soil comp action

8/2/2019 Soil Comp Action

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Geotechnical & Geoenvironmental Engineering CIVL4121 Part 2: Soil Compaction

CIVL 4121: Compaction: 1

GEOTECHNICAL

&

GEOENVIRONMENTAL

ENGINEERING

CIVL 4121

Part 2:

Soil CompactionMartin Fahey

School of Civil and Resource Engineering


What is Soil Compaction?

Compaction is the densification of soils by the application of

mechanical energy to reduce air void spaces in the three

phase soil model

it reduces the air content, but not the water content

cant compact saturated soil (almost always true)

Compaction refers to the mechanical bashing together of

unsaturated soil to form a denser soil

Do not confuse soil compaction with consolidation (long term

reduction of void ratio of a given soil).

Consolidation refers to slow squeezing out water from a saturated

soil, by application of a static load

Principal difference:

Compaction is direct & immediate

Consolidation is a time-dependent process


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Where/when is Compaction Used

Any time soil is used as a construction material, it is

compacted to improve its engineering properties:

compaction of sand pad for house foundations

compaction of soil/gravel/crushed rock/asphalt in road construction

compaction of soil in earth dams

compaction of soil behind retaining walls

compaction of soil backfill in trenches

In this unit, compaction will be referred to in:

pavement construction

dam construction

construction of clay liners for waste storage areas

construction of tailings dams

ground improvement


What does compaction achieve

At most basic level, compaction increases the dry unit weight

For soil containing (clayey) fines, well-compacted soil has

high negative pore pressures (suctions)

high effective stress, even when at ground surface

high strength

Good compaction results in:

higher stiffness (less compressible = less settlement)

higher strength = higher bearing capacity

reduced permeability (more later...)


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Type of Soil and Compaction Equipment

The desired level of compaction is best achieved by matchingthe soil type and the compaction method. Other factors mustbe considered as well, such as compaction specifications and

job site conditions

Clayey soils:

At the water content required for construction, clayey soil tends to bein the form of semi-dry hard clods

These need to be broken up (kneaded) to force the soil into a denserpacking (otherwise the compacted soil will still consist of clods withlarge voids between them)

The kneading action of a sheepsfoot roller (combined with

vibration) is the best means of doing thisGranular soils: The particles require a shaking or vibratory

action to move them; vibrating rollers (or vibratory platecompactors for small scale) are usually the best choice


Types of Compaction

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

Pressure alone (from the weight of the roller)

Vibration (+ pressure)

Kneading working the soil to break up lumps

Impact

Wide variety of field compaction equipment, so correct

choice of equipment (or mix of equipment) is vital forachieving the required result at the best possible cost (=usually in the minimum possible time)

Smooth-wheeled steel drum rollers

Pneumatic tyred rollers

Sheepsfoot rollers

Impact rollers

Vibrating rollers

Hand-operated vibrating plate and rammer compactors


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Smooth-Wheeled Steel Drum Rollers

Self-propelled or towed steel rollers ranging from 2 - 20

tonnes

Suitable for: well-graded sands and gravels; silts and clays of

low plasticity

Unsuitable for: uniform sands; silty sands; soft clays


Pneumatic-tyred Rollers

Usually a container on two axles, with rubber-tyred wheels.

Wheels aligned to give a full-width rolled track.

Dead load (water) is added to give masses of 12-40 tonnes.

Suitable for: most coarse and fine soils.

Unsuitable for: very soft clay; highly variable soils.


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Sheepsfoot Roller ('tamping roller' ; pad-foot roller)

Self propelled or towed units, with drum fitted with

projecting club-shaped feet high contact stress,

kneading action (and sometimes vibrating as well)

Mass range from 5-8 tonnes

Suitable for: fine grained soils; sands and gravels, with

>20% fines; good for breaking down soil clods

Unsuitable for: very coarse soils; uniform gravels


Impact Roller

Compaction by static pressure, combined with the impact of

the 5-sided roller

Higher impact energy breaks up soil clods, achieving better

compaction (like a sheeps-foot roller in some ways)


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Impact Roller,

Mandurah area

Effective to 2-4 m depth

(?) in Perth

Three-sided version


Vibrating Drum Roller

Vibratory compactor: Fitting a vibrating mechanism to a

drum (or sheepsfoot) roller can increase its efficiency for

many soils. It also levels and smoothens any rutting that may

have been caused by tyre-roller.

Sheepsfoot roller may also have vibration mechanism


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Vibration Mechanisms

Same direction of rotation

gives forward-backward

vibration (as well as

vertical) discomfort to

operator? Counter-rotating masses

vertical vibration only

Vibrating mechanisms consist of internal rotating eccentric

masses typically rotating at up to 30 Hz


Plate and Rammer Compactors

Vibrating plate compactors

used for smaller confined areas

common in house construction in Perth sand

Rammer compactors used for backfilling (trenches)


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Factors Affecting Field Compaction (Soils)

Soil type: Grain size distribution, shape of the particles,

specific, gravity, quantity of clay in the soil

Water content (CRUCIAL)!

Compaction Effort: Controlled by the type of the equipment,

thickness of the lift, and properties of the soil or mix.

Layer (Lift) thickness: For soil, the thinner the layer is the

better compaction, but more costly.

Number of passes of the equipment and its speed: For soil,

more passes lead to better compaction results

Mix properties: Aggregate gradation, surface texture, and

angularity of the particle surfaces.

Environmental Effects (for asphalt): Air temperature,

humidity, wind, temperature of the surface under the mix


LABORATORY COMPACTION

Aim of laboratory compaction:

Simulate field procedures, aid in

the control of placement

conditions.

Two common types of test:

Standard compaction test, steel

rammer dropped on loose soilplaced in a mold

Modified compaction test

similar, but heavier rammer, and

more layers used

AS 1289 5.1.1 & 5.2.1 1993


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Standard (or Proctor) Compaction

Mould is 105 mm diam. x 115.5 mm high (1 litre) & removable collar

Hammer is 2.7 kg, drop height 300 mm

Soil placed in mould in 3 layers, each compacted using 25 blows

Total energy delivered = 596 kN.m/m3

Layers judged so that at the end of compaction, soil is just above the top

of the lower mould

Remove collar

Strike off excess

Weigh mould

determine wet density

get water content get dry density

Repeat at different water contents

Plot dry density versus water content


Modified Compaction

Standard compaction test too light to represent modern

field compaction equipment (Standard test is from 1930s)

Modified compaction test uses:

heavier hammer (4.9 kg)

greater drop height (450 mm)

same mould (1 litre)

5 layers

25 blows per layer

Energy = 2703 kN.m/m3

Standard = 596 kN.m/m3

4.5 times more energy

Otherwise, procedure is the

sameVarious hammers

Automatic compaction machine


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Maximum Possible Compaction

Compaction involvesdriving out the air

Curves showsmaximum possible dryunit weight for givenwater content fordegree of saturation(Sr) = 100%, 95%,90% and 85%

These represent

maximum possibledensity for zero airvoids (ZAV), and airvoids (A) of 5%, 10%and 15%

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0 5 10 15 20

Water Content w (%)

DryDensity

d

(t/m3)

Sr = 1

Sr = 0.95

Sr = 0.9Sr = 0.85

For G s = 2.65

( )( ) sssr srd G.wA1GA1

G.wS

GS

+=+=


Actual Compaction Curve (Example)

For a given compactionenergy, curve achievedshows:

a maximum dry densityd max (MDD) correspondingto an optimum moisturecontent (OMC)

near saturation on the wetside of OMC(Sr = 95% in this case)

low degrees of saturation onthe dry side of OMC

(dry unit weight d ratherthan dry density d can beplotted)

1.4

1.5

1.6

1.7

1.8

1.9

2

5 10 15 20

Water Content w (%)

DryDensity

d

(t/m3)

For Gs = 2.65

Air voids (%) 15 10 5 0

ZAV

MDD = 1.87 t/m3

O

MC=14.4%

Optimum Moisture Content (OMC): The moisture content at which the

maximum possible dry density is achievedfor a particular compaction energy

or compaction method


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Reasons for the Shape of the Curve

On the dry side of OMC, clayey soil shows high suction, hard

strong lumps, difficult to break down = difficult to compact

Increasing the water content reduces the suction, softens the

lumps, lubricates the grains = easier to compact

As water content increases, higher dry densities result, until

we start approaching full saturation (say at Sr = 90-95 %)

Now nearly impossible to drive out the last of the air

further increase in water content results in reduced dry

density (curve follows down parallel to the maximum

possible density curve the Zero Air Voids curve) (Note, values of MDD and OMC depend on the compaction

energy they are not unique soil properties)

For sand, suction at low water contents also prevents

compaction (but not if completely dry)


Cohesionless soil (clean sand)

Cohesionless soils d max achieved either completely dry, orcompletely saturated

at low water content, grains held together by suction (water at grain

contacts only)

this prevents compaction

Laboratory test for d max for sand requires fully saturatedsample, and involves vibration saturated sand in mould

weight on top (=5 kPa)

vibrate for certain time

measure d max


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MDD and OMC depend on input energy

As compaction energy increases, MDD (d max) increases andOMC reduces (curves constrained by ZAV line: parallel to

ZAV line)

Modified

Standard


Compaction affects soil structure

Soil tends to be more flocculated on the dry side; more

dispersed on the wet side

A: flocculated; C: dispersed

More compactive effort tends to disperse the soil

E more dispersed than A

It is these different structures,

in conjunction with the different

dry densities, that give different

properties at different points

on the compaction diagram

Soil structure as defined here

is referred to as soil fabric


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Compaction and Permeability

Lowest permeability for clayeysoils compacted wet of OMC

Where permeability (rather thanstiffness or strength) isimportant, could be best tocompact wet of OMC

More likely to undergo shrinkageif allowed to dry = crackingpossible, leading to grossreduction in overall permeability

Balance between lowpermeability and avoidance ofshrinkage cracking is aprimary concern in dam (core)construction, and in clay linersfor waste disposal areas


Suctions

Compacted clay samples shownegative pore pressure (suction)

depends on type of compaction &moulding water content

one of the contributing factors to soilstrength and stiffness (suction =effective stress = strength)

will see later that keeping water awayfrom compacted subgrade is importantfactor in life of pavement

Amount of total shrinkage oncomplete drying varies withmoulding water content and type ofcompaction

has implications for cracking of damcores and clay liners for waste storageareas

1 psi ~7 kPa


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Range of OMC & MDD for various soils

Different soils show different

compaction curves, even for

the same compaction energy

Slight changes in soil from a

borrow area can change the

compaction characteristics

Frequent checking of the

compaction curve essential, to

ensure that the correct target

values are being used


Shear strength of compacted samples

For same compaction energy, dry samples are much stronger

than wet samples, even though dry density may be less

Dry samples more brittle

if deformation crackingWet samples ductile

may be an advantage

can tolerate movement

integrity of dam cores, liners

MF


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Slide 28

MF1 Martin Fahey, 6/07/2006


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Achieving the desired outcome

In many cases (e.g. road construction), aim is to get densest possible state

aim for high % of Modified MDD, and close to OMC (e.g. +0 2%)

In other cases, aim might be to have lowest permeability

compact wet of OMC, perhaps accepting lower density

BUT

soil compacted wet of OMC may undergo excessive shrinkage if allowed to

dry (cracking of dam cores, clay liners

high overall permeability, even if intact permeability is low soil compacted very dry of OMC is strong, but brittle

soil compacted wet of OMC is ductile can accommodate larger

deformations without cracking

Crucial to consider not just properties as compacted, but potentialchanges in properties with time due to exposure to drying, water, etc.

CHOOSING THE CORRECT COMPACTION STRATEGY

REQUIRES CAREFUL CONSIDERATION OF ALL THESE ISSUES


Drying Back

Common technique used in road construction in WA

Compact at close to OMC, to close to MDD

Leave exposed to drying

for a period (weeks)

reduces water content,

but not density

may even increase density

achieves much stiffer,

much stronger result


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Suitability of

Soils to

Compaction

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

Rolled EarthFill Dams

CanalSections

Foundations Roadways

GroupSymbol

* if gravelly** erosion critical*** volume change critical

- not appropriate for thistype of use

Soil Type

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

- - 1 1 - - 1 1 1 3

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

- - 2 2 - - 3 3 3 -

GMSilty gravels, poorly-gradedgravel/sand/silt mixtures

2 4 - 4 4 1 4 4 9 5

GCClay-like gravels, poorlygraded gravel/sand/claymixtures

1 1 - 3 1 2 6 5 5 1

SWWell-graded sands, gravellysands, little or no fines

- - 3* 6 - - 2 2 2 4

SPPoorly-graded sands, gravellysands, little or no fines

- - 4* 7* - - 5 6 4 -

SMSilty sands, poorly-gradedsand/ silt mixtures

4 5 - 8* 5** 3 7 6 10 6

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

3 2 - 5 2 4 8 7 6 2

ML

Inorganic silts and very finesands, rock flour, silty or clay-like fine sands with slightplasticity

6 6 - - 6** 6 9 10 11 -

CL

Inorganic clays of low tomediumplasticity, gravelly clays,sandyclays, silty clays, lean clays

5 3 - 9 3 5 10 9 7 7

OLOrganic silts and organic silt-clays of low plasticity

8 8 - - 7** 7 11 11 12 -

MNOrganic silts, micaceous ordiatomaceous fine sandy orsilty soils, elastic silts

9 9 - - - 8 12 12 13 -

CHInorganic clays of highplasticity, fat clays

7 7 - 10 8** 9 13 13 8 -

OHOrganic clays of medium highplasticity

10 10 - - - 10 14 14 14 -


1.4

1.5

1.6

1.7

1.8

1.9

2

5 10 15 20

Water Content w (%)

DryDen

sity

d

(t/m3)

For Gs = 2.65

ZAV

95% MDD

OMC + 1%OMC - 1%

MDD

OMC

Meets specification

Control/Monitoring of Field Compaction

Field compaction is normallyspecified in terms of themaximum dry density obtainedfrom the laboratory

Example: Must achieve 95% ofMDD, and OMC 1%

In the diagram, tests fallinginto yellow zone meet these two

requirements often, only minimum density

ratio specified (e.g. 95%MDD)

Adequate compaction requirescompaction in layers (generally


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Sand Replacement Method Find sand loose density (lab.) - min Known weight of sand in bottle

Calibrate how much left after opening

cone on level surface (A)

Dig hole collect, weigh and dry the soil

removed (B)

Fill hole with sand weigh what is left in

bottle (C)

Slow (costly), accurate

A

B

C


Coring

Drive coring tube into ground surface using special hammer

and protective collar

Dig out the coring tube, trim the ends, weigh the contents

Obtain water content

work out the dry density

Used also for obtaining samples fordetermining the in situ CBR value

(California Bearing Ratio)

discuss later in the pavements section

100 mm

130mm

Driving collar (dolly)


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Nuclear Density Meter

Measures absorption of radiation (function of density)

Fast, accurate (after calibration)

Most common method in WA


Control of Field CompactionField Density Testing Method

Sand ConeBalloon Dens

meterShelby Tube Nuclear Gauge

Advantages* Large sample* Accurate

* Large sample* Direct readingobtained* Open graded material

* Fast* Deep sample* Under pipe haunches

* Fast* Easy to redo* More tests (statisticalreliability)

Disadvantages

* Many steps* Large area required* Slow* Halt Equipment* Tempting to accept flukes

* Slow* Balloon breakage* Awkward

* Small Sample* No gravel* Sample not alwaysretained

* No sample* Radiation* Moisture suspect* Encourages amateurs

Errors

* Void under plate* Sand bulking* Sand compacted* Soil pumping

* Surface not level* Soil pumping* Void under plate

* Overdrive* Rocks in path* Plastic soil

* Miscalibrated* Rocks in path* Surface prep required* Backscatter

Cost * Low * Moderate * Low * High


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Perth Sand Penetrometer

Developed at UWA (Glick & Clegg)

from Scala Penetrometer- similar but has cone at the

tip (used for CBR testing see later in pavements)

Widely used in Perth for compaction control in sand

house sand pads (every two-bit contractor has one)

backfilling of trenches

N = number of blows for penetration from 150 mm to

450 mm (penetration of 300 mm)

Correlations between N and percentage MDD (Glick

and Clegg, UWA)

for house pads, typically N 7 or 8 Can fit extension rods (up to 3 m) -much abused test

overburden stress increases the N value without any

increase in density

therefore, N = 8 at 3 m depth indicates a much lower

density than N = 8 at surface

6 kg sliding mass,

drop onto anvil

600 mm free

drop height

Count

number of

blows (N)

for 300 mm

penetration

Ignore first

150 mm of

penetration

Fixed anvil

Top stop

16 mm diameter bar

50 mm graduations


Intelligent Compaction

Based on vibrating drum roller being equipped with

accelerometers, to measure the ground response to

the vibrations. Not yet widely used in Australia, but

will be much more so in the future.

The principle is that the accelerations measured in

the drum depend on the ground stiffness (if you drop

something onto soft ground, the (negative)

acceleration is much lower than if you drop the same

object onto hard ground). As the ground stiffness

increases with ongoing compaction, the acceleration

response changes. By automatically logging the

ground response, and mapping this onto a 2-D plan ofthe ground surface, soft spots can be readily

identified. So, even as a simple indicator of where to

concentrate the compaction effort, the system would

be useful.

The systems in use in Europe go further. The ground

response is used to change the vibration mode of the

drum to improve compaction efficiency, and to

indicate where compaction effort should be

concentrated.

At UWA in the early 1980s, Dr Baden Clegg (since deceased) invented the CLEGG IMPACT HAMMER. This is simply a

modified compaction hammer, equipped with an accelerometer. The hammer is dropped onto the ground surface from a given

height, and the acceleration measured. The CLEGG IMPACT VALUE is an indication of ground stiffness, and hence an

indirect indication of the degree of compaction.

From the Bomag Brochure

soil comp action

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