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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Harry Williams, Geomorphology 1ROCK AND SOIL STRENGTH I. Large-scale geologic structures provide the initial framework upon which landscape development proceeds. Finer details of landscapes (i.e. individual landforms) are usually determined by DIFFERENTIAL EROSION..A DOME- LARGE SCALE STRUCTUREHarry Williams, Geomorphology 2E.g. domed strata provide rock layers dipping away radially from a central point; differential erosion produces inward-facing scarps, outward-facing dipslopes and radial strike valleysDIFFERENTIAL EROSION CREATES LANDFORMSHarry Williams, Geomorphology 3Differential erosion occurs because the surface of the earth is composed of various rock and soil types with different erosion resistance or STRENGTH. The rate and manner of landform development depends on:EROSIONAL PROCESSES (type) (power) (frequency of operation)STRENGTH OR RESISTANCE OF ROCKS AND SOILSsoilHarry Williams, Geomorphology 4Strength and StressIn the context of geomorphology, strength refers to the ability to resist being moved by erosional processes, which normally operate in a downslope direction. The force exerted by erosional processes (including gravity) is a SHEAR STRESS directed downslope and causing a mass of rock or soil to shear over the underlying material..Harry Williams, Geomorphology 5SOIL ON SLOPES. Soil = mixture of mineral grains, organic material and pore spaces filled with variable amounts of air and water. Soil development = part of the process of erosion of rock :- soil is largely derived from weathering of rock (the physical disintegration and/or chemical decomposition of rock) - usually weaker than rock - therefore more easily removed by erosional processes.Harry Williams, Geomorphology 6mountainshillscliffs ridgesSoil Parent Materials. Residual soil = soil developed on underlying bedrockTransported soil = soil developed on surficial depositsWell-drainedPoorly drainedHarry Williams, Geomorphology 7canyonsvalleysbeachesdeltasControls On Soil CharacteristicsThe characteristics of soil depend on: parent material; climate; vegetation; slope.1. Parent material: influences; a. the rate of soil development (rate of weathering)b. soil composition e.g. shales produce a lot of clay; sandstone produces sandy soilc. physical properties of soil e.g. permeability/drainage (number, size and connectivity of pore spaces); shrink-swell potential (amount of expansive clay); cohesive strength (clay content - clayey soils are "sticky" - this aids cohesion). 2. Climate: influences type and rate of weathering, amount of water moving through and over the soil; type of vegetation.3. Vegetation: influences organic content of soil, strength of soil (roots increase cohesion)4. Slope: Steeper slopes -> accelerated erosion, if rate of erosion > rate of soil development -> thin or no soil. Soils on steeper slopes also have lower water contents (lower infiltration) -> less weathering, less vegetation. Soils in low-lying areas have higher water contents, more weathering, thicker soils, more vegetation.Harry Williams, Geomorphology 8Woodbine sandstoneEagle ford shaleHarry Williams, Geomorphology 9Woodbine sandstoneEagle ford shaleHarry Williams, Geomorphology 10Rolling hills, surface erosionplainsHarry Williams, Geomorphology 11Woodbine sandstoneRock And Soil Resistance To Shear StressesDepends on:1. Frictional properties of material2. Normal load (weight pushing rock/soil mass into slope)3. CohesionFrictionDepends on hardness of shear surfaces; roughness of shear surfaces and number+area of points of contact between shear surfaces.Harry Williams, Geomorphology 12depositionalSymbolized by angle of shearing resistance (f ), which determines the coefficient of plane sliding friction tanf. (these values can be found in laboratory tests). Higher normal stress increases frictional resistance therefore for purely frictional materials (e.g. dry sand), shear resistance =dn tanf (where dn = normal stress). dnHarry Williams, Geomorphology 13Cohesionrefers to "binding together" of material e.g. chemical cementation in sedimentary rocks; surface water tension in pore spaces (i.e. sticky clay); binding root systems of plants.Therefore, TOTAL shear resistance (or strength) S = C + dn tanf - the Coulomb Equation.Mass of soil held together by rootsHarry Williams, Geomorphology 14Shear stress. Refers to the force "pushing" the material down the slope. This depends on:i) the weight of the material and ii) the downslope component of gravity (see below). Force = mass x acceleration (due to gravity; in a downslope direction).The arrow length is proportional to the magnitude of the force.dnIncreased slope decreases normal stress and increases shearing stress.Harry Williams, Geomorphology 15Effect of water on soil strength. Water affects cohesion, friction and normal stress.A. Moist soil: water films create a negative PORE WATER PRESSURE i.e. a "suction" which increases cohesion by "drawing" particles together.B. Saturated soil: POSITIVE PORE WATER PRESSURES can develop, which "push" particles apart (i.e. water in pore spaces becomes “pressurized”) - this ACTS AGAINST the normal stress, effectively reducing it. Since particles are pushed apart cohesion and friction are also reduced.To account for water pressures, the Coulomb Equation is modified thus:S = C' + (dn - Pw) Tan f' where Pw = Pore water pressureC' = Cohesion, including Pw effectTan f' = Internal friction, including PwNote: Negative Pw will increase both cohesion and friction, increasing strength. Postive Pw will decrease both cohesion and friction, decreasing strength.Harry Williams, Geomorphology 16< moist sand has cohesion, strength, can stand up.Saturated sand loses cohesion, strength, falls > downHarry Williams, Geomorphology 17The evaluation of slope stability is essentially based on a comparison of shear strength and shear stress i.e. Safety factor (F) = shear strength shear stressIf F > 1 = stable slope F = 1 = critical threshold F < 1 = failureThese calculations can be made, but only for a given set of conditions. Often, however, it is a change in "normal" conditions that causes failure.Harry Williams, Geomorphology 18Changes That Decrease Stability1. Increased water content: Increased water content can change the strength and stress ratio in a


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UNT GEOG 3350 - soilstrength08

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