BFC 21702 GEOTECHNICS 1 Chapter 3: Compaction N. Sivakugan
TABLE OF CONTENTS 3.1 General Principles 3.2 Standard Proctor Test 3.3 Modified Proctor Test 3.4 Factors Affecting Compaction 3.5 Structure of Compacted Soil 3.6 Field Compaction
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3.1 What is compaction? A simple ground improvement technique, where the soil is densified through external compactive effort. Compactive effort
+ water =
) d
Compaction Curve
( y t i s n e d y r D
Soil grains densely packed - good strength and stiffness - low permeability
d, max
optimum water content
Water content
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Compaction Curve What happens to the relative quantities of the three phases with addition of water? air water solid
) d
( y t i s n e d y r D
difficult to expel all air
lowest void ratio and highest dry density at optimum w
Water content
Zero Air Void Curve ) d
- corresponds to 100% saturation
( y t i s n e d y r D
Zero air void curve (S=100%)
S<100% S>100% (impossible) All compaction points should lie to the left of ZAV curve
Water content
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Line of Optimum
) d
( y t i s n e d y r D
Compaction curves for different efforts
Line of optimum
Water content
3.2 & 3.3 Laboratory Compaction Test (after BS1377:Part 4: 1990) - to obtain the compaction curve and define the optimum water content and maximum dry density for a specific compactive effort.
Standard Proctor:
hammer
Modified Proctor:
• 3 layers
• 3 to 5 layers
• 27 blows per layer
• 27 blows per layer
• 2.5 kg hammer
• 4.5 kg hammer
• 300 mm drop
• 450 mm drop
1000 ml compaction mould
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3.4 Effect of Compactive Effort ) d
( y t i s n e d y r D
E2 (>E1)
Increasing compactive effort results in: Lower optimum water content Higher maximum dry density
E1
Water content
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3.4 Effect of Types of Soil
) d
3.5 Compaction and Clay Fabric
( y t i s n e d y r D
c i r b a f d e s r e p s i d e r o m
Higher water content or higher compactive effort gives more dispersed fabric.
more dispersed fabric
Water content
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3.6 Field Compaction Different types of roller
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3.6 Compaction Control Test Compaction specifications
d
Compare! d,field = w
wfield
? =?
compacted ground
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Core-cutter method Details of the core-cutter apparatus, which is suitable for cohesive soils, are given in figure following After the cutter has been first pressed into the soil and then dug out, the soil is trimmed to the size of the cutter and both cutter and soil are weighed; given the weight and dimensions of the cutter, the bulk density of the soil can be obtained.
Figure: Core cutter for clay soil (Source: Smith, I., 2006)
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Sand Replacement method For granular soils the apparatus shown in figure following is used A small round hole (about 100 mm diameter and 150 mm deep) is dug and the mass of the excavated material is carefully determined The volume of the hole thus formed is obtained by pouring into it with sand of known density from a special graduated container; given the weight of sand in the container before and after the test, the weight of sand in the hole and hence the volume of the hole can be determined.
Sand Replacement method (continued…) The apparatus shown in figure following is suitable for fine to medium grained soils and is known as the small pouring cylinder method. For coarse grained soils a larger pouring cylinder is used. This cylinder has an internal diameter of 215 mm and a height of 170 mm to the valve or shutter The excavated hole in this case should be about 200 mm in diameter and some 250 mm deep. This larger pouring cylinder can also be used for fine to medium grained soils.
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Figure: Sand replacement method (Source: Smith, I., 2006)
