Mass Wasting Movement of earth materials downslope, driven by Gravitational Forces. Landslides - general term for rock or soil movement. In U.S., on average, mass wasting causes 1 to 2 billion dollars in losses per year. Many mass wasting events could be prevented or their effects minimized by better site assessment and land use practices. Requirements for Mass Wasting 1. Mass in a semi-stable or unstable position. 2. Driving Force - gravity with a component in direction of possible movement. 3. Freedom to move - driving force (gravity) must exceed resisting forces (friction or particle cohesion). 4. Triggering mechanism - something that either increases driving force or decreases resistive forces. Slope Stability Stability of a slope is determined by the balance of: Shear stress - force causing movement parallel to the slope. Shear strength - resistance of the body to movement. See fig. 9.2 1
Factors Affecting Mass Wasting 1. Steepness of slope 2. Nature of the material 3. Water 4. Vegetation See Table 9.2 Steepness of Slope Pull of gravity has two components: Perpendicular acting at right angles to slope, tends to hold objects in place. Tangential acts along and down slope and causes objects to move downhill. Steeper the slope, greater the tangential component relative to the perpendicular. Hence, greater shear force on the mass. Nature of the Material Shear strength is governed by frictional and cohesive forces of the material. Crystalline rock - cohesiveness affected by: Pore spaces Intersecting sets of joints or fractures Inclined bedding or foliation 2
Nature of the Material Unconsolidated sediments Angle of repose - maximum slope at which loose, unconsolidated material remains stable. Dependent upon: Particle size Angularity of particles Degree of sorting Presence and amount of water Water Greatest angle of repose is associated with dry, large, poorly-sorted angular particles. Role of Water 1. Loading Addition of water adds weight increasing driving force. 2. Surface Tension Water thin-films adsorb onto particles and add cohesion. 3. Water Saturation Addition of too much water saturates particles and cohesion is greatly decreased. Liquifaction - transformation of regolith and soil to a fluid state by addition of water. See fig. 9.3 Role of Water 4. Increased fluid pressure compaction of water-saturated sediments increases fluid pressure at the base of a slope, greatly reduces frictional resistance along grain contacts. 5. Swelling soils certain clay-rich sediments (smectites or bentonite clay) can absorb significant amounts of water causing them to swell to many times their original volume. 3
Role of Vegetation Roots hold soil together and plants absorb water. Hence, highly vegetated slope is more stable than similar slope without vegetation. Classification of Mass Wasting Based upon three considerations: 1. Rate of the movement (cm/yr to km/hr) 2. Nature of the material (rock vs unconsolidated debris) 3. Nature of the movement (sliding as coherent unit vs flowing like a fluid) See Table 9.1 Types of Mass Wasting Falls - rapid, free-falling movement of detached bodies of rock or soil from steep slope. Slides - rapid displacement of coherent masses of rock or sediment by sliding along well defined surface. Flows - mass flows as plastic or viscous fluid. Subsidence - downdrop of surface either by slow compaction of loose sediments or catastrophic collapse into caves (e.g., karsting). See fig. 9.1 4
Falls Rockfalls - freefall of detached bodies of bedrock from cliff or steep slope (fig. 9.14. Debris falls - freefall of mixture of rock and weathered regolith and attached vegetation. Talus slopes - fan-shaped pile of rock debris from falls accumulated at base of cliff (fig. 9.15). Slides Rockslide - translational sliding along bedding plane or fracture set. Slump - rotational sliding along curved concave-up surface. Scarp - cliff formed at top of a slope when slump block rotates downward. Flows Creep - extremely slow (<1cm/yr) movement of regolith, soil and rock. Cause by expansion and contraction during freezethaw cycles (figs. 9.4, 9.5). Solifluction - movement of water-saturated debris over impermeable material. Often found in permafrost regions (fig. 9.10). Mudflow - highly fluid (up to 30% water) flows of fine-grained material. Lahars - mudflows associated with volcanic eruptions. 5
Flows (continued) Debris flow (earthflow) - regolith with grains coarser than sand that flow at 1 to 100 m/hr (fig. 9.7). Debris avalanche - rapid to extremely rapid (km s/hr) movement of turbulent mass of rock, sediment, air and water. Travel on a cushion of trapped air. Subsidence May be slow or rapid (catastrophic). Occurs in response to: Rearrangement and compaction of loose particles. Loading of sediments, rock or ice on earth s surface. Removal of supporting rocks and fluids. Predicting Mass Movement Landslide Potential (slope stability) Maps Maps constructed based upon past landslide activity and geologic studies of potentially unstable slopes. Overview map for the continental U.S. (as compiled by the U.S. Geological Survey) can be found at: http://landslides.usgs.gov/html_files/landslid es/nationalmap/national.html 6
Indications of Natural Instability History of past landslides in nearby area in the same rock units. Soil types rich in fine, silt to clay-sized particles. Downslope orientation of planes of weakness in bedrock. Slope undercutting. Earthquake tremors. Sensitive slopes subjected to intense rainfall. Human Causes of Slope Instability Oversteepening/Loading Undercutting Changes in hydrological characteristics Urbanization and changes in infiltration patterns Flood control reservoirs in steep valley Removal of vegetation See fig. 9.18 Prevention/Mitigation of Mass Wasting Slope Drainage Interceptor ditches excavated at top of slope to capture and remove runoff. Perforated pipe driven into slope. Well driven into slope (fig. 9.19). Slope Reduction Grading of steep slopes (fig. 9.20). Benching or terracing. 7
Prevention/Mitigation of Mass Wasting Engineered Methods Shot-crete seals to prevent water infiltration Retaining walls Rock bolts/anchors Cable nets and wire fences for falling rock. Interceptor ditches Rock sheds and tunnels See fig 9.21 Recognition and Safety The U.S. Geological Survey has published guidelines for the recognition of landslide prone areas and the appropriate safety precautions to take in these regions. The guidelines can be found at: http://geology.cr.usgs.gov/pub/open-filereport/ofr-00-0450/index.html 8