Soil Mechanics Lecture 3 Flashcards
What do earth dams consist of?
A core, a filter and a shoulder material
What are the properties of an earth dam?
Strong
Low permeability
Minimise settlement
What is the definition of the nature of a soil?
The essential characteristics or basic qualities of the soil
How is the nature of a soil assessed?
Through intrinsic parameters
What is the definition of the state variables of the soil?
The physical condition in which the soil exists
How is the state behaviour of a soil quantified?
State variables
i.e. dense or loose soil
What are examples of essential characteristics of soil?
Particle sizes (PSD) Mineralogy (plasticity)
What are the main methods of finding the particle size distribution?
Seiving methods and sedimentation methods
What needs to be measured for dry/ wet seiving?
Apeture size
mass retaines
percentage passing
What needs to be measured for sedimentation methods?
Sedimentation, settling velocity, dispresing agent
What are 2 examples of sedimentation methods?
Pipette/ hydrometer
What are the particle size boundaries
Clay <2 microns Silt <60 microns Sand <2mm Gravel <60mm Cobbles <20 cm Boulders >20cm
What can you use dry sieving for?
Soils containing insignificant quantities of silt and clay
What can wet sieving be used for?
Mainly coarse-grained soil
How does sieving work?
Soil sample is loaded into the top sieve with the larges apeture and then the stack of sieves is vibrated or shaken for a time period, then you measure the mass change in each sieve and calculate the percentage passing
How do you plot particle size distribution?
With percentage passing on a linear scale on the y axis, and the particle size on a log10 scale on the x
what do d50, d90 and d10 mean?
d50 = the particle size such that 50% of the particles are smaller than d50
d90 = the particle size that 90% of the particles are smaller than d90
d10 follows the same ruleas the other two
What words can be used to describe particle size distribution?
- Poorly graded/ well sorted/ uniform (ping pong balls)
- well graded/ poorly sorted
- gap - graded
- glacial till
- clay
- silty clayey sand
How do you calculate the coefficient of uniformity C_u?
C_u = d60/d10
What does a C_u <4 mean?
poorly graded
How do you calculate to coefficient of curvature C_z?
Cz = (d30)^2/(d60*d10)
What is the range of coefficient of curvature for a well graded sample?
1
What kind of soil do you use sedimentation methods on?
<425 um
What are the three main methods of sedimentation analysis?
Pippette, hydrometer, laser diffraction
What is the definition of settling velocity?
The speed with which particles of a certain diammeter settle out from a fluid
What is a dispersing agent?
it is added to soil/ water to stop particles sticking together
What does a hydrometer consist of?
A graduated stem and a glass bulb
How do you use a hydrometer?
Place it in the fluid and read the water level off the scale on the stem
Uses stokes law
What 3 phases do soils have?
voids: air + water
solids (soils)
How do you calculate density?
rho = mass/ volume (kgm^-3)
How do you calculate specific gravity?
Gs = rho_s/rho_water
How do you caclulate the water content?
w = Mw/Ms
How do you calculate the void ratio?
e = Vv/Vs
How do you calculate the bulk density?
rho = M/V (kgm^-3)
What is the min void ratio?
the densest soil packing that can be determined with a given testing method
What is the max void ratio?
the loosest soil packing that can be determined with a given testing method
What are the constituents of deposition?
Genesis
Formation of fabric
What are the constituents of Burial?
Overburden
Development of bonds
consolidation
What are the constituents of erosion?
unloading
swelling
over-consolidation
What is the definition of fabric?
The physical arrangement and organisation of particles
E.g. sizes, shapes, orientation, layering, packing
What is the definition of bonding?
Soil particles connections and their stability
e.g. Cementation, bonding nature and druability
What is the definition of structure?
fabric + bonding
What is the definition of a undisturbed sample?
Soil sample in its natural state
What is the definition of a remoulded sample?
Soil sample that has been distrubed form its natural state with some of its natural structure removed due to mechanical means
What is the definition of a reconstituted sample?
Soil sample that has all of its natural structure removed due to mechanical mixing at ~1.5 to 2 times its liquid limit
What can be used to parameterise the state of clays?
The liquidity index
I_L = (w-wp)/ (wl - wp) w = water contnet wp = water content at plastic limit wll = water content at liquid limit
What can be used to parameterise the state of sands?
The relative density
Dr = (emax - e)/(emax - emin)
e: void ration
emax: maximum void ration
emin : min void ration
What is the definition of plasticity?
Soil ability to undergo irrecoverable deformation without cracking/ crumbling
What is the definition of Plastic limit?
The lowest water content at which the soil remains deformable without cracking/ crumbling
What is the definition of liquid limit?
The lowest water content at which the soil cannot maintain its shape
How is the plasticity index calculated?
Ip = Wl - Wp
How is the liquidity index calculated?
Il = (w-wp)/(wl-wp)
What is the test for plastic limit?
Mix with spatula, adding water little by little
Weigh the sample
Roll soil into a sausage shape of a diammeter of roughly 3mm
Gradually increase the water content, when the soil breaks do the same for each of the broken pieces until the rolling thread crumbles
Reweigh the total sample when dry
What are the tests to find the liquid limit?
- Cone penetrometer
- casagrandge cup
What are the benefits of the cone penetrometer over the Casagrande cup?
- Less operator dependent
- Less subjective
Hence more repeatable/reproducible results
What are the main components of a cone penetrometer?
- Dial guage
- Stem locking mechanism
- Cone
- Metal cup
What is the method for using the cone pentrometer?
- Mix soil using metal spatulas and distilled water
- fill metal cup with the mixed soil, take care not to entrap air
- strike off extra soil to give a smooth level surface
- Lower the tip of the cone to the surface of the soil
- Lower the stem of the dial gauge to contact the cone shaft and record the initial reading
- release the cone for 5 seconds then lock
- lower the stem of the dial guage to contact the cone shaft and record the final reading
- extract a small sample to measure the water content
What is the equation for the activity of a soil?
A = Ip/c Ip = plasticity index C = clay fraction
