Soil Mechanics
Physical Properties of Soils and Compaction
page 1
Contents of this chapter :
CHAPITRE 2.
PHYSICAL PROPERTIES OF SOILS AND COMPACTION..................................1
2.1 UNITS ......................................................................................................................................1
2.2 BASIC DEFINITIONS AND TERMINOLOGY ....................................................................................2
2.2.1 VOIDS RATIO AND POROSITY ..................................................................................................2
2.2.2 DEGREE OF SATURATION .......................................................................................................2
2.2.3 UNIT W EIGHTS ......................................................................................................................3
2.2.4 SPECIFIC GRAVITY ................................................................................................................3
2.2.5 W ATER (OR MOISTURE) CONTENT ...........................................................................................4
2.2.6 EXERCISES ...........................................................................................................................4
2.3 COMPACTION ...........................................................................................................................5
2.3.1 PURPOSE OF COMPACTION ....................................................................................................5
2.3.2 COMPACTION TEST................................................................................................................5
2.3.3 EFFECTS OF SOIL TYPE ..........................................................................................................8
2.3.4 FIELD SPECIFICATIONS ...........................................................................................................8
2.3.5 EXERCISES .........................................................................................................................10
Chapitre 2.
2.1
Physical Properties of Soils and Compaction
Units
For most engineering applications the following units are used:
Length
Mass
Density1 (mass/unit volume)
Weight
Stress
Unit Weight2
metres
tonnes (1 tonne = 103 kg)
t/m3
kilonewtons (kN)
MegaPascals (MPa) 1 MPa = 1 N/mm2
kN/m3
To sufficient accuracy the density of water ρw is given by ρw = 1 tonne/m3
In most applications it is not the mass that is important, but the force due to the mass, and the
weight, W, is related to the mass, M, by the relation
W = Mg
where: g = 9.81 m/s2 is the acceleration due to gravity.
In soil mechanics calculations, for simplicity, one may use g=10m/s².
Hence the unit weight of water, γw = 9.81 º10 kN/m3
Masse volumique .Attention : density ≠ densité (En Français : rapport de la masse volumique du corps à
une masse volumique de référence. Donc densité se traduit par : relative density ou specific gravity)
2
Poids volumique
1
Soil Mechanics
2.2
Physical Properties of Soils and Compaction
page 2
Basic Definitions And Terminology
Soil is formed by the disintegration of rock under the action of various forces of nature such as
water, wind, frost, temperature changes and gravity. It may thus be considered to consist of a
3
network of solid particles which enclose voids or pores. The voids may be filled with water or air
or both.
Soil is thus a three phase material which consists of solid particles which make up the soil skeleton
voids which may be :
full of water if the soil is saturated,
full of air if the soil is dry,
partially saturated as shown in Figure 2.1.
Weights
Volumes
Air
Water
Solid
Figure 2.1: Air, Water and Solid phases in a typical soil
2.2.1 Voids Ratio and Porosity
Using volumes is not very convenient in most calculations. An alternative measure that is used is
the voids ratio4, e. This is defined as the ratio of the volume of voids, Vv to the volume of solids,
Vs, that is
e =
Vv
Vs
A related quantity is the porosity, n, which is defined as ratio of the volume of voids to the total
volume.
n =
Vv
VT
2.2.2 Degree of Saturation
3
4
vides
Indice des vides
Soil Mechanics
Physical Properties of Soils and Compaction
page 3
The degree of saturation, S, has an important influence on the soil behaviour. It is defined as the
ratio of the volume of water to the volume of voids :
S =
Vw
Vv
2.2.3 Unit Weights
Several unit weights are used in Soil Mechanics. These are the bulk, saturated and dry unit
weights.
5
The bulk unit weight is simply defined as the weight per unit volume
γ bulk =
WT
VT
When all the voids are filled with water the bulk unit weight is identical to the saturated unit weight,
γsat, and when all the voids are filled with air the bulk unit weight is identical with the dry unit
weight6, γ dry =
Ws
.
VT
2.2.4 Specific Gravity
Another frequently used quantity is the Specific Gravity of the soil, Gs, which is defined by :
Gs =
ρ
γ
W
Density of Soil
= s = s = s
Density of Water
ρ w γ w Vsγ w
It is often found that the specific gravity of the materials making
up the soil particles are close to the value for quartz, that is
Gs ≈ 2.65
For all the common soil forming minerals 2.5 < Gs < 2.8
A pycnometer is shown in Fig. 2.2. It is used in the determination
of the specific gravity of the solid particles in the field. In the
laboratory, a 1 litre jar is used together with a mechanical
shaker. The procedure for either method is, however, the same :
The mass of the empty pycnometer (m1) is found using a
balance.
A sample of the oven-dried soil is placed inside and the
combined mass (m2) is found.
5
6
Poids volumique apparent
Poids volumique sec
Fig. 2.2
Soil Mechanics
Physical Properties of Soils and Compaction
page 4
Water is added to the soil which is agitated to remove all air pockets. and when the pycnometer is
full up, its mass (m3) is measured.
Finally the pycnometer is emptied. cleaned and filled with water and its new mass (m4) found.
W
W
m1
S
S
m2
m3
m4
Exercises
1. Demonstrate that the specific gravity can be calculated by the formula :
Gs =
m2 − m1
(m4 − m1) − (m3 − m2)
2. A pycnometer having a mass of 620 g was used to determine the specific gravity of an ovendried sample of soil. If the mass of the soil sample was 980 g and the mass of the
pycnometer with the sample and filled up with water was 2112 g. determine the specific
gravity of the soil particles. The mass of the pycnometer when filled with water only was 1495
g.
2.2.5 Water (or moisture) content
The water content7, w, is a very useful quantity and it is simple to measure. It is defined as the
ratio of the weight of water to the weight of solid material. That value can be greater than 1 (100%)
in the case of peat (600%!).
w =
Ww
(x 100 if expressed in %)
Ws
Example showing the standard method of determining the water content :
3. A moist8 sample of soil in a bottle had a mass of 25.24 g and the bottle, when empty, had a mass
of 14.2 g. After drying in an oven 24 hours, the bottle and soil sample had a mass of 21.62 g. Find
the water content of the soil.
2.2.6 Exercises
4. An undisturbed specimen of clay is taken from a sampling tube the volume of which is 0.013 m3.
The weight of the specimen is 250 N and the water content is 21.1%. Calculate the dry unit
7
8
Teneur en eau
humide
Soil Mechanics
Physical Properties of Soils and Compaction
page 5
weight. If the specific gravity of the particles is 2.68, find the void ratio and the degree of
saturation.
5. The mass of an oven-dried sample of clay is 11.26 gm and its (total) volume is 5.83 cm3. If the
specific gravity of the soil particles is 2.67 determine the shrinkage limit of the soil. The shrinkage
limit is the water content the soil would have had if fully saturated at its minimum volume.
6. The dry unit weight cannot be measured directly. It is deduced from the measured bulk unit weight
and water content. Prove that γ dry =
2.3
γ bulk
(1+w)
Compaction
One important application of the physical properties is the compaction of soils.
Compaction is the application of mechanical energy to a soil to rearrange the particles and reduce
the void ratio.
2.3.1 Purpose of Compaction
Compaction :
reduces subsequent settlement under working loads.
increases the shear9 strength of the soil,
reduces the voids ratio making it more difficult for water to flow through soil. This is
important if the soil is being used to retain water such as would be required for an earth
dam.
can prevent the build up of large water pressures that cause soil to liquefy during
earthquakes.
2.3.2 Compaction10 Test
For a given quantity of soil, if the total volume is reduced (the expected result of compaction), the
dry unit weight ( γ dry =
Ws
) increases.
VT
Proctor, an American engineer, was the first to study, in 1933, the compaction process and
noticed the influence of the water content and energy of compaction on the dry unit weight. He
developed a standard compaction test still in use today. This test involves compacting soil into a
mould11 at various water contents
Standard Compaction Test
A sample of soil is compacted, at different water contents, into a mould in 3-5 equal layers, each
layer receiving 25 blows12 of a hammer of standard weight. The apparatus is shown in Fig. 2.3.
The important dimensions are :
Volume of mould
943.9 cm3
9
cisaillement
Compactage
11
Un moule
12
coup
10
Hammer mass
2.5 kg
Drop of hammer
304.8 mm
Soil Mechanics
Physical Properties of Soils and Compaction
page 6
Figure 2.3 Standard Proctor Apparatus for laboratory
compaction tests
Because of the benefits from compaction, contractors have built larger and heavier machines to
increase the amount of compaction of the soil.
It was found that the Standard Compaction test could not reproduce the densities measured in the
field and this led to the development of the Modified Compaction test not described here.
Presentation of Results
From the different compaction tests realised on the same soil, with different water contents, a
graph like Fig. 2.4 can be drawn.
From this graph we can determine the optimum water content, wopt, also called Proctor optimum,
for the maximum dry unit weight, (γdry)max.
There is a limiting dry unit weight for any water content and this occurs when the voids are full of
water. Increasing the water content for a saturated soil results in a reduction in dry unit weight.
For a saturated soil : γ dry =
Now Vs =
γ bulk
1+ w
Ws
Gs γ w
=
Ws + Ww
Ws + Ww
=
VT (1 + w)
(Vs + Vw ) (1 + w)
Vw =
Hence γ dry =
Ww
γw
=
wWs
γw
Gs γ w
Gs w + 1
The relation between the water content and dry unit weight for saturated soil is shown on the
graph in Fig. 2.5. This line is known as the zero air voids line.
Soil Mechanics
Physical Properties of Soils and Compaction
page 7
Fig. 2.4 A typical compaction test result
Fig. 2.5 Typical compaction curve showing noair-voids line
Effects of water content during compaction
As water is added to a soil (at low water content) it becomes easier for the particles to move past
one another during the application of the compacting forces. As the soil compacts the voids are
reduced and this causes the dry unit weight to increase. Initially then, as the water content
increases so does the dry unit weight. However, the increase cannot occur indefinitely because
the soil state approaches the zero air voids line which gives the maximum dry unit weight for a
given water content. Thus as the state approaches the no air voids line further water content
increases must result in a reduction in dry unit weight. As the state approaches the no air voids
line a maximum dry unit weight is reached and the water content at this maximum is called the
optimum water content.
Effects
effort
of
increasing compactive
Increased compactive effort enables
greater dry unit weights to be achieved
which because of the shape of the no
air voids line must occur at lower
optimum water contents. The effect of
increasing compactive energy can be
seen in Fig. 2.6. It should be noted that
for water contents greater than the
optimum, the use of heavier compaction
machinery will have only a small effect
on increasing dry unit weights. For this
reason it is important to have good
control over water content during
compaction of soil layers in the field.
Fig. 2.6 Effects of increasing compactive effort on
compaction curves
It can be seen from this figure that the
compaction curve is not a unique soil characteristic. It depends on the compaction energy. For this
reason it is important when giving values of (γdry)max and wopt to also specify the compaction test
procedure (for example, standard or modified).
Soil Mechanics
Physical Properties of Soils and Compaction
page 8
2.3.3 Effects of soil type
The table below contains typical values for the different soil types obtained from the Standard
Compaction Test.
Well graded sand
SW
Sandy clay
SC
Poorly graded sand
SP
Low plasticity clay
CL
Non plastic silt
ML
High plasticity clay CH
Typical Values
3
(γdry )max (kN/m )
22
19
18
18
17
15
wopt (%)
7
12
15
15
17
25
Note that these are typical values. Because of the variability of soils it is not appropriate to use
typical values in design, tests are always required.
2.3.4 Field specifications
To control the soil properties of earth constructions (e.g. dams, roads) it is usual to specify that the
soil must be compacted to some pre-determined dry unit weight. This specification is usually that a
certain percentage of the maximum dry unit weight, as found from a laboratory test (Standard or
Modified) must be achieved.
For example we could specify that field dry unit weights must be greater than 98% of the
maximum dry unit weight as determined from the Standard Compaction Test. It is then up to the
Contractor to select machinery, the thickness of each lift (layer of soil added) and to control water
contents in order to achieve the specified amount of compaction.
Accept
Reject
Fig. 2.7 Possible field specification for compaction
There is a wide range of compaction equipment. For pavements some kind of wheeled roller or
vibrating plate is usually used. These only affect a small depth of soil (20 to 30cm maximum), and
Soil Mechanics
Physical Properties of Soils and Compaction
page 9
to achieve larger depths vibrating piles and drop weights can be used. The applicability of the
equipment depends on the soil type as indicated in the table below
Equipment
Smooth
13
rollers ,
vibrating
Most suitable soils
Typical application
Well graded sand- Running surface14,
15
gravel, crushed rock, subgrades
asphalt
Least suitable soils
Uniform sands
Coarse grained soils Pavement subgrade
with some fines
Coarse uniform soils
and rocks
Weathered17
well graded
soils
Clays,
silty
clays,
uniform materials
wheeled
static
or
Rubber tyred rollers
16
rock, Subgrade, subbase18
coarse
Grid rollers
Fine grained soils with Dams, embankments,
> 20% fines
subgrades
Sheepsfoot
static
Sheepsfoot
vibratory
13
rollers19,
rollers, as above, but also subgrade layers
sand-gravel mixes
Rouleaux lisses
Surface de roulement
15
Couche de fondation, la couche la plus basse dans la composition d'une route
16
Rouleaux à pneus
17
érodé
18
Couche de base, située entre la surface de roulement et la couche de fondation
19
Rouleaux à pieds de mouton
14
Coarse soils, soils
with cobbles, stones
Soil Mechanics
Physical Properties of Soils and Compaction
Coarse soils, 4 to 8% Small patches
fines
Vibrating plates
21
page 10
clays and silts
20
All types
Difficult access areas
Tampers, rammers22
Most saturated
moist soils
and
Dry,
sands
gravels
and
Impact rollers
2.3.5 Exercises
7. In a compaction test a bulk unit weight of 16 kN/m3 was measured at a water content of 8%.
What would the water content have been if the soil had been fully saturated. Assume Gs = 2.70.
8.
On the application of the standard compaction test to a soil, the results tabulated below were
obtained. Obtain an estimate of the optimum water content, maximum dry unit weight and draw
the line of zero air voids relating dry unit weight and water content. Assume the specific gravity is
2.75.
Water content (%)
7.4
8.8
10.0
12.2
15.2
17.2
20
Plaques vibrantes
Petites réparations
22
Pilonneuses
21
Bulk unit weight (kN/m3)
18.5
19.9
20.9
21.2
20.6
20.3