METHODS OF SOIL COMPACTION

Methods of Soil Compaction During Construction

Methods of Soil Compaction During Construction

Introduction

Soil compaction is defined as the method of mechanically increasing the density of soil.  In construction, this is a significant part of the building process.  If performed improperly, settlement of the soil could occur and result in unnecessary maintenance costs or structure failure.  Almost all types of building sites and construction projects utilize mechanical compaction techniques.

Types of Compaction There are four types of compaction effort on soil or asphalt: 

Vibration

Impact

Kneading

Pressure

These different types of effort are found in the two principle types of compaction force: static and vibratory. 

Static force is simply the deadweight of the machine, applying downward force on the soil surface, compressing the soil particles.  The only way to change the effective compaction force is by adding or subtracting the weight of the machine.  Static compaction is confined to upper soil layers and is limited to any appreciable depth.  Kneading and pressure are two examples of static compaction. 

Vibratory force uses a mechanism, usually engine-driven, to create a downward force in addition to the machine's static weight.  The vibrating mechanism is usually a rotating eccentric weight or piston/spring combination (in rammers).  The compactors deliver a rapid sequence of blows (impacts) to the surface, thereby affecting the top layers as well as deeper layers.  Vibration moves through the material, setting particles in motion and moving them closer together for the highest density possible.  Based on the materials being compacted, a certain amount of force must be used to overcome the cohesive nature of particular particles.

Results of Poor Compaction

Both illustrations above show the result of improper compaction and how proper compaction can ensure a longer structural life.

Every soil type behaves differently with respect to maximum density and optimum moisture.  Therefore, each soil type has its own unique requirements and screen or separate the different grain sizes. Soil classification is categorized into 15 groups, a system set up by AASHTO (American Association of State Highway and Transportation Officials).  Soils found in nature are almost always a combination of soil types.  A well-graded soil consists of a wide range of particle sizes with the smaller particles filling voids between larger particles.  The result is a dense structure that lends itself well to compaction. 

RELATIVE DESIRABILITY OF SOILS AS COMPACTED FILL

GroupSymbol

 

*  if gravelly**  erosion critical***  volume change critical-  not appropriate for this type of use 

Soil Type

Relative Desirability for Various Uses(1=best; 14=least desirability)

Rolled Earth Fill Dams

Canal Sections

Foundations

Roadways

GW

Well-graded gravels, gravel/ sand mixes, little or no fines

-

-

1

1

-

-

1

1

1

3

GPsand

Poorly-graded gravels, gravel/ mixtures, little or no fines

-

-

2

2

-

-

3

3

3

-

GM

Silty gravels, poorly-graded gravel/sand/silt mixtures

2

4

-

4

4

1

4

4

9

5

GC

Clay-like gravels, poorly graded gravel/sand/clay mixtures

1

1

-

3

1

2

6

5

5

1

SW

Well-graded sands, gravelly sands, little or no fines

-

-

3*

6

-

-

2

2

2

4

SP

Poorly-graded sands, gravelly sands, little or no fines

-

-

4*

7*

-

-

5

6

4

-

SM

Silty sands, poorly-graded sand/ silt mixtures

4

5

-

8*

5**

3

7

6

10

6

SC

Clay-like sands, poorly-graded sand/clay mixtures

3

2

-

5

2

4

8

7

6

2

ML

Inorganic silts and very fine sands, rock flour, silty or clay-like fine sands with slight plasticity

6

6

-

-

6**

6

9

10

11

-

CL

Inorganic clays of low to mediumplasticity, gravelly clays, sandyclays, silty clays, lean clays

5

3

-

9

3

5

10

9

7

7

OL

Organic silts and organic silt-clays of low plasticity

8

8

-

-

7**

7

11

11

12

-

MN

Organic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts

9

9

-

-

-

8

12

12

13

-

CH

Inorganic clays ...