Compaction Oct2010

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-1-Soil Compaction (Dr. P McMahon)

SOIL COMPACTIONIntroduction

Generally, soil consists of solid particles, water and air;

Compaction of the soil, usually by mechanical means, reduces the air voids with the aim of: -

Achieving a state of increased unit weight Increasing the shear strength of the soil Reducing the permeability Making the soil less susceptible to settlement under loading(Self weight and especially repeated loading e.g. traffic)

In order to achieve any of these aims when constructing dams, embankments, retainingwalls, roads or runways, strict control of the construction methods is required.

If compaction is not carried out thoroughly during construction, future loading of the soil(when it forms part of a permanent structure) will cause further undesirable settlement withassociated deformation and likely damage.

Definitions

Compaction

Compaction is the process of increasing the density of the soil by packing thesolid particles closer together by reducing the volume of the air.

The degree of compaction is measured in terms of dry density, d. This represents the

mass of solid particles in a given volume of soil, i.e. 1m3. Compaction should aim to

achieve as high a dry density as possible.

Laboratory compaction tests and field tests provide values for soil density, , and water

content, w.

Dry density can be calculated from soil density and water content as shown below:


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Content, AV

AV =Volume of air

=VA

=VV - VW

Total volume V V

Where;VA = volume of air voids

VW = volume of waterV = total volume of soil

VA is usually expressed as a % but used in equations as a ratioe.g. 5% air voids = 0.05

AV =e w GS

1 + e

The air voids content, Av, of a soil after laboratory or on-site compaction is an important

parameter:

If all the air could be expelled by compaction the soil would be fully saturated and the drydensity would be the maximum possible value for the given water content. However, thisdegree of compaction is not attainable in practice.

The maximum possible value of dry density is referred to as the zero air voids dry densityor the saturated dry density(i.e. mass of solids when there is only water in the soils voids)and can be found from: -

NOTE: AV and w are expressed as ratiose.g. 5% air voids = 0.05

W = 1000kg/m3

The relationship between zero air voids dry density and water content produces a curve,known as the zero air voids line or the saturation line.

Curves relating to dry density at air contents of 5% and 10% with water content can alsobe drawn, see next page.

d =Gs(1 - AV)W

(1 + w Gs)


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NOTE: 1.463 Mg/m3

= 1463 kg/m30.5 kg/m3

These curves enable the air content at any point on the experimental dry density/ watercontent curve to be found by inspection.

Laboratory compaction tests

Since the degree of compaction is influenced by the compactive effort applied there arethree laboratory tests which attempt to reflect the compactive effort available on site: -

1. BS Light Compaction Method [2.5kg rammer] (cohesive soils), or2. BS Heavy Compaction Method [4.5 kg rammer] (cohesive soils)3. Vibrating hammer methods (granular soils).

Cohesive soils

Light Compaction Test Heavy Compaction Test

Rammer 2.5 kg 4.5 kgDrop 300 mm 450 mmNo. of Layers 3 5No. of blow per layer 27 27


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NOTE: 1.621 Mg/m3

= 1621 kg/m3

Note: A greater compactive effort results in a higher max dry density and a lower optimum

water content, however, the values of air content at max dry density are approx the same.

Granular soilsThe vibrating hammer method consists of an electrical vibrating hammer with a tamperattached. The soil is compacted in three layers, each layer is subjected to vibration fromthe hammer under a constant downward force for 60 seconds. The test is intended toreflect the compaction obtainable on site using vibratory plant (applies to granular soils/fill).

Class example 1In a BS compaction test the following data was collected:

Water content % 5 8 10 13 16 19

Soil density kg/m3 1870 2040 2130 2200 2160 2090

Specific gravity, Gs, of the soil particles was found to be 2.7

a) Draw a graph of dry density against water content and from it determine themaximum dry density and optimum water content.

b) On the same axes, draw the d/w curves for zero and 5% air content and hence

determine, by inspection, the air content at maximum dry density.(HINT: Try water content, w, values of 10, 14, and 18 %)

Ans: 1955 kg/m3 ; 12% ; 4%


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Specifications for compaction

NOTE: Specifications for Earthworks in the UK exclusively refer to the MANUAL OFCONTRACT DOCUMENTS FOR HIGHWAY WORKS VOLUME 1 SPECIFICATION FOR HIGHWAYWORKS Series 600 Earthworks and Notes for Guidance NG600 (often referred to as MCHWEarthworks Documents used to be called the Department of Transport Specification forRoads and Bridge Works).1. End - Product Specification

A required property of the compacted soil (eg a minimum density or a maximum aircontent) is specified.Requires in situ density tests and laboratory water tests.

The two most common approaches are: -a) relative compaction

The required dry density of the earth fill must be greater than a certain proportion(90, 95, 100%) of the dry density obtained from a laboratory test - usually the BSlight (2.5 kg) or vibrating hammer test.

b) air voids contentThe maximum air voids content Av of the earth fill is quoted eg 10% for the bulkof an earth embankment, with 5% for the top layers to provide a good foundationsupport or sub-grade for a highway.

End product specification requires in situ samples of soil density to be taken in order toobtain values of dry density or air void content. Suitable methods include:-

Core cutter methods for cohesive soils ) Soil density & Sand replacement method for granular soils ) Water content Nuclear moisture/density gauge

for a wide range of materials (gives approximate water content)

See Class Example 2

2. Method Specification

A specification developed by the Department of Transport which specifies for majorprojects, a method of compaction to be used. Based on the results of previouslaboratory tests.

The type of plant, layer thickness and number of passes are specified. (see attached

extracts from MCHW Specification Documentation. Specifically refer to Tables 6/1, 6/2and 6/4 (although ensure that you always read the preceding text prior tom using theTables in practise).

See Class Example 3


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Class example 2: pre-compaction testingA highway embankment is to be constructed from clay, imported from anadjacent site. A laboratory light compaction test has been carried out on asample of the clay and the resulting plot is shown below.

The specification for site compaction requires that the final compacted dry

density of the clay be at least 95% of the maximum value achieved in thelaboratory test. After compaction by field plant, an undisturbed sample ofthe compacted clay, obtained by core cutter, weighed 2072g. The corecutter was 127 mm long with a diameter of 102 mm. After drying, thesample weighed 1806 g. Specific gravity of soil particles is 2.70.

Determine whether the compaction specification has been satisfied.

Fig. Q1

1550

1600

1650

1700

1750

1800

1850

1900

8 10 12 14 16 18 20

Moisture content %

Drydensity

kg/cum


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Class example 3: post-compaction testinga) A landfill site is to be lined with a compacted clay soil. A sample of the clay

has been subjected to a light compaction test in the laboratory and theresulting plot is shown below.

i) Describe the laboratory test procedure

ii) If the specific gravity of the clay particles was found to be2.68, determine the void ratio of the clay at maximum drydensity.

b) The specification for the compaction of the clay states that at least 95% ofthe maximum dry density achieved in the laboratory is obtained on site.After site compaction a core sample is taken and the following daterecorded.

Core size: 101.6 mm diameter203 mm long

Natural mass: 3702 gMass after drying 3358 g

For the compacted clay, determine;i) Soil densityii) Water contentiii) Dry densityiv) Whether the specification has been satisfied

Fig. Q2

1800

1850

1900

1950

2000

2050

6 8 10 12 14 16

Moisture content, %

Dr

ydensity,

kg/cum

Compaction Oct2010

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