderstorm and 50mm of rain fell in 30
mins.
the tailing dam failed and flood Merriespruit when 600,000m of liquid slurry flowed
4km away from dam.
nearest houses were 300m downslope of dam and when wave of water and slime
reached them it was 2.5m high. widespread devastation and environmental
damage, 17 killed and 80 houses destroyed.
inadequate systems for drainage water from dam were blamed for the disaster.
Case study: railway landslide in Cumbria 1995
common on railway cuttings.
line from settle to Carlisle goes through an area of carboniferous shales and
sandstones and alternating permeable and impermeable are prone to slippage.
18:55 on 31st January 1999, train was derailed by a landslide on this line at aisgill.
dark and heavy rain.
train was hit by a train travelling in the opposite direction.
conductor of 1st train was fatally injured.
Case study: California and Los angles
slides and flows are common, damaging infrastructure. LA suburbs are very
affected.
intense rainfall: downtown LA has annual precipitation of 385mm, mainly in winter
and spring with heavy rain in winter storms. coast gets less than the hilly suburbs.
great variation each year. heavy rain on dry ground leads to mudflows and loading
of the ground resulting in landslides.
soft poorly consolidated rocks: geology consists of young Neogene and Pleistocene
marine sediments deposited between 15m and 1 million years ago.
steep relief: LA rises from sea level to 1547m in the form of a basin. central city is
flat and outer suburbs are hilly.
road and housing construction: LA (pop of 3.88m 2013) has grown rapidly outwards
into hilly districts prone to mass movements. construction here can add load to
unstable slopes and road cuttings may be unstable
oil and water extraction: groundwater extraction for water supply and petroleum
extraction have caused ground subsidence. 1963 a dam collapse because of this in
the Baldwin hills.
Earthquakes: san Andreas fault system and other active faults in area can trigger
landslides on slopes affected. can also cause liquefaction of the ground.
Examples: April 18, 1906: major EQ in san Fran triggered landslides, including
devils slide in san Mateo county. still active today. Jan 3-5, 1982: landslides in san
Fran bay areas killed 25 and caused 66milion in damaged. Jan 10, 2005: mudslide
in la Conchita killed 10 and destroyed 18 homes.
mudflows in southern California December 2010: area received half its annual
rainfall and some streets flooded. governor declared state of emergency for 6
communities. evacuations and authorities on alert for landslides and mudflows.
100’s evacuate in suburbs of LA, concerns for homes in steep slide valleys
previously affected by wildfires. heavy rain caused a landslides on heavily used
section of interstate 10 covering 3 lanes near Pomona. in highland district 104km
east of LA 2 rivers overflowed, swamping 20 homes in mud. in Silverado canyon,
orange county 25-30 people were evacuated from mountain homes. homes in
mountains blocked by boulders and mud as rescue workers helped residents seek
shelter. officials ordered 232 homes to be evacuated at the bottom of large
hillsides in La Canada Flintridge and la crescent in the suburbs of LA.
attempts to reduce mass movement: hazard maps showing past landslide features
which are likely to be re-activated, done by the California geological survey,
indicate areas of liquefaction and EQ triggered landslides are significant enough to
require a more detailed site evaluation prior to developments. before 1995 site
evaluations were optional but now a legal requirement. landslide inventory and
hazard maps are available on California geological survey website. new maps cover
62mile areas known as ‘quadrangles. systems like rock bolts netting and shotcrete
aren’t appropriate for stabilising slopes in soft poorly consolidated rocks when form
many slopes. failures still happen often outside the built-up area, but system is
focused on reducing risk to property and human life where slope failures are result
of human activity. hard to produce statistics to evaluate system success but little
doubt it’ll have a significant effect.