2 - Effective stress

Question 1

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

Answer 1

Due to physical location and external forces.

Question 2

What is the pressure within the pore water called?

Answer 2

The pore pressure

Question 3

What 2 things can pore water be?

Answer 3

Either static or seeping

Question 4

What is static pore pressure?

Answer 4

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

Question 5

What is seeping pore pressure?

Answer 5

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

Question 6

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

Answer 6

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

Question 7

What does pore pressure, u equal?

Answer 7

u = yw X d

or

Pore pressure = unit weight of water X depth of water

Question 8

What is hw?

Answer 8

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

Question 9

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

Answer 9

The water table

Question 10

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

Answer 10

Positive

Question 11

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

Answer 11

The pore pressure is negative

Question 12

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

Answer 12

Pore pressure will equal zero

Question 13

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

Answer 13

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

Question 14

How are pore pressured traditionally measured?

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

Question 17

What are the benefits of using a standpipe?

Answer 17

They are cheap and easy to install.

Question 18

What are the negatives of using a standpipe?

Answer 18

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

Question 19

What does the Casagrande Piezometer consist of?

Answer 19

A filter tip joined to a riser pipe

Question 20

What are the benefits of using a Casagrande Piezometer?

Answer 20

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

Question 21

What is a negative of using a Casagrande Piezometer?

Answer 21

Remote readings are impossible, as manual labour is required.

Question 22

How does a pneumatic piezometer work?

Answer 22

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.

Question 23

What are the advantages of using a pneumatic piezometer?

Answer 23

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

Question 24

What is the disadvantage involved with using a pneumatic piezometer?

Answer 24

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

Question 25

What does a vibrating wire piezometer compromise of ?

Answer 25

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.

Question 26

Where can a vibrating wire piezometer be used?

Answer 26

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

Question 27

What are the negatives of using a vibrating wire piezometer?

Answer 27

Expensive and require electrical protection.

Question 28

What do pressure transducers consist of?

Answer 28

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.

Question 29

What are the benefits of using a pressure transducer?

Answer 29

Easy to utilise and allow high frequency changes.

Question 30

What are the negatives of using a pressure transducer?

Answer 30

Expensive and the reading can 'drift' over time.

Question 31

What are geostatic stresses?

Answer 31

Stresses caused by gravity acting on the soil or rock.

Question 32

Why are geostatic stresses always present in soil?

Answer 32

Because there is always gravity.

Question 33

What are induced stresses?

Answer 33

Stresses caused by external loads, such as foundations

Question 34

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?

Answer 34

Continuous, Homogeneous, Isotropic, Linear, Elastic

Question 35

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

Answer 35

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

Question 36

What does the friction between soil grains control?

Answer 36

A lot of the soil behaviour

Question 37

What is the friction between soil particles controlled by?

Answer 37

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

Question 38

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

Answer 38

There is millions, therefore we take an effective load.

Question 39

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

Answer 39

The presence of water

Question 40

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

Answer 40

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.

Question 41

What does the effective stress (') equal?

Answer 41

The total stress - water pressure (u)

Question 42

What is Terzaghi's principle?

Answer 42

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

Question 43

What does change in effective stress equal?

Answer 43

Change in total stress - change in water pressure

Question 44

What will a change in effective stress cause?

Answer 44

A change in volume and strenth

Question 45

What will a change in total stress cuase?

Answer 45

Will often not cause a change in effective stress

Question 46

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

Answer 46

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.

Question 47

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

Answer 47

The permeability.

Question 48

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

Answer 48

A change in the horizontal effective stress.

Question 49

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

Answer 49

The coefficient of the lateral earth pressure, K