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Table
1 Cohesive Materials - Table 2
Granular Materials - Table 3
Bituminous Materials
Ensuring
adequate compaction has been achieved will prevent shrinkage
and settlement, it will also allow for better load spreading
capabilities and stability.
Load
Spreading
Air voids in the soil result in weakness and inability
to carry heavy loads. Mechanical compaction locates the
particles closer together and reduces the percentage of air
voids. The lower the percentage of air voids in the soil the
denser the material which will allow for higher load
spreading capabilities.
Stability
If the compaction is carried out unevenly, the load
spreading capabilities of the soil is reduced in the areas
which have not been compacted sufficiently. This can cause
surface cracking or uneven settlement.
Shrinkage
Where there is a high percentage of air voids water will
accumulate in these voids. The result is that the soil will
swell during wet periods and shrink during dry spells. This
can be excessive where compaction has not been sufficiently
carried out
Settlement
Where compaction has not been adequate, excessive water
will accumulate in the voids. During cold spells this water
can freeze. On freezing the water expands as ice and can
cause heave commonly known as frost heave. In warmer weather
excessive voids are left which will allow the upper layers
to settle. Settlement can also be caused by inadequate
compaction.
Material
Compactability
Several factors effect the compactability of the material.
These are primarily, type of material, aggregate or particle
size, grading and moisture content.
HAUC
Specification for Compaction
The following tables set out the optimum compaction passes
using the equipment shown in the left column. Cohesive and
granular materials compaction passes are based on the
optimum moisture content of the material. It is essential to
ensure that the compaction equipment used complies with the
specification.
A twin drum
roller should be the preferred equipment to compact
bituminous materials, some twin drum rollers only have
vibration on one drum and should be treated as a single drum
roller.
The use of a
light weight plate is not permitted as it is only suitable
for block paving work and would not provide enough
compactive force to adequately compact the materials.
All compaction
equipment should be regularly checked and serviced to ensure
that it is operating at the manufacturers recommended
operating frequency.
Compaction
Passes
The sequence of operations used to compact materials
cannot be over emphasised as poor compaction will lead to a
failure of the reinstatement due to settlement beyond the
intervention limits given in the 'Specifications'. All bound
and unbound layers should start at the edges of the
reinstatement and a pattern followed that will complete a
series of parallel overlapping passes working towards the
centre. The first pass given to the material should be on
low operating frequency and the subsequent passes on full
frequency (full throttle). When using a roller the first
pass should be a 'dead roll' without the vibration engaged
to smooth the materials prior to using the vibration passes
from the tables below. Care should be taken not to compact
surfacing materials near or over highway ironwork, which may
disturb it, cracking any mortar bed and allowing the ingress
of water. In this instance hand tool should be used.
The layer
thickness shown in the tables below are 'compacted'
thickness and sufficient surcharge should be used to achieve
this.
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