Cyclic behavior of sand and cyclic triaxial tests

1. Cyclic Behavior of Sand
and Cyclic Triaxial Tests
Hsin-yu Shan
Dept. of Civil Engineering
National Chiao Tung University

2. Causes of Pore Pressure Buildup
due to Cyclic Stress Application
Stress are due to upward propagation of
shear waves in a soil deposit during
earthquake
Structure of the cohesionless soil tends to
become more compact
Transfer of stress to the pore water
Reduction in stress on the soil grains

3. Soil grain structure rebounds to the extent
required to keep the volume constant
Volume reduction and soil structure rebound
determines the magnitude of the increase in
pore water pressure increase
As the pore water pressure approaches a
value equal to the applied confining pressure
the sand begins to undergo deformations

4. If the sand is loose:
Pore pressure increase suddenly to a value equal
to the applied confining pressure
The sand will rapidly begin to undergo large
deformations with shear strains exceeding around
20% or more
If the sand will undergo unlimited deformations
without mobilizing significant resistance to
deformation it can be said to be liquefied

5. If the sand is dense:
It may develop a residual pore water pressure (a
peak cyclic pore pressure ratio of 100%)
When the cyclic stress is reapplied on the next
stress cycle, or if the sand is subjected to
monotonic loading
The soil will tend to dilate
Pore pressure will drop if the sand is undrained
The soil will ultimately develop enough resistance
to withstand the applied stress
Large deformation will develop during the process

6. Effect of Partial Drainage
There will be some drainage in the field
Add some margin of safety against cyclic
mobility or liquefaction
To ignore the effect of partial drainage is on
the conservative side

7. Evaluating Liquefaction or
Cyclic Mobility Potential
Methods based on observation of
performance of sand deposit in previous
earthquake
Method based on stress conditions in field
and laboratory determinations of stress
conditions causing cyclic mobility or
liquefaction of soils

8. Observation Method
Based on the location of the points
representing the data set (N1, τ/σ’0) relative
to the curve representing the lower bound for
sites where liquefaction occurred
N1 is the corrected SPT-N value
0σ
τ
′
= cyclic ratio causing liquefaction

9. d
av
r
g
a
0
0max
0
65.0
σ
σ
σ
τ
′
≈
′
amax = maximum acceleration at the ground surface
σ0 = total overburden pressure on sand layer under
consideration
σ’0 = effective overburden pressure on sand layer
under consideration
rd = a stress reduction factor varying from a value of
one at the ground surface to a value of 0.9 at a
depth of 10 m

10. Experience with the Method
The lower bound curve is strongly supported
by abundant data from Japan and China
Works satisfactorily with the data from 921
earthquake
Conservative for earthquakes with lesser
magnitudes involving shorter duration of
shaking

11. Limitations of the Method
Need for additional reliable data points to
better define the lower bound of causing
cyclic mobility or liquefaction at high values of
τav/σ’0
Need to understand more about the
significant factors affecting cyclic mobility or
liquefaction
Duration of shaking, magnitude of earthquake

12. Penetration resistance may not be an
appropriate index of the cyclic mobility
characteristics of soils
The standard penetration resistance of a soil
is not always determined with reliability in the
field and its value may vary significantly
depending on the boring and sampling
conditions

13. Factors Affecting the Cyclic
Mobility Characteristics of Sand
Density or relative density ↑
Grain structure or fabric ↑
Length of time the sand subjected to
sustained pressures ↑
Value of K0 ↑
Prior seismic or other shear strains ↑

14. Factors Affecting the N Value
The use of drilling mud vs. casing for
supporting the walls of the drill hole
The use of a hollow stem auger vs. casing
and water
The size of the drill hole
The number of turns of the rope around the
drum

15. The use of a small or large anvil
The length of the drive rods
The used of nonstandard sampling tubes
The depth range over which the penetration
resistance is measured

16. Evaluating Liquefaction or
Cyclic Mobility Potential
Methods based on observation of
performance of sand deposit in previous
earthquake
Method based on stress conditions in
field and laboratory determinations of
stress conditions causing cyclic mobility
or liquefaction of soils

17. Methods Based on Field/Lab
Stress Conditions
An evaluation of the cyclic stresses induced
at different levels in the deposit by the
earthquake shaking
A laboratory investigation to determine the
cyclic stresses which, at given confining
pressures representative of specific depths in
the deposit, will cause the soil to develop a
peak cyclic pore pressure ratio of 100% or
undergoes various degrees of cyclic strain

18. Compare the results of the two evaluation:
The cyclic stresses induced in the field with the
stresses required to cause a peak cyclic pore
pressure ratio of 100%
An acceptable limit of cyclic strain in
representative samples in the lab

19. 5 Basic Procedures Need to be
Developed
Suitable analytical procedures for evaluating
stresses developed in an earthquake
Suitable procedure for representing the
irregular stress history produced by the
earthquake by an equivalent uniform cyclic
stress series
Suitable test procedure for measuring the
cyclic stress conditions causing a peak pore
pressure ratio of 100% or intolerable level of
strain in the soil sample

20. Understanding of all the factors having a
significant influence on the cyclic mobility or
liquefaction characteristics of soils
Understanding of the effects of sample
disturbance on the in-situ properties of
natural deposits

21. Methods for Evaluating Stresses
Induced by Earthquake Shaking
Ground response analysis that neglects the
pore pressure buildup
Procedure that takes into account the pore
pressure generated in the soil
Simplified procedure based on a knowledge
of the maximum ground surface acceleration
Deconvolution of a known ground surface
motion

22. Ignoring pore pressure build up during
earthquake may not be particularly significant
May lead to somewhat conservative results in
some cases

23. Converting Irreg. Stress His. into
Equiv. Unif. Cyclic Stress Series
Because it is usually more convenient to
perform lab tests using uniform cyclic stress
applications than to reproduce the actual field
stress history
There were three methods can be used and
their differences have little effect on the final
results

24. Three basic methods:
By estimation from a visual inspection of the
irregular time history involved
By a weighting procedure for individual stress
cycles – use an experimentally-determined pore
pressure response
A cumulative damage approach based on Miner’s
law and involving the natural period of the deposit
and the duration of earthquake shaking

25. Suitable Test Procedures
Cyclic simple shear tests
Multidirectional shaking in simple shear tests
Cyclic triaxial compression tests

26. Cyclic Direct Simple Shear
DSS Roscoe-type
Four plates
Pure shear is applied to horizontal and vertical
plane
Difficulties
Preparation of representative samples
Development of uniform shear strains throughout the
samples
Application of uniform stress conditions
Avoidance of stress concentrations

27. Very long and shallow samples
Stress concentrations are limited to small areas at
the ends
Longer samples less affected by the stiffness
of the walls of the sample container

28. Cyclic Triaxial Compression
Tests
Equipment less complicated and more
available than DSS
Do not reproduce correct initial stress
conditions for NC soils or in a simple shear
test

29. Other limitations
Stress concentrations at the cap and base
A 90° rotation of the direction of major principal
stress during the two halves of the loading cycle
Necking may develop and invalidate the test data
beyond this point in the test
Intermediate principal stress does not have the
same relative value during the two halves of the
loading cycle
Difficult to achieve a high degree of accuracy for
stress ratio not representative of field values

30. Cyclic triaxial stress ratio is higher than that
for simple shear condition
triaxial3shearsimple
2 





=





′ σ
σ
σ
τ dc
r
c
h
c
cr = correction factor ranging from 0.5 – 1.0,
increases with K0

31. Factors Influencing Cyclic Mobility
or Liquefaction Characteristics
Grain characteristics
Relative density
Method of soil formation (soil structure)
Period under sustained load
Period under sustained load

32. Previous strain history
Increase the stress ratio
Lateral earth pressure coefficient and
overconsolidation
Larger K0 higher stress ratio
Cyclic mobility and liquefaction
characteristics of in-situ deposits
Disturbance lower the stress ratio