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CHAPTER V

Mechanical Modification

Methods of Compaction

• Laboratory Procedure

• Shallow Surface Compaction

• Deep Compaction Techniques

• Hydromechanical Compaction

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SOIL COMPACTIONIt is the most obvious and simple way of

increasing the stability and supporting capacity of soil (improving the physical properties of soil)

Soil compaction is the process of increasing the unit weight of soil by forcing solids into a tighter state and reducing air.

This is accomplished by applying static or dynamic loads to the soil.

Soil densification include compaction andconsolidation.

Compaction is one kind of densification that isrealized by rearrangement of soil particles

without outflow of water.

Soil is compacted by removing air and waterfrom its pore space. There exists a certain amount of moisture that a soil can have where a maximum unit weight is obtained.

It does not involve fluid flow, but with moisture changing.

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Consolidation is another kind of densification with fluid flow away.

Consolidation is primarily for clayeysoils.

Water is squeezed out from its pores under load.

Types of Action

* Ramming

* Vibration

* Static Rollers

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RammingRammers have a low frequency (800blows/minute), and a higher stroke (1.5" to 3.5“ height).

As the machine operates the ramming action breaks down the soil, pushes the particles closer together, and forces air

out of the voids.

Vibration

Vibratory machines are distinguished bytheir high frequency (2,000 to 6,000blows/minute) and a low amplitude.

The vibratory action sets the soil particles in motion, which rearranges them into adenser packing.

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Compaction improves characteristics of soils:

1- Increases Strength2- Decreases permeability3- Reduces settlement of foundation4- Increases slope stability of

embankments

Soil Compaction can be achieved either by static or dynamic loading

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Laboratory ProcedureGenerally before any soil compaction occurs,

various samples of the proposed embankment or fill are analyzed in the Lab.

One such lab test is Proctor compaction test.

The Proctor compaction test is a test that compacts the soil material at various moisture contents.

There are two Proctor tests that are defined by the American Association of Highway Officials and American Standard Testing Material:

• The Standard Proctor test (ASTM D-698, AASHTO T-99), and

• The Modified Proctor test (ASTM D-1557 and AASHTO T-180).

Laboratory Proctor Compaction Test

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The Standard Proctor Test is generally used for fills requiring the minimal sub-grade compaction, such as

• small parking lots, and • building structures.

While the Modified Proctor is generally used for fills that will support large loads, such as:

• Roadways, • Airport Runways, and • Concrete Parking Aprons.

Relative Compaction or Degree of Compaction

Relative Compaction= ρd(field)/ρd(max)lab

ρd(field): Dry density achieved in the field.

ρd(max)lab: Max. dry density (from a Proctor compaction test)

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Generally a contractor is given a percentage of optimum compaction that must be attained in the field.

This is referred to as the Relative Compaction, R(%) or Degree of Compaction.

Relative compaction generally range from 90% -95% as compared to the modified proctor

and 95% - 100% as compared to the standard Proctor.

Generally granular fills are required to be compacted to 95% - 100% in the field.

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NUCLEAR DENSITY TEST

It consists of a special device that send outgamma rays and detects how they travelthrough the soil.

The amount of gamma rays received back into the device correlates with the unit weight of the soil.

It also measures the moisture content of the soil in a similar way using alpha particles.

Shallow Surface Compaction

Shallow compaction in the field is accomplished by rolling or vibrating.

Field compaction of soils is mainly done with various types of rollers.

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The four most common types of rollers are:

1. Sheepsfoot rollers, used mainly for Clayey and Silty Soils.

2. Smooth-drum rollers, Used primarily for Granular Soils.

3. Vibratory Rollers, Used primarily for Granular Soils.

4. Pneumatic Rubber-tired rollers for clay soil.

Sheepsfoot Roller

This roller compacts all fine-grained materials, including materials that will break down or degrade under the roller feet,

but it will not compact cohesionless granular materials.

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The number of passes necessary for this type of roller to obtain the required densities must be determined for each type of soil encountered.

The roller compacts from the bottom up and is used especially for plastic materials.

The lift thickness for sheepsfoot rollers is limited to 6 inches (15cm) in compacted depth.

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Steel-Wheeled Roller

Although extensively used, it is normally operated in conjunction with one of the other three types of compaction rollers.

It is used for compacting granular materials in thin lifts.

Probably its most effective use in subgrade work is in the final finish of a surface,

following immediately behind the blade, forming a dense and watertight surface.

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Steel-Wheeled Roller

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Rubber Tired Rollers

Tamping-Foot RollerA tamping-foot roller is a modification of the sheepsfoot roller. The tamping feet are trapezoidal pads attached to a drum.

Tamping-foot rollers are normally self-propelled,

and the drum may be capable of vibrating.

The tamping-foot roller is suitable for use with a wide range of soil types.

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TampingTamping--Foot RollerFoot Roller

Pneumatic rubber-tired rollerFor compacting clay soil.

Clays are more difficult to compact than sands and gravels, because they must be brought to the right range of water content before they can be compacted to high densities.

Static pressure, as exerted by the wheels of this rubber-tired roller, compacts clays well.

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Pneumatic rubber-tired roller

Vibratory Plates are designed for consistent compaction performance.

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Rammers

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SOME SIMPLE RULES ON COMPACTION AND CONSOLIDATION

If your fill is a cohesive / clay soil, it must NOT be too wet, and it must NOT be too dry, a reasonable "rule of thumb" for cohesive (clay) soils is the moisture content range +/- 2% of the Plastic Limit.

Lay soil fills in thin layers, maximum 200mm, compact each fully before placing next layer.

Granular fills must be well graded, particle size well distributed through the range of the material.

Not single sized particles.

Consult appropriate specification to choose compaction plant and numbers of passes / blows.

Ensure correct compaction plant is on site.

When compacting bituminous materials ensure they are above minimum rolling temperatures.

Maximum thickness of 40mm nominal size bituminous roadbase is 150mm, (with a BIG roller).

Do not trust any testing apparatus that requires calibration, satisfy yourself it is working correctly by using other testing methods alongside the instruments that need calibration.

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