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), is given by:

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.

.

Where: H is the new height of the sample at any applied stress (see figure below)

i Mass

Volume

M

H As

0

1.

........................................( )

Strain at any applied stressChange in soil sample height

Original height( ) ,

H H

Ho

o

...............................( )2

Soil Compression Test

HHo

F

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, ...... (4)

Substituting Eqn (3) into Eqn (4),

These equations were first derived by Stone and Ekwue (1995).

b at any stressM

V

M

H A

.

b

bi i

b

M

H A

From Eqn and

0 1

11

1

( )

( ),

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)

  .........................(6)Also Soil particle density is:

Note: Soil particle density can be taken as 2.65 gm/cm3 for most mineral soils.

eV V

V

V

Vs

s s

1

ss

sb

s

s

b

s

s s s

s

b

M

Vand dry bulk density

M

V

i eM

VxV

M

V

V

From Eqn e

,

. .

( ), ....( )6 1 7

Soil Compression Index

2.1.3 SOIL COMPRESSION INDEXSoil compression index, Cc is defined as:

Where: e1 and e2 are void ratios at two applied stresses

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 soil particle density is 2.65 gm/cm3.

Ce e

c ( )

log( / )2 1

2 1

1 2and .

Example Contd.

Applied stress Bulk density Strain Void ratio (kPa) (gm/cm3) 10 1.09 0.21 1.43 40 1.20 0.28 1.21

60 1.24 0.31 1.1480 1.27 0.32 1.09100 1.29 0.33 1.05150 1.34 0.36 0.98200 1.37 0.37 0.93400 1.45 0.41 0.83500 1.47 0.42 0.80600 1.49 0.42 0.78800 1.52 0.43 0.741000 1.55 0.45 0.71

 Solution: Using stresses of 10 kPa and 100 kPa

38.0)10/100(log

)43.105.1(

cC

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

i)    (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.  

Higher compactive force   

d 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) 1700 1880 2010 1940 1860 Moisture content(%) 5.1 10.4 14.4 19.6 24.7

Plot the curve of dry density against moisture content and hence find the maximum dry density and the optimum(critical) moisture content.

Solution

Solution: r = rd

1 + m where: rd = dry bulk density, r = Wet Density, m = Moisture Content

  m 0.051 0.104 0.144 0.196 0.247 r 1.70 1.88 2.01 1.94 1.86

(gm/cm3) rd 1.62 1.7 1.76 1.62 1.49

(gm/cm3)

Solution Concluded

Compaction Curve For Piarco Sand

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

0 10 20 30

Moisture Content (%)

Dry

bu

lk d

en

sit

y (

gm

/cm

3 )

From graph, Maximum Dry Density = 1.76 gm/cm3 and Optimum(critical) Moisture Content = 14.5%.