Capital Equipment News August 2016

THE BASICS OF SOIL COMPACTION – Oscillation

I n its previous column (CEN May 2016) Wirtgen discussed Compaction Methods namely, Static and Dynamic. It further noted that Dynamic Compaction could be divided into Vibration and Oscillation.

Oscillation

What then is oscillation, introduced by Hamm in 1983, and what are the benefits?

During oscillation, the drum is equipped with two eccentric imbalance shafts rotating in sync. The imbalanced weights on the two shafts are arranged opposite each other and force the drum to rotate rapidly, alternating in a forwards-backwards movement. In contrast to the vibrating drum, the oscillating drum does not lose contact with the surface and in effect ‘massages’ the material to achieve compaction. During oscillation compaction, both the forward and backward ro- tating movements of the roller drum transmit shearing forces into the soil or asphalt. This ‘double frequency’ and permanent contact of the rollers’ own static weight causes the material to compact more quickly. On the other hand, the vibration drum executes an up and down movement and each time the imbalance weight rotates, it transmits a vertical force to the material along with the static weight of the machine one vibration at a time. As with vibration, the compaction performance of the oscillation is based on well matched amplitudes and frequency. However, an ad- vantage of the oscillation technology is its self-regulating amplitude which is regulated by the material that is being compacted. This means that as compaction causes the material to become more rigid, the amplitude decreases automatically and over-compaction is avoided. Compaction has evolved from the 1800s when rollers were made of stone or cast iron and drawn by man or beast during construc- tion. Today, we are faced with physical, environmental, human and statutory challenges during construction work. Therefore having the right equipment and choice of technology is important. Of even greater importance is knowing when and how to apply this technology. b

Vibration

During compaction, the vibration of the roller drum is generated by the rotating imbalance weight whose rotation speed determines the vibration frequency. The imbalance weight consists of a fixed part and a moveable weight. The position of the moveable weight depends on the direction of rotation of the exciter shaft. The effective weight of the imbalance weight increases or decreases depending on the direction of rotation. This allows the roller drum to vibrate at two different amplitudes.

Oscillating technology has an effect on the operator, machine, compaction and the surrounds:

Operator & Machines

Compaction

Soil Compaction

Asphalt Compaction

Surrounds and Environment

Less vibratory impact provides better driver comfort

High compaction through combination of static and horizontally applied shear forces Constant contact with the surface, load and shear forces

No particle destruction due to high impact forces

Effective compaction at lower temperatures

Lower impact level so less effect on already compacted surfaces in adjacent areas

Lower noise levels

Good surface compaction and avoidance of unravelling from over compaction

Good surface sealing and finish Lower impact and noise levels so less effect on surrounding buildings, underground services and inner city neighbourhoods

are continually applied to achieve density quicker

Fewer resonances so less stress on the machine structure

Fewer passes means lower fuel consumption

Good course bonding

No particle destruction on existing material during hot-on-cold seam compaction

Risk-free compaction in multi-story carparks or on concrete structures

CAPITAL EQUIPMENT NEWS AUGUST 2016 16