A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
compacting effort of using 2.5kg (or 4.5kg) rammer in laboratories. In case
the compaction test results indicate values exceeding 100%, it only means
that the in-situ compaction is more than that being carried out in
laboratories which is treated as the basic criterion for satisfactory degree of
soil compaction. Therefore, the soil results are acceptable in case
compaction test results are over 100%. However, excessive compaction
poses a risk of fracturing granular soils resulting in the reduction of soil
strength parameters.
3. Are there any differences in the methods of compaction between
clayey soil material and sandy material?
As suggested by Lars Forssblad (1981), the three main actions of
compaction are static pressure, impact force and vibration. Different
compactors contain one or more modes of these actions. For example,
vibratory tampers perform mainly by the principle of impact while vibratory
rollers work with principle of static pressure and vibration.
For sandy soils, vibration is adequate for normal compaction because the
action of vibration sets the soil particles in motion and friction forces
between soil particles are virtually demolished. During this vibration motion,
the soil particles rearrange themselves to develop a dense state.
For normal soils, it is necessary to combine the action of vibration together
with static pressure to breakdown the cohesion forces between soil
particles in order to allow for better compaction. The static pressure of
vibratory machines is adopted to exert a shearing force to eliminate the
cohesion in clayey soils.
4. For compaction of free-draining sands or gravels, what is the
optimum moisture content to achieve maximum density? (C2)
The compaction curve of sandy materials is totally different from that of
clayey materials. For sands or gravels, there are two situations of
maximum density, namely the completely dry condition and the complete
water saturation. For moisture content of sands and gravels between these
two states, the dry density obtained is lower than that obtained in the
above-mentioned states. The presence of capillary forces account for the
difficulty of compaction sand at water contents between virtually dry and
saturated state. They are formed in partially filled water void between soil
particles and perform as elastic ties cementing soil particles together.
Reference is made to Lars Forssblad (1981).
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