Effect of soil loading/unloading on its acoustic behavior

The aim of the study was to understand the changes of p-wave velocity and acoustic emission (AE) as a result of soil loading/unloading. The soil was dune sand (poorly graded - SP index and three its fractions as follows: 2.36-0.6 mm, 0.6-0.3mm and 0.3-0.075 mm). The dried sand samples were studied using odometer instrument with simultaneous measurement of load and stain level. The stress range was 0-15 kPa.

Each sample was loaded and unloaded while measuring primary wave speed and AE activity (number of hits) at each loading/unloading stage. The resonance frequency of ultrasonic sensors was 75 kHz, while measurement of AE was restricted by digital filtering in the range 300-500 kHz to avoid the effect of ultrasonic wave excitation on the AE signal.

The experimental results show that the increase of stress level causes the rise of p-wave speed in the range of 514-630, 490-624, 521-637, 476-563 m/s for natural dune sand of 2.36-0.6, 0.6-0.3, 0.3-0.075 mm fractions, respectively. Note that the coarser the sand particle size, the more intense the non-linear response to sand unloading. Analysis of the wave velocity vs. the change in deformation shows a gradual increase in the p-wave velocity value with increasing deformation, as well as a sharp decrease with relaxation of deformation (unloading branch).

The analysis of AE behavior indicates gradual increase of the AE activity with increase of stress and strain levels for all studied samples, while a minor AE activity during samples unloading is also observed.

Finally, one can see the difference in the behavior of the two elastic parameters. In the stress increase branch: gradual increase in p-wave speed and sharp excitation of AE activity. In the stress relaxation branch: abrupt decrease in p-wave speed and minor excitation of AE signals.