Compaction
Chapter 3
Compaction
 To improve the density and other properties of soil
 Increases the solid density improves strength
 Lowers its permeability
 Reduces future settlement
Maximum Dry Density
 Compaction requirements are measured in terms of dry density
 Example
 Clay soil may not reach 2000kg/m2
 Granular soil may have a dry density above clay value before
compaction
 Dry density will vary with its water content
 Water acts as a lubricate to the grains of soil when compact
 Air is forced out of the soil – leaving more space for soil solids
 However when most of the air has been removed the dry density
does not increase much further
Compaction curve
 First step is to determine maximum dry density that can be
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expected for a soil under certain compactive effort and water
content
Compaction curve is also called moisture density curve or proctor
curve
Dry desity is plotted against water content
Curve is drawn through the test points
The top of the curve represents
 Maximum dry density for the soil with the test compactive effort
 The corresponding water content – know as the optimum water
content (wo)
Standard compaction test
 Soil is compacted in a 10cm diameter mold have a volume of
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943.9cm3
The soil is placed in three layers
Each layer compacted 25 times with a 2.5 kg hammer falling 30cm
Usually test is conducted on only material that passes no 4 sieve
If larger grains are to be included a larger mold is used
In the test – a sample of soil is mixed with water and then
compacted
 Mass of the compacted sample is measured
 And water content id determined
Compaction Curve Example
 Pg 99 -100
 To aid in the drawing the mositure density curve - a
maximum theoretically possible density – zero air voids
(Zav) CURVE IS PLOTTED
 Rules for zav curve
 No point can be above the zav line – therefore errors are
obvious
 The slope of the moisture density curve on the wet side of
optimum moisture content is parallel to the zav curve – this is
helpful in sketching the curve where test results are erratic
Compaction Curve
 Moisture density curve is different for each soil
 Granular well graded soils generally have fairly high
maximum densities at low optimum moisture contents
 Clay soils have lower densities
 Since compaction equipment has become better over time
 Modified compaction test
 4.5 kg hammer falling 45cm on each of five soil layers in the same mold
 Maximum dry density obtained in the this test is higher
Compaction Curve
 Compaction of soils varies – construction requirement are
usually specified as a percent of maximum dry density
 Example would be 95%
 If the maximum dry density from the test was 2000kg/m3 at
an optimum water content of 11% the required field density
would be 95% of 2000 or 1900kg/m3 – moisture content
should be as close as possible to 11% which reduces the
required compactive effort
 Table 3-1 on pave 103 give typical values for aashto
Field Density
 Quality control of compaction involves measuring field density
 And comparing field results to lab results
 Usually made with a nuclear densometer – however other test are
available
 Sampling method
 Sample of compacted material is dug out of a test hole in the soil layer
being checked
 Dry mass of the soil removed and the volume of the hole are measure
 Field dry density equals the dry mass divided by the volume originally
occupied by the sample
 Two basic methods are used to measure the volume of the hole –
balloon filled with a liquid and a sand cone apparatue
Balloon apparatus
 The volume of the sample hole is found by forcing a liquid
filled balloon into the test hole
 Rubber membrane allows the fluid to fill all the cavities in
the test hole
 Volume of fluid required to do this is read on a scale on the
apparatus
Sand Cone apparatus
 Volume of the test hole is obtained from the mass of loose sand
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required to fill the hole
The sand is uniform medium sand that has an essentially constant
loose density when poured into a container
The cone acts a a pouring funnel
The mass of the sand container with an attached cone is obtained
The container is then inverted over a test hole – value at the small
end of the cone is open and sand flow into test hole
When flow stops the valve is closed – mass of the sand container
with its cone is measured
Example 3-4 page 105
Quality control
 Problem 3-5 page 105
Nuclear method
 Main method used today
 Test only takes a minute
 Gamma rays are emitted into the soil – these rays are
absorbed by the soil in amounts that vary according to the
density of the soil
 Neutron radiation is also emitted – these neutrons lose
energy due to collision with hydrogen atoms in the soils
 A detector measures the amount of moderated neutrons – thus
moisture content can be obtained
Compaction operations and quality
control
 In highway and other types of construction
 The layer of si9l 15-30 cm in depth that provides the foundation
for the pavement structure
 Fill material used to bring the soil surface up to the subgrade
level
 Spread in layers 15-30 cm thick – each layer compacted
 Backfill materials in trenches opened for utility
 Layers of granular material used in the pavement
 The asphalt layer
Compaction equipment
 Rate of vibration of 1500 to 2500 cycles per minute
 Mass of drum from 2-15 tonnes (2-17 tons)
 Very effective for gravel snad and silt soils
 Layers .5 -1 m in depth usually require 3-5 passes
 Padded wheel or sheep foot
 Most effective for clay and silty clay soil
 Pressure exerted by the pad can be 1000-5000kpa
 Compact layers 15-30 cm in 3-5 passes
 Pneumatic tire rollers
 15cm layers
 Smooth drum – non vibrating rollers
 Finished or sealing passes
Deeper Compaction Methods
 Last slide use to less then 1 meter deep
 Vibrocompaction methods can compact much greater depths
 Large vibrator
 Dynamic compaction