Laurence d. wesley characterisation and classification of tropical residual soils

1. CHARACTERISATION AND
CLASSIFICATION OF RESIDUAL SOILS
Laurie Wesley
University of Auckland

2. Rock
Residual soil
Sedimentary soil
Sea or lake level
Delta
deposits
Transport by stream
and river
Erosion by rainfall
and runoff
Pressure
Voidratio
A
B
C
Deposition, forming
a very soft soil
Consolidation process
increases the strength
of the soil
Uplift and erosion may
result in slight swelling
Voids
Voids
Voids
Voids
Condition at
deposition
(Point A)
Eventual condition
(Point B and C)
Point B Point C
Solids
Solids
Solids
Solids
Parent rock
Physical and chemical weathering
converts rock into soil
Residual soil
(a) Simplified representation of the formation
of residual and sedimentary soils
FORMATION OF RESIDUAL AND SEDIMENTARY SOILS:
Erosion, transportation, sedimentation and consolidation is a sorting
process that gives sedimentary soils a degree of uniformity that is
absent from residual soils

3. SOIL MECHANICS, SEDIMENTARY
SOILS, AND RESIDUAL SOILS
• Soil mechanics grew up in Northern Europe and
America, from the study of sedimentary soils
• Behavioural framework or “theoretical basis” of
soil mechanics is based almost entirely on the
properties of sedimentary soils.
• Textbooks and geotechnical engineering courses
deal almost exclusively with sedimentary soils
(even in countries with predominantly
residual soils)

4. • Many basic principles are applicable to both
groups:
eg Principle of effective stress, Mohr-Coulomb
failure criterion, Darcy’s Law and Laplace
equations etc
• But some concepts and principles are not
applicable to both groups, especially those
based on stress history and its terms normal
consolidation and over-consolidation

5. A Terzaghi quote:
“However, as soon as we pass from steel and
concrete to earth, the omnipotence of theory ceases
to exist.
In the first place, the earth in its natural state is never
uniform.
Second, its properties are too complicated for
rigorous theoretical treatment.
Finally, even an approximate mathematical solution
for some of the most common problems is extremely
difficult”.

6. Significant Characteristics of Residual Soils
1) Generally more heterogeneous
2) Stress history, concepts of normal consolidation and
over-consolidation are not relevant
3) Residual soils derived from volcanic materials may
contain very unusual clay minerals.
4) Some soils are highly structured and are not strictly
particulate – their particles disintegrate when disturbed
or remoulded
5) Empirical correlations between soil properties valid for
sedimentary soils may not be valid for residual soils

7. Significant Characteristics of Residual Soils (contd.)
6) Water table may be deep and much of the action of
interest to geotechnical engineers my occur above the
water table
7) Generally of much higher permeability than sedimentary
soils
8) Slopes in residual soils are generally much steeper than
those in sedimentary soils.
9) Field behaviour should be observed in preference to
looking at laboratory results
10) Some residual soils may be partially saturated, but this
is not generally the case in the wet tropics or in
temperate climates (like New Zealand)

8. Influence of topography on chemical weathering
Water table
Well drained hilly and mountainous areas:
Downward seepage results in deep weathering
and soils tend to have good engineering properties
Downward seepage
Poorly drained, flat, low lying areas:
Absence of vertical drainage results in
shallow weathering and soils of poor
engineering properties

9. WEATHERING
PROFILE
- Little (1969)
based on granite
- not always valid
for other soils

10. Soil:
clay or silt
Soil: clay or silt
Completely
weathered
Highly
weathered
Moderately
weathered
Slightly
weathered
Fresh rock Fresh rock Fresh rock
Fresh rock:
inter-bedded
sandstone and
clay-stone
Weathered
rock
Soil: Inter-bedded
clay, silty clay, silt,
and dense silty sand
(a) Gradual weathering
profile - typical of
weathered granite
(b) Sharp transition
from rock to soil
- typical of weathered
basalt
(d) Stratified nature of parent rock
reflected in soil profile - typical
of weathering of soft sedimentary
rock, especially sandstone.
(c) Uniform layers, degree of
weathering not necessarily
related to depth - typical of
volcanic ash
Silty clay layers,
almost homogeneous
but distinguished by
slightly different
colouring
1
2
3
4
5
6
Various weathering profiles

11. Weathered Granite

12. Weathered
sandstone

13. Weathered Schist

14. Cutting in weathered sandstone, Malaysia

15. Another cutting in
weathered sandstone,
Malaysia

16. Weathered volcanic ash layers

17. Weathered
lahar flows and
ash layers

18. Conceptual pictures of soil micro-structure
(a) “Normal” clay (b) Cemented structure (c) Honeycomb structure
Plate-like
clay particles
Silt or fine
sand particles
Silt or fine
sand particles
Bonding at
contacts
Weak
skeleton
Void
space

19. Tropical red clay
- no structure,
not sensitive

20. Weathered volcanic ash
highly structured and extremely sensitive.

21. Conceptual views of special clay minerals
found in volcanic soils
Allophane spheres
Imogolite threads
(b) halloysite(a) allophane and imogolite

22. Volcanic ash (non-crystalline)
allophane and immogolite
halloysite
kaolinite
sesqui-oxides
(geothite and gibbsite)
laterite
Decreasingsilicacontent
Increasingironandaluminiumcontent
Assumed
weathering
sequence of
volcanic ash
- a condition
for the
formation of
allophane is
that the
parent
material be
non-
crystalline

23. Index tests: particle size and Atterberg limits
- usefulness of ???
Particle size in clays
-not very informative
Atterberg limits and natural water content
- very useful, as a guide to engineering
properties

24. Usefulness of Atterberg Limits
1. Position of the soil on the Plasticity
Chart – indicates inherent properties
2. Relationship of natural water
content to the Liquid and Plastic
limit (LI) - indicates state of the soil
in the ground

25. Three soil types on the Plasticity Chart
- in the case of volcanic soils, the Chart is not a reliable
basis for rigorous classification
0 40 80 120 160 200 240
Liquid Limit
80
40
PlasticityIndex
A-line
60
20
100
Weathered sedimentary soils
Red volcanic clays
Volcanic ash (allophane)

26. The Plasticity Chart as a very good indicator of soil
properties
Volcanic ash soils
(allophane)
A-Line
0 50 100 150 200 250
150
100
50
Liquid Limit
PlasticityIndex
High activity clays
(montmorillonite)
Tropical red clays
(halloysite)
Poor engineering
properties (clay)
Silty clay
Good engineering
properties (silt)

27. Rice fields in Indonesia – volcanic ash soil – very high LL,
well below the A-line (allophane clay)

28. Slip failure on very gentle slope, Java, Indonesia
Soil: “Black cotton” clay from sedimentary rocks - old
shales. High LL, well above A-line (montmorillinite)

29. Correlations using Atterberg limits
- which is the best parameter for correlations? LL or PI?
0 50 100
50
25
Liquid limit
PlasticityIndex
A B
C
Clay
Silt
Silty clay
A-line
CH
CL
MH or OH
ML or OL

30. Residual friction angle versus Plasticity Index
0 20 40 60 80 100
40
30
20
10
Plasticity Index
Residualfrictionangle,(degrees)φr
/
Clays in general
Volcanic ash clays

31. Friction angles in relation to the A-line on the Plasticity Chart
40
30
20
10
Frictionangle()φʹ′
ΔPI = PI - 0.73(LL -20)
Above A-lineBelow A-line
Distance above or below the A-line:
A-line
Peak
Residual
Peak average
Residual average

32. LL
PL
wn
en
emax
emin
LL-PL
LL - PL
e-emaxmin
e - emax min
e-emaxn
e - emax n
w-PLn
w - PLn
0
1
1
0
= Liquidity Index
=
Density Index
= Relative Density
=
Density Index
CLAY SAND
Non-compact (loose)state
Compact (dense) state
Voidratioorwatercontentas
measuresofcompactness
Watercontent
Voidratio
Density or “Compactness” Indexes for clay and sand

33. Significance of “Density Indicies”
• Clay with low LI is likely to be a strong
material without a “yield” pressure – will
not be a difficult material to excavate and
compact – and vice versa.
• Sand with a high RD is likely to be
“strong” and of low compressibility, and
unlikely to liquefy in an earthquake – and
vice versa.

34. VOLCANIC ASH (ALLOPHANE) CLAYS
- SOME OF THEIR SPECIFIC PROPERTIES
Properties described here are from experience
in New Zealand and Indonesia where volcanic
ash clays are mainly andesitic and very young
- older volcanic ash soils, or those from different
type of ash (rhyolitic), may have quite different
properties

35. Some examples of construction in
allophane clays

36. Cipanunjang Dam, West
Java, Indonesia – built 1928

37. Flood control dam
New Plymouth, New Zealand built 1985

38. Cut slope – 8m
high at 60
degrees
Note absence
of erosion on
the face
Cutting at Derajat Geothermal Project, Indonesia

39. Mt Telomoyo, Central Java
- access road to micro-wave
station

40. Kamojang
geothermal power
station, Indonesia,
- excavation for a raft
foundation

41. Basic properties of allophane clays in Indonesia
Water content PL and LL
0 50 100 150
Water content, PL and LL
0 50 100 150 200
S (kPa)
60 100 140
u
S (kPa)
60 100 140
u
Sensitivity
0 1 2 3
2
4
6
8
10
Depth(m)
Depth(m)
5
10
15
LL
w
PL
LL
w
PL
20
25
30

42. Influence of allophane content on water content and Atterberg limits
250
150
50
Hallosite (%)
10 30 50 70 90
NaturalwatercontentandAtterberglimits(%)
Liquid limit
Natural water content
Plastic limit
Note: The total percentage of
allophane plus halloysite
is about 90%. The remaining
10% is made up of coarser
particles of varying composition.
200
100

43. Peak and
residual
strength of
allophane
clays
Indonesian samples
New Zealand samples
Residual strength from
ring shear tests:
Peak
strength
from
triaxial tests
c
=
20kPa,
=
40
/
o
φʹ′
0 100 200 300 400 500 600
Normal effective stress (kPa)
500
400
300
200
100
Shearstress(kPa)

44. Some comments on clays derived from rhyolitic ash

45. Soil profile exposed in a slip in rhyolitic ash
S1
S2
S3
S6
S10
S8
S7
S9
S4
S5
Clay, stiff to hard, dark brown
(Hamilton andesitic ash-Paleosol)
Clay, stiff to hard, dark brown ( Paleosol)
Clay, stiff to hard, dark brown (Paleosol)
Sandy silt, loose, non-plastic,
pale yellowish brown
(Rotoehu ash)
Clay, firm, high sensitivity,
pale yellowish brown
Silty clay, firm, extremely sensitive, pale yellowish brown
Clay, firm, high sensitivity, pale yellowish brown
Probable ground surface
prior to slip of May, 2005
0 2m 4m 6m 8m 10m 12m 14m
10m
8m
4m
Rhyoliticashlayers
6m
2m
Disturbed samples

46. Some
basic
properties
of rhyolitic
ash layers
- note the
very high
sensitivity
in some
layers
Elevation(m)
Natural water content and
Atterberg Limits (%)
Liquidity Index
Undrained shear
strength (kPa)
50 100 150 20040 60 80 100
Liquid Limit
Plastic Limit
Natural water content
Undisturbed
Remoulded
0 20 40 60 80 100
Sensitivity
0 0.5 1 1.5 2.0 2.5
Liquidity Index
Sensitivity
S1 Sand, non-plastic
S2
S3
S4
S5
S6
S7
S8
S9
S10
8
6
4
2

47. Another typical
slip in rhyolitic
ash layers

48. The distinctive properties of ash of rhyolitic origin
Rhyolitic ash contains a much higher proportion of
silica than andesitic ash, which is more resistant to
weathering processes and fragments of silica are likely
to be found in the weathered ash.
These silica remnants may explain the “fragile”
sensitive nature of the rhyolitic layers.
Weathering is always more intense at the surface, which
presumably accounts for the denser, more plastic
“paleosol” layers

49. Some comments on slope stability

50. Residual slope behaviour compared to sedimentary soils
1. Slopes are steeper – often stable at 45 degrees
2. Slope failures unlikely to be on deep-seated, circular
arc, failure surfaces.
3. Value of cʹ′ likely to be significant, and contributes to
the long term stability.
4. Negative pore pressure above water table may play a
significant role in maintaining stability.
5. The extent to which stability can be estimated by
analytical methods is often very limited because of the
variability of the soil and uncertainty with respect to
both strength parameters and pore pressures

51. 6. Slips and landslides in residual soils are usually
triggered by heavy rainfall. Earthquakes can also be a
trigger.
7. However, the true cause of the failure is often human
activity. Slopes have been steepened, or infiltration
increased by removal of vegetation cover etc.
We cannot control rainfall, but we can control
our own activities – if we want to minimise the
risk of landslides, we need to control our own
activities

52. Shallow circular
slide (very common)
Large translational slide
(common)
Deep seated
circular slide
(very unlikely)
Failure modes in residual soils

53. (a) random discontinuities
- indeterminate influence on stability
(b) regular discontinuities
- quantifiable influence on stability
Slopes containing discontinuities

54. Many residual soils will
not give a clearly
defined Mohr-Coulomb
failure line.
- but others, such as
allophane clays show a
narrow range.
0 200 400 600 800 1000
Normal stress (kPa)
600
400
200
Shearstrength(kPa)
c
= 54kPa
= 34
/
o
φʹ′
c = 5kPa
= 25
/
o
φʹ′
(a) “Middle clay“. from weathered sandstone
0 100 200 300 400 500
Normal stress (kPa)
300
200
100
Shearstrength(kPa)
c = 34kPa
= 35
/
/
o
φ
c = 14kPa
= 34
/
/
o
φ
(b) Volcanic ash clays (allophane)
from Indonesia and New Zealand

55. Results of
back analysis
of slope
failures in
Hong Kong
compared with
results of
triaxial tests
(a) Granite soils
150
100
50
150
100
50
0 50 100 150 200
Effective normal stress (kPa)
0 50 100 150 200
Effective normal stress (kPa)
c = 5 kPa,
= 35
ʹ′
φʹ′
o
c =
10 kPa,
=
37
ʹ′
φʹ′
o
Shearstress(kPa)Shearstress(kPa)
(b) Volcanic soils
Each point represents
one slide

56. Influence of permeability on
short and long term
behaviour of a cut slope in
sedimentary and residual
soil:
With a sedimentary soil the
water table reaches a new
steady state
With a residual soil, the
pore pressure state varies
with seasonal effects and
sudden intense storms.
Time
Time
Time
PorepressureEffectivestressSafetyfactor
Endofconstruction
Longterm
Long term steady state
- typical of low permeability
(sedimentary) clays
Fluctuating water table
- typical of many residual clays
Sedimentary clays
Residual clays
P
Potential failure
surface
Storm
events Seasonal
influence

57. Possible Classification Systems
for Residual Soils:
(a) Methods based on the weathering
profile
(b) Methods based on pedalogical
classification systems
(c) Methods intended for local use on
specific soil types only

58. Methods based
on weathering
profile:
Profile from Little
(1969), based on
experience with
weathered granite

59. Note the six weathering categories:
VI Soil
V Completely weathered
lV Highly weathered
lll Moderately weathered (rock 80 to 90%)
ll Slightly weathered
l Fresh rock

60. Limitations of this method
• describes the weathering profile of igneous rocks in the
tropics (Little’s intention)
• does not provide any comparative information between
soils from one rock type and another
• the profile shown in the Little figure is relevant only to
certain types of rocks
• other types of rock produce different profiles.

61. Use of Pedalogical Group Names
Various group names have been borrowed from
pedalogical systems used by soil science.
They have crept in randomly - for convenience
The first and best known is probably laterite
The three most commonly used at present are
probably lateritic soil, latosol, and black cotton
soil

62. Three common pedalogial groups
Rigorous pedological namesCommonly
used
names
Dominant
clay
minerals
Important
properties
Lateritic soils
Latosols
Red clays
Halloysite
Kaolinite
Gibbsite
Geothite
Large group
- highly
variable
Volcanic
ash soils
Andosols
Unique -
Very high
water content
- altered by
drying
Black cotton
soils
Black clays
Tropical
black earths
Grumusols
FAO
US Soil
taxonomy French
Andosols
Oxisols Ferralitic
soils
Eutropic
brown
soils of
tropics on
volcanic
ash
Allophane
and
minor
halloysite
Ferralsols
Andepts
Vertisols Vertisols Vertisols
Smectite
(montmorillinite)
Problemsoils
high shrink/
swell,
low strength

63. British Geological Society System
• A very complex system based on pedologial
methods
• Almost no clear connection between
classification groups and engineering
properties
• Probably best ignored for engineering
purposes

64. Methods for specific local use
• Toncer and Lohnes describe a method for use with
lateritic soils in Hawaii and Puerto Rico.
• Pender describes an adaptation of Little’s method for
use with NZ weathered greywacke
• Wirth and Zeiglet describe a system specifically
developed for one project - the Baltimore subway project
These approaches are very practical and very useful

65. A USEFUL GROUPING SYSTEM
The two components of residual soil that
give them distinctive characteristics
are:
1) Mineralogical composition
2) Structure
These can be used as the basis for a
grouping system for residual soils

66. Three main groups on basis of
mineralogical composition:
Group A: Soils without strong mineralogical influence
Group B: Soils with strong mineralogical influence from
conventional (usual) clay minerals commonly found in
sedimentary soils.
Group C: Soils with strong mineralogical influence from
special clay minerals not found in sedimentary soils.

67. Each group can be sub-divided on
basis of soil structure.
Types of soil structure:
(a) Macro structure – joints, fault lines,
bedding planes, - features visible to the
naked eye.
(b) Micro structure – bonding or
cementing between particles – not
visible to the naked eye.

68. FIN
GRACIAS POR SU ATENCION
This is a kiwi - our
national bird
and an endangered
species
This is a kiwi fruit

69. These also are Kiwis