Textbook - soil mechanics Flashcards
What does the term soil mechanics refer to?
The mechanical properties of soil
What does the term geotechnical engineering refer to?
the application of those mechanical properties to design and construction
What is the principle of effective stress?
sigma_z’ = sigma_z - u
simga_z' = effective pressure sigma_z = total pressure u = pore water pressure
What does the equation:
delta P = 2T/r
represent?
The pressure difference on either side of a surface
T= tensile force
r = radius of curved membrane
What makes up voids?
air and water
What is the equation for unit weight (gamma)?
rho*g
How do we calculate the specific gravity?
Gs = rho_solids/rho_water
What is the equation for density?
rho = mass/volume
so for water - it would be mass of water over volume of water
For solids it is mass of solid/ volume of solid
This can be rewritten as rho_s = Gs*rho_w
What does the symbol e stand for in geotechnics?
the void ratio
What is the equation for the void ration?
vol. voids/ vol. solids
Vv/ Vs
(Vw + Vg)/Vs
What does the specific volume represent and what is its symbol?
The volume occupied by unit volume of solid, v
What is the equation for specific volume?
v = total volume/volume of solid
= (Vv + Vs)/Vs = 1 + e
What is the equation and symbol for the total volume fraction?
f_vol = vol solid/total vol
= Vs/(Vs + Vv) = 1/v
= 1/(1+e)
What is the equation and symbol for porosity?
n = vol voids/ total vol
= Vv/(Vv+Vs)
=e/(1+e) = (v-1)/v
What is the equation for degree of saturation?
Sr = vol. liquid/ vol voids
=Vw/Vv
= Vw/(Vw+Vg)
What is the equation for the air void ration, a?
a = vol. gas/ total vol.
= (1-Sr)e/(1+e)
What is the equation for water content?
w = mass of water/mass of dry soil
= Mw/Ms
how can you express water content in volumetric ratios?
w = Mw/ Ms
= rho_wVw/(rho_sVs)
= 1/GsSre
Using volumetric ratios, how do you find the mass of water and the mass of solids?
Mw = Srerho_2Vs
Ms = rho_sV_s
= Gsrho_wVs
What is the equation for the bulk density, rho?
(Sr*e+Gs)/1+e *rho_w
What is the name of the process of increasing the density of soils?
compaction
How do you work out the density of an on site soil sample?
Dig a hole in the ground, and measure the mass of the sample
Put a thin membrane in hole and fill with water from a container of known volume or mass
Measure change in volume or mass
How can we make compaction easier?
By increasing the moisture content up to a point
What are the 4 methods of transport for rocks, to allow them to make up soils?
Gravity (landslide or debris flow)
Ice
Water
Air
What is the effect of glacier transport on rocks?
It smooths them, carries a range of particle sizes
What happens as the speed of water transporting rocks drops?
The coarser rocks are deposited
What are the defining features of clay minerals?
Appear as large packets of alumino-silicate molecules, characterised by large surface area and a larg ratio of lateral dimmensions to thickness
What are three of the most common clay minerals?
Laolinite
illite
montmorillonite
All decompositition producs of feldspar and volcanic ash
What is the symbol and equation or the coefficient of uniformity?
Cu = d60/d10
What does a low value for the coefficient of uniformity mean?
poorly grade
ping pong balls = 1
What shape has the lowest specific surface?
a sphere
What are the properties of rain and shingl;e?
compact shape
low specific surface
What causes clay soils to have high void ratios?
The electorstatic forces between adjacent particles
What is the first order model in seiving?
assuming particles are spherical, as particles of all shapes can pass through square holes
What are the units for viscosity?
kg m^-1 s^-1
can also be written in Pa s
As the viscosity of water is 1 MPa s
What do you need to work out the properties of a clay or fine particle soil?
A dilute, well mixed suspension
This allows us to think of the soil particles as non interacting spheres
Therefore we add a deflocculating agent to ensure that electrostatic forces do not clump
What is the process of measuring the properties of a clay or other fine particle soil?
- Start with a shaken and stirred well mixed solution
- stand the the tube and allow them to settle
- take samples from the solution, at the same depth at regular time periods
- Then dry the sample in an oven to detmine the quantity of the material that has been extracted
- This allows us to compute a particle size distribution
- see Stokes law
How do we determine the minimum void ratio emin?
Through a standard vibratory procedure
How do we determine the maximum void ration emax?
Put sample in a large tube and invert numerous times
What does Bernoullis equation describe?
the one dimensional flow of an incompressible fluid along a streamline
What kind of flow is usually in soil?
slow, massively subcritical, likely laminar
What is bernoulis equation?
E = u + rho_wgz+1/2 * rho_w*v^2
where u is the fluid pressure
What is the equation for energy head?
H = u/(rho_w*g) + z + 1/2 v^2/g
What is Poiseuille’s equation?
tau = viscosity *dv/dz
Where tau is the viscous shear stress
What does Posueuille’s equation define?
That the velocity varies smoothly from th edge of the tube
This means the water is being pulled forward by the water at the centre of a tube, and backwards by the water at the sides
What does poiseuille’s equation mean for fluid mechanics?
That the ease of water flow through a void will be proportional to the square size of that opening
What is the expression for permeability?
K = kviscosity/ (rho_wg)
this is the absolute or specific permeability
What is the expression for the seepage velocity?
v_s = vbar*(1+e)/e
= vbar/n
How is the Reynould’s number for flow through soil defined?
Re = rho_wvbard50/viscosity
What direction does flow of waterneed to be to artificially increase the stresses in soil?
downwards
What is an aquifer?
A water-bearing level that usually has some spatioal connectivity to an artificial or natural water source which controls the water pressure
What is an annulus?
A ring shapes object, structure or region
If you have two adjacent tubes, and you measure the head in the right hand one, with a soil sample in the middle, what happens when you increase, decrease and level the water levels in both tubes?
Increase: Water level in right hand tube will go down - +ve pressure gradient
Decrease: Water level in right hand tube will go up, -ve pressure gradient
Level: water level will be the same, no pressure gradient
What is the definition of a pile?
A long, slender, stiff structural member which is used to transfer loads from some surface structure through more or less soft soils to a certain depth in the ground
What is the effect of a pile foundation?
The surface load is shared with the soil over the length of the pile by the generation of shear stresses (tau) (shaft resistance) and by the end bearing stress (sigma_b)
What are the disadvantages of shallow foundations?
They are subject to overturning
What is a shear box?
A labatory test device which is used to force a failur surface to form withing a soil sample.
The soil sample is conatined within a split box
How does a shear box test work?
The shear and split boxes are alligned and filled with soil.
The soil is loaded through a platen with a force P which generates a vertical stress (P/A A is the cross sectional area)
There may be a pore pressure, although it is difficult to retain any pore pressure in a moderatley permeable soil in the shear box
A load is then appliedforcing a relative displacement and the area of contact reduces
How many degrees of freedom does the shear box have?
2 - normal and shear forces
How does sliding ocur in many geotechnical systems?
Sliding on a clear slip surface, mobilising the shear strength of the surface
What is cheaper, a pile or spread foundation?
spread
