Pressurizing the soil causes air to escape from the pores forming a strong soil base – a significant part of the construction process.
The rolling of a road roller :
close-up of a road roller wheel compacting the road
We are quite familiar with the road rollers. The compact roller or road roller as it is commonly called is extensively used for paving roads. It is the simplest example that can be cited for understanding the compaction process. The heavy wheels of the roller apply direct pressure on a specific layer hence densifying the soil and enhancing its load-bearing capacity, leaving behind a smooth surface.
The soil under stress:
pictures depicting the difference between compacted and uncompacted soil
When the soil is subjected to stress, the pore size decreases, and the voids that were occupied by air get replaced by the soil particles. The particles come together and form a strong soil base. In case of improper compaction, the soil settles which results in structural failure.
The determinants of compaction:
The mechanical process largely depends on three factors – the type of soil, the moisture content in it, and the effort required for compaction.
The fine sand or silt that is of a coarse texture crumbles easily when dry, but when moist the granular soil possesses no plasticity. Clayey soil or cohesive soil crumbles with more difficulty when dry.
Moisture thus plays a significant role in compaction. What if a paved road turns into liquid during the rainy season?
Moisture: the game changer
The moisture acts as a lubricant for the soil. It is easier to slide the soil particles together in the presence of sufficient moisture. Less moisture content results in inadequate compaction. Too little moisture or too much moisture does not work well. In the presence of large moisture content, the soil is left behind with water-filled voids. To keep it simple, we can say that a water-saturated state is best for the compaction of the soil.
Also matching the soil with the proper method of compaction is commendable.
The static and vibratory forces:
There are two basic types of compaction – static and dynamic.
Static compaction, as the name suggests – is the direct application of force on the required surface. The amount of force is altered by making changes in the machine weight. Kneading and pressure are categorized under static compaction. Dynamic compaction or vibratory force makes use of an engine-driven mechanism by creating a downward force in addition to the machine’s static weight. Vibrations set the soil particles in motion which brings them close together for the best possible density.
The choice of equipment:
The compaction machines are designed with precision to match the necessities of the soil. Two factors largely determine the impact a compact machine creates – its frequency and its amplitude. frequency shall be defined as the number of vibrations a machine can produce within a minute while the amplitude is the maximum movement a vibrating body could produce. equipment types may range from rammers and vibratory plates to the huge rollers, which have been in use since the early 20th century in the form of steam rollers. rollers have several varieties from smooth rollers to Padfoot, tamping, and pneumatic rollers. The smooth rollers make use of static force and work best on granular soil. Padfoot and tamping rollers work best on compact soils with more cohesiveness.
Significance of mechanical compaction:
The stiffness of chemically modified soils can be attributed to soil compaction. The friction arising out of the interlocked particles adds to the shear strength of the soil. With an enhanced load-bearing capacity, the process also results in a dense soil layer which prevents soil settling, shrinking, and expanding thereby avoiding structural failures. Compaction provides stability to the soil base by preventing water seepage and swelling. It also protects the layer from frost damage.
Natural compaction of soil
The natural compaction of soil is attributed to rain and negatively affects vegetation. The plant roots find it difficult to penetrate deeper into the soil and the sprouting shoot is resisted by the thin crust layer formed by rain. Compacted soils thus cause stunted plant growth.