COMPACTION - TM-5-3895-349-14-P0823

COMPACTION

Soil compaction equipment has developed from the

SOIL:

steam operated, smooth-wheel rollers of 19th century

The four basic types of soils with reference to compaction

France to the modern specialized equipment of today.

are gravel or rock fill, sand, silt and clay. Gravel and. sand

Compaction requirements have also progressed to

are non-cohesive soils with high permeability. The shear

complement modern technology.

Vehicles using

strength of cohesionless materials is essentially controlled

highways today may have six axles, 20,000 pound (9000

by five factors:

kg) axle loads and tire pressures of 150 psi (11 kg/ sq.

a) Mineralogical composition.

cm). Increases in airline traffic and capacities also

b) Size and gradation of the individual particles.

compounded the need for improved compaction

c) Shape of the individual particles.

procedures and stricter control.

d) Void ratio or dry density.

e) Confining pressure.

The shearing strength of a compacted cohesive soil is

primarily affected by:

a) water content.

b) gradation.

c) dry density.

d) soil structure.

MOISTURE CONTENT:

The most significant factor influencing the compaction of a

soil Is its moisture content. Optimum moisture content may

range any- where from 4% for rock fill to 24% for a clay,

depending upon soil type and specifications to be met. The

moisture requirement is reduced for the higher density

requirement of the Modified Proctor as compared to the

Standard Proctor.

MOISTURE CONTENT vs SOIL TYPE

There are three major purposes of compaction:

a) To Increase the potential strength and bearing

Sllty

capacity of the base. Studies have shown that the most

Sand

Important stress Induced by a load on a pavement

Sllty

and

foundation Is the shear, and not the compressive stress,

Clay

Sllt

Clay

Gravel

b) To Increase the volumetric stability and avoid

Standard Proctor

17%

14%

12%

10%

settling or localized compaction by future loads,

Modified Proctor

14%

11%

c) To minimize the permeability and prevent the

deterioration of strength due to Infiltration and saturation

Optimum moisture content Is that moisture at which the

of surface waters.

greatest density of a soil can be obtained through

Compactability depends upon:

compaction. A soil's optimum moisture content should be

a) The soil-preparation, type and moisture content.

determined by soil laboratory test using the selected Stan-

b) Compaction equipment-type and size.

dard or Modified Proctor test procedures. To produce best

c) Number and speed of passes.

field compaction results, experience has proven that when

d) Lift thickness.

Standard Proctor densities are to be achieved field

e) Foundation characteristics.

moistures should be at or slightly above optimum. Likewise

best results when compacting to Modified Proctor densities

may be achieved at or slightly below optimum. Care must

be exercised in controlling moisture, as too high a moisture

content will prevent

K300

Rev. 760501

300-1.1