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Soils > Everything > Flashcards

Everything Flashcards

1
Q

What is the preconsolidation pressure for a normally consolidated soil?

A

It is the same as the initial effective stress (σp’ = σv0’)

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2
Q

What is residual strength?

A

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).

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3
Q

What is the infinite slope derivation?

A

Draw it (page 134) of notes

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4
Q

Difference between compaction and consolidation?

A

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.

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5
Q

Derive the equation for a critical hydraulic gradient.

A

ic = dH/L = y-yw/yw

page 14 of notes

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6
Q

If you have a kf line, how to get C’ from the intercept?

A

Intercept = c’cos(phi’)

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7
Q

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?

A

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.

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8
Q

For slope stability, which operation (loading or cutting) is critical in the long term vs short term?

A 
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.
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9
Q

OCR =

A

OCR = σmax/σtoday = σp’/σv0’

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10
Q

Piezometer

A

Measures pore water pressure using a PVC pipe with a porous end.

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11
Q

Stand pipe

A

Measures the level of the water table using a slotted PVC pipe

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12
Q

Total head

A

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.

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13
Q

Methods of improving long term stability of an excavation?

A
  • ->Geometry:
  • Reduce slope angle
  • Add a toe weight at the bottom of the cutting
  • Reduce slope height
  • ->Drainage:
  • Install drains
  • Use vegetation
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14
Q

Stress path

A

A graph used to plot successive stress states (e.g. t-s’)

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15
Q

Failure envelope

A

A line drawn on a stress path plot or Mohr’s circle diagram, above which a soil is a failure.

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16
Q

Phi’ = 0 principle.

A

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.

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17
Q

When is the slope of an ESP on a t-s’ plot = 45 degrees?

A

When it is a drained triaxial test.

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18
Q

Cv = k/Mv*yw, what is Mv = ?

A

Mv = d(strain)/d(stress’) = dEv/dσv

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19
Q

Derivations of Rankine coefficients?

A

Draw mohr’s circles

20
Q

How to prevent soil boiling?

A

Use a filter (increase weight)

21
Q

Permeability test for sands?

A

Constant head permeability test

22
Q

Permeability test for clays?

A

Falling head permeability test

23
Q

Definition of effective stress

A

The average stress that soil particles exert on each other

24
Q

Normally consolidated soil vs over consolidated + graph

A

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).

25
Q

For normally consolidated soils, what is the preconsolidation pressure = to

A

The initial vertical’ stress (σ’v0)

26
Q

What is an isochrone?

A

It shows the variation of u with changes in z at a given time (graph on pg 49).

27
Q

What are the assumptions made in Terzaghi’s consolidation theory?

A
  1. Fully saturated
  2. Water is incompressible
  3. Soil grains are incompressible
  4. 1-D flow of water
  5. Darcy’s law is valid (v=ki)
28
Q

What does a Mohr’s circle represent?

A

Moh’rs circle is a diagram which shows how the normal and shear stresses within a soil element vary with orientation.

29
Q

Which direction is shear postitive for a soil element?

A

Anticlockwise

30
Q

What is shear strength?

A

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

31
Q

What are the three types of triaxial test?

A

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).

32
Q

What condition is assumed for sands and gravels (loose grained soils)?

A

Assume drained condition

33
Q

What condition(s) are assumed for clays (cohesive soils)

A

Short term - Assume undrained
Long term - Assume drained
(Low permeability so it takes a long time for water to leave).

34
Q

What governs the failure of a soil

A

The effective stresses

35
Q

b = to what if a soil is saturated

A

1

36
Q

What if soil parameter a is +ve/-ve

A
  • a is positive, soil is contractant

- a is negative, soil is dilatant

37
Q

Comparison of shear strength in clays and sands

A

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)

38
Q

How do OC/NC clays behave when sheared?

A

OC clays behave like dense sands
NC clays behave like loose sands
(Graph is the same shape).

39
Q

FoS for slope stability = ?

A

FoS = Tf/Tmob

40
Q

What are slope stability assumptions?

A
  • Soil moves as a rigid block
  • Assume a single failure surface
  • Plane-strain conditions
  • Localisation of shear stresses
41
Q

Why are Taylor’s Stability charts better than undrained arc analysis?

A
  • Taylor’s gives critical slip surface

- More accurate and conservative

42
Q

What is the assumption of the Ordinary (Swedish) method?

A

-Resultant of interslice forces acts parallel to the base of the slice

43
Q

What is the assumption of the Bishop method?

A

-Resultant of interslice forces is horizontal

44
Q

What is the graph of moisture content vs dry density?

A

Pg 147 of notes

45
Q

What is the graph of moisture content vs strength?

A

Pg 148

46
Q

Other ways to retain a soil other than a wall?

A

-Anchors

Soils (1 decks)

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