4 - parameters for design A course of lectures.pdf

University of sao Paulo

1. 1 October 2016
4 Parameters
4 – Parameters for Design
University of Sao Paulo – Polytechnic School
October 2016
Professor John Atkinson
A Course of Lectures in Geotechnical Engineering

2. 4 Parameters
2 October 2016
1 – Basic Soil Behaviour
2 – Geological Origins of Engineering Soils
3 – Simple Analyses with Pencil and Paper
4 – Parameters for Design
Topics
Fundamental components of an undergraduate course

3. 4 Parameters
3 October 2016
4 – Parameters for Design
4.1 Basis of design
4.2 Parameters from description
4.3 Parameters from tests
principles and errors
basic laboratory and in situ tests
interpretation of shear tests

4. 4 Parameters
4 October 2016
Before you start
What is critical?
collapse or movement or seepage
immediate or long term
What analyses are proposed?
simple pencil and paper
standard tables and charts
numerical analyses
How will parameters be determined?
from description and classification
from in situ tests
from laboratory tests

5. Structure
Critical
Condition
Analysis Parameter Factor
Slope
Drained
stability
Limit
equilibrium
’
c u F
s = 1
Foundation
on clay
Drained
settlement
Limit
equilibrium
s
u (peak) LF = 3
Foundation
on sand
Drained
settlement
Limit
equilibrium
peak
strength
LF = 3
Foundation
Drained
settlement
Elastic E’ or M’
Relate to
strain
Parameters and Factors
October 2016
4 Parameters
5

6. 4 Parameters
6 October 2016
Methods for determination
From description and classification
From laboratory tests
From in situ tests
Types of soil parameters
Universal – same for all soils
Material parameters – depend only on the grains
State dependent parameters – depend on the grains and current state

7. October 2016
4 Parameters
7
Strength parameters for slopes
The long term (drained) state will nearly always be
worse than a short term (undrained) state.
Critical states
Peak states
•
For a safe slope use:
’
cv and c’ = 0
worst credible pore pressure
F
s  1.0
Stress on the failure
planes in the ground
Be careful of residual strength

8. October 2016
4 Parameters
8
Strength for Limiting Foundation Settlement
Laboratory test



d
Dense
Loose
Peak
Design
= = Factor to limit settlement – based on peak strength = 0.3

d

p

d

c
Foundation settlement

d
B

B
 
d 
d 
c 
c
Dense
Loose

9. October 2016
4 Parameters
9
Settlement of foundations – elastic analyses. 



I
E
1
q
B
2
a
a

B
q q
a q
c

a
B
= 0.1%
For a drained state: E’ and ’
For an undrained state: E
u and 
u = ½
Select a value of E’ or E
u related to the
settlement and the mean strains in the ground
10
-4
10
-3

10
-2
10
-1

1 10
E
s
E
o
1
Strain %
Typical strains
in the ground.
0.25
0.5

10. October 2016
4 Parameters
10
Permeability and drainage.
Very difficult to measure and very large range
Soil

Gravel

Sand

Silt

Clay
Sizes

>2mm

2mm to 0.06mm

0.06 to 0.002mm

<0.002mm
k m/s

>10
-2


10
-2
to 10
-5


10
-5
to 10
-8


<10
-8

Time for flow of 1m

<1 minute





>3 years

11. October 2016
4 Parameters
11
Principles of Design.
For stability: CS strength + F
s = 1.
For movement: peak strength + L
f = 0.3
or E related to strain.
Drainage: sand and gravel - drained
silt and clay - undrained

12. 4 Parameters
12 October 2016
4 – Parameters for Design
4.1 Basis of design
4.2 Parameters from description
4.3 Parameters from tests
principles and errors
basic laboratory and in situ tests
interpretation of shear tests

13. Soil Description.
The nature of the grains.
Material parameters.
State: stress and water content.
State dependent parameters.
Structure, fabric, bonding.
October 2016
4 Parameters
13

14. 4 Parameters
14 October 2016
Description of the grains
Fraction %
0.002 0.02 0.2 2 20
0
750
50
25
100
Particle size mm
Coarse grained
Fine grained
Well graded
Grading
Mineralogy
Grain shape
Plasticity
Bonding

15. Critical state friction angle ’
cv
PI
36
o

24
o

12
o

’
Rounded sand: 32
o

Angular sand: 35
o

Carbonate sand: 40
o

(Muir Wood 1990)
October 2016
4 Parameters
15

16. Compressibility C
c related to PI.
log ’
wc
PI
LL
PL
x 100
C
c C
c=
PI .G
s
200

October 2016
4 Parameters
16

17. 4 Parameters
17 October 2016
State is a combination of
stress and water content
State

18. dense loose
clays
LL PL (LI)
sands
minimum
density
maximum
density
(D
r)
stiff and
strong
soft and
weak
State and behaviour at engineering stresses
October 2016
4 Parameters
18

19. Undrained Strength and Water Content
s
u kPa

wc
LL
PL
1.5 150
CSL
(Muir Wood, 1990)
LI
s
u kPa
October 2016
4 Parameters
19

20. October 2016
4 Parameters
20
0.1 1.0
0.1
1.0
p’
p’
e
q’
p’
e
Critical states
Peak states
q′
p′
e
=A
p′
p′
e
b

A and b are
material parameters
Peak strength – curved envelope - normalised

21. October 2016
4 Parameters
21
Peak strength – linear envelope
τ
γ
τ
p
σ’
w
Stress in the ground
τ
σ’
c’
ϕ’
p
τ′
p=c
′
+σ
′
tanϕ′
p
c’ and ϕ’
p depend on the
Stresses in the tests

22. October 2016
4 Parameters
22
Peak strength – linear envelope
τ′
p=c
′
+σ
′
tanϕ′
p
c’ and ϕ’
p depend on the
Stresses in the tests
τ
γ
τ
p
σ’
w
Stress in the ground
τ
σ’
c’
ϕ’
p

23. October 2016
4 Parameters
23
Peak strength and dilation
’




’
= tan(’ + )
Peak angle of dilation  proportional
to state parameter 
log ’

’
c


’
•
•
•
 = ’tan(
c’+ ) •
wc
Dilation 
•
•

24. Peak Strength and Dilation
’
c
’
p = ’
c - 
(Been and Jefferies, 1986)
October 2016
4 Parameters
24

25. October 2016
4 Parameters
25
Stiffness related to movements
G’
o for very small strain
measured in dynamic tests
10
-4
10
-3

10
-2
10
-1

1 10
G’

G
o
1
Shear strain %
Typical strains
in the ground.
0.25
0.5
Very
small
strain

26. 4 Parameters
26 October 2016
4 – Parameters for Design
4.1 Basis of design
4.2 Parameters from description
4.3 Parameters from tests
principles and errors
basic laboratory and in situ tests
interpretation of shear tests

27. Experimental Process – Who does what?
Observe gauges.
Graphs of stress and strain
Derive parameters for strength and stiffness
Determine characteristic or design values
Calibrations and corrections – by the laboratory
Definitions and mechanics – who does this?
Judgement and factors – by the engineer
October 2016
4 Parameters
27

28. October 2016
4 Parameters
28
Sources of error in observations
Noise
Drift
Fault
Calibration
h
w
Output (e.g. mV)

True value
Recorded u
time
True u
Recorded u






Linear calibration
Observations

29. October 2016
4 Parameters
29
Repeatability – do you believe it?


 


Accuracy: difference between recorded value and true value:
but what is the true value.

 
 


30. October 2016
4 Parameters
30
Accuracy
Resolution = precision: size of bullet hole; how many significant figures?
Scatter – can the data be averaged?
Accuracy: difference between recorded value and true value (not as a %)
Can the laboratory quote the accuracies of all the values of stress and strain?


 

 What do you
believe if you
don’t know
the target?

 
 


31. Standard laboratory strength and stiffness tests
Oedometer
Triaxial stress path Triaxial
Direct shear
October 2016
4 Parameters
31

32. Standard in situ tests
SPT
N blows
CPT
Cone and
sleeve
stresses
Shear vane
Torque
Shear strength
Shear wave
Travel time
Velocity
October 2016
4 Parameters
32

33. Experimental Process – Who does what?
Observe gauges.
Graphs of stress and strain
Derive parameters for strength and stiffness
Determine characteristic or design values
Calibrations and corrections – by the laboratory
Definitions and mechanics – who does this?
Judgement and factors – by the engineer
October 2016
4 Parameters
33

34. 4 Parameters
34 October 2016
Δu



Dense sand or stiff clay
Loose sand or soft clay
Typical data from undrained tests
on saturated remoulded samples
with same water content.
Determine values for strength
and stiffness.
Who should so this? The laboratory
or the engineer?
From graphs to parameters

35. 4 Parameters
35 October 2016
Interpretation of undrained test data.


’

’
= tan’


G
t

max = s
u
log
’

’
cv
’
p

36. 4 Parameters
36 October 2016
Interpretation of test data.
Can you state the reliability and
accuracies of the parameters
you are using?
Drained ’ or undrained s
u: does the
structure get safer with time?
Peak strength or cv strength?
To prevent collapse cv strength with
FoS  1
To limit deformations peak strength
with load factor 0.3 (FoS = 3)
Strange observations
Unusual soil?
Errors?

37. Structure
Critical
Condition
Analysis Parameter Factor
Slope
Drained
stability
Limit
equilibrium
’
c u F
s = 1
Foundation
on clay
Drained
settlement
Limit
equilibrium
s
u (peak) LF = 3
Foundation
on sand
Drained
settlement
Limit
equilibrium
peak
strength
LF = 3
Foundation
Drained
settlement
Elastic E’ or M’
Relate to
strain
Parameters and Factors
October 2016
4 Parameters
37

38. 4 Parameters
38 October 2016
4 - Summary
It follows that the designer must determine the design parameters
from the stress – strain test results.
Parameters obtained from laboratory or in situ tests should be
compatible with the basic soil description.
Design parameters depend on the structure and the design
criteria – drained or undrained, prevent collapse or limit deformation.
There is much that can, and does, go wrong with laboratory
and in situ testing – do you believe the data you are given?

39. 4 Parameters
39 October 2016
Where have we been?
1 Basic soil behaviour
2 Geology
3 Simple analyses
4 Design Parameters
What else should a geotechnical engineer be able to do?