Revision:
Soil = function (climate, organisms, relief (landscape), parent material, time)
Hans Jenny (1941)
Regolith = “all surficial materials above fresh bedrock”
(Ollier & Pain 1996)
Definition of a “soil” varies according to the science
Pedology - soil profile… A horizon, B horizon, C horizon… soil physics, soil chemistry, soil biology
Agriculture – soil as a resource… soil fertility, soil health… topsoil, subsoil
Geomorphology – soil series, transported soil, residual soil
When is a soil a rock?
When is a rock a soil?
Engineering soil:
“…a natural aggregate of mineral grains that can be separated by such gentle
mechanical means as agitation in water. Rock on the other hand, is a natural
aggregation of minerals connected by strong and permanent cohesive forces…
the boundary between rock and soil is necessarily an arbitrary one.”
(Karl Terzaghi)
Engineering soil:
Loose, aggregated, low strength, inelastic.
Rock:
Compact, solid, high strength, some elasticity
Mechanical properties of soils = function (soil mineralogy,
particle size, particle shape, moisture, fabric, organic content)
Soil mineralogy: clay minerals have the most influence.
Kaolin clays
- 1:1 clays (1 silica : 1 gibbsite)
- Low CEC
- Low water absorption
- Low plasticity
Illite clays
- 2:1 sheets
- Intermediate CEC
- Intermediate water absorption
- Intermediate plasticity
Montmorillonite clays
- 2:1 sheets
- High CEC
- High water absorption
- High plasticity
Engineering soil description and classification
Origins:
Residual soil = product of in situ weathering of the underlying parent rock
Transported soil = transported from their place of origin to where they are now
Engineering soil description and classification
Particle
Boulder
Cobble
Gravel
Coarse
Medium
Fine
Sand
Coarse
Medium
Fine
Silt
Clay
Size
>200 mm
60 mm – 200 mm
2 mm – 60 mm
20 mm – 60 mm
6 mm – 20 mm
2 mm – 6 mm
2 mm – 60 mm
2 mm – 600 mm
600 mm – 200 mm
200 mm – 60 mm
60 mm – 2 mm
< 2 mm
Coarse grained
Fine grained
Particle Size Distribution
Determination of particle size distribution:
• In coarse grain soils …... By sieve analysis
In fine grain soils
…... By hydrometer analysis
or particle sizer or sedimentation (pipette)
hydrometer
stack of sieves
sieve shaker
soil/water suspension
Sieve Analysis
Hydrometer Analysis
Engineering soil classification
Coarse grained soils
Sieve analysis testing gear
Engineering soil classification
Fine grained soils
Hydrometer
Laser particle sizer
100
80
% Passing
hydrometer
sieve
60
fines
sands
gravels
40
20
D
30
D10 = 0.013 mm
D30 = 0.47 mm
D60 = 7.4 mm
0
0.001
0.01
0.1
1
10
Grain size (mm)
Grain Size Distribution Curve
• can find % of gravels, sands, fines
define D10, D30, D60.. as above.
100
Engineering soil classification
Sand, poorly graded
(uniform)
Gravel, well graded
Sand, gap graded
Clay
Silt
Sand
Gravel
Engineering soil classification
Colour:
- Relatively unimportant as it is not an mechanical property
- Stick to primary colours (red-brown clay, pale yellow sand, grey silt, etc.)
- Can be useful to indicate:
• Organic matter
• Saturation
• Filled ground (imported soils)
• Deleterious materials
Behaviour:
Cohesive = sticks together. Usually contains clay minerals.
Non-cohesive = friable, flows like sand.
Can stick together when wet (apparent cohesion)
Engineering soil classification
Soil structure
Soil fabric
“card house” fabric
Engineering soil classification
Consistency:
Cohesive soils
Term
Abbreviation
UCS (kPa)
VS
<25
Soft
S
25 – 50
Easily penetrated 5 cm by thumb
Firm
F
50 – 100
Can be penetrated 5 cm by thumb with
moderate effort
Stiff
S
100 – 200
Readily indented by thumb but
penetrated only with great effort
VS
200 – 400
Readily indented by thumb nail
H
≥ 400
Very soft
Very stiff
Hard
Tactile test
Easily penetrated 5 cm by fist
Indented with difficulty by thumb nail
UCS = Unconfined Compressive Strength measured in kiloPascals (kPa)
– dealt with in later lectures
Engineering soil classification
Consistency:
Non-cohesive soils
Term
Very loose
Loose
Medium dense
Dense
Very dense
Abbreviation
Relative density (%)
Tactile test
VL
< 15
Ravelling
L
15 – 35
Easy shovelling
MD
35 - 65
Hard shovelling
D
65 - 85
Picking
VD
> 85
Hard picking
Relative density = relative to maximum dry density achieved in a standard soil
compaction test – dealt with in later lectures
Engineering soil classification
Sensitivity:
Term
Insensitive
UCS in situ : UCS remoulded
<2
Medium sensitive
2–4
Sensitive
4–8
Very sensitive
8 – 16
Quick
> 16
Thixotropy:
Looses strength under vibration.
Engineering soil classification
Behaviour when saturated:
Slaking soils:
Dispersive soils:
Engineering soil classification
Moisture:
Term
Abbreviation Tactile
Dry
D
Looks and feels dry
Moist
M
Feels and looks moist, but no fee water on hands when
remoulding
Wet
W
Feels and looks wet, free water on hands when
remoulding
Moisture content is given by:
Wt  Wd
% 
 100
Wd
Where:
% is the percentage moisture
Wt is the initial weight of the soil
Wd is the dry weight of the soil
Engineering soil classification
Consistency indices:
Atterberg Limits
Liquid limit – the  above which the soil behaves as a liquid
Plastic limit – the  below which the soil ceases behaving as plastic
Shrinkage limit – the  below which no further volume change occurs
Sticky limit – the  below which the soil looses its adhesive quality
Cohesion limit – the  below which the soil particles no longer stick together
Liquid Limit (LL) - Plastic Limit (PL)
= Plasticity index (PI)
Linear shrinkage
Free swell
Engineering soil classification
Liquid limit testing devices
Engineering soil classification
An international unified soil classification (USC) was developed to for use in
airfield construction (Casagrande 1948) and dam construction (1952).
The USC provides a common terminology for soils based on their mechanical
behaviour.
Component
Symbol
Qualifier
Symbol
Gravel
G
Well graded
W
Sand
S
Poorly graded
P
Silt
M
Silty
M
Clay
C
Clayey
C
Organic
O
Low plasticity
L
Peat
Pt
High plasticity
H
The primary division is made between:
• Coarse grained soils >50% of the soil is >60 mm
• Fine grained soils >50% of the soil is <60 mm
Engineering soil classification
For coarse grained soils:
If ≤5% is <60 mm, then the soil is either GW, GP or SW, SP
If 5% < 60 mm ≤ 12% then it carries a dual classification
If >12% <60 mm then it’s GM, GC or SM, SC
For fine grained soils:
Use the plasticity chart
Engineering soil calculations
Va
Vv
Vw
air
Mw
water
Vt
Mt
Vs
solids
Ms
Density: Bulk density, Wet density & unit weight
Mt

Vt
Density: Dry density
expressed in g/ml or t/m3
Ms
d 
Vt
expressed in g/ml or t/m3
Engineering soil calculations
Particle density: Specific Gravity (Ms & Vs)
Ms
s 
Vs
usually expressed as g/ml
Soil particle density is equivalent to the specific gravity of the solids,
which is the ratio of the unit weight of solids to the unit weight of water
g s
s
Gs 

g w  w
Material
Particle density (g/ml)
quartz
2.65
kaolinite
2.4
basalt
2.7 – 3.0
silt
2.67
silty clay
2.75
sandy clay
2.70
peat
1.50 – 2.15
Engineering soil calculations
Degree of Saturation
Water (moisture) content
Mw
w
 100 
Ms
expressed as a percentage
Porosity
Vv
n
 100
Vt
Vw
S
 100
Vv
expressed as a percentage
Void Ratio
Vv
e
Vs
expressed as a percentage
0  n 1
0e