Measuring soil strength

Question 1

How do soils generally fail?

Answer 1

shear when the shear
forces acting on the soil reach the shear strength

Question 2

What does soil shear strength =?

Answer 2

Soil shear resistance

Question 3

What is the strength of a material?

Answer 3

The greatest stress (shear stress) it can maintain

Question 4

What is soil shear strength derived from?

Answer 4

Cohension and friction

Question 5

What are the characteristics of soil cohesion?

Answer 5

Stress independent
Cementation between sand grains
Electrostatic attraction between clay particles

Stickiness

Question 6

How is factor of safety calculated?

Answer 6

Shear resistance / shear force

Question 7

How does factor of safety calculation define slope stability?

Answer 7

If less then or close to one slope is unstable

Question 8

Is resistance stress dependant?

Answer 8

Yes

Question 9

What is the equation for stress?

Answer 9

Stress (sigma) = force (N)/ area (m2)

Question 10

What are 2 methods for emasuring direct shear?

Answer 10

Shear box - force on top in box with known area allows stress to be known when a load cell moves at a set rate (but need to reverse to continue - know remolded strength)

Ring shear - small sample rotated round - find resdiual strength - stress is the rate of turn

Question 11

What will happen to normally consolidate material during shear?

Answer 11

WHen they moe the soil compacts

Question 12

WHat happens to heavily compacted soil during shear?

Answer 12

Rise up and dilate - as particles move apart

Question 13

What happens to loosely compacted soils during shear?

Answer 13

Densification

Question 14

WHat will happen to stress when a soil dilates?

Answer 14

Will be a high peak before “collapse” to residual

Question 15

What is the strength of densely packed sands?

Answer 15

stronger (then loosely) but weaken due to dilatancy as they shear

Question 16

What is the strength of loosely compacted sands?

Answer 16

weaker (than dense), but strain leads to increased
density (tighter packing)

Question 17

What will happen to the strength of sands as strain increases?

Answer 17

Strength converges to a critical value

Question 18

Why might multiple tests be done for failure criteria?

Answer 18

Get peak stress and allow a stress envelope to be developed which can be comapred to mohr circle to see if failure will occur

Question 19

What is the angle of internal friction for sand?

Answer 19

34* depending on grain size (relatively steep angle)

Question 20

What is the angle of internal friction like for clays? (Smectite)

Answer 20

5* as particles finer and flatter

Question 21

What happens to mohr circles as loading increases?

Answer 21

Mohr circle increases in diameter until failure occurs and the failure envelope is met

Question 22

Will soil above the water table be drained or undrained?

Answer 22

Drained as water can flow

Question 23

Will soil be drained or undrained when below the water table?

Answer 23

Undrained as nowhere to flow

Question 24

What is effective stress?

Answer 24

The stress that controls changes in the volume and strength of a soil

Terzaghi (1925-36)

Question 25

How do you calculate effective stress?

Answer 25

sigma - pore pressure

Question 26

What happens to the mohr circle and failure envelope as water pressure increases in soil?

Answer 26

Effective stress lowered and failure more likely as less frictinal strength is mobilised

Question 27

What is drained soil?

Answer 27

pore fluid is free to move in or out of the soil without affecting the soil properties. High permeability (e.g. sandy) soils and/or slow loading (give
the water time to move

Question 28

What is undrained soil?

Answer 28

no movement of pore fluid within the soil, interaction between solid particles and pore fluid. Low permeability soils and/or rapid loading

Question 29

What are the parameters of drained triaxial experiment?

Answer 29

Drainage allowed during cell pressure application
Increase deviatoric stress keeps cell pressure constant, sample fluid pressure constant
Water can drain so test ran slower to prevent the development of pore presssure

Question 30

What sediment does drained soil strength apply to?

Answer 30

Sediments which can contain pore fluid but that pore fluid can move and change volume
Volume can change during strain: grains incompressible but pore fluid can escape

Question 31

When will drainage occur in the undrained triaxial water experiment?

Answer 31

Drainge in consolidation

Question 32

When will drainge occur in the draine dtriaxial water experiment?

Answer 32

Drainage in consolidation and shear

Question 33

What will happen because water cant drain in the undrained triaxial experiment?

Answer 33

Pore water pressure increases and can be measured

Question 34

When does sediment liquefy?

Answer 34

when grains no longer in contact and strength drops to zero

Question 36

What happens in undrained densely and loosley compacted soils?

Answer 36

Dense - prevention of dilation (drop pore pressure and increase strength)
Loose - Contraction prevented (increse pore pressure and loss of strength)

Question 37

What will ultimately lead to liquefaction?

Answer 37

as no volume chnage during shearing of loose sands

Question 38

What are the stress paths of drained, undrained and liquefied?

Answer 38

Drained - No pore water pressure (diagonal line)
Undrained - Development of pore water pressure (efefctive stress decrease to envelope)
Liquefaction - increase to failure enevelope and decrease to 0 for normal and shear stress