What is compaction?
A simple ground improvement technique,
where the soil is densified through external
compactive effort.
Compactive
effort
+ water =
Why compact soils?
 Increases strength
 Decreases permeability
 Reduces settlement
 Reduces shrinkage
Applications:
Roads
Foundations
Embankments
Dams
Aircraft runways
Parking areas
Paving
Retaining walls
Rammed earth structures
Etc. etc.
Theory:
Ralph R. Proctor (circa 1933) related compaction to four variables:
Dry density
Moisture content
Compactive effort
Soil type
Laboratory tests
Mould (standard dimensions)
Hammer (standard cross-section area, weight, drop)
Method (standard number of layers and number of drops for each layer)
Mositure content
Dry density (d)
Compaction Curve
Soil grains densely packed
- good strength and stiffness
- low permeability
d, max
optimum
water content
Water content
Compaction Curve
Dry density (d)
What happens to the relative quantities of the three phases
with addition of water?
air
water
soil
difficult to expel all air
lowest void ratio and
highest dry density at
optimum w
Water content
Dry density (d)
Zero Air Void Curve
- corresponds to 100% saturation
Zero air void curve (S=100%)
S<100%
Gs  w
Eq :  d 
1  wGs
S>100% (impossible)
All compaction points should lie
to the left of ZAV curve
Water content
Dry density (d)
Effect of Compactive Effort
E2 (>E1)
Increasing compactive
effort results in:
• Lower optimum
water content
• Higher maximum dry
density
E1
Water content
more dispersed fabric
Dry density (d)
Compaction and Clay Fabric
Higher water content or higher
compactive effort gives more
dispersed fabric.
more dispersed fabric
Water content
Dry density (d)
Line of Optimum
Compaction curves
for different efforts
Line of optimum
Water content
Laboratory Compaction Test
- to obtain the compaction curve and define the
optimum water content and maximum dry density for a
specific compactive effort.
Standard Proctor:
hammer
Modified Proctor:
• 3 layers
• 5 layers
• 25 blows per layer
• 25 blows per layer
• 2.7 kg hammer
• 4.9 kg hammer
• 300 mm drop
• 450 mm drop
1000 ml compaction mould
Compaction Control
-a systematic exercise where you check
at regular intervals whether the
compaction was done to specifications.
e.g., 1 test per
1000 m3 of
compacted soil
• Minimum dry density
• Range of water content
Field measurements (of d) obtained using
• sand cone
• nuclear density meter
simple stress (Axial stress)
Stress (Pascal) =
Force (Newton)
Area (square metre)
Mass x gravity
Area
Reaction force
Soil mechanics: stress
Stress due to the weight of soil above
s =gh
v
s2
s3
sv = vertical stress (kPa)
Stress ellipsiod
s3
Horizontal stresses
s2
g = unit weight of soil (kN/m3)
h = depth (m)
Soil mechanics: stress
Stress due to the weight of soil above
s =gh
v
s3
sv = vertical stress (kPa)
s2
g = unit weight of soil (kN/m3)
h = depth (m)
s2
s3
Horizontal stresses
Soil mechanics: stress & strain
Circular failure surface
due to shearing of the soil
Barham River valley
Apollo Bay
1987
Moorabool River valley
Gheringhap
2001
shear
stress
Normal stress (s)
Shear stress (t)
Charles-Augustin de Coulomb
1736 - 1806
Coulomb Equation
t = c + s tanf
t = shear stress
c = cohesion
s = normal stress
f = angle of shearing resistance
shear stress
The groundwater in the pore spaces creates an uplift pressure
– the pore water pressure – to the shear plane.
The pore water pressure relates to the pressure head caused
by the weight of water and rock above
Water table
The normal stress (s )is countered by the pore water pressure (u) and
the result (s – u) is called the effective stress (s’)
Mohr - Coulomb Equation
t = c’ + s’ tanf’
s
u
t = shear stress
c’ = effective cohesion
s’ = effective stress
f’ = effective angle of shearing resistance
Slope mechanics: rainfall as a trigger of instability
Raising the watertable increases the
pore-water pressure and reduces the
effective stress, which in turn lowers
the soil’s shear strength and causes a
shear failure
Pore water
pressure
Water table
Rainfall
event
time
Soil mechanics: stress
Stress due to the weight of soil above
s =gh
v
s2
s3
sv = vertical stress (kPa)
g = unit weight of soil (kN/m3)
h = distance (m)
Stress ellipsiod
s3
Horizontal stresses
s2
Soil mechanics: strain
Strain is the change in shape caused by the application of stress
Kinds of strain
Strain ellipsoid
Oblate (s1 > s2 = s3)
Prolate (s1 = s2 > s3)
Triaxial (s1 > s2 > s3)
Introduction to Consolidation
When a saturated clay is loaded externally,
GL
saturated clay
the water is squeezed out of the clay over a long time
(due to low permeability of the clay).
Soil Consolidation
settlement
This leads to settlements occurring over a long time,
time
which could be several years.
In granular soils…
settlement
Granular soils are freely drained, and thus the
settlement is instantaneous.
time
Laboratory testing
consolidation