Case study
California and Los Angeles- Attempts to reduce mass movement
Slides and flows are common in California, damaging roads, railways, pipelines, electricity
cables and other infrastructure. The suburbs of LA are particularly affected.
Causes:
● Intense rainfall- Downtown LA has an annual precipitation of 385mm. Heavy
rainfall on dry ground can lead to mudflows and loading of the ground resulting in
landslides.
● Soft, poorly consolidated rocks- The geology of the area consists of relatively
young neogene and pleistocene marine sediments.
● Steep relief- LA rises from sea level to 1547m (Mount Lukens) in the form of a
basin. The central parts of the city are flat but the outer suburbs are hilly (e.g. San
Fernando valley, Santa Monica mountains, Mount Washington, Boyle heights and
San Pedro)
● Road and housing construction- LA has grown rapidly outwards into hilly districts
prone to mass movements. Constructions in these hilly districts can add load to
unstable slopes and road cuttings in these areas may also be unstable.
● Oil and water extraction- Groundwater extraction for water supply and petroleum
extraction have both caused ground subsidence. In 1963 in the Baldwin Hills a dam
collapsed as a result of this.
● Earthquakes- The San Andreas fault system and other active faults in the area can
trigger landslides on slopes affected by the factors listed above. Earthquakes can
also cause liquification of the ground.
Attempts to reduce mass movement:
● Producing maps that show past landslide features which are likely to be
re-activated. Site evaluations are now a legal requirement before developing.
● Landslide inventory and related hazard zone maps, new landslide inventory maps
cover 62 square mile areas.
● Systems such as rock bolts, netting and shotcrete are not appropriate for stabilising
slopes in the soft, poorly consolidated rocks which form many of the slopes.
Effectiveness of attempts:
● Slope failures still happen frequently outside the built-up area but the system is
focused on reducing the risk to property and human life where slope failures are the
result of human activity.
, ● Difficult to produce hard statistics to evaluate the success of the system but it is
believed that it has had a significant effect.
Types of mass movement
Talus creep= The slow movement of fragments along a scree slope.
Slip plane= The junction between two layers along a bedding line, or the joint between
two rock types at the point beneath the surface where shear stress is greater than shear
strength.
(heaves, slumps, flows, falls, slides in order of general speed)
Heaves (soil creep): Slow movement of material where soil particles are heaved to the
surface by wetting, heating and freezing of water. This reduces friction between particles,
California and Los Angeles- Attempts to reduce mass movement
Slides and flows are common in California, damaging roads, railways, pipelines, electricity
cables and other infrastructure. The suburbs of LA are particularly affected.
Causes:
● Intense rainfall- Downtown LA has an annual precipitation of 385mm. Heavy
rainfall on dry ground can lead to mudflows and loading of the ground resulting in
landslides.
● Soft, poorly consolidated rocks- The geology of the area consists of relatively
young neogene and pleistocene marine sediments.
● Steep relief- LA rises from sea level to 1547m (Mount Lukens) in the form of a
basin. The central parts of the city are flat but the outer suburbs are hilly (e.g. San
Fernando valley, Santa Monica mountains, Mount Washington, Boyle heights and
San Pedro)
● Road and housing construction- LA has grown rapidly outwards into hilly districts
prone to mass movements. Constructions in these hilly districts can add load to
unstable slopes and road cuttings in these areas may also be unstable.
● Oil and water extraction- Groundwater extraction for water supply and petroleum
extraction have both caused ground subsidence. In 1963 in the Baldwin Hills a dam
collapsed as a result of this.
● Earthquakes- The San Andreas fault system and other active faults in the area can
trigger landslides on slopes affected by the factors listed above. Earthquakes can
also cause liquification of the ground.
Attempts to reduce mass movement:
● Producing maps that show past landslide features which are likely to be
re-activated. Site evaluations are now a legal requirement before developing.
● Landslide inventory and related hazard zone maps, new landslide inventory maps
cover 62 square mile areas.
● Systems such as rock bolts, netting and shotcrete are not appropriate for stabilising
slopes in the soft, poorly consolidated rocks which form many of the slopes.
Effectiveness of attempts:
● Slope failures still happen frequently outside the built-up area but the system is
focused on reducing the risk to property and human life where slope failures are the
result of human activity.
, ● Difficult to produce hard statistics to evaluate the success of the system but it is
believed that it has had a significant effect.
Types of mass movement
Talus creep= The slow movement of fragments along a scree slope.
Slip plane= The junction between two layers along a bedding line, or the joint between
two rock types at the point beneath the surface where shear stress is greater than shear
strength.
(heaves, slumps, flows, falls, slides in order of general speed)
Heaves (soil creep): Slow movement of material where soil particles are heaved to the
surface by wetting, heating and freezing of water. This reduces friction between particles,
