Measurement of shear strength

Question 1

test method requires that:

Answer 1

• Deflection or strain can be measured
• Deflection or strain rate can be measured
• Total stress can be measured
• Pore pressure can be measured or accounted for interpretation

Question 2

Two stages of loading

Answer 2

application of normal and shear stress

Question 3

Tests to measure shear strength

Answer 3

1) Direct shear test

2) Triaxial test
- Unconsolidated Undrained (UU)
- Consolidated Undrained (CU)
- Consolidated Drained (CD)

Question 4

Direct shear test method

Answer 4

• Soil sample prepared in box
• Vertical load (normal stress) is applied through a metal platen and time allowed for water to drain from specimen
• Shear force is applied by moving one half of the box relative to the other to cause failure in the soil specimen
• New soil samples prepared in same way and multiple tests performed (with different loads)

Question 5

Influence of Dr (relative density)

Answer 5

high Dr, high shear stress, densely packed particles have a larger horizontal displacement, expansion as sheared towards failure

low Dr, low shear stress, loosely packed particles, no peak observed, compression change in sample height

Question 6

Advantages of direct shear test

Answer 6

simple, fast, ease of specimen preparation for coarse grained soils

Question 7

Disadvantages of direct shear test

Answer 7

• drainage conditions cannot be controlled (u unknown, effective stress and friction angle unknown)
• soil fails along predefined plane
• shear stress on the failure plane is not uniform
• cross sectional area of the sample does not remain constant

Question 8

cell pressure drainage conditions - consolidated soil specimen

Answer 8

some pore water will drain out, specimen will experience a volume change, wait for drainage to finish. note that specimen is still saturated

Question 9

cell pressure drainage conditions - unconsolidated soil specimen

Answer 9

drainage valves are closed, no water drains out, no specimen volume change.

Question 10

Loading drainage conditions - drained loading

Answer 10

Some pore water will drain out, Specimen will experience volume change during loading

Question 11

Loading drainage conditions - undrained loading

Answer 11

No water drains out, No specimen volume change during loading

Question 12

sliding plane failure

Answer 12

failure along pronounced plane

Question 13

barrel failure

Answer 13

failure along multiple planes

Question 14

Triaxial test advantages

Answer 14

• Specimens are subjected to (approximately) uniform stresses and strains
• The complete stress-strain-strength behaviour can be investigated Drained and undrained tests can be performed
• Pore water pressures can be measured in undrained tests, allowing effective stresses to be determined
• Different combinations of cell pressure and axial stress can be applied

Question 15

UU test - unconsolidated undrained

Answer 15

No drainage during cell pressure application, No drainage during application of deviator load, total stress soils with clay content, pore pressure at the start of each test will be equal to the cell pressure that is applied

Question 16

CU test - consolidated undrained

Answer 16

Drainage during cell pressure application, No drainage during application of deviator load, effective stress all soils

Question 17

CD test - consolidated drained

Answer 17

Drainage during cell pressure application, and Drainage during application of deviator load, effective stress all soils

Question 18

sensitivity

Answer 18

classification for fine grained soils, reduction in su from initial undisturbed to remoulded state (loss of strength and structure within the soil mass)

Question 19

consolidation

Answer 19

where pore pressure reduces and volume change occurs from loss of water and rearrangement of solid phase

Question 20

total vs. effective stress

Answer 20

effective stress parameters are used when chnage in pore pressure during loading = 0.
- freely draining soils, volume change will occur quickly

total stress parameter used when change in pore pressure during loading not = to 0
- low permeability soils
- rate of loading greater than rate at which seepage occurs