MODULE 3

Question 1

Mohr’s circle positive/negative axis

Answer 1

• Soil unable to take tensile loads so compression plotted as positive

Question 2

isotropic stress state

Answer 2

σ1 = σ3

Question 3

Mohr’s effective circle centre

Answer 3

s' = (σ1 - u) + (σ3 - u) / 2
s' = (σ1' +σ3') / 2

Question 4

Mohr’s effective circle radius

Answer 4

t’ = t

Question 5

CSL

Answer 5

critical state line

Question 6

soil denser than critical (shear box results)

Answer 6

• if a saturated soil is denser than critical and it is sheared, the particle tries to move apart to allow for rearrangement (volume expansion or dilation)
• if shear takes place too quickly water will not be able to flow to new voids fast enough so that excess negative pore water pressure is generated.
• results of negative pore water pressure is to increase the effective stress, resulting in a temporary increase of soil strength

Question 7

soil looser than critical (shear box results)

Answer 7

• if a saturated soil is looser than critical state and it is sheared, we have volume compression or contraction
• if shear takes place too quickly, water will not be able to drain out fast enough so positive pore water pressure generated
• result of positive pore water pressure is to decrease the effective strength = temporary decrease of soil strength

Question 8

settlement time for saturated granular soil

Answer 8

high permeability - instantaneous (time-independent)

Question 9

settlement time for fine graded soil

Answer 9

low permeability - time-dependent (CONSOLIDATION)

Question 10

Swelling

Answer 10

compression: reduction in volume due to an increase in effective stress

Question 11

Dilation

Answer 11

contraction: reduction in volume due to shearing

Question 12

Odeometer Tests

Answer 12

• measures 1D compression of soil
• specifically used for soils of low permeability because it is concerned with time-dependent deformation response of soils

Question 13

Steps of Odeometer Tests

Answer 13

1. Initially the increase in total vertical stress is taken as increase in PWP of the same magnitude
2. As excess PWP dissipates (due to water flowing out from the pores) more total stress is transferred to the soil skeleton, resulting in an increase in effective vertical stress and reduction in soil volume
3. Once PWP has completely dissipated, all the change in vertical load is taken up as increase in effective stress

Question 14

end-state data

Answer 14

after pore pressures have dissipated = long-term

Question 15

transient state data

Answer 15

during consolidation process = short-term

Question 16

NCL

Answer 16

normally consolidated line

• soil particles rearrange (sometimes break) under applied load
• PLASTIC deformation

Question 17

UL-RL line

Answer 17

unloading-reloading line

• soil particles do not need to rearrange to take the applied stress
• deformation is due to elastic compression of particles

Question 18

C(c)

Answer 18

compression index = 2.3 x λ(o)

Question 19

C(s)

Answer 19

swelling index = 2.3 x k(o)