Geol 33 Environmental Geomorphology

J Bret Bennington

Mass Wasting

Weathering rock, sediment, and soil at the surface of the Earth sometimes become unstable and move or collapse with devestating effect. Slower, more gradual movement also occurs that can damage surface structures such as buildings and roads.

Mass Wasting: refers to the movement of Earth materials downslope due to the pull of gravity.

Mass wasting includes very slow, often imperceptable movement of soil and rock called creep, and the sudden, catastophic movement of landslides.

Unfortunately, because of the pervasive spread of human habitation, many structures are built in unstable areas. According to the USGS, more people in the United States died from landslides during the last three months of 1985 than were killed during the last 20 years by all other geohazards (including earthquakes and volcanic eruptions). In general, landslides average 25 lost lives and 1.5 billion dollars in damage per year. This is almost triple the average rate for tornados, floods, hurricanes, and earthquakes combined!

Causes: Mass wasting is caused by gravity. On a mass of material gravity exerts a force downward proportional to the amount of mass. If the material is resting on a slope, the force has two distinct components:

normal force - the force perpendicular to the slope.

shear force - the force parallel to the slope in the downhill direction.

The steeper the slope, the greater will be the shear force relative to the normal force. Resisting the shear force is the shear strength - which can be understood as the degree to which the material sticks to the slope and resists deforming, sliding, or flowing. Shear strength is controlled by a number of variables such as friction, which itself is proportional to the normal force, as well as many other variables such as the roughness, cohesion, and dryness of the material.

Also important is the internal cohesiveness of the material. Even if the mass is firmly rooted to the slope, it can still flow downhill if it is incohesive enough to deform internally.

Water

Water is critical to the process of mass wasting.

Too little water in a mass of material such as sand prevents the grains from sticking together - a lack of cohesion that decreases shear strength.

Enough water to coat the grains will cause a great increase in internal cohesion, increasing shear strength.

If water is added to completely saturate the pore spaces, the weight of the mass will be greatly increased, increasing the shear force. Also, the pore water now acts as a lubricant between grains and the pore pressure helps force the grains apart. This decreases the shear strength.

Because of the effect of water on slope stability, many mass wasting events are triggered or hastened by heavy or extended rainfall.

Types of Mass Wasting

Creep

Creep is the slow, continuous movement of soil or unconsolidated sediments over extended periods of time. Often, the rate of creep is less than a centimeter per year and can only be detected over many years by looking for its effect on the landscape. Creep occurs when the shear force is only slightly greater than shear strength.

Creep is caused by repeated freeze-thaw cycles that slowly inch material downslope (during freezing, particles are elevated perpendicular to the slope, but during thaws they fall straight down to a new position lower on the slope). Creep can also be caused by a buildup of pore water that allows material to begin to flow under the influence of gravity.

Creep causes fence posts, utility poles, walls, and other structures to lean over time. Eventually the lean topples the structures and they must be rebuilt. In some cases, creep can be slowed or prevented by installing drainage pipes in soils that drain them and keep pore pressures low.

Colluvium is the geological material deposited by creep - consisting of an unsorted mix of talus blocks and clay similar to glacial till. In general, most hillslopes are covered by a blanket of colluvium with occasional outcrops of bedrock.

 

Landslides

A landslide is a movement of rock or debris down a slope along one or more distinct failure surfaces. Landslides range in speed from 1 m/day to as much as 300 km/hr. There are several different types of landslides:

Rockfall

Rockfalls occur where steep cliff of rock are eroding, causing material to fall vertically. This is a relatively common hazard along highways where there are steep roadcuts. The rockfall debris accumulates at the base to form talus or scree. Talus accumulates and forms a slope at the angle of repose. This angle is determined by the size and shape of the talus particles, but is typically between 30° and 40°. Large rockfalls are accompanied by blasts of displaced air that can reach hurricane speeds and level trees in the vicinity of the fall. For example, on July 10, 1996 a mass of granite 500 ft by 30 ft by 20 ft fell 1500 ft from a valley wall in Yosemite National Park. Hitting the valley floor at 160 mph it generated an air blast that leveled about 2000 trees.

 

Rockslide

Rockslides occur when a mass of bedrock comes loose from the underlying bedrock and slides down a slope. These are common wherever layers of sedimentary rock happen to be tilted parallel to the slope of the landscape and where a weak layer of rock such as a shale forms a sliding surface. As a rockslide progresses downslope it may break up and begin to tumble, forming a rock avalanche.

The conditions needed for a rockslide can sometimes be created by poorly planned construction of highways and roadcuts. Undercutting tilted strata to create a roadbed may leave a mass of bedrock without support and poised in slide down onto the road. In the late 1970’s just such an event occurred on the east side of the Blue Ridge mountains along Interstate 64 in Virginia. A number of motorists were crushed to death when a large mass of rock suddenly cut loose and slid down onto the highway.

Slumping

Slumping involves the rotational movement of a material downward and outward along a curved shearing plane.

Earthflow

An earthflow describes debris that moves downslope as a viscous liquid. This usually occurs on hillsides with thick soil or debris covers that have been destabilized by saturation, usually after heavy rains. Often, the saturated material will break free from the underlying bedrock and move in large sheets held together by the overlying vegetation. As these sheets flow they may break up into smaller slump blocks.

Humans can trigger earthflows by overwatering lawns or by installing septic systems that drain into unstable slopes. For example, a man in Los Angeles went away on vacation and forgot to turn off his sprinkler system. He returned to find that his lawn and his house had slide downslope and spread out over the highway below.

Earthflows are also triggered by undercutting the base of the slope, which effectively steepens it. This can occur along the coast due to wave erosion, or because of bulldozers cutting the base of a hill to create a road or railroad bed, or to create level lots for additional houses downslope.

Mudflow

Mudflows are similar to earthflows except that the debris that is flowing tends to be more fluid. Mudflows tend to occur where there is less vegetation so that loose sediment, soil, and rock is not covered. Mudflows are triggered by heavy rainfall or by the sudden melting of glaciers due to volcanic eruptions (lahars). They flow much slower than water does, but they are very viscous which allows them to move very large objects such as boulders, cars, and houses.

Southern California is particularly prone to earthflows and mudflows because of the unhappy coincidence of several factors.

Debris Avalanche

A debris avalanche is a very rapidly moving mass of rock, sediment, air, and water. These usually begin as avalanches on steep mountain slopes that gain speed and momentum as they tumble down the mountain, sweeping up additional debris. In a debris avalanche, the material moves as a chaotic, tumbling mass, accompanied by a deafening roar and moving at speeds up to several hundred mph. There is some evidence to suggest that the falling debris creates a layer of compressed air at its base that allows the mass to move without being slowed much by friction.

 

Devestating Mass Wasting Events

The Andes Mountains of South America’s west coast have been the scene of several catastrophic episodes of mass wasting. The Andes host a lethal combination of high peaks, steep valleys, volcanoes, glaciers, and earthquakes that sometimes combine to create extraordinary movements of rock and debris.

Yungay, Peru

In May of 1970, an earthquake offshore of Peru caused tremors that dislodged a half mile wide slab of glacial ice near the peak of Nevado Hascaran. The ice mass avalanched down the steep slopes, breaking off rock, scooping up debris, and bulldozing small lakes. By the time is reached the base of the mountain, the mass of debris was as tall as a ten story building and is estimated to have contained 50 million cubic meters of debris. This debris fell 12,000 feet and traveled 9 miles in less than four minutes, moving at over 200 mph. The main mass of material flowed along a valley, blocking the Santa River and burying the village of Ranrahica and its 1,800 inhabitants. A smaller mass of material was forced up the side of the valley where it topped a low ridge, became airborn, and fell vertically on top of the larger, market town of Yungay. Because the avalanche occurred on Sunday afternoon, the traditional market time, more than 17,000 people were in Yungay. Along with the town, almost all were instantly buried in the debris.

Nevada del Ruiz, Columbia

On November 13, 1985, an eruption of the Columbian volcano Nevada del Ruiz triggered the melting of glaciers and a gigantic mudflow composed of water, volcanic ash and debris. This volcanic lahar raced down the mountain at a speed of over 100 mph, burying the town of Armero and killing over 25,000 of its inhabitants.

Vaiont Dam, Italy

The world’s worst dam disaster occurred in Vaiont, Italy on October 9th, 1963. This occurred in spite of the fact that no actual damage was incurred by the damn itself or its supporting structures. What did occur was a massive rockslide that caused over 238,000,000 cubic meters of rock to rapidly slide into the reservoir, creating a wave over 300 ft high that splashed across the reservoir and over the top of the dam, flooding and destroying towns up and down the river valley. Over 2600 lives were lost in the 7 minutes it took for the slide and the flood to occur.

The rockslide occurred because the rock strata along the sides of the reservoir were steeply dipping and inclined toward it. Weak layers within the rock provided a surface for sliding and the reservoir itself caused in increase in pore pressure in the groundwater within the rock, lubricating the bed surfaces and increasing the shear stress. Engineers monitoring the reservoir know that the rock mass was creeping as much as 25 cm per day, but did not realize that is was moving as such a large mass. The day before the disaster rates as high as 100 cm per day were recorded, but no warnings or evacuation were called for. After the disaster, those responsible were put on trial and one of the chief engineers on the project committed suicide.

 

Preventing landslides

The key to preventing damage from landslides is to identify and avoid developing landslide prone areas such as steep, unstable hillsides. However, if some of these areas must be developed then building codes should require extensive efforts to insure slope stabilization:

Good slope engineering is expensive and the temptation to cut corners is great. However, landslide damage is far more expensive and estimates have shown that for every dollar spent on slope stabilization, between 10 and 2000 dollars are saved over the long term. For example, a landslide in Utah in 1983 dammed a river and caused flooding of the town of Thistle, a railroad, and a major US highway. Total damage was about $200 million. The slide was triggered by a high water table due to high precipitation and was a reactivation of an older slide that had a well known history of movement. Estimates suggest that the slide could have been predicted to be imminent and could have been prevented for about $300,000 worth of drainage engineering. Benefit to cost ratio: about 100:1.