Introduction to Soil Engineering

1
Introduction to Soil Engineering

• D. A. Cameron
• 2007

2
Particle Interactions

• Coarse soils v. Fine soils
• sand and gravel v. silt and clay
• STRENGTH DERIVED FROM
• Friction, interlock v.
• physico-chemical interaction

3
Fine - Grained Soils

• Cohesion
• Apparent cohesion
  ? apparent tensile strength,
• arising from
• electrostatic forces
• (are stronger, the finer the particle)

4

• Clays form from weathering and secondary
  sedimentary processes
• Clays are usually mixed

5
Properties of the clay minerals

• When mixed with a little water, clays become
  plastic i.e. are able to be moulded
• SO, moisture affects clay soil engineering
  properties

6
Properties of the clay minerals

• Can absorb or lose water between the silicate
  sheets
• negative charge attracts H2O
• When water is absorbed, clays may
• Expand !
• water in spaces between stacked layers
• Montmorillonite most expandable
• Kaolinite the least

7
Illite v Montmorillonite Different forms
of bonding between these minerals

• Illite - main component of shales and
  other argillaceous rocks
• - nett negative charge
• Montmorillonite
• - greater nett negative charge

8
Clay Minerals capacity for water

• i) Kaolinite (China clay)
  Water absorption, approximately 90
• ii) Montmorillonite (Bentonite, Smectite)
  Water absorption, approximately 300 - 700
• iii) Illite
  Intermediate water absorption

9
In Summary

• The basic building blocks of clays are small
• Si, O, H and Al are the chief ingredients
• Different combinations of sheets form the basic
  micelles of clay minerals
• Clay mineral properties vary due to the nature of
  bonding of the sheets between micelles

10
Engineering Soil Classification
11
The Soil Phases
PHASE DIAGRAM
THE SOIL SYSTEM
The Soil System
12
New Terms
Density ? rho Unit weight ?
gamma
e.g. ?water ?w 1 t/m3 or 1 g/cc ?
?w 9.81 kN/m3
Soil varies between ? 15 - 21 kN/m3
13
Other densities

• Soil dry density, ?d
• Particle density, ?s

Mass of soil / total volume
14
Introduction to soil terms, contd

• Particle densities range between 2.6 and 2.7 t/m3
• Moisture content, w
• based on mass of watermass of solids ( dry soil)

15
Moisture and Density

• where, w water content (just a ratio, not !)

16
More soil terms.

• Void Ratio, e
• Degree of saturation, SR

17
VOID RATIO
V Vs Vw Va
Mw, Vw
Ms, Vs
Solids
18
Soil Consistency

• DENSITY of granular soils
• loose, dense, or very dense
• STRENGTH of fine-grained soils
• soft, firm, stiff or hard

19
Unified Soil Classification System (USCS)

• Based on...
• Particle size
• - gravel, sand, silt, clay fractions
• Particle size distribution
• - grading
• Plasticity

20
Symbols of the USCS coarse grained
21
Defining Particle Sizes
Grain size (mm)
0.002
0.2
2.36
20
200
0.075
0.6
6.0
63
Basic Soil Type
F M C
F M C
CLAY
SILT
SAND
GRAVEL
COBBLES BOULDERS
Fine-grained soil
Coarse-grained soil
22
Sieve Analysis - coarse soils
Gravel (G)
Sand (S)
Silt (M)
23
Particle Size Distribution Terms
P - Poorly graded (uniform sizes)
W - Well graded Good mix of sizes
P - Poorly graded Missing range of sizes
24
Fine-grained Soils

• Too fine for sieving
• Sedimentation and/or laser equipment?
• Even then, sizes say nothing about clay
  mineralogy and potential soil behaviour!

?Fine-grained soils are defined by how plastic
they are
25
Symbols for Fine Grained Soils
26
Consistency Limits of Fine Soils

• Defining water contents
• 1. LIQUID PHASE
• - fluid, low shear resistance
• 2. PLASTIC PHASE
• - easily moulded
• 3. SOLID PHASE
• - strong, resists deformation

27
ATTERBERG LIMITS
solid
liquid
The plastic zone
Max.
Moisture content
0
28
CONSISTENCY LIMITS
Change in Volume
PL
LL
Moisture content ()
29
The Plasticity Chart
PLASTIC INDEX ()
Example LL 75 PL 32
LIQUID LIMIT ()
30
Field Tests of the USCS for fine-grained
soils

• Dry strength
• relative strength of a dry ball of soil
• prepared at PL
• Toughness
• near PL when remoulded
• Dilatancy
• volume change upon shearing
• prepared at LL

31
Interpretation of Field Tests

• Dry strength is low for O and M soils of low
  plasticity
• Dry strength increases with plasticity
• Dry strength is greater for clay soils
• Toughness increases with plasticity
• Silts are dilatant but clays are not!
• dilation increase in volume (with shearing)

32
Classification of Mixed Soils

• Wet sieve on 0.075 mm sieve
• gt 50 retained? coarse
• Sieve on 2.36 mm sieve
• lt 50 retained? Sand
• Sieve for fines
• lt 5 SP or SW (fines insignificant)
• gt12 SC or SM (plasticity?)

33
SUMMARY

• Soil classification for engineering purposes is
  based on
• 1. Fundamental particle sizes
• AND
• 2. Particle size distributions
• OR
• 3. Soil plasticity
• (LL, PI, LS and/or field tests)

34
Soil Stresses

• Dead weight stresses
• Pore water pressures
• steady state
• no flow
• water table
• Effective stress

35
VERTICAL STRESSES ? ?z force from weight of
prism above soil (area of soil in x-y plane)
?z
z
?x
z
?z
x
y
36
The dead weight stresses are termed
TOTAL soil stresses
37
PORE WATER PRESSURES, ? u in a soil mass with
a water table, are due to the dead weight of
water u ?wzw
GL
Saturated zone
z
u
u
z
x
y
38

• Concept of EFFECTIVE stress
• Terzaghi 1923
• PWP reduces the stress felt by the soil in a
  saturated soil system (with no air voids)

39
Diameter of tube, d
Height of rise fn(d)
40
Dead weight soil stress- total vertical stress
80 kPa
152 kPa
?v
41
Dead weight soil stress- effective vertical
stress
0 m
? 16 kN/m3
2 m
? 18 kN/m3
5 m
? 20 kN/m3
9 m
166 kPa
?v
u
42
Effective Stress Distribution
0 m
? 16 kN/m3
2 m
? 18 kN/m3
5 m
? 20 kN/m3
9 m
?v? ?v - u
43
Alternative approach effective unit weight, ??
? - ?w
0 m
?? 16 kN/m3
2 m
?? 8.2 kN/m3
5 m
?? 10.2 kN/m3
9 m
?v? ?v - u
44
COMPACTION OF SOIL The Process

• Expulsion of AIR
• - air void volume, Va, reduced
• - moisture content is unchanged or constant

45
The Purpose of Compaction

• increase
• STRENGTH
• STIFFNESS
• DURABILITY

• decrease
• PERMEABILITY

46
Earthwork Applications

• Earth dams, Levee banks, Road subgrades,
  Pavement layers, Subdivisions, etc
• Water retaining structures stability with low
  permeability
• Roads - reduce pavement thickness by increasing
  strength
• Subdivisions - reduce footing stiffness by
  increasing foundation strength stiffness

47
Laboratory Soil Compaction

• Compaction of all soil materials, except clean
  gravels and sands
• - achieved by falling weight hammers of known
  mass and drop height
• ? under constant energy

48
AS1289 - Standard or Modified?

• Standard Compaction
• light compaction (low energy),
•  
• (b) Modified Compaction
• heavy compaction (high energy),
• (thinner lifts)

49
Laboratory compaction testing- relevance?

• How does the soil respond when compacted on site?
• So, the laboratory method, which best replicates
  the field compaction equipment on an earthworks
  job, must be chosen

50
The Compaction Curve

• For a particular soil and compactive effort
  ........
• There is a unique relationship between the dry
  density that can be achieved and the moisture
  content of the soil
• Warning NA to clean sands and gravels

51

• Removal of all air voids is impractical
• - ?d max at an air voids ratio, A ? 5
• (A Va / V )
• w at ?d max is termed the
  OPTIMUM MOISTURE CONTENT (OMC)
• lt OMC, the soil is stiff and dry
• Its difficult to re-orientate particles 
• gt OMC, the soil is too deformable
• flows when compacted

52
The Shape of the Compaction Curve
A 5?
Dry Density
Moisture content
53
INFLUENCE OF SOIL TYPE ON COMPACTION CURVE
Sand with some fines
Dry Density
Zero air voids line
Clay
Moisture content
Constant compaction energy
54
Influence of Compaction Energy
Modified Compaction
Dry Density
Standard Compaction
Moisture content
55
Influence of Compaction Energy

• The same effect is realised on earthworks
  projects by
• Increasing the mass of compactors
• Compacting in thinner lifts
• Passing over each layer more
• number of passes

56
Compaction and permeability
B
Dry Density or permeability
C
A
kmin
Moisture content
57
Compaction Practice

• Compacted in thin layers or LIFTS
• (100 to 200 mm for fine grained soil)
• Silts and Clays - need relatively long duration
  loading
• Sands and Gravels - vibration has greatest effect

58
Specification of Compaction of Clean Sands
Gravels

• Maximum compaction when either
• bone dry or saturated
• Capillarity resists compaction
• Compaction defined in terms of maximum and
  minimum dry densities
• ?d max and ?d min

59
Description of coarse-grained soil
60
Specification of Compaction

• AS3798 Guidelines on Earthworks for Commercial
  and Residential Developments
• Dry Density Ratio, RD
• Ratio of desired dry density to the maximum
  achievable by the chosen laboratory method,
• e.g. 95 (Standard Compaction)
• or 98 (Modified Compaction)

61
Notes on specification

•  

Sometimes moisture contents for compaction need
to be tightly specified.. Why? What if a soil
on site is too wet for compaction?
62
SUMMARY

• Granular soils specified by density index
• Most soils specified by dry density ratio,RD
• Compaction curve, ?d max and OMC
• Not unique depends on compactive effort
• Field compaction curves
• Passes, lift thickness, equipment
• Field tests for density
• Penetration testing
• Sand replacement
• Nuclear density

63
WATER SEEPAGE water pressures

• Water flows from points of high to low TOTAL
  head
• WATER HEADS
• head of water x ?w water pressure, u
• Total head elevation head pressure head
• i.e h hT he hp

64
Darcys Law

• q kiA
•  
• where q rate of flow (m3/s)
• i hydraulic gradient
• A area normal to flow direction (m2)
• k coefficient of permeability (m/s)

65
Hydraulic Gradient, i
Area of flow, A
Flow rate, q
Length of flow, l
66
Hydraulic Conductivity

• Coefficient of permeability or just
  permeability
• SATURATED soil permeability

67
TYPICAL PERMEABILITIES

• Clean gravels gt 10-1
  m/s
• Clean sands, sand-gravel 10-4 to 10-2 m/s
• Fine sands, silts 10-7 to 10-4
  m/s
• Intact clays, clay-silts 10-10 to 10-7
  m/s

68
Measuring Permeability

• A Laboratory
• Constant head test
• Falling head test
• Other

A Laboratory How good is the sample?
B Field Need to know soil profile (incl. WT)
boundary conditions

• B Field
• Pumping tests
• Borehole infiltration
• tests

69
Lab Test 1 Constant head test

• Cylinder of saturated coarse grained soil
• Water fed under constant head
• elevated water tank with overflow
• Rate of outflow measured
• Repeat the above after raising the water tank

70
Test 2 Falling head permeameter

• For fine sands, silts, maybe clays
• Rate of water penetration into cylindrical sample
  from loss of head in feeder tube
• Must ensure
• no evaporation
• sufficient water passes through
• A slow procedure

71
Drawdown test

• Needs
• a well-defined water table
• and confining boundary
• Must be able to
• pull down water table
• and create flow
• (phreatic line uppermost flow line)

72
Flow Lines shortest paths for water to exit
Phreatic surface
Equipotential lines
Flow tube
73
The Flow Net - FLOW LINES
Run ? parallel to impervious boundaries
(impermeable walls or cut-offs) and the
phreatic surface The Phreatic surface is the
top flow line 2 consecutive flow lines constitute
a flow tube
74
The Flow Net - EQUIPOTENTIALS

• Are lines of equal total head
• The total head loss between consecutive
  equipotentials is constant
• Equipotentials can be derived from boundary
  conditions and flow lines

75
Flownet Basics

• Water flow follows paths of maximum hydraulic
  gradient, imax
• flow lines and equipotentials must cross at 90?

76
Since ?q is the same, ratio of sides will be
constant for all the squares along the flow tube
5 Flow Lines
M
Equi- potential lines
Impervious boundary
77
Flownet Construction
78
Flow Net Calculations

• Total flow for Nf flow channels, per unit
  width is  

But only for curvilinear squares!
79
Critical hydraulic gradient, ic

• The value of i for which the effective stress in
  the saturated system becomes ZERO!
• Consequences
• no stress to hold granular soils together
• ? soil may flow ?
• boiling or piping EROSION!

80
Likelihood of Erosion
GRANULAR SOILS chiefly! When the effective stress
becomes zero, no stress is carried by the soil
grains Note when flow is downwards, the
effective stress is increased! So the erosion
problem and ensuing instability is most likely
for upward flow, i.e. water exit points through
the foundations of dams and cut-off walls
81
Key Points

• Heads in soil
• Darcys Law
• Coefficient of permeability
• Measurement of permeability
• Flownets
• Flownet rules
• Seepage from flownets
• Piping, boiling or erosion
• Critical hydraulic gradient