First Term Checklist

Question 1

What are total stress, effective stress and pore water pressure?

Answer 1

Total stress is the sum of external stresses acting on the soil
Pore water pressure is the outward pressure exerted by incompressible water trapped in the space between soil particles
Effective stress is the stress experienced by the soil subtracting the pore pressure from the total stress

Question 2

How is effective stress (sigma’) calculated?

Answer 2

Total stress (sigma) - pore water pressure (u)

Question 3

What is the boundary particle size between coarse and fine grained soils?

Answer 3

75 microns

Question 4

How does soil change volume?

Answer 4

Since soil particles are incompressible, volume change occurs by the reduction in size of voids between particles. Depending on the contents and sizes of the voids, the time for this process can vary

Question 5

How do you plot a graph of e - log sigma’

Answer 5

Plot e on the y axis and sigma’ on a log scale on the x axis

Question 6

Define consolidation

Answer 6

The delayed volumetric response to a change in pressure due to gradual movement of water

Question 7

Define overconsolidation

Answer 7

A state in which a soil is under a lesser stress than it has previously experienced and therefore has a lower Void ratio than would be expected for its type and load

Question 8

How is the overconsolidation ratio calculated?

Answer 8

The highest stress the soil has experienced divided by the current stress

Question 9

What is the relationship between a Mohr’s circle for total and effective stress?

Answer 9

The effective stress circle is identical in size to the total stress but it is located closer to zero and closer to the failure line

Question 10

What is dilation?

Answer 10

Where shear acting on a soil with uneven particles causes these particles to rotate in such a way that the volume of the soil increases

Question 11

Why are fine grained soils assumed to remain saturated above the water table?

Answer 11

The small particles cause small voids wherein water surface tension causes a capillary effect which draws water above the water table

Question 12

Is total stress is constant, how do pore pressure and effective stress change during consolidation?

Answer 12

Initially pore pressure rises, causing effective stress to decrease. Over time pore pressure settles back to its original value and effective stress increases

Question 13

What does it mean if soil is drained?

Answer 13

Water is considered to flow out of a unit of soil instantaneously when pressure is applied

Question 14

Why are undrained conditions only considered to exist in fine grained soils?

Answer 14

The small voids in fine grained soils prevent water from freely flowing out, whereas in coarse grains this is not the case

Question 15

What is at rest Earth pressure?

Answer 15

Lateral pressure exerted due to the horizontal expansion of soil as it compresses vertically under its own weight

Question 16

Why are overconsolidated soils less compressible than non consolidated soils?

Answer 16

The past consolidation has already decreased the volume of voids in the soil, meaning there is a smaller amount left by which the volume can decrease further

Question 17

What is the difference between drained and undrained shearing?

Answer 17

In drained conditions, water flows in or out to allow volume change
In undrained conditions, volume cannot change so excess pore water pressure is generated

Question 18

How does overconsolidation ratio affect horizontal effective stress calculation?

Answer 18

The formula for coefficient of Earth pressure K0 is multiplied by the sqrt of OCR

Question 19

Why does a higher OCR result in higher horizontal stresses?

Answer 19

Particles rearrange during consolidation causing horizontal contact points to become stronger and locking in higher horizontal effective stress

Question 20

Define critical state

Answer 20

The point at which shearing will occur without any further changes in effective stress, sheer stress or volume

Question 21

Why does critical state occur?

Answer 21

After sufficient shearing all the initial packing geometry of the soil has taken place

Question 22

How does vertical effective stress affect the critical state?

Answer 22

Soil with higher effective stress will finish at a denser critical state

Question 23

How does the stress path travel on a graph of e - sigma’ in drained conditions?

Answer 23

Vertically up or down to the critical state line

Question 24

How does the stress path travel on a graph of sheer stress against effective stress in drained conditions?

Answer 24

Vertically up. If the soil is initially densely packed, the path may travel up to the peak stress and then down to the critical state line

Question 25

How does soil pack density affect volume change to critical state?

Answer 25

Loose soil is reduced in volume to the critical state. Dense soils increase in volume to the peak then decrease to the critical state

Question 26

Where does peak strength occur

Answer 26

Only in dense soils

Question 27

Why is peak stress reached?

Answer 27

Because additional energy is required to increase the soil volume during dilation

Question 28

How is the angle of dilation psi defined?

Answer 28

tan psi = - (volumetric stain epsilon/shear strain gamma)

Question 29

How does the angle of dilation affect peak strength calculation?

Answer 29

It is added to the failure angle phi’ crit

Question 30

How does progressive failure occur?

Answer 30

As the load increases the stress at the toe of the slope reaches peak stress. After surpassing peak stress the stress at the toe is redistributed to the rest of the failure surface. The stress keeps being redistributed up the slope until the entire surface has failed

Question 31

How does effective stress affect peak strength?

Answer 31

As effective stress increases the maximum dilation angle and hence the peak failure angle decrease

Question 32

How are stress paths to the critical state different in undrained shearing?

Answer 32

They are horizontal on the E-sigma’ graph and curved on the tau-sigma’ graph

Question 33

How is undrained strength calculated

Answer 33

The radius of the Mohr’s circle

Question 34

Why is the ratio of undrained strength to effective stress constant?

Answer 34

The stress paths are parallel

Question 35

How does over consolidation ratio affect undrained strength?

Answer 35

As OCR increases, undrained strength increases

Question 36

What happens in undrained shearing when failure does not occur?

Answer 36

Consolidation, which moves the stress path away from the failure line

Question 37

Why does strength change at very high shear strains in clay?

Answer 37

As strains get higher and higher the platy clay particles align and create a polished low friction surface with very low strength