GEOTECHNOLOGY I COMPREHENISIVE CLASS NOTES 2024

Technical University of Mombasa


DCE 2106: GEOTECHNOLOGY I


Lecturer : Jackson Muruda
Email :


Session Four: Soil Compaction
SOIL COMPACTION
Compaction is a process of packing together the soil particles by reducing the air voids. This
is useful in improving the soil properties. The most common examples where soil may
have to be compacted are the road pavement and backfill after below ground
construction.
Compaction is carried out by mechanical means such as rolling, ramming and vibration
and cause compression of the soil by expelling air from the voids. It is not possible to
remove water from the voids by compaction, but the addition of water to a slightly moist
soil facilities compaction by reducing surface tension.
The state of compaction is measured by the dry density (d )
Ws
where d 
V
W
but  
V
Ww
m  Ww 
mWs ws
W  Ww  Ws  mWs  Ws  Ws(1  m)
Ws
 (1  m)  d (1 
m) V
or

d 
1m
Therefore to determine the compaction of a soil it is normal to find its bulk density and
moisture content to be able to determine dry density.




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, For a particular comparative effort it has been found that when a soil has a low water
content it is not possible to reduce the air voids very much since there is insufficient
water in the voids to act as a lubricant enabling the particles to pack together. Thus the
dry density is comparatively low.
If the water content is high it is possible to reduce the few air voids but, because of the
presence of a large amount of water in the voids, the dry density will again be low.
Between these two extreme, there is a water content known as the optimum moisture
content at which the dry density reaches its maximum. This is not a fixed figure however
but depends on the amount of compactive effort applied. Increasing the comparative
effort has the effect of reducing the value of the optimum water content and increasing
the value of the dry density.
The laboratory test can be used to predict the optimum water content and dry density
in the field, it is important therefore that the energy used in the laboratory is equivalent to
that likely to be applied by the compaction plant. Two types of tests are carried out in the
lab.
i) Standard compaction test
ii) Heavy compaction test
The second one is used to predict O.M.C and D.D for heavier compaction equivalent.
1. Standard Compaction Test
The standard proctor test (as it is commonly called) is a method of finding the
optimum moisture content for compaction of a soil. A cylindrical mould 0.001m 3 in
volume is filled with a soil sample in three layers, each layer being compacted by 27
blows of a standard hammer (weight 2.5kg. length of drop 300mm each blow).
2. Heavy Compaction Test (modified AASHO test)
A heavy compaction test for soils subjected to greater compactive effort uses a 4.5kg
hammer dropping 450mm on to five layers of 27 blows.
From the curves obtained, the optimum moisture content can be read off at the point
of maximum compaction.



Heavy compaction

Max dry Standard compaction
density


Optimum moisture content




AIR VOIDS LINES
For comparison a curve is plotted on the protoctor graph for the ideal case in which the soil
is saturated and is known as the zero air voids line or saturation line.




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