As discussed in Sec. 5.5, several variables control the vibratory compaction and densification of
granular soils. They include:
1. Characteristics of the compactor
- Mass
-Size
-Operating frequency and frequency range
-Amplitude of vibration
2. Characteristics of the soil
- Initial density
-Grain size distribution
- Grain shape
- Water content
3. Construction procedures:
- Number of passes of the roller
- Lift thickness
- Modification of the vibrator frequency during compaction
-Towing speed
The compactor characteristics influence the stress level, density increase, and depth of influence of the
dynamic force. For example, as shown in Fig. 5.15, when oscillation is added to a static component,
the density is significantly increased. The influence of operating frequency for various soil types is
shown in Fig. 5.16. Note how a peak in the density-frequency curve develops for most of the soils, even
clays. The frequency at which a maximum density is achieved is called the optimum frequency. It is a
function of the compactor-soil system, and it changes as the density increases during the process of com-
paction. It is desirable for a compactor to have the capability to vary its operating frequency and to have
the range required to obtain maximum density. However, the peaks are gentle, and, on a percentage
basis, a wide frequency range is not all that important.
Soil conditions are also important. The initial density in particular strongly influences the final
density. For example, the upper 300 mm of medium dense sand may never become denser than the
initial density, whereas dense sands will become looser in the upper 300 mm.This phenomenon is also
illustrated below.