Effective stress and seepage Flashcards
How dow we characterise soils?
- Particle size distribution curves
- Atterberg limits
- Moisture
- Colour
- Fabric
- Strength
- Compressibility
What do water content profiles show?
Measuring mass of water in contrast to mass of solids
What is the Volume of Voids?
Total volume - volume of solids
What is the formula of volume of solids?
V_s = M_s/G_sρ_w
What is void ratio calculation?
e = V_v/V
What is porosity formula?
n = V_v/V
How can we relate n and e?
n = e/(1+e)
How can we relate e and v?
v = 1 + e where v is specific volume
What isa saturated soil?
All voids completely full with water
What is an unsaturated soil?
Some voids contain water
What is usually the unit weight of soil?
20
What are the main focuses in structures?
- Stability
- Deformations -> need factor of safety
What is soils factor of safety?
3
How do we assess loads?
Understand the response of the materials, often in terms of their Youngs modulus
What is effective stress equal to?
σ’ = σ - u
What are changes in stress due to?
Compression, distortion and a change in shearing resistance are exclusively due to changes in the effective stress
What is the angle of repose?
Angle at which soil rests at and is a strength characteristic
What is pore water pressure expressed interns of when its negative?
suction
How do you calculate the suction?
s = u_a - u_w
, where u_a = pore air pressure
When you add more water to a soil what happens?
Radius of curvature of the meniscus increases and surface tension decreases
What is another formula for suction?
s = 2T_s/r
How can the total stress be found?
Determined by the thickness multiplied by the bulk unit weight for all material layers
Why will it take longer for capillary action to occur in fine grained material?
Because it has lower permeability
In hydrostatic scenarios what does not occur?
Seepage
What is a perched water table?
Where water lies on a stratum of low permeability above the level of the normal water table
What happens when pore pressure goes up?
effective stress goes down
What does effective stress control?
Strength
What is the phreatic surface?
The level at which pore water pressure is zero. Above this surface, pore water pressure is negative.
What type of material is sand?
Free draining material
What are the permeability properties of London Clay, Lambeth Group, and Chalk?
London Clay: Low permeability.
Lambeth Group (Lower part): Quite permeable (aquifer).
Chalk: Impermeable but fractured, increasing permeability.
What is steady-state seepage?
It occurs when pore pressures remain constant and no soil deformations happen during long-term water flow through the ground.
What is Darcy’s Law?
q=Aki
How do you calculate flow velocity using Darcy’s Law?
v=ki
What is the hydraulic gradient?
The rate of change of total head (h) with distance in the direction of flow.
In seepage what controls the flow?
The hydraulic driving and the gradient
What is the quick condition in soils?
Occurs when effective stress ( σ_v’ ) approaches zero, causing soil particles to float, leading to soil instability and piping.
How does water flow in a system?
From high head to low head
How can we visualise the distribution of pore pressure?
Using an imaginary standpipes
What is the critical hydraulic gradient (i_crit)?
i_crit = 𝛾_s/ 𝛾_w -1
What are the assumptions for two-dimensional flow through a porous medium?
- Homogeneous, porous, saturated, constant volume medium.
- Fluid is homogeneous and incompressible.
- Flow is steady and continuous (steady state).
What is the Laplace equation for seepage through a porous medium?
v = -kdh/dl
What is a flow net?
A graphical representation of flow lines and equipotential lines that intersect at right angles. It helps visualize the flow of water through soil
How is flow through a system calculated using a flow net?
q = kHN_f/N_d
What the k value for clay?
< 10^-8 m/sec
What is the k value for sand?
10^-2 to 10^-5 m/sec
What does the manometer experiment illustrate about soil permeability?
The experiment shows that free-flowing or draining soils allow water to flow through easily, while fine soils may block or slow down the flow, causing phenomena like piping.
What is pore pressure (
u) and how is it calculated?
Pore pressure u is the pressure exerted by water within the soil and is calculated as the height of water in the standpipe multiplied by the unit weight of water (𝛾_w): u = z𝛾_w
What is a manometer?
Two tubes connected by a rubber tube at the base. One of the has free water the other has material at the bottom with a plug with holes
What are the basic element of the flow nets?
- Anywhere on Equipotential line is H constant and they represent a drop
- Flow lines enclose flow channels
- Intersections between EPLs and FLs always at right angles
What is the head drop in the system equal to?
H = N_d * Δh
Why must flow lines not intersect?
Flow stops flowing through them
What does a narrower flow channel mean?
Faster the flow velocity
