Soil Mechanics I
CE-225
Lecture 11 (Compaction) i
(Hafiz) Asif Arshid
BSc, UET Lahore (2003-07)
MSc, Saitama University, Japan (2012-14)
1
Introduction
• The Soil is used as a basic construction material in
many projects such as:
– Retaining walls,
– Highways, Embankments,
– Airports,
– Dams, Dikes, etc.
• The advantages of using soil are:
– It is generally available everywhere,
– It is durable and it will last for a long time,
– It has a comparatively low cost.
2
Introduction
• The soils at a given site are often less than ideal for
the intended purpose.
• They may be weak, highly compressible, or have a
higher permeability then desirable from engineering
or economical point of view.
• Engineering properties of such soils can be improved
or stabilized.
• Stabilization is usually mechanical or chemical.
• Mechanical stabilization is called compaction.
3
Compaction
• In most civil engineering projects, whenever soils are
imported or excavated and re-applied, they are compacted.
• The compaction is a ground improvement technique, where
the soil is densified through external compactive effort.
• The degree of compaction is measured by dry unit weight d.
Compactive
effort
+ water =
4
Compaction
5
Compaction is the densification of soils by the application of
mechanical energy to reduce air void spaces in the three phase
soil model
• it reduces the air content, but not the water content
• can’t compact saturated soil (almost always true)
Loose soil
Compacted soil
Air
Air
Water
Water
Solids
Solids
Compaction Advantages
• As compaction increases, the following
occurs:
– Increase in soil strength
– Increase in bearing capacity
– Decrease in potential for settlement
– Control of undesirable volume changes
– Reduction in hydraulic conductivity
6
Compaction vs Consolidation
7
Compaction – General Principles
When water is added
 The particles
develop larger and
larger films
around them.
 Water lubricates
the particles
 Water helps
moving particles
and orient them
into denser
configuration
8
Optimum
moisture
content
Drd
r = rd (at w = 0)
When peak density is reached
 Water starts to replace soil particles
 Since rw << rs, the dry density curve start to fall down
Compaction – General Principles
• Water acts as a lubricant
• Too much water
– takes up space
– does not allow bonding
• Too little water
– same compactive effort, lower compaction
• Optimum moisture content (OMC): The moisture
content of the soil at which maximum density can be
achieved for a given amount of compactive effort.
• OMC of fine-grained soils is higher than coarsegrained soils.
9
Standard proctor test
• Developed by R.R. Proctor (1933).
• The compaction is a function of
–
–
–
–
Dry density (rd)
Water content (w)
Compactive effort (energy E)
Soil type (gradation, presence of clay
etc.)
• Equipment & methods
– ASTM D 698
– 1/30 ft3 (943.3 cm3) mold, dia 4-in
(101.6 mm)
– 5.5 lb (2.45 kg) hammer
– 12-in (305 mm) drop
– 3 layers of soil
– 25 blows per layer
10
Standard Proctor Test - Procedure
• The soil is mixed with varying amounts of water to
achieve different water contents.
• For each water content, the soil is compacted by
dropping a hammer 25 times onto the confined soil.
• The soil is in mold will be divided into three lifts.
• Each Lift is compacted 25 times.
• This is done 4-6 times from dry-to-wet.
Layer or lift # 3
Layer or lift # 2
25 Blows/Layer
Layer or lift # 1
11
Standard Proctor Compaction
Test results
12
Standard Proctor Compaction
13
Test results
Zero-air-void (ZAV)
curve corresponds to
100% saturation
All compaction points
should lie to the left
of ZAV curve
(because S > 100% is
not possible)
S<100%
14
20
Dry unit weight  d (kN/m3)
Gs = 2.69
19
18
17
S = 100%
60%
70%
16
80%
90%
15
5
10
15
20
Moisture content, w (%)
25
15
Effect of Compaction Energy
E
 No of   No   Weight   Height

 
 
 
 blows    of
   of
   of
 per layer   layers   hammer   drop

 
 
 
16





Volume of mold
25  3  5.5 1
E
 12,375 ft  lb/ft 3  592.5 kJ/m 3
1 30
Effect of Compaction Energy
 As energy of
compaction increased,
max d of compaction
is also increased.
 As the energy of
compaction is
increased, the
optimum moisture
content is decreased to
some extent.
17
Common compaction curves encountered
Bell-shaped
Dry unit weight d
Clayey soils (LL = 30~70)
One & one-half peaks
LL <30
Double-peaked
LL <30 or LL>70
Odd-shaped
LL>70
Water content (w)
18