compaction of soil

1. Chapter Seven
Soil Compaction
BY . DUMESSA GUDISSA (M.Sc.)
June, 2016 G.C
MADDA WALABU UNIVERSITY
Soil Mechanics – I
Lecture Note

2. 1. INTRODUCTION
• Soil compaction is the densification- reduction in void ratio-of a soil
through the expulsion of air.
• This is normally achieved by using mechanical compactors, rollers,
and rammers with the addition of water.
• Soil compaction is the least expensive method of improving soils.
• Thus, compaction results in an increase in the density of the
soil, improves the engineering properties of soils, increases the
shear strength of the soil & consequently the bearing capacity,
reduces the compressibility & permeability of soil.
 Definitions of Key Terms
• Compaction - is the densification of soils by the expulsion of air.
• Maximum dry unit weight(𝜸𝐝𝐦𝐚𝐱) is the maximum unit weight that
a soil can attain using a specified means of compaction.
• Optimum water content (𝑾𝒐𝒑𝒕) is the water content required to
allow a soil to attain its maximum dry unit weight following a
specified means of compaction.

3.  The difference b/n compaction & consolidation is as follows even
though both cause reduction in volume.
1. Compaction is a rapid process by which a reduction in volume
takes place by mechanical means where as consolidation is a
gradual process of volume reduction by static and uniform load.
2. Compaction is the reduction in volume of a partially saturated soil
mass which takes place as a result of expulsion of air from the
voids at the same water content where as consolidation is the
reduction in volume of a saturated soil mass as a result of expulsion
of water from the soil.
3. Compaction is done artificially to increase the engineering
properties of the soils where as consolidation takes naturally when
soils are subjected to static loads.
1. INTRODUCTION

4. Basic concepts
• 𝜸𝒅 =
𝑮𝒔
𝟏+𝒆
𝜸𝒘 =
𝜸
𝟏+𝒘
=
𝑮𝒔
𝟏+
𝒘𝑮𝒔
𝑺
𝜸𝒘
• The extreme-right-hand term was obtained by replacing 𝒆 by 𝒆 =
𝒘𝑮𝒔
𝒔
• How can we increase the dry unit weight?
• by reduce the void ratio, that is
𝒘
𝑺
must be reduced since 𝑮𝒔 is constant.
• The theoretical maximum dry unit weight is obtained when
𝑺 = 𝟏 𝑺 = 𝟏𝟎𝟎% , that is, 𝒆𝒎𝒊𝒏 = 𝒘𝑮𝒔

5. 2. Compaction Test In The Lab
 The laboratory test generally used to obtain the maximum dry unit
weight of compaction and the optimum moisture content is called the
Proctor compaction test(Proctor, 1933).
 Objective of compaction test: determining the optimum moisture
content and maximum dry density achievable with a given compactive
effort.
1. Standard Proctor Test method
 In the Standard proctor test, the soil is compacted in a mold that has a
volume of 944 cm3.
 The diameter of the mold is 101.6 mm (4 in.)
 During the laboratory test, the mold is attached to a baseplate at the
bottom and to an extension at the top (Figure a).
 The soil is mixed with varying amounts of water and then compacted
in three equal layers by a hammer(Figure b) that delivers 25 blows to
each layer.

6.  The hammer has a mass of 2.5 kg(6.5 lb) and has a drop of 305
mm (12 in.).
 Figure c is a photograph of the laboratory equipment required for
conducting a standard Proctor test.
2. Compaction Test In The Lab

7. 2. Compaction Test In The Lab

8. 2. Modified Proctor Test
 With the development of heavy rollers and their use in field
compaction, the standard Proctor test was modified to modified
Proctor test which better represent field conditions.
 For conducting the modified Proctor test, the same mold is used
with a volume of 944 cm3 (1/30 ft3), as in the case of the standard
Proctor test.
 However, the soil is compacted in five layers by a hammer that has a
mass of 4.54 kg(10 lb).
 The drop of the hammer is 457 mm (18 in.).
 The number of hammer blows for each layer is kept at 25 as in the
case of the standard Proctor test.
 The compaction energy for this type of compaction test can be
calculated as 2700 kN-m/m3 (56,000 ft-lb/lb3).
 Because it increases the compactive effort, the modified Proctor test
results in an increase in the maximum dry unit weight of the soil.
 The increase in the maximum dry unit weight is accompanied by a
decrease in the optimum moisture content.
2. Compaction Test In The Lab

9. 3. Interpretation of Proctor Test Results
 the optimum water content and the maximum dry unit
weight of soils is very important for construction
specifications of soil improvement by compaction.
 Specifications for earth structures (embankments, footings,
etc.) usually call for a minimum of 95% of Proctor maximum
dry unit weight.
 This level of compaction can be attained at two water
contents—one before the attainment of the maximum dry
unit weight, or dry of optimum, the other after attainment of
the maximum dry unit weight, or wet of optimum.
 Normal practice is to compact the soil dry of optimum.
Compact the soil wet of optimum for swelling (expansive)
soils, soil liners for solid waste landfills, and projects where
soil volume changes from changes in moisture conditions are
intolerable.

11. Example
1. The wet mass of one of the standard Proctor test samples is 1806
grams at a water content of 8%. The volume of the standard Proctor
test sample is 9.44 × 10−4𝑚3. Determine the bulk and dry unit
weights.
2. The results of a standard compaction test are shown in the table below.
a. Determine the maximum dry unit weight and optimum water content.
b. What is the dry unit weight and water content at 95% standard
compaction, dry of optimum?
c. Determine the degree of saturation at the maximum dry density
d. Plot the zero air voids line.
Water content (%) 6.2 8.1 9.8 11.5 12.3 13.2
Bulk unit weight (kN/m3) 16.9 18.7 19.5 20.5 20.4 20.1

12. 4. Benefits of Soil Compaction
The benefits of compaction are:
1. Increased soil strength
2. Increased load-bearing capacity.
3. Reduction in settlement (lower compressibility).
4. Reduction in the flow of water (water seepage).
5. Reduction in soil swelling (expansion) and collapse (soil
contraction).
6. Increased soil stability.
7. Reduction in frost damage.
Improper compaction can lead to:
1. Structural distress from excessive total and differential settlements.
2. Cracking of pavements, floors, and basements.
3. Structural damage to buried structures, water and sewer pipes, and
utility conduits.
4. Soil erosion.