2 - geology and soils A course of lecturespdf

University of Sao Paulo

1. 2 Geological origins
1 October 2016
2 Geological Origins of Engineering Soils
University of Sao Paulo – Polytechnic School
October 2016
Professor John Atkinson
A Course of Lectures in Geotechnical Engineering

2. 2 Geological origins
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

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Basic Principles
How do the geological processes lead to the engineering properties?
The Key Question
The ground is formed by natural processes in an environment
governed by tectonics, climate and topography.
Engineering properties are determined by how the ground
was formed.
Nothing is magic and there is no divine intervention.

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2 – Geological origins of soils and rocks
2.1 Surface processes
2.3 Soil behaviour related to description
2.2 Objective description of soil

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Basic Near Surface Processes
Weathering Erosion
Transport Deposition
It is all physics and chemistry

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Soils
Rocks
Failure on pre-existing joints
Bonded grains + cracks
Material properties not
significant; joints dominate.
Unbonded grains
Failure generates slip planes
Material properties dominate.
Basic Differences between Soils and Rocks

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Residual or sedimented soil
Progressively weathered rock
Fresh rock
Weathering - chemistry
Depths vary
Weathering depends on environment.

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Elevation
Environment = climate + topography
cold
temperate
hot dry
hot wet
Latitude
Mountain
Plain
Coast
Ocean
Topography

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Climate determines weathering product
Hot wet
Hot dry
Glacial
Temperate
Weathering
Chemical
Physical
Not much
Products
Silt – sand - gravel
Clay and quartz sand

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Climate determines weathering product
Chemical
Clay
Quartz
sand
Physical
Broken
rock

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Tectonics – Continental Drift
1cm per year = 10km in 1 million years
Everywhere has been
somewhere else

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Basic Principles and Consequences
Residual soils and sedimented soils (unconsolidated sediment)
depend more on the environment and climate and less on the
parent rock.
Most geotechnical engineering is in “unconsolidated sediments”
(residual soils and sedimented soils).
Very little age-based (capital letter) geology is relevant to
geotechnical engineering.

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Transport and sorting.
Gravel
Sand
Silt
Clay
Fast flow Slow flow Lake
Erosion
Deposition
Fluvial environment
River flow speed varies with
position (inside and outside
bends) and with season
Wind has the same effect
but the size-velocity
relationship is different
Glaciers move all sizes

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Deposition Environments.
Deserts
Glacial
Coast

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Deposition Environments – Rivers

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Keywords
Processes determine what is formed.
Processes
Environment
Climate and topography control the processes.
Tectonics
Everywhere has been somewhere else.

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The Question to Ask.
Where should I go in the World today to see this soil or rock being
created; and what has happened to it since?
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2 – Geological origins of soils and rocks
2.1 Surface processes
2.3 Soil behaviour related to description
2.2 Objective description of soil

20. What to do in practice
Relate what you are looking at to the geological processes
that formed it. Where did the material come from? How was it
transported? What was the environment of deposition? What
happened since?
Describe soil in the field
Describe a soil sample in the office
Interpret standard descriptions in borehole logs
Revise when lab tests results come in
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21. Basic Description
Soils Rocks
Bonding and fabric Structure Joints
State Dense - loose Weathering grade
What it is The grains Geological name(s)
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22. A
0.002 0.006 0.02 0.06 0.2 0.6 2 6 20 60
0
75
50
25
100
Fraction smaller
than the size %
C
Silt Clay Gravel Sand
Not visible Visible
A = largest size
C = smallest size
Grading – distribution of sizes
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23. How to estimate grading – always draw a grading curve
By inspection By sedimentation
Interpret the borehole log
description
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Fraction smaller
than the size %
0.002 0.006 0.02 0.06 0.2 0.6 2 6 20 60
0
750
50
25
100
Particle size mm
Coarse grained
Fine grained
Well graded
Grading dominates drainage
Grading

25. Coarse grained soils – sand and gravel
Visual inspection
Shape and surface texture
rough and angular
smooth and rounded
Colour related to mineralogy
colourless – quartz
white – carbonate
red, yellow, grey – same as parent rock
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Estimate plasticity; add water so state changes from crumbly to flowing;
add little water – low plasticity
add much water – high plasticity
Fine grained soils – silt and clay
Be careful with silty clay and clayey silt
small fraction of montmorillonite or
large fraction of kaolinite
Activity A =
PI
% clay
< 0.5 = kaolinite
> 5 = montmorillonite

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2 – Geological origins of soils and rocks
2.1 Surface processes
2.3 Soil behaviour related to description
2.2 Objective description of soil

28. Strength of coarse grained soils
Dry the soil and pour
it into a heap; i = ’
c
Constant volume (large strain) strength
Smooth rounded grains; ’
c = 30
0

Rough angular grains; ’
c = 33
0

Carbonate sand; ’
c = 40
0

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29. If you have Atterberg limits
Constant volume ’
cv Compressibility C
c
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30. If you have Atterberg limits and water content
Always calculate the liquidity index;
at liquid limit LI = 1 and at plastic limit LI = 0
Undrained strength In the ground
LI
1 0
Depth
50m
Truly normally
consolidated
OC
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31. Manipulation – undrained strength
Flows 2
Extrudes between fingers <10
Easily moulded 10 – 20
Moulded with strong pressure 20 – 40
Indented by thumb 40 – 75
Indented by thumbnail 75 – 150
Difficult to mark with thumbnail >150
Result Undrained strength s
u kPa
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32. Grading and drainage
Coarse grained – visible grains
normal construction is drained
effective stress parameters
Fine grained – grains not visible
normal construction is undrained followed by consolidation
total stress strength and consolidation parameters
Well-graded – coarse and fine grains
Are there enough fine grains to fill the spaces between the
coarse grains?
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33. What about fabric (layers + bonding)?
Bonding; submerge in water and
wait until pore pressures equalise.
Will bonding survive the strains
Imposed by the works?
Layers; examine outcrop
or split sample
Influences permeability
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34. Geology - Engineering
1 Describe the soil grains, how they are packed (loose or
dense) and fabric.
2 Are these descriptions compatible with the geological
processes that formed the soil?
3 What can be deduced about engineering behaviour and
parameters from these descriptions?
4 Do lab and field test results agree with these deductions?
If not, why not?
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35. Key Message
Engineering properties must result from how the soils were
formed by natural geological processes.
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2 – Geological origins of soils and rocks
2.1 Surface processes
2.3 Soil behaviour related to description
2.2 Objective description of soil