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Civil - Soil Mechanics - Soil Classification And Compaction


   Posted On :  10.07.2016 09:42 pm

In construction of highway embankments, earth dams and many other engineering structures, loose soils must be compacted to improve their strength by increasing their unit weight; Compaction - Densification of soil by removing air voids using mechanical equipment; the degree of compaction is measured in terms of its dry unit weight.





In construction of highway embankments, earth dams and many other engineering structures, loose soils must be compacted to improve their strength by increasing their unit weight; Compaction - Densification of soil by removing air voids using mechanical equipment; the degree of compaction is measured in terms of its dry unit weight.


1 Objectives for Compaction


Increasing the bearing capacity of foundations;

Decreasing the undesirable settlement of structures;

Control undesirable volume changes;

Reduction in hydraulic conductivity;

Increasing the stability of slopes.


In general, soil densification includes compaction and consolidation.


Compaction is one kind of densification that is realized by rearrangement of soil particles without outflow of water. It is realized by application of mechanic energy. It does not involve fluid flow, but with moisture changing altering.


Consolidation is another kind of densification with fluid flow away. Consolidation is primarily for clayey soils. Water is squeezed out from its pores under load.



It is a gradual process of reduction of Volume under sustained, static loading.


It causes a reduction in volume of a saturated soil due to squeezing out of water from the soil.


Is a process which in nature when saturated soil deposits are subjected to static loads caused by the weight of the building





It is a rapid of reduction of volume mechanical mean such as rolling , tamping , vibration.


In compaction, the volume of partially saturated soil decreases of air the voids at the unaltered water content


Is an artificial process which is done to increase the density of the soil to improve its properties before it is put to any use.


2 Compaction Effect


There are 4 control factors affecting the extent of compaction:


  Compaction effort;

  Soil type and gradation;

      Moisture content; annd

      Dry unit weight (dryy density).




3 Effect of Water on Compaction


In soils, compaction is a funnction of water content


Water added to the soil duri ng compaction acts as a softening agent on t he soil particles


  Consider 0% moistu re - Only compact so much

  Add a little water - compacts better

  A little more water - a little better compaction

  Even more water Soil begins to flow


What is better compactionn?


The dry unit weight (?d) inccreases as the moisture content increases to a point


Beyond a certain moisture content, any increase in moisture content ten ds to reduce the dry unit weight



4 Standard Proctor C ompaction Test


The standard was originall y developed to simulate field compaction in the lab




Find the optimum moisture content at which the maximum dry unit weig ht is attained ASTM D 698




Standard Proctor; 1/30 ft3 mold 5.5 lb hammer; 12� drop

3 layers of soil; 25 blows / l ayer


Compaction Effort is calc ulated with the following parameters


Mold volume = 1/30 cubic foot Compact in 3 layers




5.5 lb hammer 12" drop




1.  Obtain 10 lbs of soil pass ing No. 4 sieve


2.   Record the weight of the Proctor mold without the base and the (c ollar) extension, the volume of which is 1/30 ft3.


3.  Assemble the compactio n apparatus.


4.   Place the soil in the mold in 3 layers and compact using 25 well dist ributed blows of the Proctor hammer.


5.  Detach the collar without disturbing the soil inside the mold


6.  Remove the base and determine the weight of the mold and compacte d soil.


7.   Remove the compacted soil from the mold and take a sample (20-30 grams) of soil and find the moisture content


Place the remainder of th e molded soil into the pan, break it down, and thoroughly remix it with the other soil, plus 100 additional grams of water.

Zero-air-void unit weight:


At certain water content, w hat is the unit weight to let no air in the voids

It is clear that in the above equation, specific gravity of the solid and thewater density are constant, the zero-air-void density is inversely proportional to water content w. For a given soil and water content the best possible compaction is represented by the zero-air-voids curve. The actual compaction curve will always be below. For dry soils the unit weight increases as water is added to the soil because the water lubricates t he particles making compaction easier. As moree water is added and the water content is larg er than the optimum value, the void spaces be come filled with water so further compac tion is not possible because water is a kind like incompressible fluid. This is illustrated by t he shape of the zero-air-voids curve which decre ases as water content increases.


Compaction Curve


Compaction curve plotted ?d vs. w.The peak of the curve is the Maxim um Compaction (?d max) at Optimum Moisture Content (wopt )




Plot of dry unit weight vs m oisture content


Find ?d (max) and w and Plot Zero-Air-Void unit weight (only S=100% )



5 Effect of Compacti on Energy


With the development of heavy rollers and their uses in field compaction, the Standard Proctor Test was modified t o better represent field compaction


As the compaction effort increases,


The maximum dry unit w eight of compaction increase; the optimum moisture content decreases to some extend Compaction energy per unit volume.




6 Compaction adopte d in the field


i) Tampers.


A hand operated tamper consists of block iron, about 3 to 5 Kg mass, a ttached to a wooden rod. The tamper is lifted for about 0.30m and dropped on the soil to be compressed. Mechanical Tampers operatted by compressed air or gasoline power.


ii) Rollers


a)  smooth wheel rollers


b)  pneumatic tyred rollers


c)  Sheep- foot rollers.


a) Smooth wheel rollers


Smooth wheel rollers ar e useful finishing operations after compaction of fillers and for compacting granular base causes of highways.


b) Pnumatic tyred roller s


Pneumatic tyred rollers u se compressed air to develop the required in flation pressure.The roller compactive the soil primarily by kneading action. These rolle srs are effecting for compacting cohesive as we ll as cohesion less soils.


c) Sheep foot rollers


The sheep foot roller con sists of a hollow drum with a large number of small projections (known as feet) on its surf ace. The drums are mounted on a steel frame. The drum can fill with water or ballast incre ases the mass. The contact pressure is gener ally between 700 to 4200 KN/m2.


Tags : Civil - Soil Mechanics - Soil Classification And Compaction
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