Lecture 3Compaction of Soils

In the construction of highway embankments, earth dams, and many other engineering structures, loose soils must be compacted to increase their unit weights.

Compaction increases the strength characteristics of soils, which increases the bearing capacity of foundations constructed over them.

Compaction also decreases the amount of undesirable settlement of structures and increases the stability of slopes and embankments.

Compaction, in general, is the densification of soil by removal of air, which requires mechanical energy.

The degree of compaction of a soil is a function of its dry unit weight.

General Concept

Phase diagrams showing the changes in soil as it moves from its natural location to a compacted fill.

Note that the volume of solids does not change during the process.

Changes in volume as soil is excavated, transported, and compacted. The numerical values are examples and would be different for each soil.

Definition

Soil compaction is defined as the method of mechanically increasing the density of soil by

reducing volume of air.

Solids

Water

Air

Solids

Water

Air

Compressed

soil

Load

Soil

Matrix

gsoil (1) = WT1

VT1

gsoil (2) = WT1

VT2

gsoil (2) > gsoil (1)

Principles of Soil Compaction

Beyond OMC, excess water results in lower density.

Water acts as a lubricating agent and replaces the voids, therefore results in higher density

Dry Side:

Principles of Soil Compaction, cont.

When water is added to the soil during compaction, it acts as a softening agent on the soil particles. The soil particles slip over each other and move into a densely packed position.

The dry unit weight after compaction first increases as the moisture content increases. When the moisture content is gradually increased and the same compactive effort is used for compaction, the weight of the soil solids in a unit volume gradually increases.

Beyond a certain water content , any increase in the moisture content tends to reduce the dry unit weight.

This phenomenon occurs because the water takes up the spaces that would have been occupied by the solid particles.

The moisture content at which the maximum dry unit weight is attained is generally referred to as the optimum moisture content.

Standard Proctor Test (ASTM D-698) or (AASHTO T-99)

In the Proctor test, the soil is compacted in a mold that has a volume of 944 cm3. The diameter of the mold is 101.6 mm (4 in.).

The soil is mixed with varying amounts of water and then compacted in three equal layers by a hammer that delivers 25 blows to each layer.

The hammer has a mass of 2.5 kg (5.5 lb) and has a drop of 30.5 mm (12 in.).

For each test, the moist unit weight of compaction, g, can be calculated as:

W = Weight of the compacted soil in the moldVm = Volume of the mold

mV

Wg

Mold and hammer for a Proctor compaction test.

In the standard test we compact the soil in three layers, (as shown),

while in the modified test we compact the soil in five layers.

Standard Proctor Test Results for a Silty Clay Soil

For each test, the moisture content of the compacted soil is determined in the laboratory.With the known moisture content, the dry unit weight can be calculated as:

The values of gd determined from the above equation can be plotted against the corresponding moisture contents to obtain the maximum dry unit weight and the optimum moisture content for the soil.

wd

1

gg

Moisture Content

Physical Characteristics of Soils

Particle Size

Particle Size Distribution

Geometry of Particles

Percent Fines in the Mix

Plasticity of Fines

Compaction Energy

Factors Affecting Compaction

Typical Compaction Curves for Various Soil Types

Note that for sands, the dry unit weight has a general tendency first to decrease as moisture content increases and then to increase to a maximum value with further increase of moisture.

The initial decrease of dry unit weight with increase of moisture content can be attributed to the capillary tension effect.

At lower moisture contents, the capillary tension in the pore water inhibits the tendency of the soil particles to move around and be compacted densely.

Various Types of Compaction Curves in Soils

Lee and Suedkamp (1972) studied compaction curves for 35 soil samples.

They observed that four types of compaction curves.

Effect of Compaction Energy on the MD Curve

Two major effects of the increasing compaction effort on the same soil:

1. As the compaction effort is increased, the maximum dry unit weight of compaction is also increased.

2. As the compaction effort is increased, the optimum moisture content is decreased.

Compaction Energy in Standard Proctor Test

In SI Units:

In English Units:

The compaction energy per unit volume used for the standard Proctor test can be calculated as:

With the development of heavy rollers and their use in field compaction, the standard Proctor

test was modified to better represent field conditions.

For conducting the modified Proctor test, the same mold is used with a volume of 944 cm3, as

in the case of the standard Proctor test.

Soil is compacted in five layers by a hammer that has a mass of 4.54 kg (10 lb).

The drop of the hammer is 457 mm (18 in.).

The number of hammer blows for each layer is kept at 25 as in the case of the standard

Proctor test.

Because it increases the compactive effort, the modified Proctor test results in an increase in

the maximum dry unit weight of the soil. The increase in the maximum dry unit weight is

accompanied by a decrease in the optimum moisture content.

Modified Proctor Test (ASTM D-1557) or AASHTO T-180

Example

The laboratory test results of a standard Proctor test are given in the following table.

I. Determine the maximum dry unit weight of compaction and the optimum moisture content.

II. Calculate and plot gd versus the moisture content for degree of saturation, S = 80%, 90%, and 100% . Assume Gs = 2.7.

Solution-Step 1

Calculation of the dry unit weight at different moisture contents to establish the MD curve.

You can then graphically determine the OMC and gd (max).

mV

Wg

wd

1

gg

Solution-Step 2

Calculation of the dry unit weight for various degrees of saturation to establish the ZAV curve.

S

wG

G

s

wsd

1

gg

Final Solution

Gyratory Compactor Vibratory Compactor Impact Compactor

Application of the Compaction Energy in the Laboratory

Velocity and Displacement Plots for Impact Hammer Method

Layer Separation Resulted from the Impact Compaction in Volumetric Shrinkage Test

Evidence of layer Separation along the Compaction Plane in Fine Grained Soils

Specimen Blanket Image Digital Image

Compaction and the Clay Structure

If clay is compacted with a moisture content on the dry side of the optimum, as represented by point A, it will possess a flocculent structure. This type of structure results because, at low moisture content, the diffuse double layers of ions surrounding the clay particles cannot be fully developed; hence, the inter-particle repulsion is reduced. This reduced repulsion results in a more random particle orientation and a lower dry unit weight.

When the moisture content is increased, as shown by point B, the diffuse double layers around the particles expand, which increases the repulsion between the clay particles and gives a lower degree of flocculation and a higher dry unit weight.

Compaction and the Clay Structure, Cont.

A continued increase in moisture content from B to Cexpands the double layers more. This expansion results in a continued increase of repulsion between the particles and thus a still greater degree of particle orientation and a more or less dispersed structure. However, the dry unit weight decreases because the added water dilutes the concentration of soil solids per unit volume.

At a given moisture content, higher compactive effort yields a more parallel orientation to the clay particles, which gives a more dispersed structure. The particles are closer and the soil has a higher unit weight of compaction. This phenomenon can be seen by comparing point A with point E.

Influence of Compaction Energy and Moisture Content on the Orientation of Clay Particles (Lambe, 1958)

Variation in the degree of particle orientation with molding water content for compacted Boston blue clay, after Lambe, 1958.

Effect of Compaction Energy on the Permeability of the Fine Grained Soils

The plots show the results of permeability tests on Jamaica sandy clay. The samples used for the tests were compacted at various moisture contents by the same compactive effort.

The hydraulic conductivity, which is a measure of how easily water flows through soil, decreases with the increase of moisture content.

It reaches a minimum value at approximately the optimum moisture content.

Beyond the optimum moisture content, the hydraulic conductivity increases slightly. The high value of the hydraulic conductivity on the dry side of the optimum moisture content is due to the random orientation of clay particles that results in larger pore spaces.

Unconfined Compressive Strength of a Compacted Silty Clay Soil

The specimens A, B, and C have been compacted, respectively, on the dry side of the optimum moisture content, near optimum moisture content, and on the wet side of the optimum moisture content.

The unconfined compression strength, qu, is greatly reduced for the specimen compacted on the wet side of the optimum moisture content.

Variation of gd with Organic Content (Franklin, 1973)

Franklin et al. (1973) conducted several laboratory tests to observe the effect of organic content on the compaction characteristics of soil.

The plot shows the effect of organic content on the maximum dry unit weight. When the organic content exceeds 8% to 10%, the maximum dry unit weight of compaction decreases rapidly.

Variation of Optimum Moisture Content with

Organic Content (Franklin, 1973)

Franklin’s study showed that the optimum moisture content for a given compactive effort increases with an increase in organic content.

Franklin also showed that the maximum Unconfined Compressive Strength (UCS) obtained from a compacted soil (with a given compactive effort) decreases with increasing organic content of a soil.

Therefore he concluded that soils with organic contents higher than about 10% are unstable and therefore undesirable for compaction work.

Nature of Variation of Swelling and Shrinkage of Expansive Clays

Expansive soils owe their characteristics to the presence of swelling clay minerals. As they get wet, the clay minerals absorb water molecules and expand; conversely, as they dry they shrink, leaving large voids in the soil.

Swelling clays can control the behavior of virtually any type of soil if the percentage of clay is more than about 5 percent by weight. Soils with smectite clay minerals, such as montmorillonite, exhibit the most profound swelling properties.

Inorganic clays of high plasticity, generally those with liquid limits exceeding 50 percent and plasticity index over 30, usually have high inherent swelling capacity.

In the field, expansive clay soils can be easily recognized in the dry season by the deep cracks, in roughly polygonal patterns, on the ground surface.

Expansive Soils

Polygonal pattern of surface cracks in the dry season. These cracks are approximately one inch wide at the top.

Note sewer manhole in background in the right photo.

The depth of the crack in the left photo is approximately 32 inches deep.

Open tension cracks formed due to loss of moisture and shrinkage of soil.

(1) At the beginning of the rainy season, the piers are still supported by friction with the soil. When it begins to rain, water enters deep into the soil through the cracks.

(2) After several rainfalls, the soil swells, lifting the house and piers.

(3) In the dry season, the groundwater table falls therefore the soil dries and contracts. As tension cracks grow around the pier, the skin friction is reduced.

(4) When the building load exceeds the remaining skin friction, adhesion is broken by this straining, and the pier sinks.

Damage to Homes Supported by Piers

Growth Curves

Growth curves in compaction show the relationship between dry unit weight and number of passes of a roller.

The dry unit weight of a soil at a given moisture content increases to a certain point with the number of roller passes.

Beyond this point, it remains approximately constant.

In most cases, about 10 to 15 roller passes yield the maximum dry unit weight economically attainable.

Variation of the Dry Unit Weight (gd) with the Number of Passes and Depth

The plot shows the variation in the unit weight of compaction with depth for a poorly graded dune sand for which compaction was achieved by a vibratory drum roller.

The dry unit weight of compaction increases with the number of roller passes. However, the rate of increase in unit weight gradually decreases after about 15 passes.

Relative Compaction ( %RC) and Relative Density (Dr)

In most specifications for earthwork, the contractor is instructed to achieve a compacted field dry unit weight of 90% to 95% of the maximum dry unit weight determined in the laboratory by either the standard or modified Proctor test.Relative Compaction can be calculated as:

For the compaction of granular soils, specifications sometimes are written in terms of the required relative density Dr or the required relative compaction.

100)(

)(

Labd

FielddRC

g

g

)(

(max)

(min)(max)

(min))(

Fieldd

d

dd

dFieldd

rDg

g

gg

gg

100)(

)(

Labd

FielddRC

g

gRelative Compaction (RC) in percent:

Typical Compaction Specifications

Moisture Content for Field Compaction

The compaction curves A, B, and C are for the same soil with varying compactive effort.

Let curve A represent the conditions of maximum compactive effort that can be obtained from the existing equipment. Assume that the contractor be required to achieve a minimum dry unit weight of gd(Field) =RCgd(max) .

To achieve this, the contractor must ensure that the moisture content w falls between w1 and w2. As can be seen from compaction curve C, the required gd(Field) can be achieved with a lower compactive effort at a moisture content w = w3. However, for most practical conditions, a compacted field unit weight of gd(Field) =RCgd(max) cannot be achieved by the minimum compactive effort. Hence, equipment with slightly more than the minimum compactive effort should be used. The compaction curve B represents this condition.

The most economical moisture content is between w3 and w4.

In-Situ CompactionSand Cone Method

In-Situ CompactionSand Cone Method

A small hole (6" x 6" deep) is dug in the compactedmaterial to be tested. The soil is removed andweighed, then dried and weighed again to determineits moisture content. The specific volume of the hole isdetermined by filling it with calibrated dry sand froma jar and cone device. The dry weight of the soilremoved is divided by the volume of sand needed tofill the hole. This gives us the density of thecompacted soil in lb per cubic foot. This density iscompared to the maximum Proctor density obtained inthe lab, which gives us the relative density of thecompacted soil in the field.

In-Situ CompactionDifferent Types of Nuclear Density Gauge

In-Situ Compaction

Nuclear Density Gauge (ASTM D2292)

Nuclear Density meters are a quick and fairly accurate way of determining density andmoisture content. The meter uses a radioactive isotope source (Cesium 137) at the soilsurface (backscatter) or from a probe placed into the soil (direct transmission). The isotopesource gives off photons (usually Gamma rays) which radiate back to the detectors on thebottom of the unit. Dense soil absorbs more radiation than loose soil and the readingsreflect overall density. Water content (ASTM D3017) can also be determined, all within afew minutes.

Intelligent Compaction

Intelligent Compaction (IC) measures stiffness (the ability of a material to resist deformation under a load) rather than density of the compacted soil. A strong correlation exists between stiffness and bearing capacity of foundations.

A machine equipped with an intelligent compaction system provides four basic functions:

1) Measures the stiffness of the soil.

2) Controls or guides the compaction effort in response to the measured stiffness.

3) Displays the stiffness measurement to the operator.

4) Maps and records the compaction results.

Such a system would enable the user to produce detailed plots of the soil stiffness levels, the number of roller passes, as well as the location and time of the application.

Schematic Illustration of IC Analysis Process (Mooney, 2013)

(a) Individual IC Roller-Measured Stiffness Maps

b) Stacked IC Data Maps

C) Calculated Elastic Layer Moduli

Filed Compaction

- Pneumatic rubber tired roller

Different types of rollers (clockwise from right):

- Vibratory plate

- Smooth-wheel roller

- Sheepsfoot roller

Field Compaction

Compacts effectively only to 200-300 mm;

therefore, place the soil in shallow layers (lifts)

Smooth Wheeled Roller

Field Compaction

For compacting very small areas, effective

for granular soils.

Vibrating Plates

Field Compaction

Provides kneading action; “walks out” after compaction,

very effective on clays.

Sheepsfoot Roller

Field Compaction

Provides deeper (2-3 m) compaction. e.g., airfield

Impact Roller

Grid Roller

Evaluation of Fill Materials for Compaction (Sowers, 1962)

Soil types best suited for various kinds of compaction equipment.(Adapted from Caterpillar, 1993.)

Construction Equipment

Large Dump Truck

Construction Equipment

Grader for spreading soil

Construction Equipment

Bulldozer for evenly spreading soil

Construction Equipment

Loader

Construction Equipment

Backhoe

Construction Equipment

Crawler mounted Hydraulic Excavator

Construction Equipment

Rock Breaker

Construction Equipment

Water Truck