A study of vibratory densification of sands

The change in relative density is profoundly affected about equally by acceleration and frequency, and also by gradation of sand : secondary factors include amplitude and surcharge. Most of the vibratory densification of sand takes place very rapidly (2 minutes for the ideally graded sand and 4 minutes for the uniform sand). The time rate of densification is non-uniform and decreases with increasing time. There is an optimum acceleration at which a maximum change in relative density is achieved, after which a further increase in acceleration causes a decrease in change in relative density due to the effect of a greater amount of energy when higher accelerations than an optimum value are applied. The optimum acceleration varies with grain size distribution. It was 2 g for the ideally graded sand and 3 g for the uniform sand tested in the present study. Thus, the optimum acceleration must be determined for a given type of soil. Acceleration and frequency exert a substantially greater influence on densification than displacement amplitude. Within the range of frequencies and acceleration tested, therefore, acceleration and frequency are equally effective and thus they are controlling parameters in the vibratory densification of the ideally graded sand. For uniform sand, however, acceleration plays a more dominant role than frequency and thus is the most significant parameter. Since acceleration, frequency and amplitude are interdependent, the effect of each of these parameters must be examined in light of the two remaining parameters. Surcharge increases the level of densification that can be obtained by vibration to an optimum surcharge value, and then it starts to decrease due to a greater amount of intergranular stress within the sand particles which serves to increase the friction between the particles, requiring supplemental energy for overcoming it