Chp14: Mass Wasting Mass Wasting (mass movement) -defined as the downslope movement of material under the influence of gravity -aided most by weathering -usually involve surficial material -move at different rates 1. creep-slow 2. rockfall or slide- fast Factors that influence mass wasting: when gravitational force acting on a slope exceeds the resisting force-failure occurs 1. Shear Strength of the slope: a function of the slope material’s strength and cohesion, amount of internal friction between grains, and external support of the slope 2. gravity: opposing force acting along slope and in some cases perpendicular to slope angle of repose=the steepest angle that a slope can maintain without collapsing a. Slope angle- THE major cause of mass wasting; in general, the steeper the slope, the less stable that slope is….. slopes are over steepened by- -undercutting by stream or waves -excavations for roads, houses, etc b. Weathering and Climate: Mass Wasting is more likely to occur in a loose or unconsolidated rock than in bedrock -climatic effects- depth of weathering zones varies with climate-deeper in tropics

Chp14: Mass Wasting Severity of landslides in US. Areas of greatest severity occur along coastal mountain ranges.

Shear strength of a slope depends upon: a Shear strength of a slope depends upon: a. strength and cohesion of material underlying the slope, amount of internal friction between grains, any external support of the slope. Force of gravity acts vertically but also has a component acting along slope….

Chp14: Mass Wasting c. Water Content-the more water contained in a rock or soil, the more likely it is to fail -water percolating through soil decreases the friction between grains. clay can contain lots of water (made up of platy particles) which makes clay susceptible to movement down slope d. Vegetation- vegetation absorbs water, decreases the water saturation in soils -root systems stabilize slope by binding soil particles and holding soil to bedrock -removal of vegetation adversely effects slope stability (e.g. fires followed by heavy rains...) e. Overloading- almost always result of human activity: dumping, filling or piling up material.... additional weight increases the water pressure in the soil f. Geology and Slope Stability: relationship between topography and geology if rocks underlying slope dip in same direction as the slope, mass wasting is more likely to occur - water can percolate along fractures and bedding planes - joints can act as failure planes or water pathways

Undercutting by stream erosion: a Undercutting by stream erosion: a. removes a slope’s base, which increases the slope angle. B. Which leads to slope failure- notice the curved weakness plane. C. example from Michigan

Sea Cliff north of Bodega Bay, CA undercut by waves during winter of 1997-98. as result, part of land slid into ocean, damaging homes

Chp14: Mass Wasting g. Triggering Mechanisms -vibrations from earthquakes -excessive amount of water: winter snow melt, or heavy rain -explosions or sounds: avalanches in mountains What are Different types of Mass Wasting? geologists classify them into two general categories- a. rapid mass movements-most dangerous b. slow mass movements-move a much greater volume of earth materials Falls 1. Rockfalls- rocks of any size fall through the air; occur along highways, cliffs and roadcuts talus- buildup of loose rocks and fragments at base of slope due to rockfall -result from failures along joints or bedding planes in bedrock -triggered by undercutting (human activity) or earthquakes -chemical weathering caused by water percolating along cracks in carbonate rocks responsible for rockfalls -common in mountainous areas

Rockfalls result from failures along cracks, fractures or bedding planes in bedrock- common in areas with steep cliffs.

Highway excavations disrupt equilibrium of slope-removing support and over-steepening it at point of removal. Results in landslides

Huge rockfall closed lanes on Hwy 70 near Rogers Flat, CA on July 25, 2003. Note dip of beds on right….

Chp14: Mass Wasting: a. rocks dipping in same direction as hill’s slope are very susceptible to mass wasting. Note stream at base of slope…. B. fractures dipping in same direction as slope can be enlarged by chemical weathering, which can weaken the rocks and cause mass wasting.

Chp14: Mass Wasting b. Slides -involve movement of material along one or more surfaces of failure. 2 types are recognized: 1. slumps or rotational slides- movement occurs along curved surface downward movement of material along a curved surface of rupture, characterized by backward rotation of slump block - occurs in unconsolidated material -size: small to large -causes: erosion along base of slope along highways, wet climate and tectonic activity 2. rock or block slides occur when rocks move downslope along a planar surface (e.g. bedding planes) c. Quick clays-clays that spontaneously liquefy and flow like water when disturbed -occur in deposits of fine silt and clay particles ground up by glacier action d. Solifluction: slow downslope movement of water saturated surface sediment -most common in permafrost areas (high latitudes- cold) -during warm weather the upper layer of permafrost melts, making surface sediment and water into a soggy mixture -this can flow down hill, produces a lobate topography causes problems for construction in high latitudes

Slump: material moves downward along curved surface of rupture, causing slump block to rotate backward.

Massive slump in Palisades, CA caused by wave undercutting. Cranes…

Chp14: Mass Wasting: heavy winter rains caused this large landslide in March 1995 in La Conchita, CA. 9 homes were destroyed.

Rock Slide- occurs when blocks of material move downslope along planar surface.

Rock slide in Laguna Beach, CA caused by- Rocks dipping in same direction as slope of sea cliffs. 2. Undercutting of sea cliffs by wave action. 3. Clay beds that became slippery when wet..

Turtle Mountain Rock Slide that killed 70 people….. April 29, 1903 caused By a combination of Factors: Joints that dipped in same direction as slope of Turtle Mountain. 2. A fault part way down the mountain. 3. Weak shale and siltstone beds underlying base of mountain, and 4. Mined out coal seams.

2 meter high mudslide that buried car in CA

A mudflow near Estes Park, CO

Debris flow …note large boulders that are part of flow

Earthflows form tongue-shaped masses of wet soil that move slowly downslope.

Earthflow Near Baraga, MI

Ground shaking during 1964 Alaska earthquake turned parts of Bootlegger Cove Clay into a Quick clay… Note the small scarps as the snow covered ground moved down slope.

An earthquake 65km away triggered a landslide in Peru that destroyed 2 towns and killed 25,000 people.

Distribution of permafrost areas in Northern Hemisphere. b. Solifluction flows in Alaska-typical lobate topography characteristic of these conditions

Chp14: Mass Wasting e. Creep - slowest type of flow, most widespread, responsible for moving largest volumes of material down slope - most effective in humid regions, can occur everywhere How to recognize? -tilted trees and power poles -broken streets and sidewalks -cracked retaining walls and foundations ….very difficult to control !! f. Complex movements- when several types of mass wasting occur together or in succession -debris avalanche: occur in steep mountain ranges begin as rock falls, then slides or flows down slope, picking up more material as it goes downhill

Evidence of Creep: a. curved tree trunks, displaced monuments, tilted power poles, displaced and tilted fences, roadways not in alignment, hummocky surface. Bent rock layers, tilted stone wall

Complex movement that began with slump at the head, followed by earthflow….

Chp14: Mass Wasting How do we recognize and minimize effects of Mass Wasting? - need a thorough geological investigation of the region to identify areas of high potential for failure. this report may produce slope stability maps..... Ways to mitigate impact of Mass wasting: -add drainage to hillsides: place drainage pipes in hillsides -divert surface waters: culverts, ditches, etc -plant vegetation on steep hillsides -reduce the slope: a. cut and fill method: remove material at upper slope and fill in the base. - benching: cut a series of benches or steps -build retaining walls: provide support at base of slope -rock bolts: fasten loose blocks to bedrock

Minimizing damage from rockfalls: Wire mesh covers rock face. Wire mesh fence along roadside

Mitigation procedures- Driving drainpipes that are perforated into a hillside.

Stabilize hillside: Cut and Fill Method-remove material from top of slope, add it to base of slope-lessens slope angle and provides support.

Slope stabilization…Benching…make several cuts along slope, reduce overall slope. Any failures are now small in size, material collects on benches.

a. Retaining wall Rock bolts…with wire mesh…

Chp14: Mass Wasting- SUMMARY Mass Wasting (mass movement) -defined as the downslope movement of material under the influence of gravity -aided most by weathering -usually involve surficial material -move at different rates Factors that influence mass wasting: 1. Shear Strength of the slope: a function of the slope material’s strength and cohesion, amount of internal friction between grains, and external support of the slope 2. gravity: opposing force acting along slope and in some cases perpendicular to slope angle of repose=the steepest angle that a slope can maintain without collapsing a. Slope angle- THE major cause of mass wasting; in general, the steeper the slope, the less stable that slope is slopes are over-steepened by - -undercutting by stream or waves -excavations for roads, houses, etc b. Weathering and Climate: Mass Wasting is more likely to occur in a loose or unconsolidated rock than in bedrock -climatic effects- depth of weathering zones varies with climate-deeper in tropics c. Water Content-the more water contained in a rock or soil, the more likely it is to fail d. Vegetation-vegetation absorbs water, decreases the the water saturation in soils e. Overloading-almost always result of human activity: dumping, f. Geology and Slope Stability: relationship between topography and geology; dip of beds

Chp14: Mass Wasting- SUMMARY g. Triggering Mechanisms-vibrations from earthquakes -excessive amount of water: winter snow melt, or heavy rain -explosions or sounds: avalanches in mountains What are Different types of Mass Wasting? geologists classify them into two general categories- a. rapid mass movements-most dangerous b. slow mass movements-move a much greater volume of earth materials a. Falls Rock falls- rocks of any size fall through the air; occur along highways, cliffs and road cuts b. Slides-involve movement of material along one or more surfaces of failure. 2 types are recognized: 1. slumps or rotational slides- movement occurs along curved surface downward movement of material along a curved surface of rupture, characterized by backward rotation of slump block 2. rock or block slides occur when rocks move down slope along a planar surface (e.g. bedding planes) c. Quick clays-clays that spontaneously liquefy and flow like water when disturbed -occur in deposits of fine silt and clay particles ground up by glacier action d. Solifluction: slow down slope movement of water saturated surface sediment -most common in permafrost areas (high latitudes- cold)

Chp14: Mass Wasting- SUMMARY How do we recognize and minimize effects of Mass Wasting? - need a thorough geological investigation of the region to identify areas of high potential for failure. this report may produce slope stability maps..... Ways to mitigate impact of Mass wasting: -add drainage to hillsides: place drainage pipes in hillsides -divert surface waters: culverts, ditches, etc -plant vegetation on steep hillsides -reduce the slope: a. cut and fill method: remove material at upper slope and fill in the base. - benching: cut a series of benches or steps -build retaining walls: provide support at base of slope -rock bolts: fasten loose blocks to bedrock

Point Fermin, CA landslides….wave action along base undercuts slope

Chp14: Mass Wasting