Types of Mass Movement By Tony, Ed, Steven

Introduction In mass movement of soil gravity is the force acting to move surface materials such as soil and rock. When natural slope stability is disrupted, sliding movements may occur. Usually rapid movements of soil or rock that behave separately from the underlying material and involve one distinct sliding surface are termed landslides. A slower long-term deformation having a series of sliding surfaces and viscous movement is termed creep. Such movement is the result of several factors, but more often the final act in a series of processes involving slope, geology, soil type, vegetation type, water, external loads and lateral support.

Process Generally mass movement occurs when the weight (shear stress) of the surface material on the slope exceeds the restraining (shear strength) ability of that material. Factors increasing shear stress include erosion or excavation undermining the foot of a slope, loads of buildings or embankments, and loss of stabilising roots through removal of vegetation. Vegetation removal and consequent lower water use may increase soil water levels, causing an increase in pore water pressure within the soil profile. Increased pore water pressure or greater water absorption may weaken inter-granular bonds, reducing internal friction and therefore lessening the cohesive strength of the soil and ultimately the stability of the slope.

Types of Mass movement Landslides Avalanches Rockfalls Mudslides Lahars

Landslides Landslides occur when masses of rock, earth, or debris move down a slope. They may be very small or very large, and can move at slow to very high speeds. However slow movement is also seen in the gradual downhill creep of soil on gently sloping land.

Case Study Hong Kong A very severe rainstorm occurred on the 8 th May bringing 350mm of rain during a 9hour deluge. A total of 350 landslips occurred, two of which killed three people. Foundations were displaced under blocks of flats and 1,500 people were evacuated.

A Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and river valleys. When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10m in diameter. Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second-much too fast for people to outrun. As a lahar rushes downstream from a volcano, its size, speed, and the amount of water and rock debris it carries constantly change. The beginning surge of water and rock debris often erodes rocks and vegetation from the side of a volcano and along the river valley it enters. This initial flow can also incorporate water from melting snow and ice (if present) and the river it overruns. By eroding rock debris and incorporating additional water, lahars can easily grow to more than 10 times their initial size. But as a lahar moves farther away from a volcano, it will eventually begin to lose its heavy load of sediment and decrease in size. Lahars

Case Study Nevado del Ruiz, Colombia 1985 On November 13 th 1985 Nevado del Ruiz erupted violently for the 3 rd time in 400 years. The volcano has triggered the flow of a lahar, which is a large mass of mud flow which travelled down to villages and towns 23,000 people were killed Most of Armero’s population was wiped out 4500 injured 8000 left homeless

Avalanches An avalanche is the movement of snow down valley down a mountain. There are around 1,00,000 avalanches around the world each year Avalanches are the result of weather changes, slope steepness, terrain and snow conditions. They usually occur during storms In Galtur, Austria an avalanche killed 31 people