Soil Compaction - Part 02

I mentioned before there are three parameters influencing the soil compaction such as,
1). Effect of the water content on the soil compaction
2). Effect of the compactive effort
3). Effect of the type of the compaction  
From the 1st blog we talked about the Effect of the water content on the soil compaction and about the proctor compaction test. (please check : https://civilengineeringthingsforundergrads.blogspot.com/2018/10/soil-compaction-if-void-ratio-of.html#more) 

From this blog let's talk about other 2 factors.

2). Effect of the compactive effort

The compactive effort means, the amount of energy that we use to compact unit volume of soil mass.

The moisture content vs dry unit weight relationship generally observed for standard and modified proctor compaction testing of the same soil type is shown in the following figure.

It is evident from the above figure that the maximum dry unit weight is increased and the optimum moisture content is reduced when the compaction effort increased.

It is also noticed that an increased in the compactive effort produces a very sizeable increase in dry unit weight for soil when it is compacted at water contents drier than optimum water content ( C to D )

On the other hand, when soil is compacted, wet of the optimum water content, an increase in the compactive effort produces only a small increase in the dry unit weight. ( E to F )

Line KL shows the dry unit weight with zero air void (100% saturated) corresponding to the respective water contents. the zero air void unit weight ( gamma zav) corresponding to the water content w, is given by the following equation.





where,
Gamma zav = The zero air void unit
Gamma w = bulk / wet unit weight
Gs = Specific gravity ( soil = 2.65)

Note : This is the KL line equation

The voids in the soil are not totally filled with water and in a partially saturated state. regardless of how soil is compacted, how much it is compacted and how high the water content of pre compacted soil is, compaction can never drive out all the air from the void spaces and invariably partially saturated soil is produced in the 'as compacted' state. therefore, all the dry unit weight vs moisture content curves will always be to the left of the zero air void line.

3). Effect of the type of compaction

Field compaction of soil done using different types of rollers,
(1). Smooth wheel / Steel drum roller
(2). Pneumatic tyre roller
(3). Sheeps foot roller
(4). Vibratory roller (vaccer)

Approximate field method to determine the optimum moisture content

First prepare a small soil clot (ball) using hand and palm, then drop it down from a height of about 1.5 m on to a solid surface.

If the soil clot is separated into individual particles (spread) the water content is less than optimum.

If the soil clot breaks into two or three pieces without spreading, the water content is close to the optimum.

If the soil clot does not break but deforms as a single unit the water content is more than optimum.

Field compaction of soil

The field compaction is generally carried out in layers until the required height of the compacted soil is reach.

There are two main factors that are to be correctly selected for effective field compaction of soil. they are layer thickness (lift) of the soil and the number of roller passes for each layer.

Basically degree of compaction should be 95%. that's means dry unit weight of the field soil should be higher than, 95% of the maximum dry unit weight of the corresponding soil.

The dry unit weight of a soil at a given moisture content will increase up to a certain point with the number of roller passes. beyond this point it will remain approximately constant. in most cases about 10 to 15 roller passes yield the maximum dry unit weight economically attainable.  (graph 01)

Considered about graph 2, dry density of the ground surface is less than mid level. reason for that : in the ground surface the lateral resistance is very low than mid level. because of that density is low.

In bottom of the graph the density is reducing. reason for that : the load supplied for the top is distributed / spread within layer area. there for pressure will reduce because of this density will lower than mid.

Compaction quality assurance 

Quality assurance during the compaction process is very important to achieve the required compaction level of the soil fill. in most specifications for earth works, it is specified to achieve a certain degree of compaction ( generally 90% to 95%) in terms of the maximum dry unit weight determined from in the laboratory by either the standard or modified proctor test. the degree of compaction (R%) is defined as follows,

The quality assurance testing should be continue during the compaction process as an when a certain volume of soil is compacted.

Commonly used methods to determine the field unit weight of soil are (field density testing methods),
1). Sand cone method
2). Rubber balloon method
3). Core cutter method
4). Nuclear method