Compaction

Principles of Compaction

uncompacted compacted uncompacted compacted

Compaction of soils is achieved by reducing the volume of voids. It is assumed that the compaction process does not decrease the

volume of the solids or soil grains·

14

Water Content

Dry Density

Effect of Energy on Soil Compaction

Higher

Energy

Increasing compaction energy Lower MC and higher dry density

In the field

increasing compaction energy =

increasing number of passes or

reducing lift depth

In the lab

increasing compaction energy

= increasing number of blows

Structure of Compacted Clay Dry

Dens

ity

Water Content

Flocculated Structure or

Random

Dispersed Structure or

parallel

Intermediate structure

Lambe and Whitman, 1979

Particle Arrangement Dry side more random

Water DeficiencyDry side more deficient; thus imbibes more water,

swells more, has lower pore pressure

Structure

Wet Side

Dry Side

Effect of Swelling

Dry

Dens

ity (d)

Water Content (w)

OMC

Holtz and Kovacs, 1981

Higher

Swelling

Potential

Higher

Shrinkage

Potential

• Swelling of compacted clays is greater for those compacted dry of optimum. They have a relatively greater deficiency of water and therefore have a greater tendency to adsorb water and thus swell more.

Wet Side

Dry Side

Compaction Results-Explanation

Dry

Dens

ity (d)

Water Content (w)

OMC

Below womc

Dry of Optimum

•As the water content increases, the particles develop larger and larger water films around them, which tend to “lubricate” the particles and make them easier to be moved about and reoriented into a denser configuration.

Hammer Impact

•Air expelled from the soil upon impact in quantities larger than the volume of water added.

At womc

The density is at the maximum, and it does not

increase any further.

Above womc

Wet of Optimum

Water starts to replace soil particles in the mold, and since

w<<s the dry density starts to decrease. Hammer Impact

Moisture cannot escape under impact of the hammer. Instead, the entrapped air is energized and lifts the soil in the region around the hammer.

Wet side

Entrapped

air

Dry side

Escaping air

Holtz and Kovacs, 1981

Engineering Properties Summary

Dry side Wet side

Permeability

Compressibility

Swelling

Strength

Structure More random More oriented (parallel)

More permeable

More compressible in high pressure

range

More compressible in low pressure

range

Swell more, higher water deficiency

Higher

*Shrinkage more

Properties

Zero Air Voids (ZAV) Curve

“the maximum theoretical density of a soil at various water contents” with no air left in the soil water mixture

2.0

1.9

1.8

1.7

1.6 0 5 10 15 20 25

Water content w (%)

Dry

dens

ity

( kg

/cum

)

Line of

optimums

"Zero Air

Voids"

ZAV:The curve represents the fully saturated condition (S=100%).

ZAV cannot be reached by compaction.

Line of Optimum: A line drawn through the peak points of several compaction curves at different compactive efforts for the same soil will be almost parallel to a 100 % S curve

Entrapped Air: is the distance between the wet side of the compaction curve and the line of 100% saturation.

Zero-Air-Void

Points from the ZAV curve can be calculated from:

Holtz and Kovacs, 1981

Degree of Saturation Degree of Saturation

Standard Proctor

Modified Proctor

100% 60% 80%

sewGs but

wmoist

d

1

Ꝩd = GꝨw / 1+ e

Typical Compaction Curve for Cohesionless Sands & Sandy Gravel

Air dry

Complete saturation

(increasing) Water content

(inc

reasing

) Dens

ity

bulking The low density that is obtained at low water content is due to capillary Forces resisting arrangements of the sand grains.

Example

Example 3 (Cont.)

Field Compaction

Most of the compaction in the field is done by means of

ROLLERS.

The most common types are:

1. Smooth-wheel rollers (smooth-drum roller)

This type of roller consists of a large steel drum in front and one or two wheels or drum on the rear end. Depending upon the number of wheels on the rear, it can be of following two types: Tandem rollers (having one wheel at rear and one wheel in front) weight of tandem roller varies from 2 to 8 tonnes Three wheeled rollers (having two wheel at rear and one in front) weight 8 to 10 tonnes. The weight of the roller can be increased by filling the inside space of the drum with water or wet sand. This is called ballasting. Uses : Used for roadwork or pavement construction work. for compacting sand, gravels Not suitable for cohesive soils like clay

2. Pneumatic rubber-tired rollers

rubber-tyred wheels.

•Wt = 12-40 tonnes.

•Suitable for: coarse and fine soils.

Has several rows of four to six closely spaced tyres.

3. Sheepsfoot rollers Sheepsfoot rollers consist of a steel drum on which round or rectangular protrusions known as ‘lugs’ or ‘feet’ are fixed. The drum’s weight can be increased by ballasting with water, damp sand, They are commonly used for compacting fine-grained soils such as silty clays. They are often used for compacting soils in earth dams, embankments and subgrade layers (soil) in pavements, road and railway projects

4. Vibratory rollers Vibratory roller consist two smooth drums with the vibrator. One is fixed at front and other one on rear side of vibratory roller. Vibrations are generated by the rotation of an eccentric shaft inside. Used for compacting granular base courses (layer of pavements in highway works)

5. Rammers

Rammer compactor is used for compacting small area and areas which are not accessible. This equipment is light weight and can be hand or machine operated. Base size of rammer 15cm x 15 cm or 20cm x 20cm https://www.youtube.com/watch?v=85R5AUQgWWk

Field compaction control It is process of checking the density and moisture content during compaction by rollers

Which are compaction specifications? Compaction specifications are (The parameters used to ensure effective compaction in the field include)

(a) relative compaction and (a) placement water content.

1) How much should be relative compaction? 2) How much should be placement water content

(a)Relative Compaction:

cohesive soils

• Relative compaction of 95% of laboratory standard Proctor test can be achieved using either sheep’s foot rollers or pneumatic tired rollers.

cohesionless soils

• Relative compaction of the order of 98%-100% or even more of modified Proctor test can be achieved using vibratory rollers or pneumatic tired rollers.

Placement water content less than OMC (dry of optimum) may be specified for highway embankments of cohesive soils to achieve higher shear strength(magnitude of the shear stress that a soil can sustain) and lower compressibility. (capability of a soil to decrease in volume when subjected to a mechanical load.) High shear strength

Low compressibility

Similarly, outer shells of earth dams are compacted at a placement water content dry of optimum (less than OMC) to achieve higher shear strength, higher permeability, and lower pore pressures.

High shear strength

High permeability

Low pore pressure

• Compaction of subgrades below pavements and foundation soils below floors may be done at a placement water content more than the OMC (wet of optimum) to prevent excessive swelling

• Similarly, the core (impervious wall in the dam) of an earth dam is compacted at a placement water content more than the OMC (wet of optimum) to reduce the permeability and swelling.

Less permeability

Soil permeability is the property of the soil to transmit water and air

Swelling : soil swell in volume when subjected to moisture.

How to Control Compaction of Soil in field? Compaction control is done by measuring the dry density and the water content of compacted soil in the field Dry density The dry density is measured by core cutter method and sand replacement method Water content For the measurement of water content, oven drying method, sand bath method, calcium carbide method etc are used. Proctor needle is also used for this.

Proctor needle

Proctor needle test is completed in two parts: (i) Plotting of a calibration curve in the laboratory

Plotting of calibration curve: 1. Compact soil at given moisture content in standard

proctor mould in the laboratory

2. Force a suitable proctor needle in the compacted soil at the rate of 12.5 mm per second to a depth not less than 75 mm. 3. Read the penetration from the calibrated stem and calculate penetration resistance per unit area by dividing the area of the needle point. 4. Procedure is repeated with different moisture content. 5. Plot a calibration curve between penetration resistance and moisture content as shown in figure

calibration chart

(ii) Determining the penetration resistance of soil in the field.

Measure penetration resistance of compacted soil in field For this penetration resistance read water content from the calibration chart

https://www.slideshare.net/DrAbdulmannanOrabi/lecture-5-soil-compaction-67797618