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
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
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
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
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