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Soils Mechanics and Engineering Geology > Effective stress and seepage > Flashcards

Effective stress and seepage Flashcards

1
Q

How dow we characterise soils?

A
  • Particle size distribution curves
  • Atterberg limits
  • Moisture
  • Colour
  • Fabric
  • Strength
  • Compressibility
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2
Q

What do water content profiles show?

A

Measuring mass of water in contrast to mass of solids

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3
Q

What is the Volume of Voids?

A

Total volume - volume of solids

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4
Q

What is the formula of volume of solids?

A

V_s = M_s/G_sρ_w

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5
Q

What is void ratio calculation?

A

e = V_v/V

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6
Q

What is porosity formula?

A

n = V_v/V

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7
Q

How can we relate n and e?

A

n = e/(1+e)

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8
Q

How can we relate e and v?

A

v = 1 + e where v is specific volume

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9
Q

What isa saturated soil?

A

All voids completely full with water

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10
Q

What is an unsaturated soil?

A

Some voids contain water

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11
Q

What is usually the unit weight of soil?

A

20

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12
Q

What are the main focuses in structures?

A
  • Stability
  • Deformations -> need factor of safety
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13
Q

What is soils factor of safety?

A

3

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14
Q

How do we assess loads?

A

Understand the response of the materials, often in terms of their Youngs modulus

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15
Q

What is effective stress equal to?

A

σ’ = σ - u

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16
Q

What are changes in stress due to?

A

Compression, distortion and a change in shearing resistance are exclusively due to changes in the effective stress

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17
Q

What is the angle of repose?

A

Angle at which soil rests at and is a strength characteristic

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18
Q

What is pore water pressure expressed interns of when its negative?

A

suction

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19
Q

How do you calculate the suction?

A

s = u_a - u_w
, where u_a = pore air pressure

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20
Q

When you add more water to a soil what happens?

A

Radius of curvature of the meniscus increases and surface tension decreases

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21
Q

What is another formula for suction?

A

s = 2T_s/r

22
Q

How can the total stress be found?

A

Determined by the thickness multiplied by the bulk unit weight for all material layers

23
Q

Why will it take longer for capillary action to occur in fine grained material?

A

Because it has lower permeability

24
Q

In hydrostatic scenarios what does not occur?

A

Seepage

25
Q

What is a perched water table?

A

Where water lies on a stratum of low permeability above the level of the normal water table

26
Q

What happens when pore pressure goes up?

A

effective stress goes down

27
Q

What does effective stress control?

A

Strength

28
Q

What is the phreatic surface?

A

The level at which pore water pressure is zero. Above this surface, pore water pressure is negative.

29
Q

What type of material is sand?

A

Free draining material

30
Q

What are the permeability properties of London Clay, Lambeth Group, and Chalk?

A

London Clay: Low permeability.
Lambeth Group (Lower part): Quite permeable (aquifer).
Chalk: Impermeable but fractured, increasing permeability.

31
Q

What is steady-state seepage?

A

It occurs when pore pressures remain constant and no soil deformations happen during long-term water flow through the ground.

32
Q

What is Darcy’s Law?

A

q=Aki

33
Q

How do you calculate flow velocity using Darcy’s Law?

A

v=ki

34
Q

What is the hydraulic gradient?

A

The rate of change of total head (h) with distance in the direction of flow.

35
Q

In seepage what controls the flow?

A

The hydraulic driving and the gradient

36
Q

What is the quick condition in soils?

A

Occurs when effective stress ( σ_v’ ) approaches zero, causing soil particles to float, leading to soil instability and piping.

37
Q

How does water flow in a system?

A

From high head to low head

38
Q

How can we visualise the distribution of pore pressure?

A

Using an imaginary standpipes

39
Q

What is the critical hydraulic gradient (i_crit)?

A

i_crit = 𝛾_s/ 𝛾_w -1

40
Q

What are the assumptions for two-dimensional flow through a porous medium?

A
  1. Homogeneous, porous, saturated, constant volume medium.
  2. Fluid is homogeneous and incompressible.
  3. Flow is steady and continuous (steady state).
41
Q

What is the Laplace equation for seepage through a porous medium?

A

v = -kdh/dl

42
Q

What is a flow net?

A

A graphical representation of flow lines and equipotential lines that intersect at right angles. It helps visualize the flow of water through soil

43
Q

How is flow through a system calculated using a flow net?

A

q = kHN_f/N_d

44
Q

What the k value for clay?

A

< 10^-8 m/sec

45
Q

What is the k value for sand?

A

10^-2 to 10^-5 m/sec

46
Q

What does the manometer experiment illustrate about soil permeability?

A

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.

47
Q

What is pore pressure (
u) and how is it calculated?

A

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

48
Q

What is a manometer?

A

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

49
Q

What are the basic element of the flow nets?

A
  • 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
50
Q

What is the head drop in the system equal to?

A

H = N_d * Δh

51
Q

Why must flow lines not intersect?

A

Flow stops flowing through them

52
Q

What does a narrower flow channel mean?

A

Faster the flow velocity

Soils Mechanics and Engineering Geology (3 decks)

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