Learning Objectives. Your goals for studying this chapter are: Understand where landslides occur. Understand the warning signs of landslides.

Size: px

Start display at page:

Download "Learning Objectives. Your goals for studying this chapter are: Understand where landslides occur. Understand the warning signs of landslides."

Randolph Sanders
5 years ago
Views:

1 Learning Objectives Landslides are common problems across the country and in many parts of the world. Next to flooding, they are the most likely natural hazard you may have to deal with in your lifetime. Your goals for studying this chapter are: Understand where landslides occur. Understand the warning signs of landslides. Understand what to do in the event of a landslide. Understand the basic types of landslides and related terminology. Understand the geologic and man-made causes of landslides. Understand landslide mitigation measures and how they are best applied. (Utah Geological Survey)

That angle is called the angle of repose, and it is 34 degrees for dry sand, which is a little steeper than a 2:1 gradient (2 feet horizontally for every 1 foot up).

2 Factors in Landslides (BYUI) Children learn about mass movement (or landsliding, or mass wasting) first by playing with sand. We quickly learn that dry sand cannot be made steeper than a certain angle. That angle is called the angle of repose, and it is 34 degrees for dry sand, which is a little steeper than a 2:1 gradient (2 feet horizontally for every 1 foot up). Coarser materials are stable up to a slightly steeper angle. Slopes made of loose materials are naturally unstable if they are steeper than the angle of repose. We also learn from playing with sand that moist sand is more stable than dry sand, and can be made into steeper slopes such as sand castles. Water that coats the sand grains holds the sand grains together by surface tension the inherent (but small) strength that the surface of a liquid has. It is that same surface tension that allows some insects to walk on water. In sand, it holds the grains together. But do you remember what happens when the sand is fully saturated that is, when it is completely full of water? It flows because water completely fills the pore spaces and there is no more surface tension. Saturated materials are weak, and prone to landsliding. These diagrams illustrate the angle of repose. Moist sand is more stable than dry or saturated sand, and can sustain steeper slopes. In developed areas, we try to avoid slopes approaching the angle of repose. The general rule is to keep slopes below a 2:1 gradient.

Clay plays a prominent role in landslide hazards. It is very weak, gets slippery when wet, holds water for a long time, and it blocks the flow of groundwater.

3 Clay plays a prominent role in landslide hazards. It is very weak, gets slippery when wet, holds water for a long time, and it blocks the flow of groundwater. In the example shown here, a significant clay layer in the ground blocked the downward flow of groundwater, causing the materials above it to become saturated and weak. The clay itself is very weak and slippery, and so the entire mass slid down the very slight slope. The force that drives all mass movement is gravity. Materials are held in place by the forces that resist gravity and shear friction and internal strength. Anything that reduces friction or internal strength will tend to cause mass movement. (NationalPost.com)

Steep slopes Soft materials are unstable at slopes greater than the angle of repose, which is a little steeper than 2:1.

4 Factors that increase the likelihood of landslides: Heavy rain or rapid snow melt These can saturate slopes, causing them to lose strength. Weak materials Materials with little internal strength tend to move more easily. These include shale, silt, sand, gravel, and talus (steep piles of loose rock, usually below cliffs). Steep slopes Soft materials are unstable at slopes greater than the angle of repose, which is a little steeper than 2:1. Anything that steepens a slope, like erosion or excavation, increases the probability of landslides. Saturation Materials are most stable when they are most, not completely dry or completely saturated. Lack of vegetation Plant roots hold the surface materials of a slope together like a blanket, stabilizing them. Landslides are common after wildfires have removed the vegetation. Clay layers Clay, as we all know, is slippery when wet. It also blocks the infiltration of water, causing layers above them to become saturated and weak. Some clays also expand greatly when they are wet, which can destabilize the slope. Climate Semiarid climates like southern California have a brief wet season followed by months of low precipitation. As a result, slope materials can go from completely dry to saturated in a very short time, which destabilizes them. In wet climates, surface materials can be deeply weathered (broken down chemically) into weaker materials that are unstable. Tilted layers When soil or bedrock layers are tilted the same direction as the slope, any removal of layers from the bottom of the slope will remove support from the higher layers. When this happens, the higher layers are only held in place by friction and internal strength, which makes them vulnerable to landslides. Earthquakes can shake unstable materials loose, causing landslides. These landslides in Alaska were caused by earthquakes. The landslides came to rest on top of glaciers, which then slowly carried the material downstream. (USGS)

5 (USGS) This landslide in Brazil was caused by heavy rains. The slope is steep and made of weak, weathered soils, so landslide risk here is high.

BEFORE In the Dolomite Mountains near Longarone, Italy in 1963, a

The tilted bedrock layers had been undercut by erosion of the river.

weakening them, and removing support for the tilted layers up higher.

6 BEFORE In the Dolomite Mountains near Longarone, Italy in 1963, a landslide fell into a newly filled Vaiont reservoir, instantly pushing all the water over the dam. The flood killed over 2500 people in the towns downstream. The tilted bedrock layers had been undercut by erosion of the river. Filling the reservoir forced water at high pressure into the rock layers, weakening them, and removing support for the tilted layers up higher. Held in place only by friction, the mass slid on a weak, wet clay layer into the reservoir. AFTER (AGU) Photo tour of the Vaiont landslide (AGU) (Penn. State U.)

Landslide Mitigation (BYUI) The general principle behind landslide mitigation is stabilization. Slopes that are too steep are shallowed or strengthened.

7 Landslide Mitigation (BYUI) The general principle behind landslide mitigation is stabilization. Slopes that are too steep are shallowed or strengthened. Loose rocks can be bolted or cemented into place. Erosion that could over-steepen a slope is controlled. Saturation is prevented with drainage systems. In this section, we look at examples of landslide mitigation. The diagram at left shows some ways that slopes can be stabilized to reduce risk from landsliding. Landslide mitigation measures include the following: 1) slope reduction 2) surface and subsurface drainage 3) terracing & rock walls 4) plants with deep root systems 5) buttressing putting a heavy rock blanket on a soft slope (TVA.gov)

A soft slope before (left) and after (right) it was stabilized with

The rock walls are permeable, allowing water to flow out of the hill

(city of Seattle) Another soft slope, this one stabilized with a

8 A soft slope before (left) and after (right) it was stabilized with terraces and rock walls. The rock walls are permeable, allowing water to flow out of the hill without allowing the mud beneath to flow. (city of Seattle) Another soft slope, this one stabilized with a blanket of large rocks. The weight of the rocks holds the soft soils in place, while allowing water to drain from the slope. (city of Seattle)

9 (city of Cambridge, MA) The slope above was stabilized using webbing made of coconut fibers. Below, the slope was stabilized by reducing the slope and planting grass with an aggressive root system. (state of California)

In addition, a buttress or blanket of heavy, loose rock can hold soft materials in place and prevent landsliding.

10 (state of California) (above) Steep slopes must be graded to be no steeper than 2:1 for every two feet horizontally, the slope rises only 1 foot. In addition, a buttress or blanket of heavy, loose rock can hold soft materials in place and prevent landsliding. The rock blanket must be permeable to allow water to drain out of the slope a saturated slope is an unstable slope! The photos below show an effective and attractive application of slope reduction and vegetation planting. (city of Seattle)

Text from Utah Geological Survey, modified and supplemented by BYU-Idaho faculty Public Information Series #58 Homeowner's guide to recognizing and reducing landslide damage on their property

11 Text from Utah Geological Survey, modified and supplemented by BYU-Idaho faculty Public Information Series #58 Homeowner's guide to recognizing and reducing landslide damage on their property Landslides are common in all 50 states. Landslide hazards are greatest near or on steep slopes or along water bodies. And REMEMBER - Homeowner's insurance typically does not cover landslide damage!

12 Human factors that lead to landslides include: Grading or excavation that removes material from the base, loads material at the top, or otherwise alters a slope. Addition of water to a slope from agricultural or landscape irrigation, roof downspouts, poor drainage, septic-tank effluent, canal leakage, or broken water or sewer lines. Areas that are generally prone to landslides are: Existing landslides. Steep natural slopes, particularly in weak geologic materials. Steep construction-related cut or fill slopes. Areas in or at the mouths of drainages (such as canyons). Slopes below leaking canals or ponds. Developed hillsides where septic-tank soil-absorption systems are used and landscapes are irrigated. Below cliffs or hills with outcrops of fractured rock.

Features that might indicate landslide movement are: Springs, seeps, or saturated ground in previously dry areas.

Unusual bulges or elevation changes in the ground, pavements, or sidewalks.

Tilting telephone poles, trees, retaining walls, or fences.

Rapid increase or decrease in stream water levels, possibly accompanied by increased turbidity (soil content).

13 Features that might indicate landslide movement are: Springs, seeps, or saturated ground in previously dry areas. Cracks in snow, ice, soil, or rock. Soil pulling away from foundations. Offset fence lines. Unusual bulges or elevation changes in the ground, pavements, or sidewalks. Decks and patios tilting and/or moving relative to the main house. Tilting telephone poles, trees, retaining walls, or fences. Excessive tilting or cracking of concrete floors and foundations. Broken water lines and other underground utilities. Rapid increase or decrease in stream water levels, possibly accompanied by increased turbidity (soil content). Sticking doors and windows, and visible open spaces indicating walls and frames out of plumb. Appearance of bare spots on slopes. Intermittent creaking, snapping, or popping noises from a house. Sunken or down-dropped roads. Ground crack. Damaged sidewalk. Foundation damage. Door frame out of plumb.

What a homeowner can do to reduce the likelihood of landslides: Minimize landscape irrigation - overwatering on bluff-tops is a common cause of landslides.

14 What a homeowner can do to reduce the likelihood of landslides: Minimize landscape irrigation - overwatering on bluff-tops is a common cause of landslides. Drain water from surface runoff, downspouts, and driveways well away from unstable slopes and landslides. Make sure water and sewer lines do not leak. Avoid removing material from the base of slopes or adding weight at the top. Contact a geotechnical consultant for professional advice. What to do if you suspect imminent landslide danger: Contact your local fire, police, or public works department. Inform neighbors. Evacuate.

Landslide Mitigation (BYUI)

Landslide Mitigation (BYUI) The general principle behind landslide mitigation is stabilization. Slopes that are too steep are shallowed or strengthened. Loose rocks can be bolted or cemented into place.