Based on part of the GeotechniCAL reference package
by Prof. John Atkinson, City University, London
Back to Soil Mechanics

Compaction


Compaction is a process that brings about an increase in soil density or unit weight, accompanied by a decrease in air volume. There is usually no change in water content. The degree of compaction is measured by dry unit weight and depends on the water content and compactive effort (weight of hammer, number of impacts, weight of roller, number of passes). For a given compactive effort, the maximum dry unit weight occurs at an optimum water content.


Compaction

Compaction purposes and processes

Compaction is a process of increasing soil density and removing air, usually by mechanical means. The size of the individual soil particles does not change, neither is water removed.

Purposeful compaction is intended to improve the strength and stiffness of soil. Consequential (or accidental) compaction, and thus settlement, can occur due to vibration (piling, traffic, etc.) or self-weight of loose fill.

 


Compaction purposes and processes

Compaction as a construction process

Compaction is employed in the construction of road bases, runways, earth dams, embankments and reinforced earth walls. In some cases, compaction may be used to prepare a level surface for building construction.

Soil is placed in layers, typically 75 mm to 450 mm thick. Each layer is compacted to a specified standard using rollers, vibrators or rammers.

Refer also to Types of compaction plant and Specification and quality control

 


Compaction purposes and processes

Objectives of compaction

Compaction can be applied to improve the properties of an existing soil or in the process of placing fill. The main objectives are to:

 


Compaction purposes and processes

Factors affecting compaction

A number of factors will affect the degree of compaction that can be achieved:

 


Compaction purposes and processes

Types of compaction plant

Construction traffic, especially caterpillar-tracked vehicles, is also used.

In the UK. further information can be obtained from the Department of Transport and handbooks on civil engineering construction methods.


Types of compaction plant

Smooth-wheeled roller

 


Types of compaction plant

Grid roller

 


Types of compaction plant

Sheepsfoot roller


Types of compaction plant

Pneumatic-tyred roller


Types of compaction plant

Vibrating plate


Types of compaction plant

Power rammer


Compaction

Laboratory compaction tests

The variation in compaction with water content and compactive effort is first established in the laboratory. Target values are then specified for the dry density and/or air-voids content to be achieved on site.


Laboratory compaction tests

Dry-density/water-content relationship

The aim of the test is to establish the maximum dry density that may be attained for a given soil with a standard amount of compactive effort. When a series of samples of a soil are compacted at different water content the plot usually shows a distinct peak.


Dry-density/water-content relationship

Explanation of the shape of the curve

For clays
Recently excavated and generally saturated lumps of clayey soil have a relatively high undrained shear strength at low water contents and are difficult to compact. As water content increases, the lumps weaken and soften and maybe compacted more easily.

For coarse soils
The material is unsaturated and derives strength from suction in pore water which collects at grain contacts. As the water content increases, suctions, and hence effective stresses decrease. The soil weaken, and is therefore more easily compacted.

For both
At relatively high water contents, the compacted soil is nearly saturated (nearly all of the air has been removed) and so the compactive effort is in effect applying undrained loading and so the void volume does not decrease; as the water content increases the compacted density achieved will decrease, with the air content remaining almost constant.

 


Dry-density/water-content relationship

Expressions for calculating density

A compacted sample is weighed to determine its mass: M (grams)
The volume of the mould is: V (ml)
Sub-samples are taken to determine the water content: w
The calculations are:

Worked example

A compacted soil sample has been weighed with the following results:
Mass = 1821 g Volume = 950 ml Water content = 9.2%
Determine the bulk and dry densities.

Bulk density r = 1821 / 950 = 1.917 g/ml or Mg/m

Dry density rd = 1.917 / (1+0.092) = 1.754 Mg/m

 


Laboratory compaction tests

Dry density and air-voids content


A fully saturated soil has zero air content. In practice, even quite wet soil will have a small air content

The maximum dry density is controlled by both the water content and the air-voids content. Curves for different air-voids contents can be added to the rd / w plot using this expression:

The air-voids content corresponding to the maximum dry density and optimum water content can be read off the rd/w plot or calculated from the expression (see the worked example).

Worked example

Determine the dry densities of a compacted soil sample at a water content of 12%, with air-voids contents of zero, 5% and 10%. (Gs = 2.68).


Laboratory compaction tests

Effect of increased compactive effort

The compactive effort will be greater when using a heavier roller on site or a heavier rammer in the laboratory. With greater compactive effort:

 

 


Laboratory compaction tests

Effect of soil type

 

 


Laboratory compaction tests

Interpretation of laboratory data

During the test, data is collected:
  1. Volume of mould (V)
  2. Mass of mould (Mo)
  3. Specific gravity of the soil grain (Gs)
  4. Mass of mould + compacted soil - for each sample (M)
  5. Water content of each sample (w)

Firstly, the densities are calculated (rd) for samples with different values of water content, then rd / w curve is plotted together with the air-voids curves.

The maximum dry density and optimum water content are read off the plot.

The air content at the optimum  water content is either read off or calculated.


Interpretation of laboratory data

Example data collected during test

In a typical compaction test the following data might have been collected:
Mass of mould, Mo = 1082 g
Volume of mould, V = 950 ml
Specific gravity of soil grains, Gs = 2.70
Mass of mould + soil (g) 2833 2979 3080 3092 3064 3027
Water content (%) 8.41 10.62 12.88 14.41 16.59 18.62

For method of determining water contents see Soil Description and Classification

 


Interpretation of laboratory data

Calculated densities and density curve

The expressions used are:

Bulk density, r (Mg/m) 1.84 2.00 2.10 2.12 2.09 2.05
Water content, w 0.084 0.106 0.129 0.144 0.166 0.186

Dry density, rd (Mg/m)

1.70 1.81 1.86 1.851 1.79 1.73


Interpretation of laboratory data

Air-voids curves

The expression used is:

Water content (%) 10 12 14 16 18 20
rd when Av = 0% 2.13 2.04 1.96 1.89 1.82 1.75
rd when Av = 5% 2.02 1.94 1.86 1.79 1.73 1.67
rd when Av = 10% 1.91 1.84 1.76 1.70 1.64 1.58

The optimum air-voids content is the value corresponding to the maximum dry density (1.86 Mg/m) and optimum water content (12.9%).


Compaction

Specification and quality control

The degree of compaction achievable on site depends mainly on:

  • Compactive effort: type of plant + No of passes
  • Water content: can be increased if dry, but vice-versa
  • Type of soil: higher densities with well-graded soils; fine soils have higher water contents
    End-result specifications require predictable conditions
    Method specifications are preferred in UK.


    Specification and quality control

    End-result specifications

    Target parameters are specified based on laboratory test results:

    Optimum water content working range, i.e. 2%
    Optimum air-voids content tolerance, i.e. 1.5%

    For soils wetter than wopt, the target Av can be used, e.g.
    10% for bulk earthworks
    5% for important work

    The end-result method is unsuitable for very wet or variable conditions.


    Specification and quality control

    Method specifications

    A site procedure is specified giving:


    Compaction

    Moisture condition value

    This is a procedure developed by the Road Research Laboratory using only one sample, thus making laboratory compaction testing quicker and simpler. The minimum compactive effort to produce near-full compaction is determined. Soil placed in a mould is compacted by blows from a rammer dropping 250 mm; the penetration after each blow is measured.


    Moisture condition value

    Apparatus and sizes

    Cylindrical mould, with permeable base plate:
    internal diameter = 100 mm, internal height at least 200 mm
    Rammer, with a flat face:
    face diam = 97 mm, mass = 7.5 kg, free-fall height = 250 mm
    Soil:
    1.5 kg passing a 20 mm mesh sieve


    Moisture condition value

    Test procedure and plot

     


    Moisture condition value

    Example plot and determination of MCV

    After plotting Dp against the number of blows n, a line is drawn through the steepest part.

    The intercept of this line and the 5 mm penetration line give the MCV

    The defining equation is:MCV = 10 log B
    (where B = number of blows corresponding to 5 mm penetration)

    On the example plot here an MCV of 13 is indicated.


    Moisture condition value

    Significance of MCV in earthworks

    The MCV test is rapid and gives reproducible results which correlate well with engineering properties. The relationship between MCV and water content for a soil is near to a straight line, except for heavily overconsolidated clays.A desired value of undrained strength or compressibility can be related to limiting water content, and so the MCV can be used as a control value after calibrating MCV vs w for the soil. An approximate correlation between MCV and undrained shear strength has been suggested by Parsons (1981).

    Log su = 0.75 + 0.11(MCV)

     

     

     

    Produced by Dr. Leslie Davison, University of the West of England, Bristol, May 2000
    in association with Prof. Sarah Springman, Swiss Federal Technical Institute, Zurich