9/23/2013. Introduction CHAPTER 7 SLOPE PROCESSES, LANDSLIDES, AND SUBSIDENCE. Case History: La Conchita Landslide

Size: px

Start display at page:

Download "9/23/2013. Introduction CHAPTER 7 SLOPE PROCESSES, LANDSLIDES, AND SUBSIDENCE. Case History: La Conchita Landslide"

Anis O’Connor’
5 years ago
Views:

Introduction CHAPTER 7 SLOPE PROCESSES, LANDSLIDES, AND SUBSIDENCE Landslide and other

, average 25 50 deaths; damage more than $3.

movements Landslide and subsidence: occurs naturally and affected by human activities Case

mi) northwest of Los Angeles Landslide occurred on January 10, 2005 Triggered by heavy

and 1995) Debris flow up to 45 km/h (30 mph) La Conchita should not be built on the

1 Introduction CHAPTER 7 SLOPE PROCESSES, LANDSLIDES, AND SUBSIDENCE Landslide and other ground failures posting substantial damage and loss of life In U.S., average deaths; damage more than $3.5 billion For convenience, definition of landslide includes all forms of mass-wasting movements Landslide and subsidence: occurs naturally and affected by human activities Case History: La Conchita Landslide 1995 Slide La Conchita: small coastal community 80 km (50 mi) northwest of Los Angeles Landslide occurred on January 10, 2005 Triggered by heavy rainfall, reactivation along an older landslide surface (35,000 years ago, 6000 years ago, and 1995) Debris flow up to 45 km/h (30 mph) La Conchita should not be built on the landslide deposits and under the foot of the slope Potential solution: relocate people and better land use regulation 2005 Slide 1

Slope Processes (1) Slopes: The most common

talus slope or upper convex slope, a straight

(2) Dynamic evolving feature, depending upon

water, and geologic time Materials constantly

Landslides (1) Slow or rapid failure of slope:

of water present, rate of movement Rate of

avalanches Types: Creep, sliding, slumping,

2 Slope Processes (1) Slopes: The most common landforms Consists of cliff face (free face) and talus slope or upper convex slope, a straight slope, and a lower concave slope Slope Processes (2) Dynamic evolving feature, depending upon topography, rock types, climate, vegetation, water, and geologic time Materials constantly moving down the slope at varied rates Types of Landslides (1) Slow or rapid failure of slope: Slope gradient, type of slope materials, amount of water present, rate of movement Rate of movement: Imperceptible creep to thundering avalanches Types: Creep, sliding, slumping, falling, flowage or flow, and complex movement (sliding and flowage) Types of Landslides (2) 2

3 Soil Slip Debris Flow Rockfall, Yosemite Home Destroyed, 2 Lives Lost Stream Erosion Soil Slip on Steep Slope Roots too shallow! 3

4 Beachfront Erosion Japanese Landslide Tennessee Landslide Why slip surfaces are curved Driving vs. Resisting Forces can change 4

Slope Stability Safety Factor SF = Resisting Forces/Driving Forces If SF >1, then safe or stable slope If SF <1, then unsafe or unstable slope Imagine the following situation Driving and resisting

do some math Geologists need to do some math Vector diagram illustrating relationships between vectors D (the weight downslope, given by WsinΘ 2 ), N (the normal force or force perpendicular to the

5 Slope Stability Safety Factor SF = Resisting Forces/Driving Forces If SF >1, then safe or stable slope If SF <1, then unsafe or unstable slope Imagine the following situation Driving and resisting forces determined by the interrelationships of the following variables: Existing slip surface Type of Earth materials Slope angle and topography Climate, vegetation, and water Time Geologists need to do some math Geologists need to do some math Vector diagram illustrating relationships between vectors D (the weight downslope, given by WsinΘ 2 ), N (the normal force or force perpendicular to the slope), and W, the overall weight of the material in the situation. Using some trigonometry and information on the materials involved, the Safety Factor can be caluclated. FS = Safety Factor S = shear strength of the material, in this case a clay L = Length of slip plane T = Unit thickness W = weight of the overlying material SF > 1.25 is best. SF for curved slip surfaces can be estimated For a curved slip surface, using the rotational geometry and applying the idea of torque (force around a moment arm) instead of direct force. The safety factor is still the ratio of resisting to driving forces/torques. Also, note the recalculation if fluid pressure is from increased moisture content is included! Human Land Use and Landslide Urbanization, irrigation Timber harvesting in weak, relatively unstable areas Artificial fillings of loose materials Artificial modification of landscape Dam construction 5

Warning of Impending Landslides Monitoring changes Human surveillance

public awareness and education Enough time for public evacuation Stop or reroute

On October 9, 1963 2600 lives were lost due to a massive landslide and

238 million cubic meters of material slid into the nearly full reservoir, moving

The displaced water made a wave 90 (~300 feet) meters high that swept over the

6 Warning of Impending Landslides Monitoring changes Human surveillance Instrumental survey: Tilt meter and geophones Landslide warning system Info for public awareness and education Enough time for public evacuation Stop or reroute traffic flow Emergency services The Vaiont Dam Disaster The Vaiont Dam Disaster On October 9, lives were lost due to a massive landslide and subsequent flood in northern Italy. 238 million cubic meters of material slid into the nearly full reservoir, moving at speeds of nearly 60 mph. The displaced water made a wave 90 (~300 feet) meters high that swept over the dam and down the valley, drowning people and washing away homes in the valley below the dam. Monitoring for 3-years leading up to the disaster showed variable rates of slip from 1-30 cm per week. Starting in September 1963, slip increased to 25 cm per day and finally up to 1 m per day just before the fateful night. The Vaiont Dam Disaster The Vaiont Dam Disaster 6

$Adverse geology weak rocks with open fractures, sinkholes and weak clay layers inclined towards$ the reservoir. 2. Steep topography = strong driving force. 3.

the reservoir. 2. Steep topography = strong driving force. 3.

reservoir caused the water level to rise further in spite of attempts by engineers to lower it.

Photographic analysis Topographic map and detailed field check Historic data Landslide hazard

engineering knowledge before any hillside development Minimizing the Landslide Hazard (2)

7 The Vaiont Dam Disaster What caused the Vaiont Dam disaster? 1. Adverse geology weak rocks with open fractures, sinkholes and weak clay layers inclined towards the reservoir. 2. Steep topography = strong driving force. 3. As the reservoir filled, water pressure increased reducing friction and weakening the clay layers even more. 4. Heavy rains in late September increased the weight of slope materials and runoff into the reservoir caused the water level to rise further in spite of attempts by engineers to lower it. Minimizing the Landslide Hazard (1) Landslide Hazard Map Identifying potential landslides Photographic analysis Topographic map and detailed field check Historic data Landslide hazard inventory map Grading code from the least stable to the most stable Application of geologic and engineering knowledge before any hillside development Minimizing the Landslide Hazard (2) Preventing landslides Drainage control: Reducing infiltration and surface runoff Slope grading: Reducing the overall slope Slope supports: Retaining walls or deep supporting piles Avoid landslide hazards Landslide warning for critical evacuations Correcting landslides 7

8 Subsidence Form of subsurface ground failure Occurred naturally or induced by human activities Retaining Walls can Prevent Mass Movements Slow settling or rapid collapse Causes: Withdrawal of fluids (water, oil and gas, steam) or removal of solid materials (dissolution, mining) 8

9 Process of Subsidence Removal of Solid Materials (1) Sinkholes Dissolution of carbonate rocks, limestone, and dolomite Affecting most of the conterminous states Natural or artificial fluctuations in water table increasing the problem Triggering other problems: Sinkholes as waste dumping sites Removal of Solid Materials (2) Salt and coal mining Salt dissolution and pumping Active coal mines and abandoned coal mines Ground failure due to depleted subsurface pressure More than 8000 km 2 of land subsidence due to underground coal mining Lake Peigneur an unexpected subsidence incident. November 21,

10 Perception of the Landslide Hazard Landslide hazard maps not preventing development Common perception: It could happen on other hillsides, but never on this one. Infrequency and unpredictability of large slides reducing awareness of the hazards Often people taking chances and unknown risks What Can You Do? (1) Professional geologic evaluation for a property on a slope Avoid building at the mouth of a canyon, regardless of its size Consult local agencies for historical records Watch signs of little slides often precursor for larger ones What Can You Do? (2) Look for signs of structure cracks or damage prior to purchase Be wary of pool leaking, tilt of trees and utility poles Look for linear cracks, subsurface water movement Put observations into perspective, one aspect may not tell the whole story 10

9/13/2011 CHAPTER 9 AND SUBSIDENCE. Case History: La Conchita Landslide. Introduction

9/13/2011 CHAPTER 9 AND SUBSIDENCE. Case History: La Conchita Landslide. Introduction CHAPTER 9 SLOPE PROCESSES, LANDSLIDES, AND SUBSIDENCE Case History: La Conchita Landslide La Conchita: small coastal community 80 km (50 mi) northwest of Los Angeles Landslide occurred on January 10, 2005