DHARM

COMPACTION OF SOIL 445

From the figure,

Optimum moisture content = 12%

Maximum dry density = 19.5 kN/m^3

Correction for oversize fraction:

G = 2.79 for gravel n 1 = 0.09 n 2 = 0.91

Corrected maximum dry density =

G

nnG

wd

dw

.γγ

γγ

max

(^12) max+

2 79 9 81 19 5
009 195 091 279 98
...
(.... .)
××
×+××
= 20 kN/m^3
Corrected optimum moisture content
= n 1 A 0 + n 2 w 0 = 0.91 × 12% (taking A 0 as zero)
= 10.9%

SUMMARY OF MAIN POINTS

1. ‘Compaction’ of soil is defined as the process by which the soil grains are packed closer together
   by mechanical means such as tamping or vibration, in order that the dry density be increased.
   The involves the expulsion of pore air only, and thus differs from the phenomenon of consolida-
   tion, which involves the expulsion of pore water from a fully saturated cohesive soil under static
   loading.
   Compaction of a soil results in the improvement of its engineering properties such as shearing
   strenght.


2. The moisture content-dry density relationship of a soil, as established by Proctor, is such that
   peak value is reached for dry density at a moisture content, called the ‘optimum’ value. Increase
   in compactive effort will result in increased maximum dry density and decreased optimum mois-
   ture content.


3. The condition of full saturation when air in the voids is completely expelled, is called the ‘Zero
   air-voids (Saturation) condition’, and the relationship between water content and dry density for
   this condition is called the ‘Zero air-void line’. The compaction curve for any soil will always lie to
   the left of the below this line, since the air in the voids can never be fully expelled by compaction.


4. The purpose of laboratory compaction tests is to provide a guideline and a basis for control of
   compaction in the field, by giving a specification that a certain percentage of the maximum dry
   density achieved in the laboratory test shall be achieved in the construction in-situ.


5. Compaction is achieved in-situ by means of rolling or vibration, the latter being most suited for
   granular soils. Smooth-wheeled rollers, pneumatic-tyred rollers and sheepsfoot rollers are com-
   monly used.


6. Control of compaction in the field involves the determination of the in-situ unit weight and in-
   situ moisture content.
   Sand replacement method and core-cutter method are used for the determination of in-situ unit
   weight. Quicker methods such as the use of Proctor’s plasticity needle or the T.V.A. Penetrometer
   are used for the determination of both the in-situ unit weight and in-situ moisture content.