Mass Wasting Chapter 8 Landslides and Other Downslope Movements Falling Mountains Mass wasting is the term geologists use to describe downslope movement of rock soil or debris directly due to gravity Most non geologists call such features landslides although the term is not specific enough to be much use to geologists Gravity Gravity is a force that acts everywhere on the Earth s surface pulling everything in a direction toward the center of the Earth The force of gravity can be resolved into two component forces The normal force helps to hold the object in place The shear force causes a shear stress parallel to the slope and helps to move the object in the down slope direction Material Resistance to Gravity Material Resistance Forces that resist movement down the slope are grouped under the term shear strength Examples frictional resistance cohesion When the sheer stress becomes greater than the combination of forces holding the object on the slope the object will move down slope Mass Wasting Classification Schemes are based on The mechanism of movement how the material moves The type of material being moved The velocity of movement Flows Movement resembles a viscous fluid In most cases water is involved Increasing water decreases viscosity increases velocity and vice versa Flows are fast moving fluid mixtures of rock mud and water similar to wet concrete flowing down a slope Flows are called avalanches if they are particularly fast moving Flows move downslope as a dense fluid mass burying and or smashing most things in their path Because of the high density of the mass of moving material flows can carry large objects for great distances Creep Slow Flows Loose material on a slope creeps downhill almost imperceptibly at rates of a few inches to a few feet per year Results from processes that cause soil and loose rock to expand and contract such as alternating freeze thaw wet dry thermal expansion and contraction No threat to life cosmetic and structural damage Evidenced by bent trees tilted fences poles cracks in walls doors windows stick Slides Slides are defined as downslope movement of a large unit of rock or soil along a distinct surface of failure slide plane The 2 main types of slides are Rotational Slides also called Slumps and Translational Slides The difference between these two types of slides relates to the shape of the surface of failure along which the mass of rock moves Translational Slides Fast Slides A Translational Slide occurs on a relatively flat sloping surface of failure One very common cause of translational slides is dipping planes of weakness toward an open space These planes might be sedimentary bedding planes faults or fractures in the rock Rotational Slide or Slump Slow Slides A Rotational Slide or Slump occurs along a curving concave up surface of failure often spoon like in shape see a below Because the failure surface curves upward the mass rotates as it moves downhill Rotational Slides are often slow triggered by erosion at the base of the slope Falls Falls are the simplest and most common form of mass wasting Falls are vertical or near vertical fall of rock material from a cliff or steep slope Primary causes of falls are weakening of the rock in a steep area and or undercutting and oversteepening of the area Causes of Mass Wasting Mass wasting processes generally result from a combination of factors that all work together to cause a slope to fail The most important factors are Increase in slope steepness shear force of gravity Reduction of slope strength by weathering by burrowing animals or by infiltration of water Water saturation The single most important factor in most mass wasting Water saturation can result from heavy rain rapid snowmelt leaking water or sewage lines or poor drainage Water saturation weakens rock by building up pore pressure and by lubricating contacts between particles Increase in weight at top of a slope Removal of support at the base of a slope Shaking by either natural processes earthquakes or human activities explosions heavy traffic Mitigation of Mass Wasting Mitigation of mass wasting problems is accomplished by Hazard zone mapping to identify areas where movements have occurred in the past Such areas may be likely to move again Sinking foundations down into solid rock below weaker surface zones o A simple mitigation method for creep Structures will shift and slide if they are built on the creeping surface layer By sinking foundations down through the creeping surface zone and anchoring the footings of the structure in solid bedrock the structure will remain in place even if the surface creeps Building codes that limit the steepness of slopes and the types of fill used in construction o Building codes are an effective way to reduce sliding problems Many state and local governments throughout the country have adopted codes for building in hilly areas Common guidelines require a slope steepness of no more than 2 1 equals a 27 degree slope angle drainage benches placed at regular intervals to catch water runoff benched fill Drainage systems that drain water from the surface and or the subsurface Since WATER is a major factor in most mass wasting problems control of water on the surface and below ground figures prominently in most mitigation methods o Water within the ground can be controlled by hydrauger holes horizontal holes drilled into the ground and lined with perforated pipe Excess water leaks into the pipes and flows into other pipes that take it away from the slope An example is shown in this photograph o Control of surface water flow on vulnerable slopes can be done by planting vegetation and or by drainage ditches set at regular intervals on the slope Buttress fills and retaining devices to hold slopes in place Examples include retaining walls shotcrete and rock bolts o Buttress fills compacted earth that is laid onto a slope to hold it in place are a commonly used to stabilize slopes A buttress fill used together with internal and external drainage systems such as shown in the diagram below makes for a very stable slope o Retaining devices are also commonly used to stabilize slopes typically in places where the base of the slope has been cut away to create more flat building area o Retaining walls must be strong enough to replace the resisting force supplied by the original slope It must also be equipped with a water drainage system as shown here The layer of gravel in back of the wall allows water behind the
View Full Document
Unlocking...