CHAPTER TWO
SOIL
COMPRESSION
SOIL COMPRESSION



This refers to a process that describes the
decrease in soil volume under an externally
applied load.
Soil compression can involve removal of air
from soil pores called compaction or
expulsion of water from soil pores called
consolidation.
Soil compaction is more usual in agricultural
fields since soils are normally worked at
unsaturated states.
MEASUREMENT OF SOIL
COMPRESSION

Soil compression can be measured in the
laboratory
using
uniaxial
confined
compression test in an oedometer, the
triaxial compression cell, or direct shear test.

Stone and Ekwue (1995, 1996) described a
simple method to measure the compression
of unsaturated agricultural soils.
MEASUREMENT OF SOIL
COMPRESSION



The soil at a known moisture content is
packed at the required bulk density into a
cylinder.
A steel plate with perforations are then
placed on top of the soil in the cylinder and
the cylinder placed on the load cell of a
compression machine.
The steel plate serves to spread the load
from the plunger of the machine to the soil.
The perforations on the steel plate provide
an exit for excess pore pressures to leave
the soil sample during compression if
present..
MEASUREMENT OF SOIL
COMPRESSION CONTD

During the test, force (F) exerted by the
loading plunger is continuously
measured as a function of decrease in
sample height () due to plunger
movement. The vertical stress () on
the sample is then F/A where A is the
area of the soil cylinder
Soil Compression Machine
Derivation of Soil Compression
Equations
The initial dry bulk density on soil packing (with no strain),
given by:
M
Mass
i

Volume

s
H0 . A
is
........................................(1)
Where: Ms is the dry mass of sample; A is the area of the soil
cylinder and Ho is the original height of soil in the cylinder.
.
Strain ( ) at any applied stress,  
 
Ho  H
Ho
Change in soil sample height
Original height
...............................(2)
Where: H is the new height of the sample at any applied stress
(see figure below)
Soil Compression Test
F
Ho
H
Derivation of Compression
Equations Contd.
Note: Since there is no lateral strain on the sample as it is
confined, axial strain is equal to volumetric strain.
From Eqn. (2), H = Ho (1 -  ) .......... (3)
Dry bulk density,
 b at any stress 
M
M

V
H. A
...... (4)
Substituting Eqn (3) into Eqn (4),
b 
M
H0 (1   ) A
From Eqn (1),  b 
i
1 
and
  1
i
b
These equations were first derived by Stone and Ekwue (1995).
Void Ratio
Void Ratio: Void ratio, e is defined as e = Vp / Vs
Where: Vp is the volume of voids = total soil volume (V) - Volume
of solids (Vs)
V  Vs V
e
 1
.........................(6)Also
Vs
Vs
Soil particle density is:
Ms
M
and dry bulk density ,  b  s
Vs
V
 s Ms V
V
i. e.

x

 b Vs M s Vs
s 
From Eqn (6), e 
s
1
b
....(7)
Note: Soil particle density can be taken as 2.65 gm/cm3 for most
mineral soils.
Soil Compression Index
2.1.3 SOIL COMPRESSION INDEX
Soil compression index, Cc is defined as:
Cc 
 (e2  e1 )
log( 2 /  1 )
Where: e1 and e2 are void ratios at two applied stresses
 1 and  2 .
Example: The following results were computed for the Piarco
sandy loam soil in a laboratory experiment. Plot the strain/stress
curve; and the soil compression curves. The initial soil bulk
density before soil compaction was 0.89 gm/cm3 and the so
particle density is 2.65 gm/cm3.
Example Contd.
Applied stress Bulk density
Strain
(kPa)
(gm/cm3)
10
1.09
0.21
40
1.20
0.28
60
1.24
0.31
80
1.27
0.32
100
1.29
0.33
150
1.34
0.36
200
1.37
0.37
400
1.45
0.41
500
1.47
0.42
600
1.49
0.42
800
1.52
0.43
1000
1.55
0.45
Solution: Using stresses of 10 kPa and 100 kPa
Cc 
 (1.05  1.43)
log (100 / 10)
 0.38
Void ratio
1.43
1.21
1.14
1.09
1.05
0.98
0.93
0.83
0.80
0.78
0.74
0.71
Bulk Density and Applied Stress
Stress/Strain Relations
Void Ratio and Applied Stress
2.2 SOIL COMPACTION



Soil Compaction is defined as the volume
change produced by momentary load
application caused by rolling, tamping or
vibration.
It involves the expulsion of air without
significant change in the amount of water in
the soil mass.
The most common causes of agricultural
soil compaction are trampling by livestock
and pressures imposed by vehicles or tillage
equipment.
Soil Compaction Contd.





While Soil compaction is desirable in most
engineering situations, it is undesirable in
agricultural fields. Improvements of engineering
properties of soils through compaction lead to
advantages such as:
i)
Reduction or prevention of detrimental
settlement of soil.
ii)
Soil strength increases and improvements of
slope stability.
iii) Improvement of bearing capacity of pavements
and
iv) The control of undesirable volume changes
caused by frost action, swelling and shrinkage.
Compaction Contd.





Compaction in agricultural fields leads to
Excess soil hardness,
Reduced soil permeability to water and
airflow and a resulting loss of crop yields.
It is not possible to remove water from the
voids by compaction, but the addition of
water to a slightly moist soil increases
compaction by reducing surface tension.
Compaction increases to a limit called the
optimum moisture content above which
further addition of water causes an increase
in voids, leading to reductions in soil
compaction.
State of Compaction







The State of Compaction of a Soil can be
Measured by
Dry Bulk Density,
Shear Strength,
Penetration Resistance or
Reductions in Soil Permeability.
To determine compaction of a soil in terms of
dry density, it is necessary to find the bulk
density and moisture content.
This is usually done using the Standard
Proctor test.
PROCTOR TEST


The Standard Proctor test is a method of
finding the optimum moisture content for
compaction of a soil.
A cylindrical mould 0.001 m3 in volume is
filled with a sieved soil sample in three equal
layers, each layer being compacted by 25 or
27 blows in a standard hammer, weight 2.5
kg, dropped from a height of 300 mm for
each blow.
Proctor Test Contd.




The mould is then trimmed and weighed, to
determine the bulk density of the soil.
Moisture content of the soil is then
determined to obtain the dry density.
The test is carried out with soil at different
moisture contents and a graph of dry density
against moisture content is plotted.
A heavy compaction test uses a greater
compactive effort from a 4.5 kg hammer
dropping 450 mm on to five soil layers in the
mould.
Typical Proctor Test Curve
Proctor Test
APPLICATIONS OF PROCTOR
TEST IN AGRICULTURE


Proctor Compaction Soil mechanics Test
can be used to index and predict with
reasonable accuracy, the compaction
behaviour of agricultural soils over a wide
range of soil moisture contents and single or
multiple passes of tyres of mechanical
equipment with varying contact pressures.
The knowledge of the moisture content and
pressure changes on dry density of a soil
could be provided in order to make
recommendations to the farmer or machine
designer.
APPLICATIONS OF PROCTOR
TEST IN AGRICULTURE CONTD.

The Proctor compaction test has
hitherto been reserved for earthwork
engineering.

In agricultural practice, it is advisable to
limit soil working below the optimum
moisture content in order not to cause
maximum soil compaction.
Factors that Affect Soil Compaction
(1)The Magnitude and Nature of Compacting forces: The
higher the Compactive effort, the higher the maximum dry density
but the optimum moisture content reduces.
i)
d
Higher compactive force
Lower compactive force
% Moisture Content
The
extent of soil compaction also varies according to
whether the force acts by impact, kneading action or
vibration etc.
Factors that Affect Soil Compaction
Contd.






ii) Moisture Content of the Soil
(see diagram
above).
iii) The Degree of Compaction of the Soil at the
time of compaction.
iv) Soil properties eg. texture, density, and
organic matter content:
Sandy soils are more compactible than clays but
clays have higher optimum moisture contents.
Organic matter reduces the maximum dry density
and increases the optimum or critical moisture
content.
This increases soil workability since it can be worked
over a wider range of moisture content without
achieving maximum compaction.
Example

Example: Standard Proctor Compaction
test carried out on a Piarco sandy soil
yielded the following results:

Bulk density(kg/m3)
1860
Moisture content(%)
24.7


1700
1880
2010
1940
5.1
10.4
14.4
19.6
Plot the curve of dry density against moisture
content and hence find the maximum dry
density and the optimum(critical) moisture
content.
Solution

Solution:

where: rd = dry bulk density,
m = Moisture Content




m
0.051
r
1.70
(gm/cm3)
rd
1.62
(gm/cm3)
r =
1+m
0.104
1.88
1.7
rd
r = Wet Density,
0.144
0.196
0.247
2.01
1.94
1.76
1.62
1.86
1.49
Solution Concluded
Dry bulk density (gm/cm 3)
Compaction Curve For Piarco Sand
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0
10
20
30
Moisture Content (%)
From graph, Maximum Dry Density = 1.76 gm/cm3 and
Optimum(critical) Moisture Content = 14.5%.