Soil Compaction

CompactionCompaction

• Compaction is the method of mechanically increasing the density of soil by removal of air.

• Dry density (d) is the measure of the degree of compaction.

Why Compaction?Why Compaction?

• Increase bearing capacity

• Increase stability of slopes of embankments

• Reduce compressibility

• Reduce permeability

• Reduce volume changes

• Prevent frost damage

Laboratory Compaction TestsLaboratory Compaction Tests

Why Test?   

Provide moisture density curve identifying optimum moisture.

Compare the degree of compaction vs. specs (Relative Compaction, R.C.).

Compaction Laboratory Tests Compaction Laboratory Tests

• Standard Proctor Test

• Modified Proctor Test

Standard Proctor TestStandard Proctor Test

wt

d

1

V

Wt sr GweS

V = 1/30 ft3

(944 cc)

s

w

W

Ww

Standard Vs. Modified Proctor CompactionStandard Vs. Modified Proctor Compaction

Standard ProctorModified Proctor

Zero Air Voids Line

S = 100%

Dry side of optimum

wopt

Wet side of optimum

Moisture-Density CurveMoisture-Density Curve

Test 1 2 3 4

w

t

d

ZAV

wt

d

1

V

Wt

s

ws

r

s

wswsZAV wG

G

S

wGG

e

G

111

Allowable MoistureAllowable Moisture

Allowable Moisture = (OMC – 3%) to (OMC + 2%)

Factors affecting CompactionFactors affecting Compaction

• Compactive Effort

• Moisture Content

• Soil Type

Compactive EffortCompactive Effort

Zero air void

Water content w (%)

Dry

den

sity

d (

Mg/

m3 )

Dry

den

sity

d (

lb/f

t3 )

Line of optimums

Modified Proctor

Standard Proctor

Holtz and Kovacs, 1981

d max

wopt

Soil TypeSoil Type

• Grain size distribution.

• Shape of soil grains.

• Specific gravity of soil solids.

• Amount and type of clay minerals.

Soil Type (cont’d)Soil Type (cont’d)

Zero air voids line

Sand, some fines

Clay

Dry Density

Moisture content

d max

OMCConstant compaction energyConstant compaction energy

Check Point MethodCheck Point Method

Check Point Method

Water content w %

wopt

Dry

den

sity

, d

d max

100% saturationLine of optimums

A

B

M

C

X

Y (No)

•1 point Proctor test

•Known compaction curves A, B, C

•Field check point X (it should be on the dry side of optimum)

Holtz and Kovacs, 1981

X

M

Relative CompactionRelative Compaction

Lab

Field

dmax

dmax

γ

γ100R.C.

rDCR .20.080..

Correlation between relative compaction (R.C.) and the relative density Dr

Typical required R.C. = 90% ~ 100%

mind,maxd,d

mind,dmaxd,

minmax

maxr ee

eeD

Field Compaction

Elephant and CompactionElephant and Compaction

Heavy Weight

Question?

The compaction result is not good. Why?

He He! I’m smart.

Types of Compaction Types of Compaction

• Vibration

• Impact

• Kneading

• Pressure

Static or Vibratory 

Compaction Equipments and Techniques Compaction Equipments and Techniques

• Smooth Wheel rollers

• Sheepsfoot Rollers

• Pad Rollers

• Pneumatic (Rubber-Tired) Rollers

• Grid Rollers

• Vibratory Rollers

Smooth-wheel roller (drum)Smooth-wheel roller (drum)

• 100% coverage under the wheel

• All soil types except for rocky soils.

• Contact pressure up to 380 kPa

• Compactive effort: static weight

• Most common use is for proof-rolling subgrades and compacting asphalt pavement.

Holtz and Kovacs, 1981

Sheepsfoot RollersSheepsfoot Rollers

• 8% ~ 12 % coverage

• Best for clayey soils.

• Contact pressure from 1400 to 7000 kPa

• Compactive effort: static weight and kneading.

Pad Roller

• About 40% coverage

• Best for compacting fine-grained soils (silt and clay).

• Contact pressure is from 1400 to 8400 kPa

• Compactive effort: static weight and kneading.

Pneumatic Rollers Pneumatic Rollers

•80% coverage under the wheel.

•Bet for Granular and fine-grained soils.

•Contact pressure up to 700 kPa.

•Compactive effort: static weight and kneading.

Grid RollersGrid Rollers

• About 40% coverage

• Contact pressure is from 1400 to 8400 kPa

• Best for compacting fine-grained soils (silt and clay).

• Compactive effort: static weight and kneading.

Vibratory CompactorsVibratory Compactors

•Compactive effort: static weight and vibration.

•Suitable for granular soils

Compaction Type Vs. Soil TypeCompaction Type Vs. Soil Type

Materials

 

Vibrating SheepsfootRollers

Static SheepsfootGrid RollerScraper

Vibrating Plate CompactorVibrating RollerVibrating Sheepsfoot

ScraperRubber-tired RollerLoaderGrid Roller

 

Lift Thickn

essImpact

Pressure(with

kneading)Vibration

Kneading(with pressure)

Gravel 12+ Poor No Good Very Good

Sand 10+/- Poor No Excellent Good

Silt 6+/- Good Good Poor Excellent

Clay 6+/- Excellent Very Good No Good

 

Compaction Difficulty Vs. Soil TypeCompaction Difficulty Vs. Soil Type

Fill Materials

  PermeabilityFoundation

SupportPavement

SubgradeExpansive

CompactionDifficulty

Gravel Very High Excellent Excellent No Very Easy

Sand Medium Good Good No Easy

Silt Medium Low Poor Poor Some Some

Clay None+ Moderate Poor Difficult Very Difficult

Organic Low Very Poor Not Acceptable Some Very Difficult

Field DensityField Density

• Destructive Testing: Sand Cone Core Cutter Rubber Balloon

• Nondestructive Testing: Nuclear Density

Sand Cone TestSand Cone Test

W1= mass of sand cone before testW2 = mass of sand cone after test W3 = mass of sand filling cone and hole (W3 = W1-W2)W4 = mass of sand filling the cone = sand*Vcone W5 = mass of sand filling the hole = W3-W4

Vhole = W5 / sand

W6 = mass of soil extracted from the whole w = moisture content of soil

W1

W2

Standard Sand with known Gs

W4

W5

wt

d

1

holeV

Wt

6

s

w

W

Ww

Sand Cone Test ProcedureSand Cone Test Procedure

• A small hole (6" x 6" deep) is dug in the compacted material to be tested. 

• The soil is removed and weighed, then dried and weighed again to determine its moisture content. 

• The specific volume of the hole is determined by filling it with calibrated dry sand from a jar and cone device. 

• The dry weight of the soil removed is divided by the volume of sand needed to fill the hole. 

• This gives the density of the compacted soil. 

 

Core CutterCore Cutter

Static Load

4 in

5 in

Suitable for cohesive soils only

wt

d

1

V

Wt

Rubber BalloonRubber Balloon

Nuclear DensityNuclear Density

• Nuclear Density meters are a quick and fairly accurate way of determining density and moisture content. 

• The meter uses a radioactive isotope source (Cesium 137) at the soil surface (backscatter) or from a probe placed into the soil (direct transmission). 

• The isotope source gives off photons (usually Gamma rays) which radiate back to the mater's detectors on the bottom of the unit. 

• Dense soil absorbs more radiation than loose soil and the readings reflect overall density. 

• Water content can also be read, all within a few minutes. 

Nuclear DensityNuclear Density