Everything Flashcards
What is the preconsolidation pressure for a normally consolidated soil?
It is the same as the initial effective stress (σp’ = σv0’)
What is residual strength?
The strength exhibited by a clay after shearing to large displacements. (draw graph).
It is the result of reorientation of clay particles.
Should be used where large displacements have occurred in the past (eg. slide).
What is the infinite slope derivation?
Draw it (page 134) of notes
Difference between compaction and consolidation?
Compaction - Soil becomes more dense by reducing air voids, usually done by mechanical plant (Increased strength, reduced permeability and compressibility).
Consolidation - Reduction of void volume by removal of water, normally due to sustained loading.
Derive the equation for a critical hydraulic gradient.
ic = dH/L = y-yw/yw
page 14 of notes
If you have a kf line, how to get C’ from the intercept?
Intercept = c’cos(phi’)
What is the behaviour of soil in shear testing? What effect does density and confining stress have on peak and critical state strength and dilation?
Dense soils - Dilate upon shearing, show a peak strength before decreasing to critical state strength at large displacements/ when being sheared at constant volume.
Loose soils - Compact upon shearing, no peak strength, have similar critical strength as dense soils. (Draw graph).
Confining stress increases peak and critical strength but reduces dilation.
For slope stability, which operation (loading or cutting) is critical in the long term vs short term?
Loading is critical in the short term, u increases initially as the pore water takes the load, meaning that FoS is low. As u decreases over time (consolidation) FoS increases. (phi' = 0) Cutting is critical in the long term, u decreases intially - increasing FoS. u then increases over time, steadily decreasing FoS. Draw graphs (pg 131). As u increases, FoS decreases.
OCR =
OCR = σmax/σtoday = σp’/σv0’
Piezometer
Measures pore water pressure using a PVC pipe with a porous end.
Stand pipe
Measures the level of the water table using a slotted PVC pipe
Total head
The sum of elevation, velocity and pressure heads, to give a representation of the total pressure at a point. As seepage velocities are usually small, velocity head can usually be ignored.
Methods of improving long term stability of an excavation?
- ->Geometry:
- Reduce slope angle
- Add a toe weight at the bottom of the cutting
- Reduce slope height
- ->Drainage:
- Install drains
- Use vegetation
Stress path
A graph used to plot successive stress states (e.g. t-s’)
Failure envelope
A line drawn on a stress path plot or Mohr’s circle diagram, above which a soil is a failure.
Phi’ = 0 principle.
Fine grained soils sometimes act as if they are purely cohesive when loaded rapidly. The pore water is not allowed to escape so the σv’ cannot change.
When is the slope of an ESP on a t-s’ plot = 45 degrees?
When it is a drained triaxial test.
Cv = k/Mv*yw, what is Mv = ?
Mv = d(strain)/d(stress’) = dEv/dσv
Derivations of Rankine coefficients?
Draw mohr’s circles
How to prevent soil boiling?
Use a filter (increase weight)
Permeability test for sands?
Constant head permeability test
Permeability test for clays?
Falling head permeability test
Definition of effective stress
The average stress that soil particles exert on each other
Normally consolidated soil vs over consolidated + graph
Normally consolidated - the current effective stress is the maximum the soil has ever experienced.
Over consolidated - the current effective stress is less than the maximum the soil has ever experienced.
(graph on page 45).
For normally consolidated soils, what is the preconsolidation pressure = to
The initial vertical’ stress (σ’v0)
What is an isochrone?
It shows the variation of u with changes in z at a given time (graph on pg 49).
What are the assumptions made in Terzaghi’s consolidation theory?
- Fully saturated
- Water is incompressible
- Soil grains are incompressible
- 1-D flow of water
- Darcy’s law is valid (v=ki)
What does a Mohr’s circle represent?
Moh’rs circle is a diagram which shows how the normal and shear stresses within a soil element vary with orientation.
Which direction is shear postitive for a soil element?
Anticlockwise
What is shear strength?
It is a soils resistance to shearing stress. Interparticle friction is the main source of strength for soils.
Tf = c’ +σv’tan(phi’) - Mohr-Coulomb failure criteria
What are the three types of triaxial test?
UU - Unconsolidated, undrained
CU - Consolidated, undrained
CD - Consolidated, drained
The first letter refers to the first stage (applying cell pressure) and the second letter applies to the second stage (apply vertical stress till failure).
What condition is assumed for sands and gravels (loose grained soils)?
Assume drained condition
What condition(s) are assumed for clays (cohesive soils)
Short term - Assume undrained
Long term - Assume drained
(Low permeability so it takes a long time for water to leave).
What governs the failure of a soil
The effective stresses
b = to what if a soil is saturated
1
What if soil parameter a is +ve/-ve
- a is positive, soil is contractant
- a is negative, soil is dilatant
Comparison of shear strength in clays and sands
Similarities:
-They are both partriculate frictional materials
Differences:
-Permeability is very low
-Electrochemical bonding leads to true cohesion
-Once critical state strength has been reached, it will then drop to residual strength (due to rupture planes forming)
How do OC/NC clays behave when sheared?
OC clays behave like dense sands
NC clays behave like loose sands
(Graph is the same shape).
FoS for slope stability = ?
FoS = Tf/Tmob
What are slope stability assumptions?
- Soil moves as a rigid block
- Assume a single failure surface
- Plane-strain conditions
- Localisation of shear stresses
Why are Taylor’s Stability charts better than undrained arc analysis?
- Taylor’s gives critical slip surface
- More accurate and conservative
What is the assumption of the Ordinary (Swedish) method?
-Resultant of interslice forces acts parallel to the base of the slice
What is the assumption of the Bishop method?
-Resultant of interslice forces is horizontal
What is the graph of moisture content vs dry density?
Pg 147 of notes
What is the graph of moisture content vs strength?
Pg 148
Other ways to retain a soil other than a wall?
-Anchors
