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Geotechnics 3 > 2 - Effective stress > Flashcards

2 - Effective stress Flashcards

1
Q

For an element of saturated soil, why is the water in the pores under pressure?

A

Due to physical location and external forces.

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2
Q

What is the pressure within the pore water called?

A

The pore pressure

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3
Q

What 2 things can pore water be?

A

Either static or seeping

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4
Q

What is static pore pressure?

A

Where the hydrostatic pressure depends simply on the depth below the water table

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5
Q

What is seeping pore pressure?

A

When the pore water seeps through the soil under a hydraulic gradient.

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6
Q

If you put a standpipe in soil with water in it, how could the pore pressure be shown?

A

As the height to which water would rise to, in a standpipe open to atmospheric pressure

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7
Q

What does pore pressure, u equal?

A

u = yw X d

or

Pore pressure = unit weight of water X depth of water

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8
Q

What is hw?

A

The pore water pressure head, or simply the pressure head.

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9
Q

What is the natural, static level of water in the ground called?

A

The water table

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10
Q

Below the water table, are the pore pressures positive or negative?

A

Positive

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11
Q

Above the water table, if the soil is saturated, is the pore pressure positive or negative?

A

The pore pressure is negative

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12
Q

In dry soil, is the pore pressure positive or negative?

A

Pore pressure will equal zero

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13
Q

Why do the pore pressures with a soil sample need to be monitored?

A

To know about changes in the water table, fluid flow and stress levels.

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14
Q

How are pore pressured traditionally measured?

A

In the drainage pipes outside a triaxial cell using a large pore pressure inducer

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15
Q

Why is it important that a pore pressure measurement system is completely free of air?

A

So that minimal pore water flow is required to register changes in pore pressure.

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16
Q

What is the simplest way of determining the ground water level and pore water pressure?

A

By observation in a borehole that is fitted with a perforated pipe.

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17
Q

What are the benefits of using a standpipe?

A

They are cheap and easy to install.

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18
Q

What are the negatives of using a standpipe?

A

Require manual effort to take readings, therefore remote readings are impossible.

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19
Q

What does the Casagrande Piezometer consist of?

A

A filter tip joined to a riser pipe

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20
Q

What are the benefits of using a Casagrande Piezometer?

A

They are economical, simple to use and have long term reliability

21
Q

What is a negative of using a Casagrande Piezometer?

A

Remote readings are impossible, as manual labour is required.

22
Q

How does a pneumatic piezometer work?

A

The piezometer is pressurized with air, and the change in air pressure is used to measure the pore water pressure. As the pressure changes, it exerts a force on the air with the chamber, which is measured by a gauge.

23
Q

What are the advantages of using a pneumatic piezometer?

A

They are sensitive, detect small changes, and do not directly contact the ground water.

24
Q

What is the disadvantage involved with using a pneumatic piezometer?

A

They are very expensive, and the tube can be vulnerable to damage.`

25
Q

What does a vibrating wire piezometer compromise of ?

A

A vibrating wire pressure transducer and a signal cable. It converts water pressure to a signal using a diaphragm, a steel wire and an electromagnetic coil.

26
Q

Where can a vibrating wire piezometer be used?

A

In a borehole, suspended in a standpipe, or embedded in a saturated soil.

27
Q

What are the negatives of using a vibrating wire piezometer?

A

Expensive and require electrical protection.

28
Q

What do pressure transducers consist of?

A

A diaphragm, strain gauges, and a data logger. Water pressure acts on the diaphragm of the transducer, and produces a signal output proportional to the pressure.

29
Q

What are the benefits of using a pressure transducer?

A

Easy to utilise and allow high frequency changes.

30
Q

What are the negatives of using a pressure transducer?

A

Expensive and the reading can ‘drift’ over time.

31
Q

What are geostatic stresses?

A

Stresses caused by gravity acting on the soil or rock.

32
Q

Why are geostatic stresses always present in soil?

A

Because there is always gravity.

33
Q

What are induced stresses?

A

Stresses caused by external loads, such as foundations

34
Q

Due to soil being a granular material, we make assumtion to formulate stresses within a soil body, we use CHILE, what does this stand for?

A

Continuous, Homogeneous, Isotropic, Linear, Elastic

35
Q

What is the total vertical stress acting on a point of soil?

A

The weight of everything above it, including the soil itself.

36
Q

What does the friction between soil grains control?

A

A lot of the soil behaviour

37
Q

What is the friction between soil particles controlled by?

A

The interphase resistance (u) and the reaction force between particles.

38
Q

Why is it impossible to measure the exact load at every particle interaction in soil?

A

There is millions, therefore we take an effective load.

39
Q

What causes a difference between total stress and effective stress in soil?

A

The presence of water

40
Q

What happens to the effective stress as the water pressure increases?

A

The effective stress decreases, as water pressure in the voids acts to push soil particles apart from each other, therefore reducing the load at the particle contacts.

41
Q

What does the effective stress (‘) equal?

A

The total stress - water pressure (u)

42
Q

What is Terzaghi’s principle?

A

That effective stress = total stress - water pressure (u)

43
Q

What does change in effective stress equal?

A

Change in total stress - change in water pressure

44
Q

What will a change in effective stress cause?

A

A change in volume and strenth

45
Q

What will a change in total stress cuase?

A

Will often not cause a change in effective stress

46
Q

What will happen immediately after the construction of a foundation on a fine soil (e.g. clay) ?

A

The pore pressure increases, but immediately begins to reduce as drainage occurs. The rate of change of effective stress under a loaded foundation will be the same as the rate of change of pore pressure.

47
Q

What is the rate of change of pore pressure controlled by?

A

The permeability.

48
Q

What will a change in vertical effective stress also cause a change in? (hint, horizontal….)

A

A change in the horizontal effective stress.

49
Q

What is the ratio of horizontal to vertical effective stress defined as?

A

The coefficient of the lateral earth pressure, K

Geotechnics 3 (1 decks)

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