Geotechnics - Lecture 1

  • Created by: cbd19
  • Created on: 18-01-20 15:15
What are ways soils are used?
Earth dams to retain water with soils that are strong, have low permeability, and minimised settlement., pipeline trenching/backfilling, retaining wall, and road embankment where a cut is made in the road and filled elsewhere
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What is the nature of soil?
Characteristic qualities
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What is the state of the soil?
The physical condition it exists in.
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What is diameter of particles?
Clay - up to 0.002mm, silt - Fine/medium/coarse - up to 0.06mm, sand - F/M/C up to 2 mm, gravel - F/M/C - up to 60mm, Cobbles up to 200 mm, Boulders larger
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What are fine grained and coarse grained soils?
Fine grained - clay and silt, coarse-grained - sand, gravel, cobbles, and boulders
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How is PSD found for coarse soils?
PSD - for coarse soils dry sieving is good if soils have minimal clay and silt, wet-sieving is used to separate clay and silt to agglomerations or clay flocs.
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For sieving, what are w, d, and w+d?
For a sieve, w is aperture size, d- wire diameter, and w+d=p which is pitch
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What are error sources with sieving?
Damage to screen cloth, leakage, sand not passing sieve to container
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As mesh diameter falls in sieving, what happens to particles found?
As the mesh diameter falls from 75mm to 65 um, smaller diameter soils are allowed through.
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What is d50 for the PSD graph?
d50 for the PSD graph which shows percent passing against the type of soil, is the diameter which is bigger than 50% of the particles in that sample
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What is well-graded, poorly graded, uniformly graded, gap graded, and glacial till?
Well graded - big range of sizes of soil particles, poorly graded- small range of sizes, uniformly graded - particles all same size/shape, gap-graded - excess or deficiency of soil sizes, glacial till - unsorted glacial sediment
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What would gap graded clayey gravelly sand show on a graph?
Gap graded clayey gravelly sand would show a constant amount of percent passing as the diameter of soil increases, showing a gap of these particle sizes
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What is glacial morraine?
Glacial morraine - glacial rock debris
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What is the coefficient of uniformity?
Cu = d60/d10, as Cu increases so does the range of sizes
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What is coefficient of curvature?
Coefficient of curvature, Cz= (d30)^2/d60 * d10, well graded soil has 1<Cz<3, gap graded soil outside this range. Represents coefficient of curvature on PSD graph
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How does hydrometer measurement work?
PSD of fine particles is determined by sedimentation analysis. With a hydrometer, place in fluid and read water level in mm, (starts at 0.995mm).
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What is Stoke's law? (used in finding PSD of fine soils with hydrometer)
Stoke's law assumes smooth spherical particles and laminar flow, NA to particles <0.2um. Says viscous drag and buoyancy is equal to force of weight, but real soils have a range of particle roughnesses and non-spherical particles.
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In context of PSD of fine particles, what's settling velocity and dispersing agent?
Settling velocity is the speed particles of certain diameters have when settling from a fluid and dispersing agent is added to soil and water to prevent particles sticking
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What's laser diffraction?
Laser diffraction shows particle flow with a pulsed light source and camera to find PSD of fine particles
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Phase diagrams, what are they?
Boxes showing proportions of air, water, and solid in a soil sample
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What's the formula for total volume and volume of voids?
V= Vv+Vsoil Vv= Vair+Vwater
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What's the formula for mass?
M=Mwater +Msoil. Mass of air is 0
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Intrinsic state variables, what is density of water, density of solids, and specific gravity?
Intrinstic = density of water=Mw/Vw, density of solids=Ms/Vs, specific gravity Gs= ps/pw.
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State variables, what's water content, void ratio and bulk density?
Water content w=Mw/Ms, void ratio e=Vv/Vs, bulk density p=M/V
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What's emin and emax? (state variables)
emin= densest soil packing, and max void ratio, emax= loosest soil packing.
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How is structure defined for soils?
Structure is the arrangement and connections of soil component through fabric (physical arrangement of particles, ie layering), and bonding (a process may be cementation
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In rock formation in situ, describe deposition of soil particles and burial.
In deposition, void ratio e is high, water content is high, stress is low. This is formation of fabric. In burial, bonds develop and overburden as well as consolidation, V changes from water, stress high, e low, w lowest.
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In final stage of soil formation, what's erosion
In erosion, there's unloading, swelling, and over-consolidation, volume changes from water content. water content increased, void ratio increased, stress decreased
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What are undisturbed, remoulded, and reconstituted soils?
Undisturbed - soil in natural state, remoulded- soil sample disturbed with some to most of natural structure removed from mechanical means, reconstituted - soil sample with natural structure removed due to mechanical mixing at 1-2x liq limit
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What's intact soil?
Intact - somewhat disturbed with some structure lost
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For clays, what's the liquidity index state variable?
IL, liquidity index= w-wP/WL-WP (water contents denoted by w) WL- water content at liq lim, WP- Water content at plastic lim
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What's a state variable, relative density, for sand's formula?
Sands, Dr (relative density)= emax-e/e max-emin
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What are Atterberg limits and plasticity?
Atterberg limits classify clays and silts. Plasticity is soil's ability to undergo irrecoverable deformation without cracking/crumbling.
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What are liquid and plastic limits?
Liquid limit (WL) is the lowest water content where soil can't contain shape. Plastic limit, WP, is lowest water content for soil to remain deformable without cracking.
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What values does liquidity index have at plastic and liquid limit, and what's plasticity index?
Liquidity index (w-WP/WL-WP), has value of 0 at plastic limit and 1 at liquid limit. Plasticity index Ip=WL-Wp
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What's a plastic limit test?
Soil shears longitudinally and transversely when rolled to 3mm diameter after 5-10 movements. Can find plastic limit but results subjective.
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What tests can find liquid limit?
To find liquid limit can use penetrometer or Cassagrande cup. Penetrometer is simple apparatus and less affected by operator. Cassagrande cup simple but subjective.
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What's the liquid limit for the penetrometer test and how is experiment prepared for?
Liquid limit - water content corresponding to 20mm penetration. Soil mixed with spatula and distilled water, placed in cup with no air trapped, and soil struck off.
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How is the penetration found for the penetrometer test before soil is tested for water content to find liquid limit?
Lower the tip of the cone to the soil's surface, lower the stem of the dial gauge to contact the soil shaft, record the initial reading. Release cone for 5s and lock cone, lower stem of dial gauge to contact cone shaft and make final reading.
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What does an A-Line chart show? (good for classification)
A-Line chart shows the plasticity index percentage against liquid limit. Clays go above the line, and silts below. Starts horizontal then linear. As liquid limit increases, plasticity does.
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What's the formula for activity? How is it found?
Activity, A= Ip/C where C is percentage of clay particles (particles with diameter <2 micrometers). Activity can be found on an Ip against clay fraction chart as gradient. London clay has A of 0.95
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What are nature and state variables of sand?
Nature variables: PSD, emax, emin, mineral composition. State variables: void ratio, water content, relative density, grain structure
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What are nature and state variables of clay?
Nature: liquid and plastic limit, plasticity index, mineral composition. State: liquidity index IL, clay structure, void ratio, and water content
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What's formula for specific gravity? (use densities)
Gs= Ps/ Pw
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what's formula for unit weight?γ
unit weight = density * gravity (water)
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formula for water content
w= Mw/ Ms
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What's formula for bulk density?
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Card 4


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Card 5


What are fine grained and coarse grained soils?


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