6 compaction

William J. Likos, Ph.D.

Department of Civil and Environmental Engineering

University of Wisconsin-Madison

GLE/CEE 330 Lecture Notes

Soil Mechanics

Compaction and Ground Improvement

Ground Improvement Methods

“Reinforcement”

• Stone Columns• Soil Nails• Deep Soil Nailing• Micropiles (Mini-piles)• Jet Grouting• Ground Anchors• Geosynthetics• Fiber Reinforcement• Lime Columns• Mechanically Stabilized

Earth (MSE)• Biological (e.g. roots)

• Compaction• Preload/Surcharge• Electro-osmosis• Compaction grouting• Blasting• Deep dynamic

compaction

• Cement• Lime Admixtures• Dewatering• Heating/Freezing• Vitrification• Biotreatment

(after Shaefer, 1997)

“Improvement” “Treatment”

Could also add “Replacement”(often not cost effective)

Some Reinforcement Methods

Jet Grouting

(Menard)

Soil Nailing

(Atlas Copco)

Fiber Reinforced Soil

Some Improvement Methods

Deep Dynamic

Compaction

Compaction

Grouting

(UC Davis)

Surcharge

with

Drainage

Some Treatment Methods

Lime Treatment

Ground

Freezing

(Max Bogl)

Microbial

Treatment

(J. Dejong)

Compaction

• Densify soil by reducing volume of voids (Vv = Va + Vw)

• We primarily reduce the volume of air (Va)

• Compaction ≠ Consolidation !!!!!• Consolidation is compression from loss of water squeezed out over time resulting from applied load.

Objectives of Compaction:(1) Decrease settlements

(2) Increase shear strength

(3) Decrease permeability

Loose

Dense

Adjust w and

Compact

High n

High e

Low n

Low e

Vv

Vs

Vt

Vs

Vt

Vv

Adding water to reach

“optimum water content

Compaction MethodsCoarse-grained soils Fine-grained soils

• Hand-operated vibration plates

• Motorized vibratory rollers

• Free-falling weight; dynamic compaction (low frequency vibration)

•Falling weight and hammers

•Kneading compactors

•Static loading and press

•Hand-operated tampers

•Sheepsfoot rollers

•Rubber-tired rollers

Labo

rato

ryF

ield

Vibration

•Vibrating hammer (BS)

(Holtz and Kovacs, 1981; Head, 1992)

Kneading

(Das, 2000)

• Sandy (non-cohesive) soils• 100% coverage• Contact pressure = 300-400 kN/m2

• Static or vibratory• “Proof” rolling (smooth surface)

Smooth-Wheeled Roller

• Sandy or clayey soils• 70 – 80 % coverage• Contact pressure = 600-700 kN/m2

• Combination of pressure and kneading• Articulated wheels find “soft spots”

(Das, 2000)

Pneumatic Rubber-Tired Roller

• Small projections for kneading action• Clayey or silty soils• Contact pressure = 1400-7000 kN/m2

(Das, 2000)

Sheepsfoot Roller

• Retaining wall backfills• Foundation backfills• Compaction close to existing structures• Usually vibratory

(Das, 2000)

Portable Compactors

Intelligent Compaction

Feedback on vibratory

compaction

(Coduto, 1999)

Applicability for Soil Types

Proctor Compaction Curve

Zero air void curve

(ZAV)

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

16

•The peak point of the compaction curve•The peak point of the compaction curve is the point with the maximum dry density d max. Corresponding to the maximum dry density d max is a water content known as the optimum water content wopt (also known as the optimum moisture content, OMC). Note that the maximum dry density is only a maximum for a specific compactive effort and method of compaction. This does not necessarily reflect the maximum dry density that can be obtained in the field.

•Zero air voids curve•The curve represents the fully saturated condition (S = 100 %). (It cannot be reached by compaction)

•Line of optimums•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, it is called the line of optimums

SwGG

e

G

s

wswsd

11

swGSeRecall:

Holtz and Kovacs, 1981

Zero Air Voids (ZAV) Curve

Standard Proctor test equipment

Das, 1998

Laboratory Compaction Procedures

(1) Several samples of the same soil, but at different water contents, are compacted according to the compaction test specifications.

(2) The total or wet density and the actual water content of each compacted sample are measured.

(3) Plot the dry unit weight gd versus water contents w for each compacted sample. The curve is called as a compaction curve.

wV

Wd

t

t

1,

Report (gd)max and wopt

Laboratory Compaction Procedures

Summary of Standard Proctor Compaction Test Specifications (ASTM D-698, AASHTO)

Das, 1998

Laboratory Compaction Procedures

Summary of Modified Proctor Compaction Test Specifications (ASTM D-698, AASHTO)

Das, 1998

Standard Proctor Test

12 in height of drop

5.5 lb hammer

25 blows/layer

3 layers

Mold size: 1/30 ft3

Energy 12,375 ft·lb/ft3

Modified Proctor Test

18 in height of drop

10 lb hammer

25 blows/layer

5 layers

Mold size: 1/30 ft3

Energy 56,250 ft·lb/ft3

23

)ft/lbft375,12(m/kJ7.592

m10944.0

)layer/blows25)(layers3)(m3048.0)(s/m81.9(kg495.2E

33

33

2

Volume of mold

Number of blows per

layer

Number of layers

Weight of hammer

Height of drop of

hammer

E =For

standard Proctor test

Effects of Soil Type

24Holtz and Kovacs, 1981; Das, 1998

Suitability of Soil Types for Construction

Type Strength Compressibility Permeability Interaction with Water

Uses Problems

Gravel High Low V. High No effect Pavement bases

Filters

Prone to caving Small clay content

affects propertiesSand High Low High Workable

over wide range

Wide range of uses

Fills (hydraulic)

Backfill

Poor at ground surface Prone to caving Prone to erosion

Low plasticity silts/clays

Low High Low Lose strength when wetted

Fills Prone to frost heave Collapse potential

High plasticity silts/clays

Low High V. Low Lose strength when wetted

Landfill covers/liners

Poor workability (sticky)

Swell/shrink potential

Organics Low High - - Landscaping Typically removed

Compaction and Soil Fabric

• Fabric – orientation and arrangement of particles (clay); has influence on soil behavior

•Soil fabric tends to be more flocculated (random) for compaction dry of optimum.

•Soil fabric tends to be more dispersed (oriented) for compaction wet of optimum.

Lambe and Whitman, 1979

Flocculated

Dispersed

Clay particles are plate-like

(e.g., kaolinite)

Engineering Behavior - Permeability

• Increasing the water content results in a decrease in permeability on the dry side of the optimum moisture content and a slight increase in permeability on the wet side of optimum.

• Increasing the compactive effort reduces the permeability since it both increases the dry density, thereby reducing the voids available for flow, and increases the orientation of particles.

From Lambe and Whitman, 1979; Holtz and Kovacs, 1981

Engineering Behavior - Strength

Samples compacted dry of optimum tend to be more rigid and stronger than samples compacted wet of optimum

From Lambe and Whitman, 1979

s1

s3

s1 – s3

Engineering Properties - Summary

29Holtz and Kovacs, 1981; Das, 1998 29

Dry side Wet side

Permeability

Compressibility

Swelling

Strength

Structure Flocculated Dispersed

More permeable

More compressible in high pressure

range

More compressible in low pressure

range

Higher

*Shrinks more

Less permeable

Higher

Lower

Field Quality Control

• Dry density and water content correlate well with the engineering properties, and thus they are convenient construction control parameters.

• Since the objective of compaction is to stabilize soils and improve their engineering behavior, it is important to keep in mind the desired engineering properties of the fill, not just its dry density and water content. This point is often lost in the earthwork construction control.

From Holtz and Kovacs, 1981

Quality Control – Relative Compaction

From Holtz and Kovacs, 1981

Control

(1) Relative compaction

(2) Water content (dry side or wet side)

100% saturation

Water content w %

wopt

Dry

de

nsi

ty, g

d

gd max

Line of optimums

90% R.C.

a c

Increase compaction

energy

b

%100..max

laboratoryd

fielddCR

QA/QC MethodsMethods

(a) Sand cone

(b) Balloon

(c) Oil (or water) method

Calculations

• Measure Wt, Vt

• Get gd field and w

• Compare d field with d max-lab and calculate relative compaction R.C.

(a)

(b)

(c)

QA/QC MethodsHoltz and Kovacs, 1981

Nuclear density meter

(a) Direct transmission

(b) Backscatter

(c) Air gap

(a)

(b)

(c)

Principles

DensityGamma radiation is scattered by the soil particles and the amount of scatter is proportional to the total density of the material. Gamma radiation is typically provided by radium or a radioactive isotope of cesium.

Water contentWater content can be determined based on neutron scattering by hydrogen atoms. Typical neutron sources are americium-beryllium isotopes.

“Borrow Pit” Problem

Borrow pit

Sandy Soil

w = 15%

e = 0.69

Compacted

Embankment

yd = 18 kN/m3

30 m X 1.5 m X 1000 m

Truck

Transport

(10 m3/truck)

volume