Slope Processes

Mass Wasting Landslide and other ground failures posting substantial damage and loss of life In the United States, average 25 to 50 deaths and up to about 100 to 150 if collapses of trenches and other excavations are included; damage more than $3.5 billion Landslide and subsidence: Naturally occurred and affected by human activities

Mass Wasting Mass wasting Comprehensive term for any type of downslope movement For convenience, definitions of landslide here includes all forms of mass-wasting movements Subsidence is another type of mass wasting except earth materials move downward in a vertical manner as opposed to along a slope

Slope Processes Slopes are the most common landform on Earth Although they appear stable, they are dynamic, evolving systems Slopes are not uniform in shape but composed of segments that are straight or curved Slopes that are straight typically form vertical cliffs Gentler slopes typically have three segments Upper convex slope Lower concave slope Straight slope (separating the two above)

Slope Processes Vertical slopes are more common with hard rock in an arid environment with little vegetation Convex and concave slopes are more common on softer rock with a humid climate and thick soils and vegetation Local conditions largely determine what type of slope is present Materials on slopes are constantly moving Imperceptibly slow Incredibly fast Slope processes help explain why valleys are typically much larger than the streams they contain

Types of Mass Wasting Earth materials may move in the course of several situations Rock fall Creep Earth flow Mud flow Debris flow Slump

Rock Fall Free fall of earth materials from a vertical cliff face

Creep Very slow flowage of rock or soil

Earthflow Rapid flowage When soil partially liquefies and runs out Typically forms a bowl-shape depression at the source area

Earth Flow

Mud flow Rapid flowage A mixture of rock, soil, and organic matter that mixes with air and water More than 50% fine material A debris flow is similar except it has less than 50% fine material

Debris Avalanche A rapid to very rapid debris flow Can cause catastrophic damage and loss of life

Debris Avalanche

Lateral Spread Occur on nearly flat surfaces Involves liquefaction of fine particles by earthquake vibrations Start suddenly then become larger in a slow, progressive manner

Subsidence May occur on slopes or flat ground Involves the sinking of earth materials below the level of the surrounding surface

Slump Almost vertical movement along a surface which translates into a lobe-shaped flow toward the bottom

Slump

Types of Slope Processes Important variables that are used to classify different slope processes include Type of movement Slope material Amount of water present Rate of movement

Types of Mass Wasting

Forces on Slopes The stability of a slope is expressed as the relationship between driving forces and resisting forces Most common driving force is the weight of material Including anything on the slope (vegetation, buildings, etc.) The most common resisting force is the strength of the earth material

Slope Stability Slope stability is computed by using a factor of safety (FS) Ratio of resisting forces to driving forces Driving and resisting forces are not permanent and can change over time These forces are determined by the interrelationships of the following Type of earth material Slope angle Climate Vegetation Water Time

Earth Materials The material of the slope affects the type and frequency of movement Rotational slides (slumps) follow a curved surface Translational slides follow a planar surface These planar surfaces can potentially include Fractures in rock layers Bedding planes Weak clay layers Foliation planes in metamorphic rock

Earth Materials The type of material composing a slope may influence the type of slope processes that occurs Weak rock (shale) will likely see creep and possibly earthflows Resistant rock (sandstone, granite)is typically much more sturdy and secure

Slope Angle Slope angle greatly affects the relative magnitude of the driving forces on slopes Driving forces increase on steeper slopes Steep slopes are associated with rockfalls and debris avalanches Gentle slopes are associated with creep

Climate Can be defined as the characteristic weather at a region over an extended time frame Climate influences the amount and timing of water that may infiltrate a slope Also influences the amount of vegetation that may be present

Vegetation Role of vegetation is complex Vegetation is significant for many reasons Provides cover that cushions the impact of falling rain Facilitates the infiltration of water into the subsurface Slows erosion of soil Root systems provide cohesion for earth materials which increases resistance to movement (similar to iron bars in concrete) Adds weight to a surface

1969 debris flow Two lives were lost in this event

Water Water is almost always involved in slope processes, either directly or indirectly Water can affect slope stability in three ways Landslides can develop when soil become saturated during rainstorms Landslides and slumps can occur months or years after water has infiltrated into a soil Water can erode the base of a slope, increasing the likelihood of a slope failure This includes rivers eroding a stream bank or waves eroding a coastal cliff

Slope failure due to river erosion

Time The forces on slopes often change over time Driving and resisting forces may change seasonally Chemical weathering, which reduces the strength of rock, takes place over time

Humans and Landslides The effect of humans on slope failures ranges from insignificant to very significant People need to learn where, when, and why slope failures occur to minimize building in hazardous areas Human activity has been shown to increase the likelihood of slope failures, including Timber harvesting Urbanization

Minimizing Hazards Minimizing the effects of landslides requires Identifying areas where landslides are likely to occur Designing slopes or engineering structures to prevent landslides Warning people of impending slides Controlling slides after they start moving The most preferential and least expensive option is to avoid development on sites where slides are likely to occur

Identifying Potential Landslides Identifying areas with a high potential for landslides is the first step in developing a plan to avoid landslides hazards Sites at risk for landslides can be identified by local geologic conditions and aerial photographs to recognize past slides Once identified, a landslides inventory should be prepared The inventory is a map that shows locations in terms of their relative activity

Preventing Landslides Preventing landslides is difficult but can be done with common sense and good engineering practices People should avoid overloading the top of slopes, cutting into sensitive slopes, placing fills on slopes, and changing water conditions on slopes Common engineering practices include Drainage control Removal of unstable slope material Construction of retaining walls

Drainage Control Drainage controls are usually effective in minimizing slope failures The objective is to divert water to keep it from running across or infiltrating into a soil The amount of water infiltration can also be controlled by covering the slope with an impermeable layer such as soil-cement, asphalt, or plastic

Drainage Controls

Grading Carefully planned grading can increase the stability of a slope In many cases, material from the upper part of the slope is removed and placed near the base of the slope In some cases, the slope may be cut into benches or steps with drainage controls to divert water

Grading

Slope Supports Retaining walls constructed from concrete, stone-filled wire baskets, or concrete, steel, or wooden piles can provide support at the base of a slope They are typically anchored well below the base of the slope, backfilled with permeable gravel, and provided with drainage controls

Slope Supports

Subsidence The very slow to rapid sinking or settling of earth materials Subsidence can be caused by several factors Withdrawal of fluids Chemical weathering Mining

Fluid Withdrawal The withdrawal of fluids from the subsurface can cause incidents of subsidence Fluids beneath the surface have a high fluid pressure that supports overlying materials If the fluid is removed, the support is also removed and subsidence can occur Example: Overpumping of groundwater in central California for agricultural purposes

Chemical Weathering The dissolution of limestone or salt by groundwater can cause overlying earth materials to subside One of the most common features that forms by this process are sinkholes Sinkholes occur as limestone is dissolved by groundwater, enlarging voids within the rock The overlying rock then collapses into the subsurface

Mining Coal mining has caused a number of issues with ground subsidence The subsidence is most common where underground mining is close to the surface Usually, 50% of the coal is left behind as pillars that support the overlying rock Over time these pillars can weaken, producing subsidence More than 8,000 square kilometers of land has subsided due to mining and subsidence can continue long after mining operations cease