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Midterm

Earth midterm 2 notes.docx

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Department
Earth Sciences
Course
EARTH 121
Professor
Keith Delaney
Semester
Fall

Description
Earth midterm 2 notes Soil and Weathering  Regolith: the layer of rock that cover nearly everywhere on the earths land surface (blanket rock)  O horizon- organic matter  A- mainly mineral, Finer particles move down to this layer  B- accumulation zone for fine particles  C- slightly altered rock  Mechanical fragmentation- disintegration  Chemical alteration- (decomposition and dissolution)  Frost wedging- repeated freeze and thaw of water in pores and fractures  Unloading- the reduction of confining stress when rocks are brought to surface because of erosion on the top layer fracturing into a skin layer  Chemical weathering- bedrock and internal material is broken down converting some parent material into different minerals, water plays a big role  Mafic minerals are least resistant to weathering  Quartz most resistant  Soil is then where slopes become steeper  Porosity = Vv/Vtotal  Void ratio= Vv/Vs  Water content= Mw/Ms  Saturation= Vw/Vv  Bulkdensity= Mtotal/Vtotal  Unit weight= mtotal*g/Vtotal  Dry strength- try to break with fingers, the siltier or sandier the easier to break  Knife Test- Cut piece with knife, if shiny the clay content is high  Teeth test- grind small fragment against teeth, gritty is a lot of silt, smooth is clay particles no silt  Clay < 2um, 2-60 um =silt, 60-2000 um=sand, gravel >10 mm  Clay minerals add to cohesive strength of soil  Plasticity- ability of soil to undergo unrecoverable deformation at a constant volume without cracking or crumbling  States of Soil: Decreasing Water content o Liquid- no strength, flows o Plastic- most common for clay/silt dominated soil o Semi solid- minor plasticity, but solid behavior o Solid- cannot be deformed without brittle behavior  Liquid limit- upper limit of water content for soil to show plastic behavior  Plastic limit- lower limit of water content to show plastic behavior  Shrinkage limit- transition from plastic to solid behavior, no volume change with further drying Earthquakes  Earthquake- sudden slip(rupture) on a fault and the result is ground shaking caused by the seismic energy released  Pwaves- compressional, rock waves vibrate PARALLEL to direction of wave propagation, fast, primary wave to recording station, can pass through solids and fluids  S waves- shearing, rock vibrates PERPINDICULAR to direction of wave propagation, slow, secondary wave to recording station, can only pass through solids  Love waves- involve only side to side motion of ground surface, cant travel through fluids  Rayleigh waves- behave like ocean waves and can cause ground to move, damage buildings  Subsidence- buried wetland soils resulting from sudden subsidence  Tsunami- some buried soilds, overlain by thin layers of sand as tsunami deposits  Shaking(liquefaction)- features include dikes, sills, and blows  Structural failure- induced by shaking, may be AMPLIFIED by foundation soils  Foundation failure- induced by seismic shaking, buildings undamaged by structural failure  Liquefaction- transformation of cohesion-less material from a solid state into a liquefied state by increased pore pressure, HOUSE OF CARDS  Chile1960- 9.5, Alaska1964 -9.2, Sumatra2004-9.1, Japan2011- 9.0, Kamchatka1952-9.0, Chile2010-8.8 Rock Properties and Testing  Surface conditions- (slopes, bedrock outcrops, bedrock type, done in the office, and on the field  Subsurface conditions- (depth to hard bedrock, water levels, rock mass quality)  RQD- sum of length of core sticks in which the length is twice as large as the diameter, as a percentage of total length of the core  Very poor 0-25, poor 25-50, fair 50-75, good 75-90, excellent 90-100  GMU(geomechanics units)- single unit for design purposes, has one set of geomechanical properties  Rock classification: o Intact rock- strength, type, etc o Rock mass- discontinuities, fissures, joints  Axial stress= force/ area  Common minerals average specific gravity is 2.7  Strength is the capacity to sustain (support): o Compressive normal stress (crushing strength) o Shear stress (shear strength) o Tensile stress (tensile strength) o Bending stress (beam strength)  Stress is force per unit area acting on a rock  Strain describes changes of points in a body relative to eachother, thus internal  Concrete only 40 mpa, not very strong  Sample must be oriented to test shear strength in desired plane  RMR(rock mass rating)- includes UCS(uniaxial compressive strength), RQD, spacing of discontinuities, condition of discontinuities, groundwater conditions, orientations of discontinuities  Q system- includes rqd, joing set number, roughness of most unfavorable joint of discontinuity, degree of alteration in weakest joint, water inflow, stress reduction factor(srf) Crustal Deformation  Deformation- all changes in form or size as well as location of a rock body  Folding(ductile), fracturing (brittle)  Compression causes bodies to shorten horizontally and thicken vertically  Tension causes bodies to lengthen horizontally and thin vertically  Shear stress causes displacements along fault zones  Structural traps- due to movement of rock
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