factors affecting compaction of soil

state and discuss different factors influencing compaction of soil in the field.

There are four main factors which influence compaction and they are as follows :-

(1) Water content  there are two theories to explain the typical water content-dry unit weight relationship. They are the Lubrication theory by proctor and the Electrical Double Layer theory by Lambe.

According to Lubrication theory at lower water contents, the soil is stiff and the soil grains offer more resistance to compaction. As the water content increases, the dry density increases and air voids are decreased till the optimum water content is reached, a stage when lubrication effect is maximum. With further increase in moisture content, however the water starts to replace the soil particles and since yw << ys” dry unit weight starts decreasing.

Lambe uses concept of soil structure and the Electrical Double Layer theory to explain the effect of water content on dry unit weight. In case of cohesive soils, there is an attractive force namely the Vander Wall’s forces which acts between two soil particles and a repulsive force which is due to double layers of adsorbed water tending to come into contact with each other. While the attractive forces remain same in magnitude, the repulsive force is directly related to the size of double layers. If the net force between the particles is attractive, flocculated structure is the result; if it is repulsive, the particles tends o move away –‘disperse’. 
At low water contents attractive forces are predominant which makes it difficult for the particles to move about when compactive effort is applied. A low dry unit weight is the consequence. As the water content is increased, the double layer expands and inter particle repulsive forces increase. The particles easily slide over one another and get packed more closely, resulting in higher dry unit weight.The maximum expansion of the double layer is at the OMC, beyond that, the addition of water does not add any further to the expansion of double layer but the water tends to occupy space which otherwise would have been occupied by soil articles. Hence a decrease in unit weight. It also explains why the shape of the compaction curve is not the typical inverted V shape in the case of soils which are not cohesive and plastic in nature.

(ii) Compactive effort : 
For a given type of compaction, the higher the compactive effort, he higher the maximum dry unit weight and low the OMC. In the above figure compaction curve B corresponds to the higher comp active effort in a MPT, comparing it with the compaction curve A for SPT,one can see the compaction curve shifts to the top and to the left when comp active effort is increased. However, the margin of increase becomes smaller and smaller even on the dry side of the OMC while on the wet side of OMC, there is hardly any increase at all. If the peaks of compaction curves for different comp active efforts are joined together a ‘line of optimums’ is obtained which is nearly parallel to zero air void line. This brings out the face that even a higher comp active effort does not result in a higher efficiency of compaction.


(iii) Types of Soil :-
(a) Coarse grained, well graded soils compact to high dry unit weight especially if they contain some fines.
(b) Poorly graded sands lead to lowest dry unit weight values.
(c) In clay soils, the maximum dry unit weight tends to decrease as plasticity increases.
(d) Cohesive soils have generally high values of OMC.
(e) Heavy clays with high plasticity have very low maximum dry density and very high OMC


(iv)Methods of Compaction :-
Ideally speaking, the laboratory test must reproduce a given field compaction procedure, because the mode of compaction does influence somewhat the shape and the position of the ‘yd’ vs ‘w’ plot. Since the field compaction is essentially a kneading type compaction or rolling type compaction and the laboratory tests use dynamic impact type compaction, one must expect some divergence in OMC and yd(max) in the two cases.





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